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Influence of Tray Rigidity and Material Thickness on Accuracy of Polyvinyl Siloxane Impressions

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INFLUENCE OF TRAY RIGIDITY AND MATERIAL THICKNESS ON ACCURACY OF POLYVINYL SILOXANE IMPRESSIONS By ALEX HOYOS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Alex Hoyos

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To my parents and my wife, who are my inspiration

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iv ACKNOWLEDGMENTS I would like to expres s thanks to Dr. Karl-Johan Sder holm, my thesis advisor, for his support and guidance during my graduate thes is research. He opened my eyes with his thoughts and his perspective about different topics in life. It has been a pleasure and an honor to work with him. I also want to thank my other committee members, Dr. Buddy Clark and Dr. Glenn E. Turner, for their time a nd disposition to crystallize this project. Special thanks go to Dr. Jaime Rueda for hi s constant motivation and moral support. Thanks go to Dr. Lucius Battle and Dr. E dgar O’Neill for accompanying and supporting me during this process. Thanks go to all th e other people in Colombia and here in the United States that have participated in my e ducation and have assisted me in pursuing my goals. I am also grateful for the financ ial support provided by the Department of Prosthodontics, University of Florida. Thanks go to Diana Mucci and to Dentsply/Caulk company for the donation of the materials used in this study. Thanks to Mr. Pete Michel and other people from the Bi oengineering Department for their participation in the construction of the mechanical devices used in the study. In addition, I can never thank my parents enough for their continuous suppor t and comprehension. Last, thanks go to my loved Paula. Her love has been the fuel that keeps my engine running.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... ..x CHAPTER 1 INTRODUCTION........................................................................................................1 2 LITERATURE REVIEW.............................................................................................3 Custom vs. Stock Trays................................................................................................4 Tray Material and Dimensional Stability.....................................................................9 Impression Techniques...............................................................................................15 Impression Technique Studies....................................................................................18 Laboratory Models Used to Test Impression Materials Accuracy.............................21 Master Model Material and Abutment Replica Design..............................................22 Methods to Seat the Tray............................................................................................24 Thermal Changes........................................................................................................33 Type of Measurements...............................................................................................34 3 MATERIALS AND METHODS...............................................................................39 Impression Groups......................................................................................................39 Impression Materials..................................................................................................40 Master Model..............................................................................................................40 Impression Making Device.........................................................................................46 Impression Procedures Seque nce and Standardization...............................................47 Measurements.............................................................................................................51 Statistical Evaluation..................................................................................................52 4 RESULTS AND DISCUSSION.................................................................................57 5 SUMMARY AND CONCLUSIONS.........................................................................75

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vi LIST OF REFERENCES...................................................................................................76 BIOGRAPHICAL SKETCH.............................................................................................81

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vii LIST OF TABLES Table page 2-1 Variables related to some of the studies that have used abutment replicas to test impression material accuracy...................................................................................25 2-2 Variables related to some of the studies that have used abutment replicas to test impression material accuracy (cont)........................................................................27 2-3 Variables related to some of the studies that have used abutment replicas to test impression materi als accuracy.................................................................................29 2-4 Variables related to some of the studies that have used abutment replicas to test impression materials accuracy.................................................................................31 2-5 Coefficient of thermal expansion of so me of the materials used in the master models (Inlay waxes listed fo r comparison purposes only).....................................35 3-1 Tray rigidity and thickness c ontrol for the different study groups...........................45 4-1 Mean and standard deviation of ten rounds of measuremen ts performed on the eleven marks of the Master model..........................................................................63 4.2 Measurements from ten impressions taken with a plastic tray and the heavy/light bodied technique (Group 1)......................................................................................63 4-3 Measurements from ten impressions take n with a plastic tray and the putty/light bodied without spacer technique (Group 2).............................................................64 4-4 Measurements from ten impressions take n with a plastic tray and the putty/light bodied with spacer technique (Group 3)..................................................................64 4-5 Measurements from ten impressions taken with a metal tray and the heavy/light bodied technique (Group 4)......................................................................................65 4-6 Measurements from ten impressions take n with a metal tray and the putty/light bodied without spacer technique (Group 5).............................................................65 4-7 Measurements from ten impressions take n with a metal tray and the putty/light bodied with spacer technique (Group 6)..................................................................66

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viii LIST OF FIGURES Figure page 3-1 Plastic stock tray used for th e study (Disposable Impression trays.........................41 3-2 Metal stock tray used in the study............................................................................42 3-3 Occlusal view of the master model. Note the stainless steel rods between abutment replicas......................................................................................................42 3-4 Frontal view of the master model.............................................................................43 3-5 Buccal view of nine of the eleven marks on the master model with their corresponding numbers............................................................................................43 3-6 Lingual view of the marks on the ma ster model with their corresponding numbers....................................................................................................................44 3-7 Impression materials used for the st udy. Aquasil Ultra heavy and light bodied and impression gun (Dentsply/Caulk Milf ord, DE), Exaflex putty (GC America Inc, Alsip, Il), and V.P.S. Tray Adhesive (Kerr, Romulus, MI)..............................44 3-8 Master model attached to an aluminum plate. Note the three stainless steel guiding pins with the thr ee plastic vertical stops.....................................................47 3-9 Metal tray secured to a second plate by a screw. Note the three holes that match the three guiding pin.................................................................................................48 3-10 Plastic tray secured to a second plate by a screw. Note the three holes that match the three guiding pins. There is one pl ate design for plastic and another for metal.........................................................................................................................4 8 3-11 Lateral view of the two plates assemb led previous to an impression procedure with the plastic tray..................................................................................................49 3-12 Lateral view of the two plates assemb led previous to an impression procedure with the metal tray. Plastic stops for the metal tray are different in length from the ones for the plastic tray......................................................................................49 3-13 Lateral view of measuring microscope....................................................................54

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ix 3-14 Close up views of measuring microscope with master model in position for measurement and measuring devices on the instrument..........................................55 3-15 Close up views of measuring microsc ope with master model in position for measurement and measuring devices on the instrument..........................................55 3-16 Lateral view of the impression devi ce with the 3 pounds weight on top while taking one of the third group impressions................................................................56 4-1 Difference between master model and impression groups in distance 1.................66 4-2 Difference between master model and impression groups in distance 2.................67 4-3 Difference between master model and impression groups in distance 3.................67 4-4 Difference between master model and impression groups in distance 4.................68 4-5 Difference between master model and impression groups in distance 5.................68 4-6 Difference between master model and impression groups in distance 7.................69 4-7 Difference between master model and impression groups in distance 8.................69 4-8 Difference between master model a nd impression groups in distance 9.................70 4-9 Difference between master model and impression groups in distance 10...............70 4-10 Difference between master model a nd impression groups in distance 11...............71 4-11 All distances mean diff erence value for group 1 (PHL) in comparison to master model........................................................................................................................71 4-12 All distances mean diff erence value for group 2 (PPL) in comparison to master model........................................................................................................................72 4-13 All distances mean diff erence value for group 3 (PSP) in comparison to master model........................................................................................................................72 4-14 All distances mean diff erence value for group 4 (MHL) in comparison to master model........................................................................................................................73 4-15 All distances mean diff erence value for group 5 (MPL) in comparison to master model........................................................................................................................73 4-16 All distances mean diff erence value for group 6 (MSP) in comparison to master model........................................................................................................................74

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x Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INFLUENCE OF TRAY RIGIDITY AND MATERIAL THICKNESS ON ACCURACY OF POLYVINYL SILOXANE IMPRESSIONS By Alex Hoyos May 2006 Chair: Karl-Johan Sderholm Major Department: Prosthodontics The objective of this study was to determine how tray rigidity and impression technique affect the accuracy of impressions made with an addition polymerizing silicone material. Metallic rim-lock trays and dispos able plastic trays were used in combination with three different impression techniques. The three techniques consisted of 1) heavy/light bodied materials in a one-step impression (HL), 2) putty impression without spacer and light body impression made in two steps (PL), 3) putty impression with 2 mm space and light body impression made in tw o steps (SP). Ten impressions of each combination technique/tray were made of a master model. The master model included two steel abutments (44 and 47) and a steel rod placed at ridge level between the two abutments. Five marks had been placed on each steel abutment. One mark placed on the steel rod in between the two a butments served as a reference point. By use of a universal measuring microscope, the x, y, z-coordinates were recorded for each of the 11 marks on the master model and the impressions. The di stances between the different marks and the

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xi reference point were calculated and compared with the master model. Using a t-test and pair wise comparisons, signi ficant differences (p<0.05) we re found between 4 of the investigated groups and the master model. A ll techniques (PL, SP and HL) used with the plastic trays had distances that were significan tly different from the master model, while for the metal trays it was only the HL techni que that resulted in a distance that was significantly shorter th an the matching distance on the master model. In conclusion, plastic trays produced less accurate impressions than metal trays. When metal trays were used, putty based impressions were dime nsionally better than heavy/light body impressions. Consequently, tray rigidity and control of the bulk of the impression material improved impression reliability.

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1 CHAPTER 1 INTRODUCTION During the past few years, there has been a tendency to use plastic stock trays rather than custom made trays to make impr essions for crowns, bridges and implants. In addition, these plastic trays have been used in combination with different materials of different viscosities. Tray rigidity and material thickness are among many variables that have been described as im portant factors in making accurate impressions (13, 14, and 15). It has been recognized that the tray should be rigi d enough to stand the forces generated during the impression procedures without distortion ( 50). Regarding the material thickness, it has been said that a thin even layer of 2 mm impression material produces the most accurate impressions (43) Two widely used impression techniques consider or completely ignore these two variab les. One uses a metal tray with putty and light bodied material in a two step technique (our group 6) (3 4), the second uses plastic stock tray with heavy and light bodied material in a one step technique (our group 1)(8). In this study, we decided to test the hypothe sis that higher tray ri gidity and better bulk impression material control (2 mm space) would produce more accurate impressions by performing intra and inter abutment measurements. We hypothesized that by using more ri gid trays and bette r controlling the impression material thickness, the precision of an impression would improve. To test our hypothesis, a master model simulating replicas of two abutments of a four unit posterior bridge was used. Two tray types and three di fferent impression material thicknesses were evaluated. Only polyvinyl siloxane impre ssion material was used. Ten impressions per

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2 group were made, and the distances between fixed points on the master model and the impressions were measured. Matching distances between the different groups were then compared by using t-test and pair wise co mparisons to determine whether significant differences (p<0.05) existed in distortion among the two tray types, the three impression thicknesses, or whether there were any inte ractions between tray types and impression thicknesses. The null hypothesis we tested was that neither tray ri gidity or impression thickness had any significant impact on the dimensional reproduction ability of a PVS impression material

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3 CHAPTER 2 LITERATURE REVIEW Impression materials are used to register or reproduce the forms and relationships of teeth and oral tissues. Their purpose is to create an exact replica of the oral tissues and then pour the impression w ith a model material such as stone, on which precise reconstructions can be made in a labor atory environment (11). Today, the most commonly used impression materials for precise reproductions are the non-aqueous elastic impression materials. These material s can be divided into two main groups – condensation and addition polymerized elas tomeric impression materials. Of these materials, the addition polymerized elastomers are more stable over time, and are therefore now the materials that are most wi dely used in fixed prosthodontics (43). The addition polymerized elastomers include a ddition silicone (PVS ) and polyether, two materials well known for their accuracy a nd dimensional stability (24, 15). Many factors affect the accuracy of imp ression materials. These factors include; impression material selection (56), impression material manipulation (8), impression tray design (46), impression retention to impressi on tray surface (48), impression material thickness (15), tray deformation potential (3), impression technique (the introduction of endogenous tension), impression removal (42) thermal changes af ter removal (23), storage condition after remova l, and material used for making the dies and its compatibility with the impression material. By considering all these variables and how they interact with each others, it beco mes clear that impression accuracy is a

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4 multifactorial phenomenon. These variables interact sometimes in a positive way and sometimes in a negative way making it quite complex to predict the final outcome. The trend to use stock trays rather than custom made trays have continuously increased during the past 20 years (7). The r eason behind that change relates to improved dimensional stability of polyether (introdu ced during the mid 60s) and PVS (introduced in the mid 70s) impression materials. Stock tr ays reduce the number of steps needed to make a final impression. In addition, between pl astic and metallic stock trays, there is a strong preference for the plastic ones, because they are disposable (don’t need to be returned from the laboratory) and don’t need to be cleaned and disi nfected after use. An important question, though, is whether there ar e any significant diffe rences in precision between impressions made with custom tray s or stock trays. Ba sed on the popularity of the stock trays, it does not seem that the transition to these trays from individual trays have made any clinical significance. Two important variables must be considered regarding tray selection. First, the space allo wed between the tray and the prepared teeth and second, the tray rigidity. The tray should be rigid enough to resi st forces developed during impression procedures without permanent deformation. Because of these considerations, as well as the design of this study, this literature review will focus on 4 main topics: 1. 1. Custom vs. stock trays 2. 2. Tray material and dimensional stability 3. 3. Impression techniques 4. 4. Laboratory models used to test impression materials accuracy Custom vs. Stock Trays Custom trays are trays specially designed for an individual case. They provide a uniform thickness of the impression material and often reduce the amount of impression

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5 material needed. Stock trays are the trays that come in predetermined sizes. They may be either in metal or in plastic. The term “stock tray “ is used in the li terature independent of whether the trays are made of metal or plastic. However, it is important to differentiate between metal and plastic stock trays, because the accuracy of metal and plastic stock trays can differ because of differences in th eir rigidity. Also, the use of stock trays, independent of tray material, results in an uneven bulk thickness of the impression material, which increases the risk for distor tion (18, 14, and 15). Therefore, custom trays are recommended to create a thin, even space between the tray and the teeth to control the impression material thickness. When such trays are made, a maximum space of 2 mm is often recommended (14). In studies by Reis bick and Matyas in 1975 (37), thicknesses in the range of 2 to 4 mm were recommended, while Asgar in 1971 suggested 3 to 4 mm (1). According to Nogawa in 1968, differen ces in thicknesses ranging between 1 and 5 did not produce significant differences, at least as long as th e impression is poured immediately after it has been made (33). Eames et al (14) studied the effects of 2, 4, and 6 mm space on the accuracy of 9 elastomeric materials and found that 2 mm space produced the most accurate impressions. De Araujo and Jorgensen (12) in 1985 studied impression material thicknesses and undercut si zes and how they affected PVS material and found that better accuracy was generated for lower impression th ickness values (1 mm) and smaller undercuts (0.5 mm). In thei r study it was clear that 1 mm PVS thickness was better than 2 or 3 mm thicknesses. Apparently, de Araujo et al findings suggest that one should use thinner rather than thicker laye rs of PVS impression materials. A potential weakness, though, with such a conclusion is that the risk for permanent deformation increases during impression removal if th e impression thickness is too thin. Tjan et al in

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6 1992 (47) used custom trays with impression material spaces of 2, 4 and 6 mm, to study changes in the precision of PVS impressions of abutment replicas for a 3 unit bridge. They found good accuracy in intra abutment dimensions for the four PVS materials used, but distortion in the inter abutment distance existed for two of the impressions poured at different times after impression making. No explanation for that finding was discussed in that article. A possible explanation behind th e somewhat conflicting results seen in the literature in relation to material thickness control; may simply be related to the magnitude of recorded undercuts. During impression removal, larger undercuts will increase the deformation of the impression mate rial. According to Hooke’s Law, stress is defined as modulus of elasticity times strain, where strain is defined as change in length divided by original length. Thus, if the materi al thickness is thin and the undercut big, extensive strain will be induced which ma y raise the stress level to a level where permanent deformation starts occurring. Acco rdingly, larger undercuts require thicker impression thicknesses. However, as that th ickness increases, material shrinkage starts playing a more significant role. Therefore, depending on the size of the undercuts, different errors (plastic deformation vs. material shrinkage) will dominate the final outcome. Consequently, the conflicting result s are likely caused by different experimental design approaches. Other studies have shown that tray sp ace does not have any effect on the intra abutment dimensional accuracy of monophasi c polyvinyl siloxane impressions, however, the inter abutment dimensional accuracy is a ffected (47). Because the dimensions of the abutment replicas are small in comparison to the whole master model, differences in distances are much smaller on the abutment replica than on the full arch impressions. As

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7 a consequence, it is much more difficult to id entify significant differences on an abutment than between abutments that are far apart, a nd the rather small intra abutment differences may not be big enough to have any clinical si gnificance. On the other hand, replications of big spans between abutments may result in impressions with small dimensional deviations. Other factors that add to such errors include va riations in tray dimensions, expansion/contraction of model material, a nd last, but probably not least important, differences in operator techniques. All th ese factors may become noticeable during measurements. Obviously, such measurable dimensional changes would have bigger impact clinically than the smaller intra abutme nt errors. A factor affecting the accuracy of the inter abutment distances is the flexibility of the tray and its dimensional change when it is removed from the master model as expressed by Gordon et al in 1990 (19). Not all studies confirm the importance of impression material thickness and tray rigidity. Valderhaug and Floystrand in 1984 (4 9) compared the accuracy of c-silicone and polyether using custom a nd metallic stock trays to cr eate impressions of master models. They did not find significant differen ces in accuracy of impressions made with custom trays or metal stock trays with the tw o materials. Questions arise from that study because it is well known from laboratory studie s that polyether is much more accurate and stable over time than c-silicone and the fact that the measurements did not show any difference between the two materials raises some doubt regarding the validity of that study. The times at which the measurements were made on the impressions were 0 h, 1 h, and 24 h after impression making, and because it is known that a low viscous polyether shrinks about 0.24% during a 24 h period and a low viscous c-silicon shrinks about 0.6% (11), one would expect to see a difference. In Valderhaug and Fl oystrand’s study (49),

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8 the authors did not find any si gnificant differences in inte r abutment distances using either tray, even thought ampl e amount of material was used (2 to 9 mm). Despite Valderhaug and Floystrand’s (49) claim that 2-9 mm thick impressi on materials layer did not have any significant impact, it is generally believed that elastomeric impressions are more accurate in uniform, thin layers of 2 to 3 mm thickness (41). A possible explanation for Valderhaug and Floystrand’s (49) claim may have been given by Bomberg et al in 1985 (2), who reported that the mean differe nce in material thickness between custom trays and stock trays is less than 1 mm, and th at variations in unifo rm impression material thickness exist in both custom and stock trays. The precision of the instruments used to measure changes could be another reason why Valderhaug and Floystrand (49) could not find any differences. Rueda in 1991 (38) did not find any clinical significant differences in impressions made from custom or plasti c stock trays. Again, the biggest distortion (>50 m) was found in the distance between left and right molars. No comments regarding custom tray rigidity were presente d in that study. A possi ble explanation could be that both custom and plastic stoc k trays had the same rigidity. Dixon et al in 1994, (13) advocated 3 to 4 mm of custom tray thickness to produce enough rigidity to stand impression forces. One factor that seems important in mo st studies where fixed partial denture abutments have been replicated is that custom trays should control the impression material bulkiness around abutments and pontic areas. Failing to do so leaves a great amount of material suitable for setting contraction and th ermal changes. If a uniform thickness of material is required with a custom tray, attention should be paid to the pontic areas so these regions receive 2 mm of impressi on material as well. This is important to

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9 consider, because this might be the reason why some studies that compare stock and custom trays impression accuracy cannot find any difference between them. Inter abutment accuracy becomes even more important when implants are used as abutments. Small abutment discrepancies are not as critical when one is working with natural dentitions, because the periodontal ligament could help to compensate for smaller impression errors. Even the die spacer used to build the fixed partial dentures could help. With implants the tolerance is lower. Burns et al in 2003 (5) evaluated the accuracy of implant impressions in vitro using rigid custom open trays and polycarbonate stock trays and found significant differences among groups in the vertical fit of the casts. Custom trays performed better. Vigolo et al found in 2004 (51) that joining the impression copings with Duralay resin for implant level impressions is more accurate than not doing so. Even though they used custom trays for the experiments, they probably were not rigid enough to avoid the distortion pr oduced during the impression procedures. The cross arch accuracy achievable with customized trays is highly desirable to ca pture multiple implant positions that will be joined by a metal superstructure. Passivity, less solder joints and/or remakes would be the immediate benefit of usi ng more precise impre ssion techniques. It can be concluded from this review th at stock trays probabl y provide sufficient accuracy for single tooth restorations, particul arly if polyvinyl siloxane or polyether are used. However, if one piece fixed partial de nture of three or more units are to be fabricated on the cast, the inter preparation and cross arch discrepancies from stock tray impressions could have a significant impact on the fit of the restoration (43, 6). Tray Material and Dimensional Stability Other important variables in impression accu racy are related to the rigidity and dimensional stability of the tray. The tray s hould be stiff and stab le enough so it does not

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10 deform during insertion and retrieval of th e tray-impression complex from the mouth. Any tray deformation, particularly elastic de formation, will result in distortion errors. Metal trays are more rigid than plastic tray s. Among plastic trays there are different levels of rigidity. Valderhaug and Floys trand (49) found no differences between impressions made with metal stock trays and rigid resin custom trays. This finding is somewhat surprising if we c onsider the differences in modulus of elasticity between metallic tray materials and plastic tray ma terials where the modulus of metal tray materials is around 50 times higher than that of plastic trays. It is possible that by using thick custom trays (4 mm), as the ones advocated by Dixon et al in 1994 (13), the rigidity of a plastic tray can be enough to withstand the forces involved in impression seating and removal, and therefore producing similar results as with metal trays. Millstein et al in 1998 (30) studied casts made from the use of three different stock trays and a custom tray. Two plastic stock trays, one meta l stock tray, and one custom tray were used. Casts produced from the custom tray were more precise and significantly different from the ones produced with the othe r two trays. Metal stock trays were more accurate than plastic trays. Tjan et al in 1981 (48) published a paper that emphasizes the importance of rigid trays for elastomeric impr essions. They reported a research project in which crowns were constructed on 15 working casts made from impressions of a full crown preparation on a typodont (plastic replica of a dental arch). Impressions were made in rigid stock trays, disposab le trays and reinforced dispos able trays. Not one of the crowns made on the cast dies produced from impressions made with disposable trays fit the master die. All ten of those made on the models from impressions made in rigid or reinforced trays were assessed as satisfactory.

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11 Gordon et al in 1990 (19) compared the dimensions of working casts made from impressions made in either custom trays (usi ng two different tray ma terials) or plastic stock trays. They found that th e plastic stock tray which was much less rigid, consistently produced casts with greater dimensional change than the two custom tray systems, and concluded that the use of plas tic stock trays should be limite d to the reprod uction of casts where great accuracy is not n eeded. Mitchell and Dammele in 1970 (29) investigated the distortion caused by various tray types with reversible and irreversible hydrocolloids, polysulfides and silicones. They found that tray form had a significant impact on the amount of impression distortion by all mate rials tested, most seriously with the polysulfide and silicone elas tomers and least with reve rsible hydrocolloid. Even irreversible hydrocolloid exhi bited distortion when used in non rigid trays. Carrotte et al (9) studied the influence of the impressi on tray on the accuracy of putty wash impressions. For that purpose an ivorine m odel with 3 crown preparations, one for a crown and two for a FPD was used. Four stock trays were tested (3 plastic and 1 metal) with two putty viscosities (heavy and soft). Master model casts were seated on duplicate models and discrepancies were measured. The metal tray and the most rigid plastic tray produced the best fitting. In relati on to metal stock trays, Heartwell et al in 1972 showed no difference in the dimensions of casts poured from irreversible hydrocolloid taken in perforated or non perforated metal rim-lock trays (20). Gordon et al in 1990 (19) found that the inter preparation distance (simulation of edentulous areas for a FPD) in casts made from polysulfide, po lyvinyl siloxane, and polyet her impressions was 45 100 m greater when stock trays were used instead of custom acrylic resin or thermoplastic trays.

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12 They also found 260 m cross arch discrepancies, wh ich they attributed to stock tray flexibility (19). Similar results in in ter abutment distance distortion have been reported (43) for hydrocolloids, C-silicones, polysulfide, and polyeth er. Stauffer in his study concluded that none of th e tested materials was capab le of producing a complete arch fixed partial denture on a cast poured from one single master impression (43). However PVS was not tested in that study. Wasell et al (52) and Saunders et al (39) in two separate studies in 1991 showed that reinforcement of stock trays improved the quality of the impression, but did not elim inate completely the distortion from tray deformation. The forces generated during impr ession procedures are such that even metal trays show changes in dimension intra arch and cross arch. Cho et al in 2004 studied cross section and cross arch changes of six disposable plastic trays and compared them with a metal stock tray (10). Impressions of a plastic model were done using a putty material. Distortion in both across arch and cross section directions was found for the six plastic trays. Metal stock trays showed significan tly less change than plastic trays (10). To reduce inaccuracies in the impressi on the aforementioned phenomenon should be eliminated or minimized as much as possible. Dimensional changes of the tray can also occur due to the custom tray material behavior. The tray must remain dimensiona lly stable over time (3). Because of that concern, the dimensional stability of auto pol ymerizing acrylic resi n tray materials has been the subject of many studies. These st udies have resulted in recommendations suggesting that the trays made of auto polymer izing acrylic resin should be made at least 20 to 24 h in advance to avoid major dimens ional changes in the material (37). Other studies suggest times from 40 min to 9 h (44, 35). It is well demonstrated on Pagniano’s

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13 study that auto polymerizing resins will k eep changing even after 24 h (35). Some authors have different recommendations under th e assumption that after the first hour of the tray resin setting, most of the dimensiona l changes have occurred. They believe that the impression can be taken but it should be poured immediately because according to them, the stone would counteract the dimensi onal changes ongoing in the tray. No other study could be found supporting this idea. In the study by Pagniano et al in 1982 (35) the linear dimensional change of acrylic resins us ed in the fabrication of custom trays was studied. Four commercial materials were tested for linear changes during 24 h. The results of this study showed that all the ma terials changed for a period up to 24 h. The most rapid linear dimensional shrinkage of all th e materials occurred in the first hour after mixing, varying from a mean change of 0.08% to 0.33%. During the first hour 50 % of the total change that occurred for th e 24 hour period had occurred in each of the materials. Pagniano et al (35) recommended to use the tray for the impression after 9 h of setting because at that time most of the total shrinkage had occu rred and once made, to pour it immediately to avoid further cha nges (in resin tray) th at would affect its accuracy. Studies on visible light curing (VLC) resins have indicated that these tray materials largely eliminate the disadvantages associated with auto polymerizing resins by improving stiffness, form, and volume stability and by reducing sensit ivity to moisture. The reason for this is basically that the VLC material is similar to the light-cured composites that instead of using an inorganic fi ller it uses an organic filler (34). The filler consists of acrylic resin beads of varying si zes that become part of an interpenetrating polymer network structure when cured. The VL C material can be us ed immediately after fabrication (34). It is important to consid er this difference among both materials because

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14 if they are used at the appr opriate time, the mechanical pr operties are quite similar and good results can be accomplished with either one. Breeding et al (3) studied the mechanical properties of one polymethyl meth acrylate, one light pol ymerizing and three thermoplastic resins used as custom tray materials. The polymethyl methacrylate resin exhibited measured mechanical property values that were significan tly higher than those of the thermoplastic resins tested. Small di fferences in mechanical property values between polymethyl methacrylate resin and li ght polymerizing resin (Triad Tru Tray) were found. Even though they were statistica lly different they were too small to be clinically important. The different thermoplas tic resins had different mechanical property values among them. The importance of the rigidity and the mech anical properties in general of the trays was highlighted in an article by Dixon et al published in 1994 (13) when they determined the amount of force needed to re move a tray with impression material from the mandibular arch. They found that the fo rce used was higher when three evenly distributed point forces were used on the tray (514 N) than when the force was placed at a single anterior poin t (224 N). The amount of force needed when using three points seems to be too much clinically. No ot her similar studies were found to compare their results. Those findings showed that th e mechanical properties of the materials such as polymethyl methacrylate resin and lig ht polymerizing resins in an appropriate thickness (2.5 to 3 mm) were good enough to resist permanent deformation when subjected to removal forces created under bot h situations. The aut hors concluded that thermoplastic resins need to be approxima tely 4 mm thick to avoid deformation during tray removal (31).

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15 In the light of today’s scie ntific knowledge it is rec ognized that the dimensional stability and the rigidity are two important characteristics to consider when fabricating customized trays. Auto polymerizing polymet hyl methacrylate resins are stable after 24 hours of fabrication and rigid enough at 2.5 to 3 mm thickness. VLC urethane dimethacrylate resins are stab le enough immediately after light curing and require the same minimum thickness to assure rigidity. Wh en a stock tray is to be used, metal trays, either perforated or non perforated, produce ac curate impressions due to their ability to resist deformation during the impression procedure. Impression Techniques Many impression techniques are used but very few are well understood and supported by research. Wstman in 1997 (56) described 9 impre ssion techniques that pretty much cover all the techniques availabl e. Below are descriptions of the 9 impression techniques. Correction Impression : Use of Aor C-silicone in combination with the correction impression technique is the most commonly used techniqu e in West Germany. After the teeth have been prepared, an impression is made with a knead paste. A perforated metal stock tray is recommende d. Flexible plastic trays are extensively deformed during impression making by the hi gh viscosity paste. As a result, an uncontrollable distortion of the entire impression occurs. In dividual trays and stock trays made of plastic are useless for this tec hnique. The success of th e correction impression depends on the first impression. The risk is de formation of the first impression when the second is made with the wash material. There is also the risk that the wash does not bond properly to the first impression. The dies produ ced with this technique are in general too

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16 small but the author suggests that th e compression impression technique produces clinically useful impressions. Double Impression : This technique is a variation of the compression impression technique. In this case, the first impression is made before the teeth are prepared. The second impression is made after the teeth have been prepared and that impression is made with a thin flowing impression material that now fills the space that has been removed during preparation. With such an approach, much lower compression is introduced during final impression making. It has also been sugge sted to make the first impression at least one day in advance to let that material sh rink completely before the final impression is made. By storing the first impression for some time before the second impression is made, the rigidity of that material is incr eased. As a result, impr ession changes decrease. (56) Segment Impressions : For this technique the firs t impression is divided into several segments. It is also a two step techni que. Like the first two techniques, this is a modification of them. After the first impression is made, escape furrows are cut to let the material escape lingually or buccally. With this technique it is al so possible to reduce stresses that are easily introduced in the correction technique. Double Mix Technique : With this technique high a nd low viscosity materials are used simultaneously. The low viscosity is pl aced on the teeth and the high viscosity material in the tray. In comparison to the correction technique, the double mix technique should have the potential of being more prec ise because it should not induce the same amount of impression and tray deformation as the correction techni que. The key problem with this technique is instead that a dela y in the complex process may result in some

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17 material setting that in turn can induce re sidual stresses. Such stresses, when relaxed, result in a somewhat smaller luminum. Besi des, this technique tends to be unreliable when it comes to forcing the material down into subgingival regions. Sandwich Technique : This technique is a variat ion of the double mix technique. However, with the sandwich technique, the low viscosity impression material is not injected with a syringe tip ar ound the prepared margins, just placed on the preparations with a mixing instrument (can be described as the “butter and bread” technique). This technique does not reproduce well subgingival regions. Hydrocolloid Impression : These impressions can be described as being in principle similar to the double mix impressi on technique. Instead of a silicone or a polyether, hydrocolloid agar is used. The key drawback with this technique is the need for special equipment and more strict prepara tion. To plasticize the material a special heater is used. The impressions are taken in specially designed water cooled trays. Advantages of the technique in clude the comfortable use, th e low material cost and the consistent outcomes. A significant limita tion, though, is the problem to record subgingival margins. Single Phase Impression : With this technique a single material viscosity is used, and some of the material is injected around the teeth. The use of st ock trays with this technique results in poor compression. This technique can be improved by use of a custom made tray. Polyether is the most common ly used material with this technique. By use of the single phase technique combined with a custom made tray and an A-silicone or a polyether impression material, it is possi ble to produce very precise impressions.

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18 Shrinkage and distortion effects caused by e ndogen stress play a minimal role with the single phase impression technique. Ring Impression (Copper bands): Ring impression with a thermoplastic compound is one of the oldest used impressi on methods. Today this technique has been replaced by more modern techniques. When the ring impression technique is used, only one tooth is imprinted in each ring impression. Optical Impressions : Since 1971 attempts have been made to generate numerical information of the teeth. The first successful attempts were introduced by Duret, Rekow, Mormann, and Brandistini (56). One of the systems available (Cer ec) consists of an intraoral camera that records the prepared tooth surface, then the computer converts that information to an x, y, z coordinate system. Then a milling machine produces the restoration from these coordinates. Impression Technique Studies Gelbard et al (17) studied the effect of two im pression materials used with three different techniques in order to address the marginal fit of metal castings. The following methods were used to make the impressions: 1) putty/ wash in one phase with metal tray, 2) copper band relined with auto polymeriz ing acrylic resin and subsequent light body elastomeric impression material, 3) c opper bands with modeling compound. Metal copings were fabricated from casts made out of the impressions, seated on master die models with pressure indicat or paste and then cemented. Master die and coping were cut in half bucco-lingually and measured under the microscope to evaluate the marginal fit. Measurements of the thickness of the cement layer were calculated manually and with a computerized method. The metal castings fit were from 38.3 m to 128.4 m for both the methods of measuring. Gelbard et al (17)

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19 claimed no superiority of any of the impression techniques. Newman et al in 1986 investigated the dimensional stability of various impressi on techniques using different impression materials. Their concern was to de termine if different techniques would cause different degrees of dimensional changes with different impression materials. Six polyvinyl siloxane materials a nd one polyether material were shown to be dimensionally stable, while the two polysulfide materials we re not stable. The si ngle viscosity custom made tray impression technique gave consis tently greater degree of error with both polysulfide and polyvinyl silo xane impression materials, wh ile the putty-wash technique gave consistently more accurate fits with ei ther a one step or a two step technique. One widely used technique is the technique that uses light body in the syringe and heavy or medium body in the tray (8). Johnson and Drennon in 1987 conducted a clinical evaluation of detail reproducti on of elastomeric impression materials. They concluded that the double mix technique produced better detail than did the single mix technique. Heavy consistencies, rather than medium, in combination with a light consistency material resulted in better details. Anothe r technique is the putty-wash technique, which was developed to compensate for the polym erization shrinkage of the condensation silicones attributed to the production of an alcohol byproduc t during polymerization (29). The two step putty/wash techniqu e used a thin layer of wash material that minimized the amount of alcohol byproduct and thereby reta ined the dimensional stability within acceptable limits (37). Although the putty wash technique was originally recommended for problems associated with polymerization shrinkage of th e c-silicones, this tec hnique has also been suggested for a-silicone impression material s (36).Two variations of the putty wash

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20 method are commonly used: the putty/wash one step technique, in which the materials polymerize in one stage and the putty/wash two step technique, in which a putty is first used alone as the initial step and then a final impression is made within the putty material by use of a silicone of lower viscosity (22). Idris et al compared the accuracy of the one step and two step technique using asilicone impression material (22). For the purpose of the study they used three stainless steel replicas of abut ment, one die without undercuts and two with undercuts. Grooves were prepared on the occlusal surfaces for measuring purposes. Impressions were take n using perforated metal trays. All impressions were poured and then measured under the microscope. Inter abutment distances increased for all but one measurement in comparison to the master model for both techniques. Almost all intra abutment distances were smalle r than the ones on the master model. An explanation for these results could be that smaller dies (smaller intra abutment distances) may create bigger inter abutment spaces. The smaller dies are supposedly the result of the hydraulic pressure created while seating the one step or two step techniques with the materials in place. Idris et al consider that the differences found between the two techniques were not of c linical significance. Di fferences of about 32 m for spans 40 mm long are not of clinical signi ficance as long as mobility exists in the periodontal ligament. The authors concluded that neith er technique result ed in dies that deviated sufficiently from the master m odel to cause clinical difficulties (22). Nissan et al studied the putty wash technique s but the two step technique was further divided in two different approaches. The three approaches were: 1) putty/wash in one phase, 2) putty/wash with 2 mm metal coping spacer in two phases, and 3) putty/wash with polyethylene spacer in two phases (32). A metal master model,

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21 containing three complete crowns fixed part ial denture abutment preparations with grooves on the occlusal surfaces, and perforat ed custom trays were used. Impressions were made at room temperature and then poured in stone. All measurements were done using a Toolmaker’s microscope. As in Idris et al study (22), Nissan et al (32) found increases in inter abutment distances (0.009% to 0.1%) and decreases in intra abutment (0.08% to 3%) distances. They found signi ficant differences among the three groups. The second group was the most accurate of the three groups. The authors criticized the one step technique because it reproduced a part of the margins in the putty material, which they claimed had not enough detail reproduction to produce a reliable casting. The question, however, is whethe r it is only putty on the margins or that the light body is so thin that it cannot be seen. Mo st putties on the market cannot reproduce details fine enough to meet the American Dental Association (ADA) specification 19. Laboratory Models Used to Test Impression Materials Accuracy Research into impression accuracy has relied heavily on in vitro tests rather than clinical evaluations. The key reason is simply that clinical studies are difficult to standardize and reproduce. For example, variab les such as tray flexing/recoiling ability and differences in impression te chniques are difficult to eliminate clinically. Also, it is easier to make measurements in the laboratory than in the mouth (53). However, the drawback with most laboratory tests is that such studies do not simulate the true oral conditions very well. Some tests provide realistic tooth morphology, arch form and temperature control but none of them mimi cs soft tissue consistency or the surface characteristics imparted by the oral fluids (5 3). The presence of oral fluids under clinical conditions is important to emphasize, because some of the “best” materials according to

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22 laboratory evaluations are al so the most hydrophobic materials. Consequently, materials that seem to be superior in a laboratory environment may still be inferior in a clinical situation because they cannot reproduce a mo ist dentin surface. Under such conditions a material that may be inferior in laboratory evaluations may still be superior in clinical situations. Being well aware of the laboratory/ clinical conflicts, the following review will target those variables that might influence the results of our study. Master Model Material and Abutment Replica Design Many models have been used by researchers to test impression materials. Stauffer, et al use an aluminum master model machined with four prepared abutment teeth of stainless steel positioned in a maxilla (43). Undercuts were not take n into consideration. All four teeth were cylindrical and parallel to each other and all vertical walls had 5 taper. Evaluations of the resulting casts we re made by comparison with a master fixed partial denture. Metal (stainless steel, ch rome steel, aluminum, and cooper) has been mainly used because it resists wear during la boratory work. Metal is also less susceptible to accidental damage and its coefficient of thermal expansion is small enough to resist master model changes during experimental simulation of oral temperatures (35-37 C). Another important consideration is the availa bility of stainless steel material at the engineering department laborator ies at different universities where similar studies can be conducted. Eames et al used stainless steel dies with 12 taper to evaluate the effect of the bulk of the impression material on the accuracy of impressions (14). Marcinak and Draughn (26) prepared two maxillary central in cisors mounted in an acrylic resin block. The distal surfaces were machined precisely parallel to each other to provide accurate measurements. Valderhautg and Floystrand (49) tested the materials using two models

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23 with stainless steel standardiz ed abutment teeth in the area of the canines and the first molars. The teeth were dr illed with a taper of 10 Two grooves 5 m wide were engraved at right angles in th e center on the occlusal surfaces. De Araujo and Jrgensen (12) used a truncated chromium steel cone die with and 8 mm base diameter and 8 mm in height. The die was undercut apical to the gingival margin. Three rings were used to create three different underc ut heights. Johnson and Crai g (24) tested four rubber impression materials using a stainless steel ma ster model simulating two full fixed partial denture abutment preparations. The prepar ations were 10 mm in height and 10 mm in diameter, and one of them had an undercut with an 8 mm diameter. Lin et al. (25) prepared four abutment teeth from a dentof orm for complete crowns with 1 mm shoulder margins by using a handpiece mounted in a para llel instrument. The height of the canine tooth preparation was 8 mm and the molar was 7 mm. Saunders et al (39) used a dentoform with prepared teeth as a master model to test th e effect of impression tray design and impression technique on the accuracy of the resulting casts. A copper plated master model was used by Wasell and Ibbetson (52) to evaluate the accuracy of polyvinyl siloxane impressions made with standa rd and reinforced stock trays. Tjan et al (47) also used stainless steel dies to create the mast er model for their study. They created a model simulating a three-unit fixed partial denture. Reference lines were inscribed on top and axial surfaces of the abutments to assess the dimensional changes. In conclusion, most of the studies have used stainless steel to create the master models. Others have used natural teeth, plastic teeth, copper plated or chromium steel models. Metal bases as well as metal dies are much more dimensionally stable to thermal changes while simulating oral

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24 environment (35 C to 37 C) than the plastic ones. On the studies reviewed, 5 to 12 taper were used as total occlusal convergen ce for the abutment preparations replicas. Undercuts are another important variable to consider when testing impression materials. Larger undercuts are mo re difficult to imprint precisely. Methods to Seat the Tray Another important variable is the way the tray is seated against the master model time after time. It is necessary to do it the same way every time an impression is made, and therefore a standardized technique is required. To verify the importance of standardization in seating the tray, a simple exercise was performed at the University of Florida, Graduate Prosthodontics clinic with three residents. All of them were asked to make an impression of a resin mandibular m odel using the heavy/light body technique. Using a scale the amount of pressure was m easured while the impression was made. The lowest amount of pressure was 2 pounds and the highest 15 pounds. No studies could be found in relation to this variable but some differences in mate rial behavior are supposed. The use of alignment pins that guide the tray during seating is a common method to achieve this goal during laborat ory experiments (43, 24, 49, 13).

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25 Table 2-1 Variables related to some of the st udies that have used abutment replicas to test impression material accuracy. Marcinak et al.19822 teeth,acrylicprep. parallel0yes Valderhaug et al.1984SS and Al4abutments10yes/no 2 models de Araujo et al.1985Cr steelsingle die5yes 26.5 degree undercut Jhonson et al.1985SS2 dies in a6yes block Lin et al.1988Plastic4abutmentsn/ano Max model Gordon et al.1990SS and Plastic2 abutmentsn/ayes 3 metal inserts Wasell et al.1991Copper plated model3 abutments copiesn/ano Dentoformof a dentoform Saunders et al.1991Plastic2 FPD abutmentsn/ano 1 inlay opposite side Tjan et al.1992SS2 FPD abutmentsn/ano mounted in a plate Hung et al.1992SS2 FPD abutmentsn/ayes mounted in a plate Gelbard et al.1994PlasticIvorine preparedn/ano tooth mounted in a dentoform Idris et al.1995SS3 abutment replicasn/ayes triangular distribution Occlusal grooves Millstein et al. 1998SS4 abutments. 2 anterior 5 no 2 posterior. U shaped bar

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26 Table 2-1. Continued AUTHORYEAR MATERIALABUTMENTSTOCUNDERCUTS Carrote et al.1998Stone dentoform1 full crown and5no Ivorine teeth2 abutments for PFM Nissan et al.20003 SS preparation 3 full crown preps n/ano replicasin small metal block Thongthammachat2002Metal master modelMultiple prepared n/ano et al.teeth. 8 reference holes, 3 reference points. Wadhwani et al.2005Modified dentoformMetal inserts and12 no Metal inserts on 1sta removable die molars and lower incisors

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27 Table 2-2 Variables related to some of the st udies that have used abutment replicas to test impression material accuracy (cont) AUTHORYEARTEMPERATUREIMPRESSION MATERIAL Stauffer et al.197635 C and 12 C2 hydrocolloids, 1c-silicone 2 polysulfides,1polyether Eames et al.1979a37 2 C 3 polysulfides, 2 polyethers 2 c-silicones Eames et al.1979b32 2 C2 polyethers, 8 c-s ilicones 2 a-silicones, 5 polysulfides Marcinak et al.198237 C5 a-silicones Valderhaug et al.198421 C1 polyether, 1c-silicone de Araujo et al.198537 C1 polysulfide, 1a-silicone Jhonson et al.198525 C1 a-silicone,1polysulfide 1 c-silicone,1polyether Lin et al.1988n/a2 polyether, 2 a-silic ones 2 polysilfides, 2 reversible hydrocolloid, 2 irreversible rev -irrev combination Gordon et al.199034 C1polyether,1 a-s ilicone,1polysulfide Wasell et al.199123 1 CPVS H/L and P/L techniques Saunders et al.199137 C PVS P/L 3 techniques: P/L 1 stage, P/L in 2 stages creating space with an scalpel, and P/L creating space with spacer Tjan et al.199222 C 2CFour brands of monophasic PVS Hung et al.1992n/aFive brands of P/L impression material. Tested in one and two stage techniques. Gelbard et al.1994n/a3 groups. 1 P/L in one stage with metal tray.2 Copper band relined with acrylic and wash material. 3 Copper band and impression compound Idris et al.1995Room temperatureP/L one stage and P/L two stages PVS material Millstein et al.1998n/aH/L techni que. 3 stock trays and 1 custom tray

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28 Table 2-2 Continued AUTHORYEARTEMPERATUREIMPRESSION MATERIAL Carrote et al.1998n/aPVS P/L Material. Putty in 2 viscosities normal and soft. One stage technique Nissan et al.2000room temperature1 step P/L, 2 step P/L with 2mm relief" 2 step P/L with polyethylene spacer Thongthammachat200235C 1 CPVS and Polyether et al. Wadhwani et al.200523 CFast set PVS and fast set polyether regular setting polyether as control

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29 Table 2-3 Variables related to some of the st udies that have used abutment replicas to test impression materials accuracy AUTHORYEARTRAYPOURING TIME Stauffer et al.1976custom5 minutes 2mm space Eames et al.1979acustom0 min 2, 4, and 6mm space Eames et al.1979bcustom 2.4mm30min and 24h space Marcinak et al.1982custom, 3mm10m, 30m, 2h space4h, 8h, 24h, 48h,96h,168h Valderhaug et al.1984custom 3mm no stone model thick.2 to 4 mm space. metal stock de Araujo et al.1985custom metal10min 1mm, 2mm, 3mm, 4mm Jhonson et al.1985custom 2mm1h,4h (2 pour) thick.3.75mm24h space Lin et al.1988custom 2mm,NS thick 3mm space Gordon et al.19902 custom types and 1 stock1 h 3mm thick, 2.5 mm space Wasell et al.19912 stock trays (plastic)24 h 2 reinforced stock trays Saunders et al.1991Plastic tray without and with 24h 3 types of reinforcement Tjan et al.1992Perforated small custom trays1h, 24h and 7 days with 2, 4,and 6mm space Hung et al.1992Perforated metal tray1h Gelbard et al.1994Metal tray and copper bandNS Idris et al.1995Metal tray1h Millstein et al.19983 plastic and 1 custom tray 0h

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30 Table 2-3 Continued AUTHORYEARTRAYPOURING TIME Carrote et al.19981 metal and 3 plastic traysNS Suupossed different rigidity for all of them Nissan et al.2000Metal custom tray1h Thongthammachat et al.2002Metal and plastic stock trays30 min, 6h, 24h, 30d 4 types of custom trays 2 to 2.5 mm Wadhwani et al.2005Plastic stock tray1h

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31 Table 2-4 Variables related to some of the st udies that have used abutment replicas to test impression materials accuracy. AUTHORYEARMEASUREMENT METHODFINDINGS Stauffer et al.1976visual inspection of metal frameworkNo interarch accuracy found for any Dial gauge measurementmaterial. Eames et al.1979aimpression at 0min ,24 h. microscope2mm space better accuracy, Cast Other impressions poured at 0 min. lifting from 4 and 6mm impressions Castings/master die tested on dieswere clinically unaceptable Eames et al.1979bimpression at 0min ,24 h. microscopewhen poured at 0min all materials other impressions poured at30 min. where accurate. At 24h a-silicones and 24h. Castings tested on dies least change Marcinak et al.19825 measurements of the die with a greatest change of 0.3% up to a micrometer.putty wash most unstable. Produced smaller dies. Valderhaug et al.1984impressions measured at 0min, 1h c-silcone and polyether behaved 24h.linear measurements made onlysimilar at different time intervals microscope used.no difference between stock and custom trays de Araujo et al.1985traveling microscope on stoneincrease of material thickness from 1 to 4 mm increased distortion. less distortion when increasing the Jhonson et al.1985traveling microscope on stoneLarger diameter of abutment for asilicone and polysulfide. Vertically where smaller in general.siliconese were least affected by double pour. more distortion with undercut Lin et al.1988master casting joined with resinPolyethers the most accurate seated on castscomplete arch impressions measured under microscope Gordon et al.1990Measurescope, measures up to 1Dies height very accurate for all materials width larger for polyether and polysufide. PVS very precise Interprep distance longer for all, the stock trays produced distortion from 45 to 100 Cross arch dimensions with stock trays distorted up to 260 Wasell et al.1991Wild photomicroscope. Measured in mmH/L technique minimum distortion with with 2 decimalsthe different trays. P/L high distortion. Reinforcement improved P/L accuracy but P/L performed better Saunders et al.1991Reflex microscope and computerNone of the trays affected the accuracy of the P/L impressions.One stage technique one distance significantly different. Two stage techniques had also one Results contradict conclusion.

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32 Table 2-4 Continued AUTHORYEARMEASUREMENT METHODFINDINGS Tjan et al.1992Microscopequantitative methodDifferences found among PVS brands Master castingsqualitative methodTray space and pouring time did not affect accuracy for individual preparations Tay space did affect inter preparation distances. Both methods did not correlate Hung et al.1992Microscope4 brands produced larger interabutment Measurment of stone castsdistances.One stage P/L technique and two were equally accurate. All materials produced larger abutment height Gelbard et al.1994Castings cut sections measured with No statisically significant differences SEMamong the techniques Idris et al.1995Toolmaker's microscope and computerIn general all interabutment distances Master model and stone casts increased but one. Intrabutment distances measureddecreased, except for two distances. Significant differences for all interabutment measurements.Not clinically significant Millstein et al. 1998Micrometer and a templateCasts from custom trays were significantly more precise than the ones from stock trays. The more flexible trays presented more distortion. Carrote et al.1998Travelling microscope. 3 master castingsMarginal adaptation for metal or plastic rigid 3 single crownstrays is very similar.From 55 to 72 For more flexible trays from 137 to 207.5 Nissan et al.2000Toolmaker's microsocpe and computerSignificant differences among the groups The second group was the most accurate Thongthammachat2002Measuring microscopeNo differences in accuracy with different et al.trays. More distortion for polyether Silicone stable up to 30 days and 4 pourings. Wadhwani et al.2005Measuring microscope No differences between disinfected and Stone castsnon disinfected conditions Significant differences among the 3 materials in 4 dimensions. Differences are not sigificant clinically

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33 The amount of load used to seat the tr ay should also be controlled. Stauffer et al used 9.8 N on top of the tray platform to seat them into position (43). Wassell and Ibbetson standardized the impression procedur e using guiding pins and an Instrom testing machine to seat the impression tray (52). The majority of the studies do not control this variable when performing the experi ments by hand seating the trays. Thermal Changes Impressions reach a temperature of approximately 33 C after being in the mouth for 5 min (23). On cooling to room temperat ure measurable dimensional changes occur as impression materials have a relatively high coefficient of thermal expansion. A similar rate and amount of impression te mperature rise should be incor porated in laboratory tests. At present there is no agreement over the best way to do this. The standardization organizations use water baths at 32 C while other workers prefer 35 C. Another approach is to use a heat source within the master mode l, but the characteristic s of heat flow into the impression has never been specified (55). Stauffer et al tempered the master model at 35 C before making the impressions to simulate the mouth temperature (42). Eames et al used a 37 + 2 C water bath to let the impressions ha rden in it (14). In a different study the same authors used 32 + 2 C water bath for the same purpose (15). Marcinak and Draughn in 1986 stored the natural teeth model at 37 C and 100% humidity just before the impression was ta ken (26). Johnson and Craig in 1985 allowed the impressions to set at room temperature ra ther that an elevated oven temperature to avoid expansion of the stainless stee l master model with heating (24). To conclude: Temperatures from room temperature (~ 20 C ) to 37 + 2 C have been used to reproduce thermal charact eristics of the oral environment.

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34 Type of Measurements Tests can broadly be divided into tw o main groups depending on whether measurements are made on the impressions themselves or on the resulting casts. In addition, measurements may either be made of the individual dies or the inter die relationships or both (53). Direct Measurements of the Impression Material : The ADA and BSI bodies both employ a scribed block which is used to form a disc of impression material. Measurements are made between the scribed lines on the block and the resulting lines on the impression discs to give an indication of time-dependent dimensional changes. In addition the scribed lines provide a measur e of surface reproduction. Light and medium bodied materials should be able to reproduce a 20 m wide line (53). A logical use of the standard block method is to inscribe the o cclusal surface of a crown preparation with engraved marks like the ones on the standard and measure them within the impression Eames et al (14) used a similar design to test the accuracy of different impression materials at different thickne sses on the impression tray. Measurements of Individual Dies: The measurement of dies poured from impressions is clinically a more realistic method of assessing impr ession accuracy than direct measurement of impressi on shrinkage (53). This is beca use the accuracy of the die will determine the final fit of the restoration. In addition it would be difficult to view microscopically the critical ce rvical part of a preparation within an impression. Type IV die stone is generally chosen because it has a minimal linear setting expansion of 0.1 %. There are some drawbacks involved in this technique. The most important drawback is that it is impossible to know whether it is the cast or the impression that causes the biggest problem. Another drawback is that the detail reproducing ability of die material is

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35 poor in comparison with the impression material causing the precision of the measurements to become questionable. To overcome that problem, different methods have been used. Thus comparisons of die m easurements with master dies have been done in the following three ways: 1. Assessment of how well castings made on each of the poured dies fit the master die. (55) 2. Assessment of how well a single master casti ng made on the master die fits each of the poured dies. (55) 3. Linear measurements with contact or no contact methods (55) Table 2-5 Coefficient of thermal expansion of some of the materials used in the master models (Inlay waxes listed for comparison purposes only) Material Coefficient x 106 /C Inlay waxes Silicone impression material Acrylic Resin Stainless steel Tooth (crown portion) 350-450* 20 76 5.5-17.6* 11.4 *Differences in values for different material composition. The first method is rarely used. It is time consuming because the number of castings necessary to run a study and at the sa me time, castings can be another source of errors that are difficult to control. In this technique the amount of lift of the cast is measured under the microscope. The lift of th e study metal casting may be influenced by the roughness of the die, the cas ting, the casting orientation an d seating pressure. On the other hand, if the stone die is oversized the cast will seat loose on the master die and no lifting will occur, even though some distortion in size wa s caused by the impression. The second method is more widely used. In th is case, if the stone die is undersized the cast will seat completely and no lift will be present. Since only one casting needs to be done it is more popular than the previous method. Eames et al (15) used this method in his study. Careful manipulation and standard ized seating pressure and orientation are

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36 necessary to avoid abrasion of the stone dies and record false lift measurements (closer fitting). The third method is basically about lin ear measurements made using contacting instruments such as caliper s, vernier calipers or dial gauges, or non contacting measurements like the ones obtained with di fferent microscopes such as the traveling microscopes, toolmaker’s microscope or the sophisticated reflex microscope (3D measurements). Non contact measurements ar e preferred over contac t methods because it prevents stone abrasion. The second method, the one that uses one master casting to fit all resulting study dies, assesses the problem from a more clinical stand point including the variable of the casting. It is also more sus ceptible to three-dimensional changes of the stone dies. On the other hand, many sources of errors are inherently present. The third method, that uses different non contact m easuring devices, only analyses linear dimensions of predetermined marks which ma y lead to loose valu able information. The ideal method would be the one that scans dir ectly the impression a nd is accurate enough to digitally compare them three dimensiona lly by volume and superimposition, in other words a combination of methods 2 and 3. Inter Die Relationships: If a fixed partial denture does not fit, the problem may be caused by distortion of th e inter die relationship as well as inaccuracies of the individual dies. Therefore, an important prere quisite of all inter die relationship studies is that the individual dies are not allowed to move within the master model. Different options to measure inte r die relationships ar e described below: 1. Master model and cast master bridge (55) 2. Master model and machined template (55) 3. Contact measurements of in ter die relationships (55)

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37 4. Non Contact measurements of inter die relationship -Two-dimensional (55) -Three-dimensional (55) The first technique is similar to that us ed for individual dies except linked master castings are use to fit the abutments. Stauffer et al (43) used a machined stainless steel master model with four abutments of 5 taper representing two canines and two molars. A surveyor was used to align the linked cas tings to the four dies during seating under standardized force. An average value of lif t of the four castings was used to assess impression accuracy. No distin ction could be made between the amount of lift due to intra abutment inaccuracies and inter abutme nt inaccuracy. In this situation the method would not give a good indication of how well a br idge might fit the abutments clinically. The second technique presents the same i ssue as the first one, and no distinction can be made between intra abutment inaccuracies and inter abutment accuracies. The third approach was used by Stauffer et al (43) who used a L shaped device that had gauges to measure changes along the x and y axes of the stone casts and compare these values with those of the master model. This device recorded differences that were difficult to rationalize clinically. The fourth approach is basically the us e of microscopes and the measuring of distances between marks on the master model and impressions. Two or three dimensional measurements can be made, depending on the instrument and the methodology employed to calculate distances. In conclusion: There are many ways to m easure intra abutment and inter abutment impression accuracy. All of them have adva ntages and disadvantages. The recommended method should be the one accurately assesses the finish line area. Dies poured from

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38 impressions produce a more realistic method of assessing impression accuracy but adds the errors caused by the prope rties of the die material. If measured with non contact devices the detail reproduction of the stones prevents the acc urate reading of established marks. Three dimensional measurements of the dies generate better assessments, particularly when non contact measurements ar e used. Such analysis of 3D digital data was used recently by Brosky et al. to determine the effect of impression tray selection on accuracy of reproduction of a dental arch (4).

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39 CHAPTER 3 MATERIALS AND METHODS This study was designed to determine the importance of material thickness control and tray rigidity on the accuracy of polyvinyl siloxane impressions. To achieve that goal, the following six impression groups we re compared in this study. Impression Groups The six impression groups were distributed as follows: 1. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY, USA) in which heavy and light bodied PV S (Aquasil, Dentsply /Caulk, Milford DE) material were used with the one phase technique. Tray adhesive was applied at least 10 min prior to the im pression (V.P.S. Tray Adhesive. Kerr Romulus, MI). 2. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY, USA) in which putty (Exaflex, GC Amer ica Inc, Alsip, Il) and light bodied PVS (Aquasil, Dentsply /Caulk, Milford DE) material were used with the two phase technique. Compression techni que. Tray adhesive was applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI). 3. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY, USA) in which putty (Exaflex, GC Amer ica Inc Alsip, Il) an d light bodied PVS (Aquasil, Dentsply /Caulk, Milford DE) material were used with the two phase technique. Space was created with a 2 mm pl astic pressure formed template (Great Lakes Company). Tray adhesive was applie d at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI). 4. Metal tray (Rim-lock trays, Dentsply-Cau lk) in which heavy and light bodied PVS (Aquasil, Dentsply /Caulk, Milford DE) material were used with the one phase technique. Tray adhesive was applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI). 5. Metal tray (Rim-lock trays, Dentsply-Cau lk) in which putty (Exaflex, GC America Inc Alsip, Il) and light bodied PVS (A quasil, Dentsply /Caulk, Milford DE) material were used with the two phase/c ompression technique. Tray adhesive was applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI).

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40 6. Metal tray (Rim-lock trays, Dentsply-Cau lk, Milford, DE) in which putty (Exaflex, GC America Inc Alsip, Il) and light b odied PVS (Aquasil, Dentsply /Caulk, Milford DE) material were with the two phase technique. Space was created with a 2 mm plastic pressure formed template (G reat Lakes Company). Tray adhesive was applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI). Impression Materials Three impression materials were used for this study. Forty light bodied consistency cartridges of Aquasil Ultra LV (lot # 050421, Dentsply /Caulk, Milford, DE) were used. Forty heavy bodied consistency cartridge s of Aquasil Ultra Heavy (lot # 050618, Dentsply /Caulk, Milford, DE) were also used. This material is described as a quadrafunctional hydrophilic a ddition reaction silicone. Four standard packages of Exaflex (lot # 0505201, GC America Inc Alsip, Il) putty material Type 0 (very high viscosity) were also used during the experiments. Adhesive was applied on all metal and plas tic trays at least 10 minutes prior to the impression. Four bottles of V.P.S Tray Adhesive (lot # 5-1082, Kerr Corporation, Romulus MI) were used. Master Model A lower arch master model was made of self curing Orthodontic Resin (DentsplyCaulk, Milford, DE) with resin teeth from can ine to canine and four machined stainless steel dies. The four stainless steel simulati ng prepared abutment teeth were embedded in region 37, 34, 44, and 47. In addition, two stainl ess steel reference posts located in the region between 37 and 34, and 44 and 47 were placed at the ridge level. The stainless steel dies were designed to simulate circul ar full crown preparations with shoulders. The molar die preparations had 6 total occlusal convergence (TOC), were 7.0 mm high, had a cervical outer diameter of 9.0 mm and a s houlder width of 1.0 mm. The premolar dies

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41 had the same TOC, height and shoulder width, but an outer diameter of 7 mm. The root portion of the stainless steel dies were 15 mm in length and had a design that well locked them inside the resin to prevent rotation a nd vertical displacement. Approximately 1 mm of the dies root portion was exposed on all four dies. The two referen ce rods consisted of two stainless steel rods, 3 mm in diameter, were inserted at the ridge level, halfway between the stainless steel replicas lo cated on each side. A diamond bur (Maxima Diamonds, 801-012C-FG, Henry Schein Inc, Me lville, NY, USA), attached to a planparallel meter (PFG 100; Ce ndres & Meraux, Bienne, Switzer land), was used to make marks on the occlusal surfaces and the shoulders of the dies as well as on the stainless steel rods between the dies. These marks under the microscope looked like targets on which the center was the refere nce for each particular mark. Figure 3-1. Plastic stock tray used for th e study (Disposable Impression trays, Henry Schein Inc, Melville, NY, USA)

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42 Figure 3-2. Metal stock tray used in the study (Rim-lock tr ays, Dentsply/Caulk, Milford, DE, USA) Figure 3-3. Occlusal view of the master model. Note the stainless steel rods between abutment replicas

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43 Figure 3-4. Frontal view of the master model Figure 3-5. Buccal view of nine of the el even marks on the mast er model with their corresponding numbers (One mark on each occlusal aspect of 44 and 47)

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44 Figure 3-6. Lingual view of the marks on th e master model with their corresponding numbers (occlusal marks on 44 and 47) Figure 3-7. Impression materials used for th e study. Aquasil Ultra heavy and light bodied and impression gun (Dentsply/Caulk Milford, DE), Exaflex putty (GC America Inc, Alsip, Il), and V.P.S. Tray Adhesive (Kerr, Romulus, MI)

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45 Table 3-1. Tray rigidity and thickne ss control for the different study groups Group Tray rigidity Thickness 1 Low High 2 Low Minimum* 3 Low Low* 4 High High 5 High Minimum* 6 High Low* High Thickness: Represents in this study the He avy/Light impression technique in which the material thickness is not cont rolled at all (stock trays used). Minimum Thickness: Represents in this study the Putty/ Light technique in which the material thickness was controlled using a two step technique without creating space for the second impression. Low Thickness: Represents in this study the Pu tty/Spacer/ Light technique in which the material thickness was controlled using a two step t echnique creating a 2 mm even space for the second impression. Low Rigidity : Represents the rigidity of the plastic stock trays High Rigidity: Represents the rigidity of the metal stock trays The eleven marks location of the metal dies for each side (right and left) were the following: Five marks on toot h 47 abutment one distal, one mesial, one buccal, and one lingual on the 1 mm shoulder, while the fifth mark was on the occlusal surface of the die. Five similarly located marks were present on tooth 44. The eleventh mark was on the metal rod in between tooth 44 and tooth 47 and served as a reference point for the other ten points. In this study only tooth 44 and t ooth 47 abutment replicas were used for the experiments.

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46 Impression Making Device Once the master model had been constr ucted according to the specifications mentioned before, the next step was to sta ndardize the way the impr essions were made. For that purpose, the master model was atta ched by two screws to a half inch thick aluminum plate. This plate was seven inches long and five inches width. Three stainless steel pins, each one with a diamet er of three eights of an inch and a height of five inches, were vertically positioned on the aluminum plat e, two in the front and one in the back of the master model. The three vertical pins on the base plat e guided a second plate to which either the metal or the plastic tray was attached. It wa s necessary to build separate plates for the metal trays and plastic trays. These plates were made in aluminum and had the same dimensions as the previously described base plate. These two top plates with their holes sliding along the rods of the base plate allowe d the top plates to slide very precisely onto the master model during the impression pr ocedures. This system controlled the positioning of the impression trays in three dimensions every time an impression was made. Three plastic stops were assembled on th e pins in order to control the seating of the tray against the model. Two different sets of vertical stops were built, one set for the metal trays and the other for the plastic trays. Sixty complete lower arch impressions in PVS material utilizing the t echniques and trays described fo r each group were made of the master model. All impressions we re made at room temperature (23 to 25 C) and kept at room temperature during the 24 h period be fore measuring them under the microscope. The humidity where the impressions were stored was between 54 and 56 %. Ten impressions per group were made.

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47 Impression Procedures Sequence and Standardization The following was the impression sequence followed for impressions made either with plastic or metal trays. An homogeneous th in layer of tray adhesi ve was applied to all the trays at least 10 min before the impressi on was made. Different weights were used during impression making because of differences in viscosities of the material as well as differences between the different impression te chniques. Initially 3 lb. was planned to be used to seat the trays to the standard pos ition. Later on during the preliminary tests it was realized that the putty/light technique needed 28 lb. to reach the standard master model tray position. Different impression procedures needed different pressu res to seat the tray into proper positions. Figure 3-8. Master model attached to an alum inum plate. Note the three stainless steel guiding pins with the three pl astic vertical stops

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48 Figure 3-9. Metal tray secure d to a second plate by a screw. Note the three holes that match the three guiding pin. Figure 3-10. Plastic tray secured to a second plate by a screw. Note the three holes that match the three guiding pins. There is one plate design for plastic and another for metal.

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49 Figure 3-11. Lateral view of the two plat es assembled previous to an impression procedure with the plastic tray Figure 3-12. Lateral view of the two plat es assembled previous to an impression procedure with the metal tray. Plastic stops for the metal tray are different in length from the ones for the plastic tray.

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50 Heavy/Light Bodied Technique – Groups One and Four: The lower plate was prepared with the stops that matched the top plate (either plastic or metallic). The tray was screwed into place and checked against the master model for proper seating. Then the top plate was taken off, the tray was loaded with the heavy bodied consistency material and simultaneously the light bodied co nsistency material was injected directly onto the abutment replicas of tooth 44 and tooth 47. The top plate was placed and guided close to reach the stops and then a 3 lb wei ght was placed on top to fully seat the tray against the master model to the pre establis hed ideal master /tray relation. This ideal master model/tray relation was built into the top plate design. When seating the top tray against the master model it was centered in close proximity to the model without touching it. After this procedure the impre ssion was left undisturbed for 10 min and then removed in one quick pulling action. Last, th e impression was inspected to verify that no bubbles were present on the marks. Putty/ Light Bodied Without Spacer Technique – Groups Two and Five : The lower plate was prepared with the stops that matched the top plat e (either plastic or metallic). The tray was screwed into place and checked against the master model for proper seating. Then the top plate was taken off, the tray was loaded with the putty material. The top plate was placed and guided close to reach the stops and then a 20 lb weight was placed on top to fully seat the tr ay against the master model to the pre established ideal master /tray relation. Af ter this procedure the impression was left undisturbed for 6 min and then removed in one quick pulling action. Then the light material was injected around the abutments a nd inside the already set putty impression. The top plate was placed and guided close to re ach the stops and then a 28 lb weight was

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51 placed on top to fully seat the tr ay against the master model to the pre established ideal master/tray relation. The impression was left undisturbed for 10 min and then removed in one quick pulling action. Last, the impression was inspected to verify that no bubbles were present on the marks. Putty/ Light Bodied With Spacer Technique – Group Three and Six : The lower plate was prepared with the stops that matche d the top plate (either plastic or metallic). The tray was screwed into place and checked against the master model with the 2 mm plastic spacer (Copyplast, Scheu DentalGmbh) in place for proper seating. Then the top plate was taken off, the tray was loaded with the putty material. The top plate was placed and guided close to reach the stops and then a 20 lb weight was placed on top to fully seat the tray against the master model to the pre established ideal master /tray relation. After this procedur e the impression was left undisturbed for 6 min and then removed in one quick pulling action. The 2 mm plastic spacer was carefully retrieved from the putty and placed aside. Then the light material was injected around the abutments and inside the already set putty impression. The top plate was placed and guided close to reach the stops and then a 3 lb weight was placed on top to fully seat the tray against the master model to the pre established ideal master /tr ay relation. The impression wa s left undisturbed for 10 min and then removed in one quick pulling acti on. Last, the impression was inspected to verify that no bubbles were present on the marks. Measurements Initially the coordinates (x, y and z) of the 11 marks on the master model were recorded. This coordinates were recorded 10 times on the master model for teeth 44 and 47 and for the reference rod using a measur ing microscope (Figures 6 and 7) (Unitron

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52 Universal Measuring Microscope, Unitron Inst ruments, Inc, Plainview, NY, USA). Same readings were made on impressions made no more than 24 h earlier. With the impressions attached to the table of the micr oscope, the coordinates for the eleven marks present on teeth 44 and 47, and the referen ce rod were recorded. Ten impressions per group were measured. Using the Pythagoras fo rmula in three dimensions and the program Microsoft Excel (Microsoft Corporation) computer program the distances between the marks on the abutment replicas and the re ference rod were calculated. The formula utilized to measure the dist ance between 2 points using the coordinates x, y and z was the following: Distance from mark 1 to 6 = 2 6 1 2 6 1 2 6 1) ( ) ( ) ( Z Z Y Y X X A total of 11 measurements per impressi on resulted from the computer calculation. Distance 1 is the distance between mark 1 on the master model and the reference mark which is mark 6. Distance 2 is the distance between mark 2 on the master model and the reference mark which is mark 6, and so on for every mark. Statistical Evaluation Six rounds of measurements of the 11 marks on teeth 44 and 47 and the reference mark on the stainless steel rod were done ini tially to determine the distances between the marks and the inherent errors associated with the measuring techniqu e. Another 6 rounds of measurements were also done of a preliminary impression for similar purposes. Based on these results a statistical evaluation us ing a subunit of the SA S program (Statistical Program) mean values and standard devi ations of master model and impression measurements were compared to determine the number of specimens needed to detect significant differences (p<0.05).

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53 According to those calculations, a mini mum of seven impressions per group would be needed to prove significant differences (p<0.05) between measurements on the master model and the studied impressions. Based on that finding, we decided to use 10 impressions per group. The coordinates of the eleven marks were recorded 10 times for the master model. The mean value for the different distances was calculated and used as the master model dimensions. The coordina tes of the eleven marks on each impression were recorded once per impression, ten impr essions per group. Comparisons between the different tray (stiffness) groups and impression thickness gr oups were conducted by use of a t-test and pair wise comparisons.

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54 Figure 3-13. Lateral view of measuring microscope (Unitron Instruments, Inc)

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55 Figure 3-14. Close up views of measuring micr oscope with master model in position for measurement and measuring devices on the instrument (silver knobs on the right image) (Unitron Instruments, Inc) Figure 3-15. Close up views of measuring mi croscope with master model in position for measurement and measuring devices on the instrument (silver knobs on the right image)(Unitron Instruments, Inc)

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56 Figure 3-16. Lateral view of the impre ssion device with the 3 pounds weight on top while taking one of the third group impressions

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57 CHAPTER 4 RESULTS AND DISCUSSION The results shown in Tables 4 -10 repr esent 11 distances expressed in microns between ten points on the abutments # 44 and # 47 and the reference point. Table 4 shows values for the master model. The refere nce point in Table 4 is represented by measurement 6 which is equal to 0 (compared to itself). Mean values, maximal values, standard deviations and minimal values ar e also shown in the tables. The distances between the different marks and the reference point were calculated and compared with the master model. The reference point was us ed in this study to be able to identify changes in space of each abutment. Using a t-test and pair wise comparisons significant differe nces (p<0.05) were found between four of the investigated groups and the master model. All techniques (PL, SP and HL) used with the plas tic trays had distances that were significantly different from the master model, while for the metal trays it was only the HL technique that resulted in a distance that was significantly shorter than the matching distance on the master model. Groups 2 and 3 used plastic tr ay /putty/light (PPL) and pl astic tray/putty /light with spacer (PSP) respectively, and were the two groups that had the largest number of distances which were significan tly different from the master model. For group 2 (PPL), the distances numbered 3, 4, 8 and 10 had valu es that differed with the master model ranging from 94.2 m on distance 4 to 161.8 m on distance 10. For group 3 (PSP), distances 1, 3, and 8 differed from the master model with a difference ranging from 75.8

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58 m to 106.2 m. Distortion found on groups 2 (PPL) a nd 3 (PSP) may be attributed to tray distortion. Group 1 consisting of plastic tray/hea vy-light material (PHL) and group 4 consisting of metallic tray/heavy-light mate rial (MHL) respectively also presented significant differences in one of the 10 distances. Group 1 (PHL) was significantly different in distance 5 with 72.6 m and group 4 (MHL) in distance 2 with 61.4 m. Changes in groups 1 (PHL) and 4 (MHL) may be related to material bulk and polymerization shrinkage. The discrepancies found on these two groups are probably still clinically acceptable for certain procedures Differences in distances up to 90 m between abutments for a fixed partial denture have been estimated as acceptable (46) due to the fact that the period ontal ligament measures from 100 m to 250 m (57). Probably even higher values than 90 m are acceptable for some patients. It means that perhaps under pressure the bridge fabricated from a s lightly differently sized cast could seat onto the abutments and fit properly against them. This amount of distortion for a multiple implant bridge/structure would have a differe nt outcome; it would probably be clinically unacceptable because its inability to adapt to the stiff implants. Vigolo et al. in their study about impression techniques for multiple im plant found that discrepancies up to 34 m were judged as acceptable and “passive” to manual and visual inspection (53). Fortunately, such big variati ons in length are found only wh en dealing with edentulous spans where the impression material bulk is big and is highly susceptible to polymerization shrinkage and thermal change s. Intra abutment dimensions are not affected enough by all impression variables to make them clinically important. It has been recognized in the past that dimensiona l changes in intra abutment dimensions are

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59 very minimal when a-silicone or polyether is used in conjunction with stock or custom trays (40, 45, and 49). Ultimately, the main goa l is to have a restorative margin sealed either for a single crown or for a multiple unit fixed partial denture. The rigidity of the tray is one of the mu ltiple factors related to impression accuracy (13). Great distortions of tr ays have been shown in a study when comparing plastic stock trays with metal trays while performing impre ssions with putty material. (10). Plastic tray flexibility was probably the cause for the distortion se en for groups 2 (PPL) and 3 (PSP) where the pressure created by the putty could have distorted initially the trays and then the pressure of the light material dur ing the second impression stage increased the distortion even more. Rigid trays have b een recommended by some authors (13, 9) in order to reduce distortion dur ing seating and removal of th e trays from the patient’s mouth. Gordon et al. found up to 100 m difference on inter abutment distances and 260 m cross arch discrepancy when using plastic stock trays. They attributed this distortion to tray flexibility. Comparable distortion was found in this study with the plastic trays when using an impression techni que with putty material (19). It is almost impossible to simulate and analyze all the variables affecting such a complex event as the impression procedure is. The complexity of impression making probably goes even further than one could possibly imagine. Local anesthetics and the time at which the impressions are made have been shown to have the most significant impact on the final clinical outcome (54). Fu rther more, materials which do not perform well in laboratory studies do sometimes very we ll in clinical studies (54). In our study many variables such as tray rigidity (stock plastic or stock metal), material bulkiness, type of impression material, tray adhesi on, tray seating pressure, pouring time and

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60 impression technique were considered. Other va riables were purposely not considered in order to isolate and simplify the studied variab les. Some of the vari ables not considered were: use of custom tray, mouth temperat ure, moisture, undercuts, other impression materials, cast production, and castings just to name some of them. In our study, the master model was designed in line with what has been done in previous studies (Tables 2.1 to 2.4). Six de gree taper stainless st eel abutments for a 4 unit fixed partial denture. The reviewed studies have used from parallel walls up to 12 taper, which probably is closer to reality. The base of the model was fabricated in plastic due to the fact that this study did not simulate oral temperature. Therefore a more stable model base such as metal was not needed. Some studies may have incorporated this potential source of error inadvertently (39). Pins to standardize tray seating are very popular among these in vitro studies (43, 52). Seating pressure is not commonly standardized, but it seems to ha ve some influence on material behavior (54). No studies were found on this specific topic. In previous studie s, weights as well as universal testing machines have been used to standardize th e forces while seating the tray against the master model time after time (43, 52). Interes tingly enough, the forces used in a previous study with an Instron testing machine closel y resemble the ones used in our study (52). The differences in weights used for the differe nt techniques were due to the fact that different techniques, materials and trays require d different levels of pressure to establish ideal master /tray relationship. As an interesting observation, the metal trays, when loaded with the putty for the first step impr ession (putty groups) al ways took little longer time to reach the stops. Plastic trays, probably due to their higher flexibility, did not show this behavior confirming the results by Cho et al. (10) regarding tr ay distortion. This

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61 behavior resembles what happens clinically when we need to apply different pressures while using different impression material viscosities. The top plates used to attach the trays (one for plastic and one for metal trays) weighted 2 lb each. This weight was not incorporated in the description of the different techniques. It is important to mention that these 2 lb were not included when we discuss tray loads. In other words, the total tray load is 2 lb higher than listed. This study measured the distances direc tly on the impressions and not on stone models like many others have done (14, 15, 27, 37, and 43). First, by measuring the impressions, errors incorporated during gypsum pouring could be avoided. Second, before this study was conducted, an impression wa s made of the master model using an asilicone H/L technique. The impression wa s then poured with type IV stone and evaluated under the microscope. The marks created with a diamond bur on the stainless steel abutments of the master model were ve ry easy to read on the impression under the microscope, but the same marks were blurred and poorly defined in the stone cast. For those reasons, stone casts measurements were not incorporated in th is study. Stone casts and metal castings resemble closely what happe ns in the dental labo ratory, but such an evaluation would introduce many more variab les and sources of errors, making it even more difficult to identify the real infl uence of the variables being studied. It is also known that temperature ch anges have great influence on impression materials and their accuracy (16). After 5 minutes in the mouth an impression can reach 33C (23). When retrieved from the mouth, the room temperature is about 23C. That is a 10C drop in temperature. Furthermore, wh en poured, the water temperature is even lower and may also influence the therma l contraction of the impression. Impression

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62 temperature changes from the mouth (37C) to room temperature (23C) was found in one study to be the dominating factor in die inaccuracy (16). Undercut is another variable that was not included in this study. Its importance regarding impression accuracy is well recogni zed (12). The greater the undercut is, the more likely a thin layer of impression mate rial will deform permanently. On the other hand, the thicker the material layer is, the mo re susceptible it becomes to polymerization shrinkage. Thin layers of 2-3 mm of impression material are accepted to produce accurate dies even in the presence of undercuts (21, 22, and 24). The instrument used for the measuremen ts is a Unitron Microscope capable of measuring down to 1 m. Coordinates were recorded for each of the eleven marks on abutment 44, abutment 47, and reference point Later the coordinates were used to calculate distances in the comput er. Coordinates x and y were ve ry easy to read in a very precise manner. The z coordinate, which was recorded with the lens scale, was much more cumbersome to determine and theref ore less precise and less reproducible. Therefore, the accuracy for the z coordinate is much lower than for the other two. A major limitation with our study is that we did not consider intra abutment measurements. It has been expressed theoretical ly that bucco-lingual dimensions of dies produced from distorted putty impressions from tray recoil are much smaller, producing oval shape dies rather than of round ones (9). However, it was very unlikely that we could have detected any si gnificant difference measuring the impressions directly. One study reported better fitting of the resul ting castings on the master model when metal or rigid plastic trays were used (9).

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63 Table 4-1 Mean and standard deviation of ten rounds of measurem ents performed on the eleven marks of the Master model Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18071.5 27.9 18032.4 18121.8 2 10 14899.0 32.5 14846.2 14948.7 3 10 15577.4 26.3 15538.3 15613.6 4 10 9186.0 46.7 9087.3 9251.2 5 10 17665.2 70.1 17567.9 17789.3 6 10 0 0 0 0 7 10 9994.1 26.1 9959.0 10036.5 8 10 14336.6 25.3 14313.2 14390.0 9 10 12629.7 35.5 12594.5 12708.0 10 10 15721.6 18.9 15694.6 15765.6 11 10 14758.2 58.3 14655.2 14861.1 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation Table 4.2 Measurements from ten impressi ons taken with a plastic tray and the heavy/light bodied technique (Group 1) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18062.1 24.3 18033.8 18104.1 2 10 14888.0 37.2 14835.4 14961.7 3 10 15579.5 25.0 15543.3 15625.8 4 10 9190.4 61.2 9124.2 9306.4 5 10 17592.6 63.5 17510.0 17738.2 6 10 0 0 0 0 7 10 9984.1 27.1 9943.3 10049.2 8 10 14302.7 29.2 14269.5 14352.9 9 10 12605.8 25.4 12575.7 12657.5 10 10 15737.9 36.6 15694.6 15822.4 11 10 14722.7 299.4 13903.0 14954.4 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation

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64 Table 4-3 Measurements from ten impressi ons taken with a plastic tray and the putty/light bodied without spacer technique (Group 2) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18043.3 128.3 17856.0 18353.9 2 10 14917.4 83.1 14855.6 15133.5 3 10 15476.4 102.2 15364.6 15737.5 4 10 9280.1 77.6 9217.1 9477.2 5 10 17623.7 126.7 17458.6 17888.1 6 10 0 0 0 0 7 10 10189.4 587.0 9775.7 11831.8 8 10 14194.4 89.5 13975.2 14264.0 9 10 12686.5 57.2 12613.6 12828.5 10 10 15559.8 299.5 147514.6 15749.9 11 10 14783.8 116.4 14492.1 14914.9 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation Table 4-4 Measurements from ten impressi ons taken with a plastic tray and the putty/light bodied with spacer technique (Group 3) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 17995.7 26.0 17955.6 18031.2 2 10 14896.5 61.0 14808.0 15025.6 3 10 15473.4 78.3 15288.1 15564.0 4 10 9163.2 45.3 9081.1 9212.8 5 10 17594.5 72.8 17475.1 17702.8 6 10 0 0 0 0 7 10 9998.4 85.6 9900.9 10217.7 8 10 14239.8 64.0 14109.6 14343.6 9 10 12627.4 23.6 12596.8 12677.3 10 10 15658.2 84.5 15447.4 15779.4 11 10 14772.1 55.8 14701.8 14912.0 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation

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65 Table 4-5 Measurements from ten impressions taken with a metal tray and the heavy/light bodied technique (Group 4) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18066.3 17.6 18029.6 18089.8 2 10 14837.7 81.4 14617.0 14898.1 3 10 15631.9 103.1 15580.6 15924.2 4 10 9164.7 24.9 9132.8 9209.7 5 10 17658.7 69.9 17542.8 17739.3 6 10 0 0 0 0 7 10 9997.0 18.5 9979.9 10044.8 8 10 14335.4 49.3 14197.5 14370.0 9 10 12802.5 504.7 12600.4 14221.4 10 10 15761.7 45.9 15730.8 15858.8 11 10 14806.3 65.1 14699.3 14916.9 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation Table 4-6 Measurements from ten impressions taken with a metal tray and the putty/light bodied without spacer technique (Group 5) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18054.3 15.1 18032.1 18079.8 2 10 14854.1 22.2 14829.6 14900.0 3 10 15579.7 19.2 15553.0 15615.0 4 10 9211.1 17.2 9186.3 9235.4 5 10 17667.0 68.4 17544.6 17744.7 6 10 0 0 0 0 7 10 9939.0 312.5 9050.8 10078.2 8 10 14356.3 19.1 14317.0 14386.6 9 10 12711.1 114.5 12627.0 12883.7 10 10 15783.0 19.6 15755.7 15808.2 11 10 14832.5 80.1 14685.5 14934.9 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation

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66 Table 4-7 Measurements from ten impressions taken with a metal tray and the putty/light bodied with spacer technique (Group 6) Distance # n Mean ( m) S.D. ( m) Minimum ( m) Maximum ( m) 1 10 18047.1 19.8 18018.9 18073.6 2 10 14886.0 24.6 14858.7 14931.1 3 10 15568.6 29.0 15525.0 15611.7 4 10 9176.4 28.6 9136.9 9218.4 5 10 17675.0 75.9 17538.7 17745.0 6 10 0 0 0 0 7 10 9991.6 20.2 9950.8 10022.9 8 10 14329.1 20.3 14291.1 14354.2 9 10 12618.1 19.4 12584.5 12654.4 10 10 15730.1 24.8 15696.0 15777.0 11 10 14800.0 69.6 14677.9 14917.2 D = Distance measured n = Number of measurements of the same distance in the master model. S.D. = Standard deviation 9.49 17.26 28.26 75.82 24.43 5.26 0 10 20 30 40 50 60 70 80 MHL PHL MPL MPS PPL PSP IMPRESSION GROUPS M I C R O N S Figure 4-1. Difference between master m odel and impression groups in distance 1. Significantly different values marked with a red star. Plastic tray groups in blu e and metal tray groups in orange.

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67 -18.37 11.07 61.39 2.58 13 44.93 -30 -20 -10 0 10 20 30 40 50 60 70 PPL PSP PHL MPS MPL MHL IMPRESSSION GROUPS M I C R O N S Figure 4-2. Difference between master m odel and impression groups in distance 2 Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange -54.48 103.96 100.98 -2.27 -2.13 8.81 -80 -60 -40 -20 0 20 40 60 80 100 120 MHL MPL PHL MPS PPL PSP IMPRESSION GROUPS M I C R O N S Figure 4-3. Difference between master m odel and impression groups in distance 3. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange

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68 21.33 22.82 -94.15 -25.11 9.58 -4.41 -100 -80 -60 -40 -20 0 20 40 PPLMPLPHLMPSMHLPSP IMPRESSION GROUPS M I C R O N S Figure 4-4. Difference between master m odel and impression groups in distance 4. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange -17.8 6.47 41.45 70.64 -9.8 72.61 -30 -20 -10 0 10 20 30 40 50 60 70 80 MPS MPL MHL PPL PSP PHL IMPRESSION GROUPS M I C R O N S Figure 4-5. Difference between master m odel and impression groups in distance 5. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange

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69 -19.53 0.25 1.01 5.52 -0.43 -0.29 -25 -20 -15 -10 -5 0 5 10 PPL PSP MHL MPS PHL MPL IMPRESSION GROUPS M I C R O N S Figure 4-6. Difference between master m odel and impression groups in distance 7. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange 33.91 142.27 96.79 -19.7 1.2 7.51 -40 -20 0 20 40 60 80 100 120 140 160 MPL MHL MPS PHL PSP PPL IMPRESSION GROUPS M I C R O N S Figure 4-7. Difference between master m odel and impression groups in distance 8. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange

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70 -56.42 2.36 11.67 -172.73 23.94 -81.35 -200 -150 -100 -50 0 50 MHL MPL PPL PSP MPS PHL IMPRESSION GROUPS M I C R O N S Figure 4-8. Difference between master m odel and impression groups in distance 9. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange -61.49 161.79 -40.14 63.39 -8.53 -16.35 -100 -50 0 50 100 150 200 MPL MHL PHL MPS PSP PPL IMPRESSION GROUPS M I C R O N S Figure 4-9. Difference between master m odel and impression groups in distance 10. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange

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71 -74.28 35.55 -13.9 -25.59 -41.46 -48.06 -80 -60 -40 -20 0 20 40 60 MPL MHL MPS PPL PSP PHL IMPRESSION GROUPS M I C R O N S Figure 4-10. Difference between master m odel and impression gr oups in distance 11. Significantly different values marked with a red star. Plastic tray groups in blue and metal tray groups in orange 0 1.01 33.91 35.55 9.49 11.07 72.61 *23.94 -16.35 -2.13 -4.41 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11 DISTANCES MEASURED M I C R O N S Figure 4-11. All distances mean differen ce value for group 1 (PHL) in comparison to master model.

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72 28.26 41.45 0 -56.42 161.79 142.27 *-94.15 *100.98 *-25.59 -19.53 -18.37 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11DISTANCES MEASURED M I C R O N S Figure 4-12. All distances mean differen ce value for group 2 (PPL) in comparison to master model. 2.58 22.82 0 2.36 63.39 96.79 *103.96 *70.64 75.82 -0.43 -13.9 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11 DISTANCES MEASURED M I C R O N S Figure 4-13. All distances mean differen ce value for group 3 (PSP) in comparison to master model.

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73 5.26 61.39 6.47 0 -0.29 1.2 -172.73 -40.14 -48.06 -54.48 *21.33 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11 DISTANCES MEASURED M I C R O N S Figure 4-14. All distances mean differen ce value for group 4 (MHL) in comparison to master model. 17.26 44.93 0 -81.35 -2.27 -17.8 5.52 -25.11 -61.49 -74.28 -19.7 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11DISTANCES MEASUREDM I C R O N S Figure 4-15. All distances mean differen ce value for group 5 (MPL) in comparison to master model.

PAGE 85

74 0 0.25 11.67 7.51 11.67 7.51 9.58 8.81 13 24.43 -9.8 -200 -150 -100 -50 0 50 100 150 200 1 2 3 4 5 6 7 8 9 10 11DISTANCES MEASURED M I C R O N S Figure 4-16. All distances mean differen ce value for group 6 (MSP) in comparison to master model.

PAGE 86

75 CHAPTER 5 SUMMARY AND CONCLUSIONS Plastic trays produced less accurate impressi ons than metal trays. When metal trays were used, putty based impressions were dimensionally better than heavy/light body impressions. Consequently, tray rigidity and material bulk control through the use of two stage techniques improved impression reliability. Future work can be done as described here As shown in other studies (9, 13, 14, and 34), this study supported claims that fact ors such as the control of the impression material thickness and the tray rigidity aff ect the impression accuracy. Custom trays have been the gold standard for many years be cause they control the thickness, but little attention has been paid to the rigidity requi red by them to perform well during impression procedures. In a future project it would be interesting to test under the same conditions described in this study the performance of cu stom trays against the Rim-Lock metal tray using putty viscosity te chnique in two steps with the metal trays. It would also be interesting to include as impr ession technique in a new proj ect the putty/light one step impression technique. This technique has been criticized in the past because supposedly some of the margins are imprinted in putty material which does not fulfill the specifications for detail reproduc tion. Is it truly putty against the margin or a few microns layer of light body material that cannot be seen by the human eye? The reason for that critique has never been supported by reliable research evidences.

PAGE 87

76 LIST OF REFERENCES 1. Asgar K. Elastic impression materials. Dent Clin North Am 1971; 15:81-98. 2. Bomberg TJ, Hatch RA, and Hoffman W. Impression material thickness in stock and custom trays. J Prosthet Dent 1985; 54:170-172. 3. Breeding LC, Dixon DL, and Moseley JP. Custom impression trays: part 1 Mechanical properties. J Pr osthet Dent 1994; 71:31-34. 4. Brosky ME, Major RJ, DeLong R and Hodges JS. Evaluation of dental arch reproduction using three-dimensional optical digitization. J Pros thet Dent 2003; 90: 434-440. 5. Burns J, Palmer R, Howie L, and Wilson R. Accuracy of open tray implant impressions: an in vitro comparison of stoc k versus custom trays. J Prosthet Dent 2003; 89:250-255. 6. Christensen GJ. Complex fixed and implant prosthodontics: Making nearly foolproof impressions. J Am Dent Assoc 1992; 123:69-70. 7. Christensen GJ. Now is the time to change to custom impression trays. J Am Dent Assoc 1994; 125:619. 8. Chee WWL and Donovan TE. Polyvinyl siloxane impre ssion materials: A review of properties and techniques. J Prosthet Dent 1992; 68:728-732. 9. Carrotte PV, Johnson A and Winstanley RB. The influence of the impression tray on the accuracy of impressions for cr own and bridgeworkan investigation and review. Br Dent J 1998; 185:580-585. 10. Cho G and Chee WWL. Distortion of disposable pl astic stock trays when used with putty vinyl polysiloxane impression materials. J Prosth et Dent 2004; 92:354358. 11. Craig RG. Restorative Dental Materials, ed 8. St Louis, CV Mosby Co, 1989, p 293. 12. De Araujo A and Jorgensen KD. Effect of material bulk and undercuts on the accuracy of impression materials. J Prosthet Dent 1985; 54:791-794.

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77 13. Dixon DL, Breeding LC, and Moseley JP. Custom impression trays: Part III – Stress distribution model. J Pr osthet Dent 1994; 71:316-318. 14. Eames WB, Sieweke JC, Wallace SW, and Rogers LB. Elastomeric impression materials: effect of bulk on accura cy. J Prosthet De nt 1979; 41:304-307. 15. Eames WB, Wallace SW, Suway NB, and Rogers BA. Accuracy and dimensional stability of elastomeric impr ession materials. J Prosthet Dent. 1979; 42:159-162. 16. Finger WJ. Significance of filler content to properties of silicone impression materials. Dent Mater1988; 4:33-37. 17. Gelbard S, Aoskar Y Zalkind M and Stern N. Effect of impression materials and techniques on the marginal fit of me tal castings. J Prosth et Dent 1994;71:1-6. 18. Gilmore WH, Schnell RJ, and Phillips RW. Factors influenci ng the accuracy of silicone impression material. J Prosthet Dent 1954; 4:94-103. 19. Gordon GE, Johnson GH, and Drennon DG. The effect of tray selection on the accuracy of elastomeric impression mate rial. J Prosthet Dent 1990; 63:12-15. 20. Heartwell CM, Modjeski PJ, Mullins EE, and Strader KH. Comparison of impressions made in perforated and non perf orated rim lock trays. J Prosthet Dent 1972; 27:494-500. 21. Hung SH, Purk JH, Ti ra DE and Eick JD. Accuracy of one step versus two step putty wash addition silicone impression technique. J Prosthet De nt 1992; 67:583589. 22. Idris B, Houston F and Claffey N. Comparison of the dimensional accuracy of one and two step techniques with the use of putty/wash addition silicone impressions. J Prosthet Dent 1995; 74:535-41. 23. Jamani K, Fayyad M. Harrington E, and Wilson HJ. Temperature changes of materials during impression taking. Br Dent J. 1988; 165:129-132. 24. Johnson GH and Craig RG: Accuracy of four types of rubber impression materials compared with tim e of pour and a repeat pour models. J Prosthet Dent 1985; 53:484-490. 25. Lin C, Ziebert G, Donegan SJ, and Dhuru VB. Accuracy of impression materials for complete arch fixed partial dentur es. J Prosthet Dent 1988; 59:288-291. 26. Marcinak CF, and Draughn DF. Linear dimensional ch anges in addition curing silicone impression materials. J Prosthet Dent 1982; 47:411-413.

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78 27. Martinez LJ, and von Fraunhofer JA. The effects of custom tray material on the accuracy of master casts. J Prosthod 1998; 7:106-110. 28. McCabe JF and Wilson HJ. Addition curing silicone rubber impression materials: an appraisal of their physical prope rties. Br Dent J 1978; 145:17-20. 29. Mitchell JV and Dammele JJ. Influence of tray desi gn upon elastic impression materials. J Prosthet Dent; 1970; 23:51-57. 30. Millstein P, Maya A, and Segura C. Determining the accuracy of stock and custom tray impression/casts. J Oral Rehab 1998; 25:645-648. 31. Moseley JP, Breeding LC, and Dixon DL. Custom impression trays: Part II – Removal forces. J Prosth et Dent 1994; 71:532-38. 32. Nissan J, Laufer BZ, Brosh T and Assif D. Accuracy of three polyvinyl siloxane putty-wash impression techniques. J Prosthet Dent 2000; 83:161-165. 33. Nogawa I. Factors influencing dimensional accu racy of indirect working model: the method by use of thiokol rubber base and silicone rubber base impression materials. Odontology 1968; 56:396. 34. Ogle RE, Sorensen SE, Lewis EA. A new visible light cure d resin system applied to removable prosthodontics. J Prosthet Dent 1986; 55:592-597. 35. Pagniano RP, Scheid RC, Clowson RL, Dagefoerde RO and Zardiackas LD. Linear dimensional change of acrylic resins used in the fa brication of custom trays. J Prosthet Dent 1982; 47:279-283 36. Phillips RW. Skinner’s Science of Dental Mate rials, ed 9. Chapter 11: Denture base resins: Technical considerations, miscellaneous resins and techniques, Philadelphia, PA, Saunders,1991, p 207. 37. Reisbick MH and Matyas J. The accuracy of highly filled elastomeric impression materials. J Prosthet Dent 1975; 33:67-72. 38. Rueda LJ, Sy-Munoz JT ,Naylor WP Goodacre CJ, and Swartz ML. The effect of using custom or stock trays on the accuracy of gypsum casts. Int J Prosthodont 1996; 9:367-373. 39. Saunders WP, Sharkey SW, Smith G McR, and Taylor WG. Effect of impression tray design and impression tec hnique upon the accuracy of stone casts produced from a putty-wash polyvinyl siloxa ne impression material. J Dent. 1991; 19:283-289. 40. Shigeto N, Murata H, and Hamada T. Evaluation of methods for dislodging the impression tray affecting the dimensional accuracy of the abutments in a complete dental arch cast. J Pros thet Dent 1989; 61:54-58.

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79 41. Shillinburg HT, Hobo S, Whitsett LD, Jacobi R, and Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence Publishing Co, 1997; p 290. 42. Smith PW, Richmond R, and McCord JF. The design and use of special trays in prosthodontics: guidelines to improve clinical effectiveness. Br Dent J. 1999; 187: 423-426. 43. Stauffer JP, Meyer JM, and Nally JN. Accuracy of six elastic impression materials used for completearch fixed partial dentures. J Prosthet Dent 1976; 35:407-415. 44. Thongthammachat S, Moore KB, Barco MT, Hovijitra S, Brown DT, and Andres CJ. Dimensional accuracy of dental cas ts: influence of tray material, impression material and time. J Prosthodont 2002; 11:98-108. 45. Tjan AHL, Whang SB, Tjan AH, and Sarkissian R. Clinically oriented evaluation of the accuracy of commonly used impression materials. J Prosthet Dent 1986; 56:4-8. 46. Tjan AHL and Whang SB. Comparing effects of tray treatment on the accuracy of dies. J Prosthet Dent 1987; 58:175-178. 47. Tjan AHL, Nemetz H, Nguyen LTP, and Contino R. Effect of tray space on the accuracy of mono phasic polyvinylsiloxane impressions. J Prosthet Dent 1992; 68:19-28. 48. Tjan AHL, Whang SB, and Miller GD. Why a rigid impression tray is so important to the putty wash impression method. J Can Dent Assoc 1981; 9:53-58. 49. Valderhaug J and Floystrand F. Dimensional stability of elastomeric impression materials in custom-made and stock tr ays. J Prosthet Dent 1984; 52:514-517. 50. Wadhwani CPK, Jhonson G, Lepe X and Raigrodski. Accuracy of newly formulated fast-setting elastomeric impr ession materials. J Prosthet Dent 2005; 93:530-539. 51. Vigolo P, Fonzi F, Majzoub Z, and Cordioli G. An evaluation of impression techniques for multiple internal connection implant prostheses. J Prosthet Dent 2004; 92:470-476. 52. Wasell RW and Ibbetson RJ. The accuracy of polyvinyl siloxane impressions made with standard and reinforced stoc k trays. J Prosthet Dent 1991; 65:748-757. 53. Wassell RW and Abuasi HA. Laboratory assessment of impression accuracy by clinical simulation. J Dent 1992; 20:108-114.

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80 54. Williams PT, Jackson DG, and Bergman W. An evaluation of the time dependent dimensional stability of eleven elastomeric impression materials. J Prosthet Dent 1984: 52:120-125. 55. Wilson TG and Kornman KS. Fundamentals of Periodontics. 1st ed. Kimberly Streams: Quintessence Publishing Co, 1996; p 20. 56. Wostman B. Zum derzeitigen Stand der Abfo rmung in der Zahnheilkunde.Berlin: Quintessenz Verlags-Gmbh, 1998.

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81 BIOGRAPHICAL SKETCH I got my dental diploma from the “Instituto de Ciencias de la Salud CES” in Medellin, Colombia, South America, in December of 1995. My undergraduate thesis was “Craniofacial and Dentoalveolar Changes with First Permanent Molars Extraction” which was part of a large longitudina l study in growth and developm ent. In 1997 I enrolled in a two and a half year specialty program in peri odontal-prosthesis at th e same university. I became a specialist in April 2000. My thesis project “SAMM-III. Analysis and Design of a Mandibular Movement Measurement Syst em” was named as the Best Dental Postgraduate Research Work 2000. After livi ng in Colombia since age 4, I decided in 2000 to come back to the United States fleei ng from the violence in this country. From 2001 to 2003, I did the Foreign Trained Dentist (FTD) program at University of Florida, obtaining the Florida dental license the same year. In 2003, I started a three year specialty program in prosthodontics with a Master of Science at the University of Florida. I received my Master of Science in prost hodontics in May 2006. Currently, I am planning to establish my dental practice in Ocala, Florida, limited to prosthodontics. I am also planning to serve as a vis iting faculty in the graduate prosthodontics program at University of Florida. My wife Paula has been my support and engine throughout all these years at school. After co mpletion of my specialty a ne w era starts in our lives.


Permanent Link: http://ufdc.ufl.edu/UFE0014360/00001

Material Information

Title: Influence of Tray Rigidity and Material Thickness on Accuracy of Polyvinyl Siloxane Impressions
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0014360:00001

Permanent Link: http://ufdc.ufl.edu/UFE0014360/00001

Material Information

Title: Influence of Tray Rigidity and Material Thickness on Accuracy of Polyvinyl Siloxane Impressions
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0014360:00001


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INFLUENCE OF TRAY RIGIDITY AND MATERIAL THICKNESS ON ACCURACY
OF POLYVINYL SILOXANE IMPRESSIONS















By

ALEX HOYOS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Alex Hoyos




























To my parents and my wife, who are my inspiration















ACKNOWLEDGMENTS

I would like to express thanks to Dr. Karl-Johan Soderholm, my thesis advisor, for

his support and guidance during my graduate thesis research. He opened my eyes with his

thoughts and his perspective about different topics in life. It has been a pleasure and an

honor to work with him. I also want to thank my other committee members, Dr. Buddy

Clark and Dr. Glenn E. Turner, for their time and disposition to crystallize this project.

Special thanks go to Dr. Jaime Rueda for his constant motivation and moral support.

Thanks go to Dr. Lucius Battle and Dr. Edgar O'Neill for accompanying and supporting

me during this process. Thanks go to all the other people in Colombia and here in the

United States that have participated in my education and have assisted me in pursuing my

goals. I am also grateful for the financial support provided by the Department of

Prosthodontics, University of Florida. Thanks go to Diana Mucci and to Dentsply/Caulk

company for the donation of the materials used in this study. Thanks to Mr. Pete Michel

and other people from the Bioengineering Department for their participation in the

construction of the mechanical devices used in the study. In addition, I can never thank

my parents enough for their continuous support and comprehension. Last, thanks go to

my loved Paula. Her love has been the fuel that keeps my engine running.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST O F TA B LE S ......................................... ......... ............ .............. .. vii

LIST OF FIGURES .................. .. .......... .............................. viii

A B ST R A C T .......... ..... ...................................................................................... x

CHAPTER

1 INTRODUCTION ............... ................. ........... ................. ... .... 1

2 LITER A TU R E REV IEW ............................................................. ....................... 3

Custom vs. Stock Trays .................................................. ........ .............. .... .4
Tray M material and D im ensional Stability ........................................ .....................9
Im pression Techniques ................................................ ........ .. .......... .. 15
Im pression Technique Studies............... ............ ............... ............... ... 18
Laboratory Models Used to Test Impression Materials Accuracy ...........................21
Master Model Material and Abutment Replica Design...........................................22
M ethods to Seat the Tray ......................................... .......... ....... ............... .24
Therm al Changes ................ ................................................... .. .........33
T ype of M easurem ents ....................................................................... ..................34

3 M ATERIALS AND M ETHOD S ........................................ ......................... 39

Im pression G roups............ ................................................................ .......... .. 39
Im pression M materials .......................................... .. .. .... ........ .. ....... 40
M a ster M o d el ......... .... .................. ..............................................................4 0
Impression Making Device........................ ................................. 46
Impression Procedures Sequence and Standardization.................. ...............47
Measurements ............ ......... .......... ...............51
Statistical Evaluation ........................ .................. ................... ........ 52

4 RESULTS AND DISCU SSION .......................................... ........................... 57

5 SUMMARY AND CONCLUSIONS.................................................................75




v









L IST O F R E F E R E N C E S ...................................... .................................... ....................76

B IO G R A PH IC A L SK E T C H ...................................................................... ..................81















LIST OF TABLES


Table pge

2-1 Variables related to some of the studies that have used abutment replicas to test
impression m material accuracy. ............................................................................25

2-2 Variables related to some of the studies that have used abutment replicas to test
im pression m material accuracy (cont) ............................................. ............... 27

2-3 Variables related to some of the studies that have used abutment replicas to test
im pression m materials accuracy ........................................... .......................... 29

2-4 Variables related to some of the studies that have used abutment replicas to test
im pression m materials accuracy. ........................................ ........................... 31

2-5 Coefficient of thermal expansion of some of the materials used in the master
models (Inlay waxes listed for comparison purposes only)...................................35

3-1 Tray rigidity and thickness control for the different study groups........................45

4-1 Mean and standard deviation of ten rounds of measurements performed on the
eleven m arks of the M aster m odel ............................................... ............... 63

4.2 Measurements from ten impressions taken with a plastic tray and the heavy/light
bodied technique (G roup 1)........................................................... ............... 63

4-3 Measurements from ten impressions taken with a plastic tray and the putty/light
bodied without spacer technique (Group 2) .................................. ............... 64

4-4 Measurements from ten impressions taken with a plastic tray and the putty/light
bodied with spacer technique (Group 3) ...................................... ............... 64

4-5 Measurements from ten impressions taken with a metal tray and the heavy/light
bodied technique (G roup 4)........................................................... ............... 65

4-6 Measurements from ten impressions taken with a metal tray and the putty/light
bodied without spacer technique (Group 5) .................................. ............... 65

4-7 Measurements from ten impressions taken with a metal tray and the putty/light
bodied with spacer technique (Group 6) ...................................... ............... 66















LIST OF FIGURES


Figure page

3-1 Plastic stock tray used for the study (Disposable Impression trays .......................41

3-2 M etal stock tray used in the study ...................................... ...................... .......... 42

3-3 Occlusal view of the master model. Note the stainless steel rods between
abutm ent replicas .................. ................................ .. .......................... .. 42

3-4 Frontal view of the m aster m odel............................................................ ........... 43

3-5 Buccal view of nine of the eleven marks on the master model with their
corresponding num bers ................................................. ............................... 43

3-6 Lingual view of the marks on the master model with their corresponding
n u m b ers .............................................................................4 4

3-7 Impression materials used for the study. Aquasil Ultra heavy and light bodied
and impression gun (Dentsply/Caulk Milford, DE), Exaflex putty (GC America
Inc, Alsip, II), and V.P.S. Tray Adhesive (Kerr, Romulus, MI) ............................ 44

3-8 Master model attached to an aluminum plate. Note the three stainless steel
guiding pins with the three plastic vertical stops ........................................... 47

3-9 Metal tray secured to a second plate by a screw. Note the three holes that match
the three guiding pin ..................... .. .. .. ................. .. ....... .. 48

3-10 Plastic tray secured to a second plate by a screw. Note the three holes that match
the three guiding pins. There is one plate design for plastic and another for
m etal. ................................................................................ 4 8

3-11 Lateral view of the two plates assembled previous to an impression procedure
w ith the plastic tray ............................................ ................. ........ 49

3-12 Lateral view of the two plates assembled previous to an impression procedure
with the metal tray. Plastic stops for the metal tray are different in length from
the ones for the plastic tray. ............................................ ............................. 49

3-13 Lateral view of measuring microscope ................. ................... ............... 54









3-14 Close up views of measuring microscope with master model in position for
measurement and measuring devices on the instrument..................................55

3-15 Close up views of measuring microscope with master model in position for
measurement and measuring devices on the instrument..................................55

3-16 Lateral view of the impression device with the 3 pounds weight on top while
taking one of the third group impressions ............... ........................ ............... 56

4-1 Difference between master model and impression groups in distance 1. ...............66

4-2 Difference between master model and impression groups in distance 2 ...............67

4-3 Difference between master model and impression groups in distance 3. ...............67

4-4 Difference between master model and impression groups in distance 4. ...............68

4-5 Difference between master model and impression groups in distance 5. ...............68

4-6 Difference between master model and impression groups in distance 7. ...............69

4-7 Difference between master model and impression groups in distance 8. ...............69

4-8 Difference between master model and impression groups in distance 9. ...............70

4-9 Difference between master model and impression groups in distance 10. .............70

4-10 Difference between master model and impression groups in distance 11. .............71

4-11 All distances mean difference value for group 1 (PHL) in comparison to master
m odel. ................................................................................7 1

4-12 All distances mean difference value for group 2 (PPL) in comparison to master
m odel. ................................................................................72

4-13 All distances mean difference value for group 3 (PSP) in comparison to master
m odel. ................................................................................72

4-14 All distances mean difference value for group 4 (MHL) in comparison to master
m odel. ................................................................................73

4-15 All distances mean difference value for group 5 (MPL) in comparison to master
m odel. ................................................................................73

4-16 All distances mean difference value for group 6 (MSP) in comparison to master
m o d el. ............................................................ ................ 7 4















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

INFLUENCE OF TRAY RIGIDITY AND MATERIAL THICKNESS ON ACCURACY
OF POLYVINYL SILOXANE IMPRESSIONS

By

Alex Hoyos

May 2006

Chair: Karl-Johan Soderholm
Major Department: Prosthodontics

The objective of this study was to determine how tray rigidity and impression

technique affect the accuracy of impressions made with an addition polymerizing silicone

material. Metallic rim-lock trays and disposable plastic trays were used in combination

with three different impression techniques. The three techniques consisted of 1)

heavy/light bodied materials in a one-step impression (HL), 2) putty impression without

spacer and light body impression made in two steps (PL), 3) putty impression with 2 mm

space and light body impression made in two steps (SP). Ten impressions of each

combination technique/tray were made of a master model. The master model included

two steel abutments (44 and 47) and a steel rod placed at ridge level between the two

abutments. Five marks had been placed on each steel abutment. One mark placed on the

steel rod in between the two abutments served as a reference point. By use of a universal

measuring microscope, the x, y, z-coordinates were recorded for each of the 11 marks on

the master model and the impressions. The distances between the different marks and the









reference point were calculated and compared with the master model. Using a t-test and

pair wise comparisons, significant differences (p<0.05) were found between 4 of the

investigated groups and the master model. All techniques (PL, SP and HL) used with the

plastic trays had distances that were significantly different from the master model, while

for the metal trays it was only the HL technique that resulted in a distance that was

significantly shorter than the matching distance on the master model. In conclusion,

plastic trays produced less accurate impressions than metal trays. When metal trays were

used, putty based impressions were dimensionally better than heavy/light body

impressions. Consequently, tray rigidity and control of the bulk of the impression

material improved impression reliability.














CHAPTER 1
INTRODUCTION

During the past few years, there has been a tendency to use plastic stock trays

rather than custom made trays to make impressions for crowns, bridges and implants. In

addition, these plastic trays have been used in combination with different materials of

different viscosities. Tray rigidity and material thickness are among many variables that

have been described as important factors in making accurate impressions (13, 14, and

15). It has been recognized that the tray should be rigid enough to stand the forces

generated during the impression procedures without distortion (50). Regarding the

material thickness, it has been said that a thin even layer of 2 mm impression material

produces the most accurate impressions (43). Two widely used impression techniques

consider or completely ignore these two variables. One uses a metal tray with putty and

light bodied material in a two step technique (our group 6) (34), the second uses plastic

stock tray with heavy and light bodied material in a one step technique (our group 1)(8).

In this study, we decided to test the hypothesis that higher tray rigidity and better bulk

impression material control (2 mm space) would produce more accurate impressions by

performing intra and inter abutment measurements.

We hypothesized that by using more rigid trays and better controlling the

impression material thickness, the precision of an impression would improve. To test our

hypothesis, a master model simulating replicas of two abutments of a four unit posterior

bridge was used. Two tray types and three different impression material thicknesses were

evaluated. Only polyvinyl siloxane impression material was used. Ten impressions per









group were made, and the distances between fixed points on the master model and the

impressions were measured. Matching distances between the different groups were then

compared by using t-test and pair wise comparisons to determine whether significant

differences (p<0.05) existed in distortion among the two tray types, the three impression

thicknesses, or whether there were any interactions between tray types and impression

thicknesses. The null hypothesis we tested was that neither tray rigidity or impression

thickness had any significant impact on the dimensional reproduction ability of a PVS

impression material.














CHAPTER 2
LITERATURE REVIEW

Impression materials are used to register or reproduce the forms and relationships

of teeth and oral tissues. Their purpose is to create an exact replica of the oral tissues and

then pour the impression with a model material such as stone, on which precise

reconstructions can be made in a laboratory environment (11). Today, the most

commonly used impression materials for precise reproductions are the non-aqueous

elastic impression materials. These materials can be divided into two main groups -

condensation and addition polymerized elastomeric impression materials. Of these

materials, the addition polymerized elastomers are more stable over time, and are

therefore now the materials that are most widely used in fixed prosthodontics (43). The

addition polymerized elastomers include addition silicone (PVS) and polyether, two

materials well known for their accuracy and dimensional stability (24, 15).

Many factors affect the accuracy of impression materials. These factors include;

impression material selection (56), impression material manipulation (8), impression tray

design (46), impression retention to impression tray surface (48), impression material

thickness (15), tray deformation potential (3), impression technique (the introduction of

endogenous tension), impression removal (42), thermal changes after removal (23),

storage condition after removal, and material used for making the dies and its

compatibility with the impression material. By considering all these variables and how

they interact with each others, it becomes clear that impression accuracy is a









multifactorial phenomenon. These variables interact sometimes in a positive way and

sometimes in a negative way making it quite complex to predict the final outcome.

The trend to use stock trays rather than custom made trays have continuously

increased during the past 20 years (7). The reason behind that change relates to improved

dimensional stability of polyether (introduced during the mid 60s) and PVS (introduced

in the mid 70s) impression materials. Stock trays reduce the number of steps needed to

make a final impression. In addition, between plastic and metallic stock trays, there is a

strong preference for the plastic ones, because they are disposable (don't need to be

returned from the laboratory) and don't need to be cleaned and disinfected after use. An

important question, though, is whether there are any significant differences in precision

between impressions made with custom trays or stock trays. Based on the popularity of

the stock trays, it does not seem that the transition to these trays from individual trays

have made any clinical significance. Two important variables must be considered

regarding tray selection. First, the space allowed between the tray and the prepared teeth

and second, the tray rigidity. The tray should be rigid enough to resist forces developed

during impression procedures without permanent deformation.

Because of these considerations, as well as the design of this study, this literature

review will focus on 4 main topics:

1. 1. Custom vs. stock trays
2. 2. Tray material and dimensional stability
3. 3. Impression techniques
4. 4. Laboratory models used to test impression materials accuracy

Custom vs. Stock Trays

Custom trays are trays specially designed for an individual case. They provide a

uniform thickness of the impression material and often reduce the amount of impression









material needed. Stock trays are the trays that come in predetermined sizes. They may be

either in metal or in plastic. The term "stock tray is used in the literature independent of

whether the trays are made of metal or plastic. However, it is important to differentiate

between metal and plastic stock trays, because the accuracy of metal and plastic stock

trays can differ because of differences in their rigidity. Also, the use of stock trays,

independent of tray material, results in an uneven bulk thickness of the impression

material, which increases the risk for distortion (18, 14, and 15). Therefore, custom trays

are recommended to create a thin, even space between the tray and the teeth to control the

impression material thickness. When such trays are made, a maximum space of 2 mm is

often recommended (14). In studies by Reisbick and Matyas in 1975 (37), thicknesses in

the range of 2 to 4 mm were recommended, while Asgar in 1971 suggested 3 to 4 mm

(1). According to Nogawa in 1968, differences in thicknesses ranging between 1 and 5

did not produce significant differences, at least as long as the impression is poured

immediately after it has been made (33). Eames et al. (14) studied the effects of 2, 4, and

6 mm space on the accuracy of 9 elastomeric materials and found that 2 mm space

produced the most accurate impressions. De Araujo and Jorgensen (12) in 1985 studied

impression material thicknesses and undercut sizes and how they affected PVS material

and found that better accuracy was generated for lower impression thickness values (1

mm) and smaller undercuts (0.5 mm). In their study it was clear that 1 mm PVS thickness

was better than 2 or 3 mm thicknesses. Apparently, de Araujo et al. findings suggest that

one should use thinner rather than thicker layers of PVS impression materials. A potential

weakness, though, with such a conclusion is that the risk for permanent deformation

increases during impression removal if the impression thickness is too thin. Tjan et al. in









1992 (47) used custom trays with impression material spaces of 2, 4 and 6 mm, to study

changes in the precision of PVS impressions of abutment replicas for a 3 unit bridge.

They found good accuracy in intra abutment dimensions for the four PVS materials

used, but distortion in the inter abutment distance existed for two of the impressions

poured at different times after impression making. No explanation for that finding was

discussed in that article. A possible explanation behind the somewhat conflicting results

seen in the literature in relation to material thickness control; may simply be related to the

magnitude of recorded undercuts. During impression removal, larger undercuts will

increase the deformation of the impression material. According to Hooke's Law, stress is

defined as modulus of elasticity times strain, where strain is defined as change in length

divided by original length. Thus, if the material thickness is thin and the undercut big,

extensive strain will be induced which may raise the stress level to a level where

permanent deformation starts occurring. Accordingly, larger undercuts require thicker

impression thicknesses. However, as that thickness increases, material shrinkage starts

playing a more significant role. Therefore, depending on the size of the undercuts,

different errors (plastic deformation vs. material shrinkage) will dominate the final

outcome. Consequently, the conflicting results are likely caused by different experimental

design approaches.

Other studies have shown that tray space does not have any effect on the intra

abutment dimensional accuracy of monophasic polyvinyl siloxane impressions, however,

the inter abutment dimensional accuracy is affected (47). Because the dimensions of the

abutment replicas are small in comparison to the whole master model, differences in

distances are much smaller on the abutment replica than on the full arch impressions. As









a consequence, it is much more difficult to identify significant differences on an abutment

than between abutments that are far apart, and the rather small intra abutment differences

may not be big enough to have any clinical significance. On the other hand, replications

of big spans between abutments may result in impressions with small dimensional

deviations. Other factors that add to such errors include variations in tray dimensions,

expansion/contraction of model material, and last, but probably not least important,

differences in operator techniques. All these factors may become noticeable during

measurements. Obviously, such measurable dimensional changes would have bigger

impact clinically than the smaller intra abutment errors. A factor affecting the accuracy of

the inter abutment distances is the flexibility of the tray and its dimensional change when

it is removed from the master model as expressed by Gordon et al. in 1990 (19).

Not all studies confirm the importance of impression material thickness and tray

rigidity. Valderhaug and Floystrand in 1984 (49) compared the accuracy of c-silicone

and polyether using custom and metallic stock trays to create impressions of master

models. They did not find significant differences in accuracy of impressions made with

custom trays or metal stock trays with the two materials. Questions arise from that study

because it is well known from laboratory studies that polyether is much more accurate

and stable over time than c-silicone and the fact that the measurements did not show any

difference between the two materials raises some doubt regarding the validity of that

study. The times at which the measurements were made on the impressions were 0 h, 1 h,

and 24 h after impression making, and because it is known that a low viscous polyether

shrinks about 0.24% during a 24 h period and a low viscous c-silicon shrinks about 0.6%

(11), one would expect to see a difference. In Valderhaug and Floystrand's study (49),









the authors did not find any significant differences in inter abutment distances using

either tray, even thought ample amount of material was used (2 to 9 mm). Despite

Valderhaug and Floystrand's (49) claim that 2-9 mm thick impression materials layer did

not have any significant impact, it is generally believed that elastomeric impressions are

more accurate in uniform, thin layers of 2 to 3 mm thickness (41). A possible explanation

for Valderhaug and Floystrand's (49) claim may have been given by Bomberg et al. in

1985 (2), who reported that the mean difference in material thickness between custom

trays and stock trays is less than 1 mm, and that variations in uniform impression material

thickness exist in both custom and stock trays. The precision of the instruments used to

measure changes could be another reason why Valderhaug and Floystrand (49) could not

find any differences. Rueda in 1991 (38) did not find any clinical significant differences

in impressions made from custom or plastic stock trays. Again, the biggest distortion

(>50 rm) was found in the distance between left and right molars. No comments

regarding custom tray rigidity were presented in that study. A possible explanation could

be that both custom and plastic stock trays had the same rigidity. Dixon et al. in 1994,

(13) advocated 3 to 4 mm of custom tray thickness to produce enough rigidity to stand

impression forces.

One factor that seems important in most studies where fixed partial denture

abutments have been replicated is that custom trays should control the impression

material bulkiness around abutments and pontic areas. Failing to do so leaves a great

amount of material suitable for setting contraction and thermal changes. If a uniform

thickness of material is required with a custom tray, attention should be paid to the pontic

areas so these regions receive 2 mm of impression material as well. This is important to









consider, because this might be the reason why some studies that compare stock and

custom trays impression accuracy cannot find any difference between them.

Inter abutment accuracy becomes even more important when implants are used as

abutments. Small abutment discrepancies are not as critical when one is working with

natural dentitions, because the periodontal ligament could help to compensate for smaller

impression errors. Even the die spacer used to build the fixed partial dentures could help.

With implants the tolerance is lower. Bums et al. in 2003 (5) evaluated the accuracy of

implant impressions in vitro using rigid custom open trays and polycarbonate stock trays

and found significant differences among groups in the vertical fit of the casts. Custom

trays performed better. Vigolo et al. found in 2004 (51) that joining the impression

copings with Duralay resin for implant level impressions is more accurate than not doing

so. Even though they used custom trays for the experiments, they probably were not rigid

enough to avoid the distortion produced during the impression procedures. The cross arch

accuracy achievable with customized trays is highly desirable to capture multiple implant

positions that will be joined by a metal superstructure. Passivity, less solder joints and/or

remakes would be the immediate benefit of using more precise impression techniques.

It can be concluded from this review that stock trays probably provide sufficient

accuracy for single tooth restorations, particularly if polyvinyl siloxane or polyether are

used. However, if one piece fixed partial denture of three or more units are to be

fabricated on the cast, the inter preparation and cross arch discrepancies from stock tray

impressions could have a significant impact on the fit of the restoration (43, 6).

Tray Material and Dimensional Stability

Other important variables in impression accuracy are related to the rigidity and

dimensional stability of the tray. The tray should be stiff and stable enough so it does not









deform during insertion and retrieval of the tray-impression complex from the mouth.

Any tray deformation, particularly elastic deformation, will result in distortion errors.

Metal trays are more rigid than plastic trays. Among plastic trays there are different

levels of rigidity. Valderhaug and Floystrand (49) found no differences between

impressions made with metal stock trays and rigid resin custom trays. This finding is

somewhat surprising if we consider the differences in modulus of elasticity between

metallic tray materials and plastic tray materials where the modulus of metal tray

materials is around 50 times higher than that of plastic trays. It is possible that by using

thick custom trays (4 mm), as the ones advocated by Dixon et al. in 1994 (13), the

rigidity of a plastic tray can be enough to withstand the forces involved in impression

seating and removal, and therefore producing similar results as with metal trays.

Millstein et al. in 1998 (30) studied casts made from the use of three different stock

trays and a custom tray. Two plastic stock trays, one metal stock tray, and one custom

tray were used. Casts produced from the custom tray were more precise and significantly

different from the ones produced with the other two trays. Metal stock trays were more

accurate than plastic trays. Tjan et al. in 1981 (48) published a paper that emphasizes the

importance of rigid trays for elastomeric impressions. They reported a research project in

which crowns were constructed on 15 working casts made from impressions of a full

crown preparation on a typodont (plastic replica of a dental arch). Impressions were made

in rigid stock trays, disposable trays and reinforced disposable trays. Not one of the

crowns made on the cast dies produced from impressions made with disposable trays fit

the master die. All ten of those made on the models from impressions made in rigid or

reinforced trays were assessed as satisfactory.









Gordon et al. in 1990 (19) compared the dimensions of working casts made from

impressions made in either custom trays (using two different tray materials) or plastic

stock trays. They found that the plastic stock tray which was much less rigid, consistently

produced casts with greater dimensional change than the two custom tray systems, and

concluded that the use of plastic stock trays should be limited to the reproduction of casts

where great accuracy is not needed. Mitchell and Dammele in 1970 (29) investigated the

distortion caused by various tray types with reversible and irreversible hydrocolloids,

polysulfides and silicones. They found that tray form had a significant impact on the

amount of impression distortion by all materials tested, most seriously with the

polysulfide and silicone elastomers and least with reversible hydrocolloid. Even

irreversible hydrocolloid exhibited distortion when used in non rigid trays. Carrotte et al.

(9) studied the influence of the impression tray on the accuracy of putty wash

impressions. For that purpose an ivorine model with 3 crown preparations, one for a

crown and two for a FPD was used. Four stock trays were tested (3 plastic and 1 metal)

with two putty viscosities (heavy and soft). Master model casts were seated on duplicate

models and discrepancies were measured. The metal tray and the most rigid plastic tray

produced the best fitting. In relation to metal stock trays, Heartwell et al. in 1972 showed

no difference in the dimensions of casts poured from irreversible hydrocolloid taken in

perforated or non perforated metal rim-lock trays (20). Gordon et al. in 1990 (19) found

that the inter preparation distance (simulation of edentulous areas for a FPD) in casts

made from polysulfide, polyvinyl siloxane, and polyether impressions was 45 100 pm

greater when stock trays were used instead of custom acrylic resin or thermoplastic trays.









They also found 260 [m cross arch discrepancies, which they attributed to stock

tray flexibility (19). Similar results in inter abutment distance distortion have been

reported (43) for hydrocolloids, C-silicones, polysulfide, and polyether. Stauffer in his

study concluded that none of the tested materials was capable of producing a complete

arch fixed partial denture on a cast poured from one single master impression (43).

However PVS was not tested in that study. Wasell et al. (52) and Saunders et al. (39) in

two separate studies in 1991 showed that reinforcement of stock trays improved the

quality of the impression, but did not eliminate completely the distortion from tray

deformation. The forces generated during impression procedures are such that even metal

trays show changes in dimension intra arch and cross arch. Cho et al. in 2004 studied

cross section and cross arch changes of six disposable plastic trays and compared them

with a metal stock tray (10). Impressions of a plastic model were done using a putty

material. Distortion in both across arch and cross section directions was found for the six

plastic trays. Metal stock trays showed significantly less change than plastic trays (10).

To reduce inaccuracies in the impression the aforementioned phenomenon should

be eliminated or minimized as much as possible.

Dimensional changes of the tray can also occur due to the custom tray material

behavior. The tray must remain dimensionally stable over time (3). Because of that

concern, the dimensional stability of auto polymerizing acrylic resin tray materials has

been the subject of many studies. These studies have resulted in recommendations

suggesting that the trays made of auto polymerizing acrylic resin should be made at least

20 to 24 h in advance to avoid major dimensional changes in the material (37). Other

studies suggest times from 40 min to 9 h (44, 35). It is well demonstrated on Pagniano's









study that auto polymerizing resins will keep changing even after 24 h (35). Some

authors have different recommendations under the assumption that after the first hour of

the tray resin setting, most of the dimensional changes have occurred. They believe that

the impression can be taken but it should be poured immediately because according to

them, the stone would counteract the dimensional changes ongoing in the tray. No other

study could be found supporting this idea. In the study by Pagniano et al. in 1982 (35) the

linear dimensional change of acrylic resins used in the fabrication of custom trays was

studied. Four commercial materials were tested for linear changes during 24 h. The

results of this study showed that all the materials changed for a period up to 24 h. The

most rapid linear dimensional shrinkage of all the materials occurred in the first hour

after mixing, varying from a mean change of 0.08% to 0.33%. During the first hour 50 %

of the total change that occurred for the 24 hour period had occurred in each of the

materials. Pagniano et al. (35) recommended to use the tray for the impression after 9 h

of setting because at that time most of the total shrinkage had occurred and once made,

to pour it immediately to avoid further changes (in resin tray) that would affect its

accuracy. Studies on visible light curing (VLC) resins have indicated that these tray

materials largely eliminate the disadvantages associated with auto polymerizing resins by

improving stiffness, form, and volume stability and by reducing sensitivity to moisture.

The reason for this is basically that the VLC material is similar to the light-cured

composites that instead of using an inorganic filler it uses an organic filler (34). The filler

consists of acrylic resin beads of varying sizes that become part of an interpenetrating

polymer network structure when cured. The VLC material can be used immediately after

fabrication (34). It is important to consider this difference among both materials because









if they are used at the appropriate time, the mechanical properties are quite similar and

good results can be accomplished with either one. Breeding et al. (3) studied the

mechanical properties of one polymethyl methacrylate, one light polymerizing and three

thermoplastic resins used as custom tray materials. The polymethyl methacrylate resin

exhibited measured mechanical property values that were significantly higher than those

of the thermoplastic resins tested. Small differences in mechanical property values

between polymethyl methacrylate resin and light polymerizing resin (Triad Tru Tray)

were found. Even though they were statistically different they were too small to be

clinically important. The different thermoplastic resins had different mechanical property

values among them.

The importance of the rigidity and the mechanical properties in general of the trays

was highlighted in an article by Dixon et al. published in 1994 (13) when they

determined the amount of force needed to remove a tray with impression material from

the mandibular arch. They found that the force used was higher when three evenly

distributed point forces were used on the tray (514 N) than when the force was placed at

a single anterior point (224 N). The amount of force needed when using three points

seems to be too much clinically. No other similar studies were found to compare

their results. Those findings showed that the mechanical properties of the materials

such as polymethyl methacrylate resin and light polymerizing resins in an appropriate

thickness (2.5 to 3 mm) were good enough to resist permanent deformation when

subjected to removal forces created under both situations. The authors concluded that

thermoplastic resins need to be approximately 4 mm thick to avoid deformation during

tray removal (31).









In the light of today's scientific knowledge it is recognized that the dimensional

stability and the rigidity are two important characteristics to consider when fabricating

customized trays. Auto polymerizing polymethyl methacrylate resins are stable after 24

hours of fabrication and rigid enough at 2.5 to 3 mm thickness. VLC urethane

dimethacrylate resins are stable enough immediately after light curing and require the

same minimum thickness to assure rigidity. When a stock tray is to be used, metal trays,

either perforated or non perforated, produce accurate impressions due to their ability to

resist deformation during the impression procedure.

Impression Techniques

Many impression techniques are used but very few are well understood and

supported by research. Wostman in 1997 (56) described 9 impression techniques that

pretty much cover all the techniques available. Below are descriptions of the 9 impression

techniques.

Correction Impression: Use of A- or C-silicone in combination with the

correction impression technique is the most commonly used technique in West Germany.

After the teeth have been prepared, an impression is made with a knead paste. A

perforated metal stock tray is recommended. Flexible plastic trays are extensively

deformed during impression making by the high viscosity paste. As a result, an

uncontrollable distortion of the entire impression occurs. Individual trays and stock trays

made of plastic are useless for this technique. The success of the correction impression

depends on the first impression. The risk is deformation of the first impression when the

second is made with the wash material. There is also the risk that the wash does not bond

properly to the first impression. The dies produced with this technique are in general too









small but the author suggests that the compression impression technique produces

clinically useful impressions.

Double Impression: This technique is a variation of the compression impression

technique. In this case, the first impression is made before the teeth are prepared. The

second impression is made after the teeth have been prepared and that impression is made

with a thin flowing impression material that now fills the space that has been removed

during preparation. With such an approach, much lower compression is introduced during

final impression making. It has also been suggested to make the first impression at least

one day in advance to let that material shrink completely before the final impression is

made. By storing the first impression for some time before the second impression is

made, the rigidity of that material is increased. As a result, impression changes decrease.

(56)

Segment Impressions: For this technique the first impression is divided into

several segments. It is also a two step technique. Like the first two techniques, this is a

modification of them. After the first impression is made, escape furrows are cut to let the

material escape lingually or buccally. With this technique it is also possible to reduce

stresses that are easily introduced in the correction technique.

Double Mix Technique: With this technique high and low viscosity materials are

used simultaneously. The low viscosity is placed on the teeth and the high viscosity

material in the tray. In comparison to the correction technique, the double mix technique

should have the potential of being more precise because it should not induce the same

amount of impression and tray deformation as the correction technique. The key problem

with this technique is instead that a delay in the complex process may result in some









material setting that in turn can induce residual stresses. Such stresses, when relaxed,

result in a somewhat smaller aluminum. Besides, this technique tends to be unreliable

when it comes to forcing the material down into subgingival regions.

Sandwich Technique: This technique is a variation of the double mix technique.

However, with the sandwich technique, the low viscosity impression material is not

injected with a syringe tip around the prepared margins, just placed on the preparations

with a mixing instrument (can be described as the "butter and bread" technique). This

technique does not reproduce well subgingival regions.

Hydrocolloid Impression: These impressions can be described as being in

principle similar to the double mix impression technique. Instead of a silicone or a

polyether, hydrocolloid agar is used. The key drawback with this technique is the need

for special equipment and more strict preparation. To plasticize the material a special

heater is used. The impressions are taken in specially designed water cooled trays.

Advantages of the technique include the comfortable use, the low material cost and the

consistent outcomes. A significant limitation, though, is the problem to record

subgingival margins.

Single Phase Impression: With this technique a single material viscosity is used,

and some of the material is injected around the teeth. The use of stock trays with this

technique results in poor compression. This technique can be improved by use of a

custom made tray. Polyether is the most commonly used material with this technique. By

use of the single phase technique combined with a custom made tray and an A-silicone or

a polyether impression material, it is possible to produce very precise impressions.









Shrinkage and distortion effects caused by endogen stress play a minimal role with the

single phase impression technique.

Ring Impression (Copper bands): Ring impression with a thermoplastic

compound is one of the oldest used impression methods. Today this technique has been

replaced by more modern techniques. When the ring impression technique is used, only

one tooth is imprinted in each ring impression.

Optical Impressions: Since 1971 attempts have been made to generate numerical

information of the teeth. The first successful attempts were introduced by Duret, Rekow,

Mormann, and Brandistini (56). One of the systems available (Cerec) consists of an

intraoral camera that records the prepared tooth surface, then the computer converts that

information to an x, y, z coordinate system. Then a milling machine produces the

restoration from these coordinates.

Impression Technique Studies

Gelbard et al. (17) studied the effect of two impression materials used with three

different techniques in order to address the marginal fit of metal castings. The following

methods were used to make the impressions: 1) putty/ wash in one phase with metal tray,

2) copper band relined with auto polymerizing acrylic resin and subsequent light body

elastomeric impression material, 3) copper bands with modeling compound. Metal

copings were fabricated from casts made out of the impressions, seated on master die

models with pressure indicator paste and then cemented.

Master die and coping were cut in half bucco-lingually and measured under the

microscope to evaluate the marginal fit. Measurements of the thickness of the cement

layer were calculated manually and with a computerized method. The metal castings fit

were from 38.3 gm to 128.4 gm for both the methods of measuring. Gelbard et al. (17)









claimed no superiority of any of the impression techniques. Newman et al. in 1986

investigated the dimensional stability of various impression techniques using different

impression materials. Their concern was to determine if different techniques would cause

different degrees of dimensional changes with different impression materials. Six

polyvinyl siloxane materials and one polyether material were shown to be dimensionally

stable, while the two polysulfide materials were not stable. The single viscosity custom

made tray impression technique gave consistently greater degree of error with both

polysulfide and polyvinyl siloxane impression materials, while the putty-wash technique

gave consistently more accurate fits with either a one step or a two step technique. One

widely used technique is the technique that uses light body in the syringe and heavy or

medium body in the tray (8). Johnson and Drennon in 1987 conducted a clinical

evaluation of detail reproduction of elastomeric impression materials. They concluded

that the double mix technique produced better detail than did the single mix technique.

Heavy consistencies, rather than medium, in combination with a light consistency

material resulted in better details. Another technique is the putty-wash technique, which

was developed to compensate for the polymerization shrinkage of the condensation

silicones attributed to the production of an alcohol byproduct during polymerization (29).

The two step putty/wash technique used a thin layer of wash material that minimized the

amount of alcohol byproduct and thereby retained the dimensional stability within

acceptable limits (37).

Although the putty wash technique was originally recommended for problems

associated with polymerization shrinkage of the c-silicones, this technique has also been

suggested for a-silicone impression materials (36).Two variations of the putty wash









method are commonly used: the putty/wash one step technique, in which the materials

polymerize in one stage and the putty/wash two step technique, in which a putty is first

used alone as the initial step and then a final impression is made within the putty

material by use of a silicone of lower viscosity (22). Idris et al. compared the accuracy of

the one step and two step technique using a-silicone impression material (22). For the

purpose of the study they used three stainless steel replicas of abutment, one die without

undercuts and two with undercuts. Grooves were prepared on the occlusal surfaces for

measuring purposes. Impressions were taken using perforated metal trays. All

impressions were poured and then measured under the microscope. Inter abutment

distances increased for all but one measurement in comparison to the master model for

both techniques. Almost all intra abutment distances were smaller than the ones on the

master model. An explanation for these results could be that smaller dies (smaller intra

abutment distances) may create bigger inter abutment spaces. The smaller dies are

supposedly the result of the hydraulic pressure created while seating the one step or two

step techniques with the materials in place. Idris et al. consider that the differences found

between the two techniques were not of clinical significance. Differences of about 32 ism

for spans 40 mm long are not of clinical significance as long as mobility exists in the

periodontal ligament. The authors concluded that neither technique resulted in dies that

deviated sufficiently from the master model to cause clinical difficulties (22).

Nissan et al. studied the putty wash techniques but the two step technique was

further divided in two different approaches. The three approaches were: 1) putty/wash in

one phase, 2) putty/wash with 2 mm metal coping spacer in two phases, and 3)

putty/wash with polyethylene spacer in two phases (32). A metal master model,









containing three complete crowns fixed partial denture abutment preparations with

grooves on the occlusal surfaces, and perforated custom trays were used. Impressions

were made at room temperature and then poured in stone. All measurements were done

using a Toolmaker's microscope. As in Idris et al. study (22), Nissan et al. (32) found

increases in inter abutment distances (0.009% to 0.1%) and decreases in intra abutment

(0.08% to 3%) distances. They found significant differences among the three groups. The

second group was the most accurate of the three groups.

The authors criticized the one step technique because it reproduced a part of the

margins in the putty material, which they claimed had not enough detail reproduction to

produce a reliable casting. The question, however, is whether it is only putty on the

margins or that the light body is so thin that it cannot be seen. Most putties on the market

cannot reproduce details fine enough to meet the American Dental Association (ADA)

specification 19.

Laboratory Models Used to Test Impression Materials Accuracy

Research into impression accuracy has relied heavily on in vitro tests rather than

clinical evaluations. The key reason is simply that clinical studies are difficult to

standardize and reproduce. For example, variables such as tray flexing/recoiling ability

and differences in impression techniques are difficult to eliminate clinically. Also, it is

easier to make measurements in the laboratory than in the mouth (53). However, the

drawback with most laboratory tests is that such studies do not simulate the true oral

conditions very well. Some tests provide realistic tooth morphology, arch form and

temperature control but none of them mimics soft tissue consistency or the surface

characteristics imparted by the oral fluids (53). The presence of oral fluids under clinical

conditions is important to emphasize, because some of the "best" materials according to









laboratory evaluations are also the most hydrophobic materials. Consequently, materials

that seem to be superior in a laboratory environment may still be inferior in a clinical

situation because they cannot reproduce a moist dentin surface. Under such conditions a

material that may be inferior in laboratory evaluations may still be superior in clinical

situations. Being well aware of the laboratory/clinical conflicts, the following review will

target those variables that might influence the results of our study.

Master Model Material and Abutment Replica Design

Many models have been used by researchers to test impression materials. Stauffer,

et al. use an aluminum master model machined with four prepared abutment teeth of

stainless steel positioned in a maxilla (43). Undercuts were not taken into consideration.

All four teeth were cylindrical and parallel to each other and all vertical walls had 50

taper. Evaluations of the resulting casts were made by comparison with a master fixed

partial denture. Metal (stainless steel, chrome steel, aluminum, and cooper) has been

mainly used because it resists wear during laboratory work. Metal is also less susceptible

to accidental damage and its coefficient of thermal expansion is small enough to resist

master model changes during experimental simulation of oral temperatures (35-37'C).

Another important consideration is the availability of stainless steel material at the

engineering department laboratories at different universities where similar studies can be

conducted. Eames et al. used stainless steel dies with 120 taper to evaluate the effect of

the bulk of the impression material on the accuracy of impressions (14). Marcinak and

Draughn (26) prepared two maxillary central incisors mounted in an acrylic resin block.

The distal surfaces were machined precisely parallel to each other to provide accurate

measurements. Valderhautg and Floystrand (49) tested the materials using two models









with stainless steel standardized abutment teeth in the area of the canines and the first

molars. The teeth were drilled with a taper of 100. Two grooves 5 [im wide were

engraved at right angles in the center on the occlusal surfaces. De Araujo and Jorgensen

(12) used a truncated chromium steel cone die with and 8 mm base diameter and 8 mm in

height. The die was undercut apical to the gingival margin. Three rings were used to

create three different undercut heights. Johnson and Craig (24) tested four rubber

impression materials using a stainless steel master model simulating two full fixed partial

denture abutment preparations. The preparations were 10 mm in height and 10 mm in

diameter, and one of them had an undercut with an 8 mm diameter. Lin et al. (25)

prepared four abutment teeth from a dentoform for complete crowns with 1 mm shoulder

margins by using a handpiece mounted in a parallel instrument. The height of the canine

tooth preparation was 8 mm and the molar was 7 mm. Saunders et al. (39) used a

dentoform with prepared teeth as a master model to test the effect of impression tray

design and impression technique on the accuracy of the resulting casts. A copper plated

master model was used by Wasell and Ibbetson (52) to evaluate the accuracy of polyvinyl

siloxane impressions made with standard and reinforced stock trays. Tjan et al. (47) also

used stainless steel dies to create the master model for their study. They created a model

simulating a three-unit fixed partial denture. Reference lines were inscribed on top and

axial surfaces of the abutments to assess the dimensional changes. In conclusion, most of

the studies have used stainless steel to create the master models. Others have used natural

teeth, plastic teeth, copper plated or chromium steel models. Metal bases as well as metal

dies are much more dimensionally stable to thermal changes while simulating oral









environment (350C to 370C) than the plastic ones. On the studies reviewed, 50 to 120

taper were used as total occlusal convergence for the abutment preparations replicas.

Undercuts are another important variable to consider when testing impression

materials. Larger undercuts are more difficult to imprint precisely.

Methods to Seat the Tray

Another important variable is the way the tray is seated against the master model

time after time. It is necessary to do it the same way every time an impression is made,

and therefore a standardized technique is required. To verify the importance of

standardization in seating the tray, a simple exercise was performed at the University of

Florida, Graduate Prosthodontics clinic with three residents. All of them were asked to

make an impression of a resin mandibular model using the heavy/light body technique.

Using a scale the amount of pressure was measured while the impression was made. The

lowest amount of pressure was 2 pounds and the highest 15 pounds. No studies could be

found in relation to this variable but some differences in material behavior are supposed.

The use of alignment pins that guide the tray during seating is a common method to

achieve this goal during laboratory experiments (43, 24, 49, 13).












Table 2-1 Variables related to some of the studies that have used abutment replicas to
test impression material accuracy.


Marcinak et al.

Valderhaug et al.


de Araujo et al.



Jhonson et al.


Lin et al.


Gordon et al.


Wasell et al.



Saunders et al.


Tjan et al.



Hung et al.


Gelbard et al.



Idris et al.



Millstein et al.


1982 2 teeth,acrylic


1984


SS and Al


1985 Cr steel



1985 SS


1988 Plastic


1990 SS and Plastic


1991 Copper plated model
Dentoform


Plastic


1992 SS



1992 SS



1994 Plastic



1995 SS



1998 SS


prep. parallel

4abutments
2 models

single die
26.5 degree
undercut

2 dies in a
block

4abutments
Max model


2 abutments
3 metal inserts

3 abutments copies
of a dentoform

2 FPD abutments
1 inlay opposite side


2 FPD abutments
mounted in a plate


2 FPD abutments
mounted in a plate

Ivorine prepared
tooth mounted in a
dentoform

3 abutment replicas
triangular distribution
Occlusal grooves

4 abutments. 2 anterior
2 posterior. U shaped
bar


0 yes


10 yes/no


5 yes



6 yes


n/a no


n/a yes


n/a no



n/a no



n/a no



n/a yes



n/a no



n/a yes



5 no












Table 2-1. Continued


AUTHOR
Carrote et al.



Nissan et al.


Thongthammachat
et al.



Wadhwani et al.


YEAR MATERIAL
1998 Stone dentoform
Ivorine teeth


2000 3 SS preparation
replicas

2002 Metal master model





2005 Modified dentoform
Metal inserts on 1st
molars and lower
incisors


ABUTMENTS
1 full crown and
2 abutments for
PFM


3 full crown preps
in small metal block

Multiple prepared
teeth. 8 reference
holes, 3 reference
points.

Metal inserts and
a removable die


TOC UNDERCUTS
5 no


n/a no


n/a no





12 no











Table 2-2 Variables related to some of the studies that have used abutment replicas to
test impression material accuracy (cont)


AUTHOR
Stauffer et al.


Eames et al.


Eames et al.


M arcinak et al.

Valderhaug et al.


de Araujo et al.

Jhonson et al.


Lin et al.


YEAR TEMPERATURE
1976 35 C and 12 C


1979a 37 2 C


1979b 32 2o C


1982 370 C

1984 21 C


1985 370 C

1985 250 C


1988 n/a


IMPRESSION MATERIAL
2 hydrocolloids, Ic-silicone
2 polysulfides,1polyether

3 polysulfides, 2 polyethers
2 c-silicones

2 polyethers, 8 c-silicones
2 a-silicones, 5 polysulfides

5 a-silicones

1 polyether, 1c-silicone


1 polysulfide, la-silicone

1 a-silicone, 1 polysulfide
1 c-silicone, 1 polyether

2 polyether, 2 a-silicones
2 polysilfides, 2 reversible
hydrocolloid, 2 irreversible
rev -irrev com bination


Gordon et al.

Wasell et al.

Saunders et al.


Tjan et al.


Hung et al.



Gelbard et al.


Idris et al.


Millstein et al.


1990 34o C

1991 23o 1 C

1991 37 o C


1992 22o C 2o C


1992 n/a



1994 n/a


1995 Room temperature


1998 n/a


1 polyether,1 a-silicone,1 polysulfide

PVS H/L and P/L techniques

PVS P/L 3 techniques: P/L 1 stage,
P/L in 2 stages creating space with
an scalpel, and P/L creating space
with spacer

Four brands of monophasic PVS

Five brands of P/L impression
material. Tested in one and two stage
techniques.

3 groups. 1 P/L in one stage with
metal tray.2 Copper band relined with
acrylic and wash material. 3 Copper
band and impression compound

P/L one stage and P/L two stages
PVS material

H/L technique. 3 stock trays and
1 custom tray











Table 2-2 Continued
AUTHOR
Carrote et al.


Nissan et al.


Thongthammachat 2002
et al.


Wadhwani et al.


YEAR TEMPERATURE
1998 n/a


2000 room temperature


35C 1 C


2005 23 C


IMPRESSION MATERIAL
PVS P/L Material. Putty in 2 viscosities
normal and soft. One stage technique

1 step P/L, 2 step P/L with 2mm relief'
2 step P/L with polyethylene spacer

PVS and Polyether


Fast set PVS and fast set polyether
regular setting polyether as control












Table 2-3 Variables related to some of the studies that have used abutment replicas to
test impression materials accuracy


AUTHOR
Stauffer et al.


Eames et al.



Eames et al.


Marcinak et al.



Valderhaug et al.





de Araujo et al.



Jhonson et al.



Lin et al.



Gordon et al.


Wasell et al.


Saunders et al.


Tjan et al.


Hung et al.

Gelbard et al.

Idris et al.

Millstein et al.


YEAR TRAY
1976 custom
2mm space

1979a custom
2, 4, and 6mm
space

1979b custom 2.4mm
space


1982 custom, 3mm
space


1984 custom 3mm
thick.2 to 4 mm
space.
metal stock

1985 custom metal
1mm, 2mm,
3mm, 4mm

1985 custom 2mm
thick.3.75mm
space

1988 custom 2mm,
thick
3mm space

1990 2 custom types and 1 stock
3mm thick, 2.5 mm space

1991 2 stock trays (plastic)
2 reinforced stock trays

1991 Plastic tray without and with
3 types of reinforcement

1992 Perforated small custom trays
with 2, 4,and 6mm space

1992 Perforated metal tray

1994 Metal tray and copper band

1995 Metal tray

1998 3 plastic and 1 custom tray


POURING TIME
5 minutes


0 min



30min and 24h



10m, 30m, 2h
4h, 8h, 24h,
48h,96h,168h

no stone model





10min



1h,4h (2 pour)
24h


NS



1 h


24 h


24h


1h, 24h and 7 days


1h

NS

1h

Oh











Table 2-3 Continued
AUTHOR
Carrote et al.


YEAR TRAY
1998 1 metal and 3 plastic trays
Suupossed different rigidity
for all of them


POURING TIME
NS


Nissan et al.


2000 Metal custom tray


Thongthammachat et al. 2002


Metal and plastic stock trays
4 types of custom trays
2 to 2.5 mm


30 min, 6h, 24h, 30d


Wadhwani et al.


2005 Plastic stock tray












Table 2-4 Variables related to some of the studies that have used abutment replicas to
test impression materials accuracy.


AUH I
Stauffer et al.


Eames et al.



Eames et al.



Marcinak et al.



Valderhaug et al.


de Araujo et al.





Jhonson et al.


Lin et al.


Gordon et al.


Wasell et al.


Saunders et al


YEhA MEASUKEMNI ME IMUU
1976 visual inspection of metal framework
Dial gauge measurement

1979a impression at Omin ,24 h. microscope
Other impressions poured at 0 min.
Castings/master die tested on dies

1979b impression at Omin ,24 h. microscope
other impressions poured at30 min.
and 24h. Castings tested on dies

1982 5 measurements of the die with a
micrometer.


1984 impressions measured at Omin, 1h
24h.linear measurements made only
microscope used.


1985 traveling microscope on stone





1985 traveling microscope on stone


1988 master casting joined with resin
seated on casts
measured under microscope

1990 Measurescope, measures up to 1p


1991 Wild photomicroscope. Measured in mm
with 2 decimals



1991 Reflex microscope and computer


NINUINGS
No interarch accuracy found for any
material.

2mm space better accuracy, Cast
lifting from 4 and 6mm impressions
were clinically unacceptable

when poured at Omin all materials
where accurate. At 24h a-silicones
least change

greatest change of 0.3% up to a
putty wash most unstable. Produced
smaller dies.

c-silcone and polyether behaved
similar at different time intervals
no difference between stock and
custom trays

increase of material thickness from 1
to 4 mm increased distortion.
less distortion when increasing the


Larger diameter of abutment for a-
silicone and polysulfide. Vertically
where smaller in general.siliconese
were least affected by double pour.
more distortion with undercut

Polyethers the most accurate
complete arch impressions


Dies height very accurate for all materials
width larger for polyether and polysufide.
PVS very precise
Interprep distance longer for all, the stock
trays produced distortion from 45 to 100p
Cross arch dimensions with stock trays
distorted up to 260p

H/L technique minimum distortion with
the different trays. P/L high distortion.
Reinforcement improved P/L accuracy
but P/L performed better

None of the trays affected the accuracy of
the P/L impressions.One stage technique
one distance significantly different. Two
stage techniques had also one .
Results contradict conclusion.












Table 2-4 Continued


AU I HO
Tjan et al.


YEAR MEASUREMENT METHOD
1992 Microscope- quantitative method
Master castings- qualitative method


FINDINGS
Differences found among PVS brands
Tray space and pouring time did not affect
accuracy for individual preparations
Tay space did affect inter preparation
distances. Both methods did not correlate


Hung et al.


Gelbard et al.


Idris et al.


Millstein et al.


1992 Microscope
Measurment of stone casts


1994 Castings cut sections measured with
SEM

1995 Toolmaker's microscope and computer
Master model and stone casts
measured


1998 Micrometer and a template


4 brands produced larger interabutment
distances.One stage P/L technique and two
were equally accurate. All materials
produced larger abutment height

No statisically significant differences
among the techniques

In general all interabutment distances
increased but one. Intrabutment distances
decreased, except for two distances.
Significant differences for all interabutment
measurements. Not clinically significant

Casts from custom trays were significantly
more precise than the ones from stock
trays. The more flexible trays presented
more distortion.


Carrote et al.



Nissan et al.


Thongthammachat 2002
et al.



Wadhwani et al. 2005


1998 Travelling microscope. 3 master castings Marginal adaptation for metal or plastic rigid
3 single crowns trays is very similar.From 55 to 72 p
For more flexible trays from 137 to 207.5p

2000 Toolmaker's microsocpe and computer Significant differences among the groups
The second group was the most accurate


Measuring microscope





Measuring microscope
Stone casts


No differences in accuracy with different
trays. More distortion for polyether
Silicone stable up to 30 days and 4
pouring.

No differences between disinfected and
non disinfected conditions
Significant differences among the 3
materials in 4 dimensions.
Differences are not significant clinically


Table 2-4 Continued









The amount of load used to seat the tray should also be controlled. Stauffer et al.

used 9.8 N on top of the tray platform to seat them into position (43). Wassell and

Ibbetson standardized the impression procedure using guiding pins and an Instrom testing

machine to seat the impression tray (52). The majority of the studies do not control this

variable when performing the experiments by hand seating the trays.

Thermal Changes

Impressions reach a temperature of approximately 330C after being in the mouth

for 5 min (23). On cooling to room temperature measurable dimensional changes occur as

impression materials have a relatively high coefficient of thermal expansion. A similar

rate and amount of impression temperature rise should be incorporated in laboratory tests.

At present there is no agreement over the best way to do this. The standardization

organizations use water baths at 320C while other workers prefer 350C. Another approach

is to use a heat source within the master model, but the characteristics of heat flow into

the impression has never been specified (55). Stauffer et al. tempered the master model

at 350C before making the impressions to simulate the mouth temperature (42). Eames et

al. used a 37 + 20C water bath to let the impressions harden in it (14). In a different study

the same authors used 32 + 20C water bath for the same purpose (15).

Marcinak and Draughn in 1986 stored the natural teeth model at 370C and 100%

humidity just before the impression was taken (26). Johnson and Craig in 1985 allowed

the impressions to set at room temperature rather that an elevated oven temperature to

avoid expansion of the stainless steel master model with heating (24).

To conclude: Temperatures from room temperature (- 200C ) to 37 + 20C have

been used to reproduce thermal characteristics of the oral environment.









Type of Measurements

Tests can broadly be divided into two main groups depending on whether

measurements are made on the impressions themselves or on the resulting casts. In

addition, measurements may either be made of the individual dies or the inter die

relationships or both (53).

Direct Measurements of the Impression Material: The ADA and BSI bodies

both employ a scribed block which is used to form a disc of impression material.

Measurements are made between the scribed lines on the block and the resulting lines on

the impression discs to give an indication of time-dependent dimensional changes. In

addition the scribed lines provide a measure of surface reproduction. Light and medium

bodied materials should be able to reproduce a 20 [im wide line (53). A logical use of the

standard block method is to inscribe the occlusal surface of a crown preparation with

engraved marks like the ones on the standard and measure them within the impression

Eames et al. (14) used a similar design to test the accuracy of different impression

materials at different thicknesses on the impression tray.

Measurements of Individual Dies: The measurement of dies poured from

impressions is clinically a more realistic method of assessing impression accuracy than

direct measurement of impression shrinkage (53). This is because the accuracy of the die

will determine the final fit of the restoration. In addition it would be difficult to view

microscopically the critical cervical part of a preparation within an impression. Type IV

die stone is generally chosen because it has a minimal linear setting expansion of 0.1 %.

There are some drawbacks involved in this technique. The most important drawback is

that it is impossible to know whether it is the cast or the impression that causes the

biggest problem. Another drawback is that the detail reproducing ability of die material is









poor in comparison with the impression material causing the precision of the

measurements to become questionable. To overcome that problem, different methods

have been used. Thus comparisons of die measurements with master dies have been done

in the following three ways:

1. Assessment of how well castings made on each of the poured dies fit the master
die. (55)

2. Assessment of how well a single master casting made on the master die fits each of
the poured dies. (55)

3. Linear measurements with contact or no contact methods (55)

Table 2-5 Coefficient of thermal expansion of some of the materials used in the master
models (Inlay waxes listed for comparison purposes only)
Material Coefficient x 10 6 /C
Inlay waxes 350-450*
Silicone impression material 20
Acrylic Resin 76
Stainless steel 5.5-17.6*
Tooth (crown portion) 11.4
*Differences in values for different material composition.

The first method is rarely used. It is time consuming because the number of

castings necessary to run a study and at the same time, castings can be another source of

errors that are difficult to control. In this technique the amount of lift of the cast is

measured under the microscope. The lift of the study metal casting may be influenced by

the roughness of the die, the casting, the casting orientation and seating pressure. On the

other hand, if the stone die is oversized the cast will seat loose on the master die and no

lifting will occur, even though some distortion in size was caused by the impression.

The second method is more widely used. In this case, if the stone die is undersized

the cast will seat completely and no lift will be present. Since only one casting needs to

be done it is more popular than the previous method. Eames et al. (15) used this method

in his study. Careful manipulation and standardized seating pressure and orientation are









necessary to avoid abrasion of the stone dies and record false lift measurements (closer

fitting).

The third method is basically about linear measurements made using contacting

instruments such as calipers, vernier calipers or dial gauges, or non contacting

measurements like the ones obtained with different microscopes such as the traveling

microscopes, toolmaker's microscope or the sophisticated reflex microscope (3D

measurements). Non contact measurements are preferred over contact methods because it

prevents stone abrasion. The second method, the one that uses one master casting to fit all

resulting study dies, assesses the problem from a more clinical stand point including the

variable of the casting. It is also more susceptible to three-dimensional changes of the

stone dies. On the other hand, many sources of errors are inherently present. The third

method, that uses different non contact measuring devices, only analyses linear

dimensions of predetermined marks which may lead to loose valuable information. The

ideal method would be the one that scans directly the impression and is accurate enough

to digitally compare them three dimensionally by volume and superimposition, in other

words a combination of methods 2 and 3.

Inter Die Relationships: If a fixed partial denture does not fit, the problem may

be caused by distortion of the inter die relationship as well as inaccuracies of the

individual dies. Therefore, an important prerequisite of all inter die relationship studies is

that the individual dies are not allowed to move within the master model. Different

options to measure inter die relationships are described below:

1. Master model and cast master bridge (55)

2. Master model and machined template (55)

3. Contact measurements of inter die relationships (55)









4. Non Contact measurements of inter die relationship
-Two-dimensional (55)
-Three-dimensional (55)

The first technique is similar to that used for individual dies except linked master

castings are use to fit the abutments. Stauffer et al. (43) used a machined stainless steel

master model with four abutments of 50 taper representing two canines and two molars.

A surveyor was used to align the linked castings to the four dies during seating under

standardized force. An average value of lift of the four castings was used to assess

impression accuracy. No distinction could be made between the amount of lift due to

intra abutment inaccuracies and inter abutment inaccuracy. In this situation the method

would not give a good indication of how well a bridge might fit the abutments clinically.

The second technique presents the same issue as the first one, and no distinction

can be made between intra abutment inaccuracies and inter abutment accuracies.

The third approach was used by Stauffer et al. (43) who used a L shaped device

that had gauges to measure changes along the x and y axes of the stone casts and compare

these values with those of the master model. This device recorded differences that were

difficult to rationalize clinically.

The fourth approach is basically the use of microscopes and the measuring of

distances between marks on the master model and impressions. Two or three

dimensional measurements can be made, depending on the instrument and the

methodology employed to calculate distances.

In conclusion: There are many ways to measure intra abutment and inter abutment

impression accuracy. All of them have advantages and disadvantages. The recommended

method should be the one accurately assesses the finish line area. Dies poured from






38


impressions produce a more realistic method of assessing impression accuracy but adds

the errors caused by the properties of the die material. If measured with non contact

devices the detail reproduction of the stones prevents the accurate reading of established

marks. Three dimensional measurements of the dies generate better assessments,

particularly when non contact measurements are used. Such analysis of 3D digital data

was used recently by Brosky et al. to determine the effect of impression tray selection on

accuracy of reproduction of a dental arch (4).














CHAPTER 3
MATERIALS AND METHODS

This study was designed to determine the importance of material thickness control

and tray rigidity on the accuracy of polyvinyl siloxane impressions. To achieve that goal,

the following six impression groups were compared in this study.

Impression Groups

The six impression groups were distributed as follows:

1. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY,
USA) in which heavy and light bodied PVS (Aquasil, Dentsply /Caulk, Milford
DE) material were used with the one phase technique. Tray adhesive was applied
at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI).

2. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY,
USA) in which putty (Exaflex, GC America Inc, Alsip, II) and light bodied PVS
(Aquasil, Dentsply /Caulk, Milford DE) material were used with the two phase
technique. Compression technique. Tray adhesive was applied at least 10 min prior
to the impression (V.P.S. Tray Adhesive. Kerr Romulus, MI).

3. Plastic stock tray (Disposable impression trays, Henry Schein Inc, Melville, NY,
USA) in which putty (Exaflex, GC America Inc Alsip, II) and light bodied PVS
(Aquasil, Dentsply /Caulk, Milford DE) material were used with the two phase
technique. Space was created with a 2 mm plastic pressure formed template (Great
Lakes Company). Tray adhesive was applied at least 10 min prior to the impression
(V.P.S. Tray Adhesive. Kerr Romulus, MI).

4. Metal tray (Rim-lock trays, Dentsply-Caulk) in which heavy and light bodied PVS
(Aquasil, Dentsply /Caulk, Milford DE) material were used with the one phase
technique. Tray adhesive was applied at least 10 min prior to the impression
(V.P.S. Tray Adhesive. Kerr Romulus, MI).

5. Metal tray (Rim-lock trays, Dentsply-Caulk) in which putty (Exaflex, GC America
Inc Alsip, II) and light bodied PVS (Aquasil, Dentsply /Caulk, Milford DE)
material were used with the two phase/compression technique. Tray adhesive was
applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr
Romulus, MI).









6. Metal tray (Rim-lock trays, Dentsply-Caulk, Milford, DE) in which putty (Exaflex,
GC America Inc Alsip, II) and light bodied PVS (Aquasil, Dentsply /Caulk,
Milford DE) material were with the two phase technique. Space was created with a
2 mm plastic pressure formed template (Great Lakes Company). Tray adhesive was
applied at least 10 min prior to the impression (V.P.S. Tray Adhesive. Kerr
Romulus, MI).

Impression Materials

Three impression materials were used for this study. Forty light bodied consistency

cartridges of Aquasil Ultra LV (lot # 050421, Dentsply /Caulk, Milford, DE) were used.

Forty heavy bodied consistency cartridges of Aquasil Ultra Heavy (lot # 050618,

Dentsply /Caulk, Milford, DE) were also used. This material is described as a

quadrafunctional hydrophilic addition reaction silicone. Four standard packages of

Exaflex (lot # 0505201, GC America Inc Alsip, II) putty material Type 0 (very high

viscosity) were also used during the experiments.

Adhesive was applied on all metal and plastic trays at least 10 minutes prior to the

impression. Four bottles of V.P.S Tray Adhesive (lot # 5-1082, Kerr Corporation,

Romulus MI) were used.

Master Model

A lower arch master model was made of self curing Orthodontic Resin (Dentsply-

Caulk, Milford, DE) with resin teeth from canine to canine and four machined stainless

steel dies. The four stainless steel simulating prepared abutment teeth were embedded in

region 37, 34, 44, and 47. In addition, two stainless steel reference posts located in the

region between 37 and 34, and 44 and 47 were placed at the ridge level. The stainless

steel dies were designed to simulate circular full crown preparations with shoulders. The

molar die preparations had 60 total occlusal convergence (TOC), were 7.0 mm high, had

a cervical outer diameter of 9.0 mm and a shoulder width of 1.0 mm. The premolar dies









had the same TOC, height and shoulder width, but an outer diameter of 7 mm. The root

portion of the stainless steel dies were 15 mm in length and had a design that well locked

them inside the resin to prevent rotation and vertical displacement. Approximately 1 mm

of the dies root portion was exposed on all four dies. The two reference rods consisted of

two stainless steel rods, 3 mm in diameter, were inserted at the ridge level, halfway

between the stainless steel replicas located on each side. A diamond bur (Maxima

Diamonds, 801-012C-FG, Henry Schein Inc, Melville, NY, USA), attached to a plan-

parallel meter (PFG 100; Cendres & Meraux, Bienne, Switzerland), was used to make

marks on the occlusal surfaces and the shoulders of the dies as well as on the stainless

steel rods between the dies. These marks under the microscope looked like targets on

which the center was the reference for each particular mark.























Figure 3-1. Plastic stock tray used for the study (Disposable Impression trays, Henry
Schein Inc, Melville, NY, USA)






























Figure 3-2. Metal stock tray used in the study (Rim-lock trays, Dentsply/Caulk, Milford,
DE, USA)


Figure 3-3. Occlusal view of the master model. Note the stainless steel rods between
abutment replicas






























Figure 3-4. Frontal view of the master model


Figure 3-5. Buccal view of nine of the eleven marks on the master model with their
corresponding numbers (One mark on each occlusal aspect of 44 and 47)






44






















Figure 3-6. Lingual view of the marks on the master model with their corresponding
numbers (occlusal marks on 44 and 47)


Figure 3-7. Impression materials used for the study. Aquasil Ultra heavy and light bodied
and impression gun (Dentsply/Caulk Milford, DE), Exaflex putty (GC
America Inc, Alsip, II), and V.P.S. Tray Adhesive (Kerr, Romulus, MI)


I' 'rj









Table 3-1. Tray rigidity and thickness control for the different study groups
Group Tray rigidity Thickness
1 Low High
2 Low Minimum*
3 Low Low*
4 High High
5 High Minimum*
6 High Low*

High Thickness: Represents in this study the Heavy/Light impression technique in

which the material thickness is not controlled at all (stock trays used).

Minimum Thickness: Represents in this study the Putty/ Light technique in which

the material thickness was controlled using a two step technique without creating space

for the second impression.

Low Thickness: Represents in this study the Putty/Spacer/ Light technique in

which the material thickness was controlled using a two step technique creating a 2 mm

even space for the second impression.

Low Rigidity: Represents the rigidity of the plastic stock trays

High Rigidity: Represents the rigidity of the metal stock trays

The eleven marks location of the metal dies for each side (right and left) were the

following: Five marks on tooth 47 abutment one distal, one mesial, one buccal, and one

lingual on the 1 mm shoulder, while the fifth mark was on the occlusal surface of the die.

Five similarly located marks were present on tooth 44. The eleventh mark was on the

metal rod in between tooth 44 and tooth 47 and served as a reference point for the other

ten points. In this study only tooth 44 and tooth 47 abutment replicas were used for the

experiments.









Impression Making Device

Once the master model had been constructed according to the specifications

mentioned before, the next step was to standardize the way the impressions were made.

For that purpose, the master model was attached by two screws to a half inch thick

aluminum plate. This plate was seven inches long and five inches width. Three stainless

steel pins, each one with a diameter of three eights of an inch and a height of five inches,

were vertically positioned on the aluminum plate, two in the front and one in the back of

the master model.

The three vertical pins on the base plate guided a second plate to which either the

metal or the plastic tray was attached. It was necessary to build separate plates for the

metal trays and plastic trays. These plates were made in aluminum and had the same

dimensions as the previously described base plate. These two top plates with their holes

sliding along the rods of the base plate allowed the top plates to slide very precisely onto

the master model during the impression procedures. This system controlled the

positioning of the impression trays in three dimensions every time an impression was

made. Three plastic stops were assembled on the pins in order to control the seating of

the tray against the model. Two different sets of vertical stops were built, one set for the

metal trays and the other for the plastic trays. Sixty complete lower arch impressions in

PVS material utilizing the techniques and trays described for each group were made of

the master model. All impressions were made at room temperature (230 to 250C) and kept

at room temperature during the 24 h period before measuring them under the microscope.

The humidity where the impressions were stored was between 54 and 56 %. Ten

impressions per group were made.









Impression Procedures Sequence and Standardization

The following was the impression sequence followed for impressions made either

with plastic or metal trays. An homogeneous thin layer of tray adhesive was applied to all

the trays at least 10 min before the impression was made. Different weights were used

during impression making because of differences in viscosities of the material as well as

differences between the different impression techniques. Initially 3 lb. was planned to be

used to seat the trays to the standard position. Later on during the preliminary tests it was

realized that the putty/light technique needed 28 lb. to reach the standard master model

tray position. Different impression procedures needed different pressures to seat the tray

into proper positions.






















Figure 3-8. Master model attached to an aluminum plate. Note the three stainless steel
guiding pins with the three plastic vertical stops




























Figure 3-9. Metal tray secured to a second plate by a screw. Note the three holes that
match the three guiding pin.


Figure 3-10. Plastic tray secured to a second plate by a screw. Note the three holes that
match the three guiding pins. There is one plate design for plastic and another
for metal.






























Figure 3-11. Lateral view of the two plates assembled previous to an impression
procedure with the plastic tray


Figure 3-12. Lateral view of the two plates assembled previous to an impression
procedure with the metal tray. Plastic stops for the metal tray are different in
length from the ones for the plastic tray.









Heavy/Light Bodied Technique Groups One and Four: The lower plate was

prepared with the stops that matched the top plate (either plastic or metallic). The tray

was screwed into place and checked against the master model for proper seating. Then

the top plate was taken off, the tray was loaded with the heavy bodied consistency

material and simultaneously the light bodied consistency material was injected directly

onto the abutment replicas of tooth 44 and tooth 47. The top plate was placed and guided

close to reach the stops and then a 3 lb weight was placed on top to fully seat the tray

against the master model to the pre established ideal master /tray relation. This ideal

master model/tray relation was built into the top plate design. When seating the top tray

against the master model it was centered in close proximity to the model without

touching it. After this procedure the impression was left undisturbed for 10 min and then

removed in one quick pulling action. Last, the impression was inspected to verify that no

bubbles were present on the marks.

Putty/ Light Bodied Without Spacer Technique Groups Two and Five: The

lower plate was prepared with the stops that matched the top plate (either plastic or

metallic). The tray was screwed into place and checked against the master model for

proper seating. Then the top plate was taken off, the tray was loaded with the putty

material. The top plate was placed and guided close to reach the stops and then a 20 lb

weight was placed on top to fully seat the tray against the master model to the pre

established ideal master /tray relation. After this procedure the impression was left

undisturbed for 6 min and then removed in one quick pulling action. Then the light

material was injected around the abutments and inside the already set putty impression.

The top plate was placed and guided close to reach the stops and then a 28 lb weight was









placed on top to fully seat the tray against the master model to the pre established ideal

master/tray relation. The impression was left undisturbed for 10 min and then removed in

one quick pulling action. Last, the impression was inspected to verify that no bubbles

were present on the marks.

Putty/ Light Bodied With Spacer Technique Group Three and Six: The lower

plate was prepared with the stops that matched the top plate (either plastic or metallic).

The tray was screwed into place and checked against the master model with the 2 mm

plastic spacer (Copyplast, Scheu Dental- Gmbh) in place for proper seating. Then the

top plate was taken off, the tray was loaded with the putty material. The top plate was

placed and guided close to reach the stops and then a 20 lb weight was placed on top to

fully seat the tray against the master model to the pre established ideal master /tray

relation. After this procedure the impression was left undisturbed for 6 min and then

removed in one quick pulling action. The 2 mm plastic spacer was carefully retrieved

from the putty and placed aside.

Then the light material was injected around the abutments and inside the already set

putty impression. The top plate was placed and guided close to reach the stops and then

a 3 lb weight was placed on top to fully seat the tray against the master model to the pre

established ideal master /tray relation. The impression was left undisturbed for 10 min

and then removed in one quick pulling action. Last, the impression was inspected to

verify that no bubbles were present on the marks.

Measurements

Initially the coordinates (x, y and z) of the 11 marks on the master model were

recorded. This coordinates were recorded 10 times on the master model for teeth 44 and

47 and for the reference rod using a measuring microscope (Figures 6 and 7) (Unitron









Universal Measuring Microscope, Unitron Instruments, Inc, Plainview, NY, USA). Same

readings were made on impressions made no more than 24 h earlier. With the

impressions attached to the table of the microscope, the coordinates for the eleven marks

present on teeth 44 and 47, and the reference rod were recorded. Ten impressions per

group were measured. Using the Pythagoras formula in three dimensions and the program

Microsoft Excel (Microsoft Corporation) computer program the distances between the

marks on the abutment replicas and the reference rod were calculated. The formula

utilized to measure the distance between 2 points using the coordinates x, y and z was the

following:

Distance from mark 1 to 6 = (X -X6)2 + (Y -Y6)2 + (Z Z6)2

A total of 11 measurements per impression resulted from the computer calculation.

Distance 1 is the distance between mark 1 on the master model and the reference mark

which is mark 6. Distance 2 is the distance between mark 2 on the master model and the

reference mark which is mark 6, and so on for every mark.

Statistical Evaluation

Six rounds of measurements of the 11 marks on teeth 44 and 47 and the reference

mark on the stainless steel rod were done initially to determine the distances between the

marks and the inherent errors associated with the measuring technique. Another 6 rounds

of measurements were also done of a preliminary impression for similar purposes. Based

on these results a statistical evaluation using a subunit of the SAS program (Statistical

Program) mean values and standard deviations of master model and impression

measurements were compared to determine the number of specimens needed to detect

significant differences (p<0.05).









According to those calculations, a minimum of seven impressions per group would

be needed to prove significant differences (p<0.05) between measurements on the master

model and the studied impressions. Based on that finding, we decided to use 10

impressions per group. The coordinates of the eleven marks were recorded 10 times for

the master model. The mean value for the different distances was calculated and used as

the master model dimensions. The coordinates of the eleven marks on each impression

were recorded once per impression, ten impressions per group. Comparisons between the

different tray (stiffness) groups and impression thickness groups were conducted by use

of a t-test and pair wise comparisons.













































Figure 3-13. Lateral view of measuring microscope (Unitron Instruments, Inc)




























Figure 3-14. Close up views of measuring microscope with master model in position for
measurement and measuring devices on the instrument (silver knobs on the
right image) (Unitron Instruments, Inc)


Figure 3-15. Close up views of measuring microscope with master model in position for
measurement and measuring devices on the instrument (silver knobs on
the right image)(Unitron Instruments, Inc)
































Figure 3-16. Lateral view of the impression device with the 3 pounds weight on top
while taking one of the third group impressions














CHAPTER 4
RESULTS AND DISCUSSION

The results shown in Tables 4 -10 represent 11 distances expressed in microns

between ten points on the abutments # 44 and # 47 and the reference point. Table 4 shows

values for the master model. The reference point in Table 4 is represented by

measurement 6 which is equal to 0 (compared to itself). Mean values, maximal values,

standard deviations and minimal values are also shown in the tables. The distances

between the different marks and the reference point were calculated and compared with

the master model. The reference point was used in this study to be able to identify

changes in space of each abutment.

Using a t-test and pair wise comparisons, significant differences (p<0.05) were

found between four of the investigated groups and the master model. All techniques (PL,

SP and HL) used with the plastic trays had distances that were significantly different

from the master model, while for the metal trays it was only the HL technique that

resulted in a distance that was significantly shorter than the matching distance on the

master model.

Groups 2 and 3 used plastic tray /putty/light (PPL) and plastic tray/putty/light with

spacer (PSP) respectively, and were the two groups that had the largest number of

distances which were significantly different from the master model. For group 2 (PPL),

the distances numbered 3, 4, 8 and 10 had values that differed with the master model

ranging from 94.2 [pm on distance 4 to 161.8 [pm on distance 10. For group 3 (PSP),

distances 1, 3, and 8 differed from the master model with a difference ranging from 75.8









gm to 106.2 gm. Distortion found on groups 2 (PPL) and 3 (PSP) may be attributed to

tray distortion.

Group 1 consisting of plastic tray/heavy-light material (PHL) and group 4

consisting of metallic tray/heavy-light material (MHL) respectively also presented

significant differences in one of the 10 distances. Group 1 (PHL) was significantly

different in distance 5 with 72.6 [m and group 4 (MHL) in distance 2 with 61.4 im.

Changes in groups 1 (PHL) and 4 (MHL) may be related to material bulk and

polymerization shrinkage. The discrepancies found on these two groups are probably still

clinically acceptable for certain procedures. Differences in distances up to 90 gm

between abutments for a fixed partial denture have been estimated as acceptable (46) due

to the fact that the periodontal ligament measures from 100 gm to 250 gm (57). Probably

even higher values than 90 [m are acceptable for some patients. It means that perhaps

under pressure the bridge fabricated from a slightly differently sized cast could seat onto

the abutments and fit properly against them. This amount of distortion for a multiple

implant bridge/structure would have a different outcome; it would probably be clinically

unacceptable because its inability to adapt to the stiff implants. Vigolo et al. in their study

about impression techniques for multiple implant found that discrepancies up to 34 gm

were judged as acceptable and "passive" to manual and visual inspection (53).

Fortunately, such big variations in length are found only when dealing with edentulous

spans where the impression material bulk is big and is highly susceptible to

polymerization shrinkage and thermal changes. Intra abutment dimensions are not

affected enough by all impression variables to make them clinically important. It has

been recognized in the past that dimensional changes in intra abutment dimensions are









very minimal when a-silicone or polyether is used in conjunction with stock or custom

trays (40, 45, and 49). Ultimately, the main goal is to have a restorative margin sealed

either for a single crown or for a multiple unit fixed partial denture.

The rigidity of the tray is one of the multiple factors related to impression accuracy

(13). Great distortions of trays have been shown in a study when comparing plastic stock

trays with metal trays while performing impressions with putty material. (10). Plastic

tray flexibility was probably the cause for the distortion seen for groups 2 (PPL) and 3

(PSP) where the pressure created by the putty could have distorted initially the trays and

then the pressure of the light material during the second impression stage increased the

distortion even more. Rigid trays have been recommended by some authors (13, 9) in

order to reduce distortion during seating and removal of the trays from the patient's

mouth. Gordon et al. found up to 100 stm difference on inter abutment distances and 260

ltm cross arch discrepancy when using plastic stock trays. They attributed this distortion

to tray flexibility. Comparable distortion was found in this study with the plastic trays

when using an impression technique with putty material (19).

It is almost impossible to simulate and analyze all the variables affecting such a

complex event as the impression procedure is. The complexity of impression making

probably goes even further than one could possibly imagine. Local anesthetics and the

time at which the impressions are made have been shown to have the most significant

impact on the final clinical outcome (54). Further more, materials which do not perform

well in laboratory studies do sometimes very well in clinical studies (54). In our study

many variables such as tray rigidity (stock plastic or stock metal), material bulkiness,

type of impression material, tray adhesion, tray seating pressure, pouring time and









impression technique were considered. Other variables were purposely not considered in

order to isolate and simplify the studied variables. Some of the variables not considered

were: use of custom tray, mouth temperature, moisture, undercuts, other impression

materials, cast production, and castings just to name some of them.

In our study, the master model was designed in line with what has been done in

previous studies (Tables 2.1 to 2.4). Six degree taper stainless steel abutments for a 4

unit fixed partial denture. The reviewed studies have used from parallel walls up to 120

taper, which probably is closer to reality. The base of the model was fabricated in plastic

due to the fact that this study did not simulate oral temperature. Therefore a more stable

model base such as metal was not needed. Some studies may have incorporated this

potential source of error inadvertently (39). Pins to standardize tray seating are very

popular among these in vitro studies (43, 52). Seating pressure is not commonly

standardized, but it seems to have some influence on material behavior (54). No studies

were found on this specific topic. In previous studies, weights as well as universal testing

machines have been used to standardize the forces while seating the tray against the

master model time after time (43, 52). Interestingly enough, the forces used in a previous

study with an Instron testing machine closely resemble the ones used in our study (52).

The differences in weights used for the different techniques were due to the fact that

different techniques, materials and trays required different levels of pressure to establish

ideal master /tray relationship. As an interesting observation, the metal trays, when

loaded with the putty for the first step impression (putty groups) always took little longer

time to reach the stops. Plastic trays, probably due to their higher flexibility, did not show

this behavior confirming the results by Cho et al. (10) regarding tray distortion. This









behavior resembles what happens clinically when we need to apply different pressures

while using different impression material viscosities. The top plates used to attach the

trays (one for plastic and one for metal trays) weighted 2 lb each. This weight was not

incorporated in the description of the different techniques. It is important to mention that

these 2 lb were not included when we discuss tray loads. In other words, the total tray

load is 2 lb higher than listed.

This study measured the distances directly on the impressions and not on stone

models like many others have done (14, 15, 27, 37, and 43). First, by measuring the

impressions, errors incorporated during gypsum pouring could be avoided. Second,

before this study was conducted, an impression was made of the master model using an a-

silicone H/L technique. The impression was then poured with type IV stone and

evaluated under the microscope. The marks created with a diamond bur on the stainless

steel abutments of the master model were very easy to read on the impression under the

microscope, but the same marks were blurred and poorly defined in the stone cast. For

those reasons, stone casts measurements were not incorporated in this study. Stone casts

and metal castings resemble closely what happens in the dental laboratory, but such an

evaluation would introduce many more variables and sources of errors, making it even

more difficult to identify the real influence of the variables being studied.

It is also known that temperature changes have great influence on impression

materials and their accuracy (16). After 5 minutes in the mouth an impression can reach

330C (23). When retrieved from the mouth, the room temperature is about 230C. That is a

100C drop in temperature. Furthermore, when poured, the water temperature is even

lower and may also influence the thermal contraction of the impression. Impression









temperature changes from the mouth (370C) to room temperature (230C) was found in

one study to be the dominating factor in die inaccuracy (16).

Undercut is another variable that was not included in this study. Its importance

regarding impression accuracy is well recognized (12). The greater the undercut is, the

more likely a thin layer of impression material will deform permanently. On the other

hand, the thicker the material layer is, the more susceptible it becomes to polymerization

shrinkage. Thin layers of 2-3 mm of impression material are accepted to produce

accurate dies even in the presence of undercuts (21, 22, and 24).

The instrument used for the measurements is a Unitron Microscope capable of

measuring down to 1 im. Coordinates were recorded for each of the eleven marks on

abutment 44, abutment 47, and reference point. Later the coordinates were used to

calculate distances in the computer. Coordinates x and y were very easy to read in a very

precise manner. The z coordinate, which was recorded with the lens scale, was much

more cumbersome to determine and therefore less precise and less reproducible.

Therefore, the accuracy for the z coordinate is much lower than for the other two.

A major limitation with our study is that we did not consider intra abutment

measurements. It has been expressed theoretically that bucco-lingual dimensions of dies

produced from distorted putty impressions from tray recoil are much smaller, producing

oval shape dies rather than of round ones (9). However, it was very unlikely that we

could have detected any significant difference measuring the impressions directly.

One study reported better fitting of the resulting castings on the master model when

metal or rigid plastic trays were used (9).









Table 4-1 Mean and standard deviation of ten rounds of measurements performed on the
eleven marks of the Master model
Distance # n Mean (jim) S.D. (jim) Minimum (jim) Maximum (jim)

1 10 18071.5 27.9 18032.4 18121.8
2 10 14899.0 32.5 14846.2 14948.7
3 10 15577.4 26.3 15538.3 15613.6
4 10 9186.0 46.7 9087.3 9251.2
5 10 17665.2 70.1 17567.9 17789.3
6 10 0 0 0 0
7 10 9994.1 26.1 9959.0 10036.5
8 10 14336.6 25.3 14313.2 14390.0
9 10 12629.7 35.5 12594.5 12708.0
10 10 15721.6 18.9 15694.6 15765.6
11 10 14758.2 58.3 14655.2 14861.1
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation

Table 4.2 Measurements from ten impressions taken with a plastic tray and the
heavy/light bodied technique (Group 1)
Distance # n Mean (jim) S.D. (jim) Minimum (jm) Maximum (jim)
1 10 18062.1 24.3 18033.8 18104.1
2 10 14888.0 37.2 14835.4 14961.7
3 10 15579.5 25.0 15543.3 15625.8
4 10 9190.4 61.2 9124.2 9306.4
5 10 17592.6 63.5 17510.0 17738.2
6 10 0 0 0 0
7 10 9984.1 27.1 9943.3 10049.2
8 10 14302.7 29.2 14269.5 14352.9
9 10 12605.8 25.4 12575.7 12657.5
10 10 15737.9 36.6 15694.6 15822.4
11 10 14722.7 299.4 13903.0 14954.4
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation









Table 4-3 Measurements from ten impressions taken with a plastic tray and the
putty/light bodied without spacer technique (Group 2)
Distance # n Mean (jim) S.D. (jlm) Minimum (im) Maximum (jim)
1 10 18043.3 128.3 17856.0 18353.9
2 10 14917.4 83.1 14855.6 15133.5
3 10 15476.4 102.2 15364.6 15737.5
4 10 9280.1 77.6 9217.1 9477.2
5 10 17623.7 126.7 17458.6 17888.1
6 10 0 0 0 0
7 10 10189.4 587.0 9775.7 11831.8
8 10 14194.4 89.5 13975.2 14264.0
9 10 12686.5 57.2 12613.6 12828.5
10 10 15559.8 299.5 147514.6 15749.9
11 10 14783.8 116.4 14492.1 14914.9
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation

Table 4-4 Measurements from ten impressions taken with a plastic tray and the
putty/light bodied with spacer technique (Group 3)
Distance # n Mean (jm) S.D. (jm) Minimum (jm) Maximum (jm)
1 10 17995.7 26.0 17955.6 18031.2
2 10 14896.5 61.0 14808.0 15025.6
3 10 15473.4 78.3 15288.1 15564.0
4 10 9163.2 45.3 9081.1 9212.8
5 10 17594.5 72.8 17475.1 17702.8
6 10 0 0 0 0
7 10 9998.4 85.6 9900.9 10217.7
8 10 14239.8 64.0 14109.6 14343.6
9 10 12627.4 23.6 12596.8 12677.3
10 10 15658.2 84.5 15447.4 15779.4
11 10 14772.1 55.8 14701.8 14912.0
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation









Table 4-5 Measurements from ten impressions taken with a metal tray and the heavy/light
bodied technique (Group 4)
Distance # n Mean (jim) S.D. (jlm) Minimum (im) Maximum (jim)
1 10 18066.3 17.6 18029.6 18089.8
2 10 14837.7 81.4 14617.0 14898.1
3 10 15631.9 103.1 15580.6 15924.2
4 10 9164.7 24.9 9132.8 9209.7
5 10 17658.7 69.9 17542.8 17739.3
6 10 0 0 0 0
7 10 9997.0 18.5 9979.9 10044.8
8 10 14335.4 49.3 14197.5 14370.0
9 10 12802.5 504.7 12600.4 14221.4
10 10 15761.7 45.9 15730.8 15858.8
11 10 14806.3 65.1 14699.3 14916.9
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation

Table 4-6 Measurements from ten impressions taken with a metal tray and the putty/light
bodied without spacer technique (Group 5)
Distance # n Mean (jm) S.D. (jm) Minimum (jm) Maximum (jm)
1 10 18054.3 15.1 18032.1 18079.8
2 10 14854.1 22.2 14829.6 14900.0
3 10 15579.7 19.2 15553.0 15615.0
4 10 9211.1 17.2 9186.3 9235.4
5 10 17667.0 68.4 17544.6 17744.7
6 10 0 0 0 0
7 10 9939.0 312.5 9050.8 10078.2
8 10 14356.3 19.1 14317.0 14386.6
9 10 12711.1 114.5 12627.0 12883.7
10 10 15783.0 19.6 15755.7 15808.2
11 10 14832.5 80.1 14685.5 14934.9
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation










Table 4-7 Measurements from ten impressions taken with a metal tray and the putty/light
bodied with spacer technique (Group 6)
Distance # n Mean (jim) S.D. (jlm) Minimum (im) Maximum (jim)
1 10 18047.1 19.8 18018.9 18073.6
2 10 14886.0 24.6 14858.7 14931.1
3 10 15568.6 29.0 15525.0 15611.7
4 10 9176.4 28.6 9136.9 9218.4
5 10 17675.0 75.9 17538.7 17745.0
6 10 0 0 0 0
7 10 9991.6 20.2 9950.8 10022.9
8 10 14329.1 20.3 14291.1 14354.2
9 10 12618.1 19.4 12584.5 12654.4
10 10 15730.1 24.8 15696.0 15777.0
11 10 14800.0 69.6 14677.9 14917.2
D = Distance measured
n = Number of measurements of the same distance in the master model.
S.D. = Standard deviation


80
70
M60
I
c 50
R 40
0 30
N
S 20
10
0


MHL PHL MPL MPS PPL PSP
IMPRESSION GROUPS


Figure 4-1. Difference between master model and impression groups in distance 1.

Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange.







67






70
61.39*
60
50 S44.93
M 40
I
c 30
R 20 13
2.58
O 10
N
S 0
-10
20
-20 18.37
-30
IMPRESSION GROUPS



Figure 4-2. Difference between master model and impression groups in distance 2

Significantly different values marked with a red star. Plastic tray groups in blue and

metal tray groups in orange



120
120 100.98* 103.96*
100
80
M 60
I
40
R 20 8.81
O 0

-20 MP PHI MPS P PSP
-40
-2.27 -2.13
-60
-60 -54.48
-80
IMPRESSION GROUPS


Figure 4-3. Difference between master model and impression groups in distance 3.

Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange




















0


-20-


-O
-40--


-60--


-80--


-100


9.58 21.33 22.82


PHL

-4.41


MHL


-2D. 11


-94.15 *


IMPRESSION GROUPS


Figure 4-4. Difference between master model and impression groups in distance 4.


Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange


/ U.04


6.47


/AlI


DDI


DQD


/Z.bl


DWI


-1/.8


IMPRESSION GROUPS

Figure 4-5. Difference between master model and impression groups in distance 5.


Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange


II


MPS


PSP


t I .t


-9.6







69






10
5.52
-0.29
0.25 1.01
M 0
I ^PSP MHL MPS PHL MPL
C -5 -0.43
R
0 -10
N
s -15

-20
-19.53
-25
IMPRESSION GROUPS

Figure 4-6. Difference between master model and impression groups in distance 7.

Significantly different values marked with a red star. Plastic tray groups in blue and

metal tray groups in orange




160 142.2/*
140
120
M 100 96.79*
I
c 80
R 60
O 40 33.91
N 7.51
20







Figure 4-7. Difference between master model and impression groups in distance 8.

Significantly different values marked with a red star. Plastic tray groups in blue and
metal tray groups in orange
0 -
-20 MHI MPS PHI PSP PPI
-40
-19.7
IMPRESSION GROUPS

Figure 4-7. Difference between master model and impression groups in distance 8.

Significantly different values marked with a red star. Plastic tray groups in blue and

metal tray groups in orange

















0 --


-50


-100


-150


IMPRESSION GROUPS


Figure 4-8. Difference between master model and impression groups in distance 9.

Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange



200
161.79 *
150
M
S100-
C 63.39
R 50 -
0
N 0-
S PHL MPS PSP PPL
-50
-40.14 -16.35 -8.53
-61.49
-100
IMPRESSION GROUPS

Figure 4-9. Difference between master model and impression groups in distance 10.

Significantly different values marked with a red star. Plastic tray groups in blue and


metal tray groups in orange


236 11.67 23.94

-PSP MPS PHL




-81.35















uu

40 -35.55

M 20
I
C 0
R PHL
O -20
N -25.59 -139
s -40
S -40 -41.46

-60

-80 -4.28
IMPRESSION GROUPS



Figure 4-10. Difference between master model and impression groups in distance 11.

Significantly different values marked with a red star. Plastic tray groups in blue


and metal tray groups in orange


200

150

100

50

0

-50

-100

-150

-200


DISTANCES MEASURED


Figure 4-11. All distances mean difference value for group 1 (PHL) in comparison to
master model.


72.61

-2.13 33.91 23.94 35.55

9.49 11.07 0 1.01

1 2 3 4 5 6 7 8 9 10 11
-4.41 -16.35
















142.27
100.98


2826 41.45

0

1 2 3 5 6 7 8 10
-18.37 -19.53 -56.42 -25.59


-94.15


200

150

100
M
I 50
C
R 0
O
N -50
S
-100

-150

-200


DISTANCES MEASURED

Figure 4-12. All distances mean difference value for group 2 (PPL) in comparison to
master model.





200

150 -- "k
75.82 103.96 96.79

M 1000.64 63.39
I 50
C 2.58 0 2.36
-0.43 M
0 1 2 3 4 5 6 7 8 9 10 11
N -50 -13.9
S
-100

-150

-200
DISTANCES MEASURED

Figure 4-13. All distances mean difference value for group 3 (PSP) in comparison to
master model.













200

150

100
M 61.39
S5021.33
I 50
C 5.26 6.47 0 -0.29 1.2
R 0
O 1 2 4 5 6 7 8
-50
N -50 -40.14 -4806
-100 -54.48

-150

-200 -179 73
DISTANCES MEASURED




Figure 4-14. All distances mean difference value for group 4 (MHL) in comparison to
master model.



200

150

100
M
44.93
I 50 172
C 0
R 0 -2.27 5.52
0 1 2 3 5 6 7 8
N -50 -
S -25.11 -17.8 -19.7 -61.49
-100 -81.35 -74.28

-150

-200
DISTANCES MEASURED

Figure 4-15. All distances mean difference value for group 5 (MPL) in comparison to
master model.







74




200

150

100
M
I 50 -24.43
C 13 8.81 9.58 0 0.25 7.51 11.67 7.51 11.67
R 0 ,
O 1 2 3 4 5 6 7 8 9 10 11
N -50
S -9.8
-100 -

-150

-200
DISTANCES MEASURED

Figure 4-16. All distances mean difference value for group 6 (MSP) in comparison to
master model.














CHAPTER 5
SUMMARY AND CONCLUSIONS

Plastic trays produced less accurate impressions than metal trays. When metal trays

were used, putty based impressions were dimensionally better than heavy/light body

impressions. Consequently, tray rigidity and material bulk control through the use of two

stage techniques improved impression reliability.

Future work can be done as described here. As shown in other studies (9, 13, 14,

and 34), this study supported claims that factors such as the control of the impression

material thickness and the tray rigidity affect the impression accuracy. Custom trays

have been the gold standard for many years because they control the thickness, but little

attention has been paid to the rigidity required by them to perform well during impression

procedures. In a future project it would be interesting to test under the same conditions

described in this study the performance of custom trays against the Rim-Lock metal tray

using putty viscosity technique in two steps with the metal trays. It would also be

interesting to include as impression technique in a new project the putty/light one step

impression technique. This technique has been criticized in the past because supposedly

some of the margins are imprinted in putty material which does not fulfill the

specifications for detail reproduction. Is it truly putty against the margin or a few microns

layer of light body material that cannot be seen by the human eye? The reason for that

critique has never been supported by reliable research evidences.
















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BIOGRAPHICAL SKETCH

I got my dental diploma from the "Instituto de Ciencias de la Salud CES" in

Medellin, Colombia, South America, in December of 1995. My undergraduate thesis was

"Craniofacial and Dentoalveolar Changes with First Permanent Molars Extraction" which

was part of a large longitudinal study in growth and development. In 1997 I enrolled in a

two and a half year specialty program in periodontal-prosthesis at the same university. I

became a specialist in April 2000. My thesis project "SAMM-III. Analysis and Design of

a Mandibular Movement Measurement System" was named as the Best Dental

Postgraduate Research Work 2000. After living in Colombia since age 4, I decided in

2000 to come back to the United States fleeing from the violence in this country. From

2001 to 2003, I did the Foreign Trained Dentist (FTD) program at University of Florida,

obtaining the Florida dental license the same year. In 2003, I started a three year

specialty program in prosthodontics with a Master of Science at the University of Florida.

I received my Master of Science in prosthodontics in May 2006. Currently, I am planning

to establish my dental practice in Ocala, Florida, limited to prosthodontics. I am also

planning to serve as a visiting faculty in the graduate prosthodontics program at

University of Florida. My wife Paula has been my support and engine throughout all

these years at school. After completion of my specialty a new era starts in our lives.