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The Effect of Incomplete Seating of the Implant Screwdriver Tip in the Abutment Screw Head, an In-Vitro Study

Permanent Link: http://ufdc.ufl.edu/UFE0042793/00001

Material Information

Title: The Effect of Incomplete Seating of the Implant Screwdriver Tip in the Abutment Screw Head, an In-Vitro Study
Physical Description: 1 online resource (66 p.)
Language: english
Creator: ALABHOOL,HAYA S
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: ABUTMENT -- ANGULATION -- DENTAL -- IMPLANT -- MATERIALS -- METAL -- SCREW -- STRIPPING
Dentistry -- Dissertations, Academic -- UF
Genre: Dental Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: 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 THE EFFECT OF INCOMPLETE SEATING OF THE IMPLANT SCREWDRIVER TIP IN THE ABUTMENT SCREW HEAD, AN IN-VITRO STUDY By Haya Alabhool May 2011 Chair: Chiayi Shen Major: Dental Sciences In implant dentistry, the stripping of the abutment screw head socket often occurs during the restorative phase of treatment. Retrieving these deformed screws can be very time-consuming and costly for both the patient and the restorative dentist. In addition, the restoration, abutment and/or the implant can get damaged during the screw retrieval process. Such problem can be avoided if there is a good fit between the driver tip and the socket of the screw head during tightening or loosening of the abutment screw. This is often accomplished when the socket is free of debris and the driver is perpendicular to the abutment screw axis. There are several common conditions that could prevent full driver seating, such as, plaque accumulation, cotton pellet remnants, or restorative resin residue in the screw head. The specific aims of this in-vitro study are to test the hypothesis that incomplete seating of the screw driver tip in the screw head socket can lead to stripping and to determine the threshold of incomplete seating when the stripping occurs in two implant systems. Also, to test the hypothesis that angulation of the screwdriver tip in the screw head socket can lead to stripping. Twenty-five Zimmer screws ( Code: MHLAS) and Twenty-five Astra tech abutment screws (Code: 24449) were used in this study. The screws were divided into two groups: the first group consisted of twenty screws of each company to study the effect of incomplete seating due to residual debris, and the second group of five screws was assigned to study the effect of angulations. All screws were tightened with the torque value recommended by the manufacturer. Each group was tightened into their correspondent mounted implant (diameter 3.7mm for Zimmer and 4.0mm for Astra). A new screwdriver was used for every five screws tested, and screwdriver tips were examined under the stereo microscope for any plastic deformations. In addition, to help understand the system, microhardness and geometries of screwdrivers and screws were studied. Within the limitations of this in-vitro study, it was shown that incomplete seating of the screwdriver in the abutment screw head as well as angulations of the screwdriver in the head socket could potentially lead to stripping.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by HAYA S ALABHOOL.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Shen, Chiayi.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0042793:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042793/00001

Material Information

Title: The Effect of Incomplete Seating of the Implant Screwdriver Tip in the Abutment Screw Head, an In-Vitro Study
Physical Description: 1 online resource (66 p.)
Language: english
Creator: ALABHOOL,HAYA S
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: ABUTMENT -- ANGULATION -- DENTAL -- IMPLANT -- MATERIALS -- METAL -- SCREW -- STRIPPING
Dentistry -- Dissertations, Academic -- UF
Genre: Dental Sciences thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: 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 THE EFFECT OF INCOMPLETE SEATING OF THE IMPLANT SCREWDRIVER TIP IN THE ABUTMENT SCREW HEAD, AN IN-VITRO STUDY By Haya Alabhool May 2011 Chair: Chiayi Shen Major: Dental Sciences In implant dentistry, the stripping of the abutment screw head socket often occurs during the restorative phase of treatment. Retrieving these deformed screws can be very time-consuming and costly for both the patient and the restorative dentist. In addition, the restoration, abutment and/or the implant can get damaged during the screw retrieval process. Such problem can be avoided if there is a good fit between the driver tip and the socket of the screw head during tightening or loosening of the abutment screw. This is often accomplished when the socket is free of debris and the driver is perpendicular to the abutment screw axis. There are several common conditions that could prevent full driver seating, such as, plaque accumulation, cotton pellet remnants, or restorative resin residue in the screw head. The specific aims of this in-vitro study are to test the hypothesis that incomplete seating of the screw driver tip in the screw head socket can lead to stripping and to determine the threshold of incomplete seating when the stripping occurs in two implant systems. Also, to test the hypothesis that angulation of the screwdriver tip in the screw head socket can lead to stripping. Twenty-five Zimmer screws ( Code: MHLAS) and Twenty-five Astra tech abutment screws (Code: 24449) were used in this study. The screws were divided into two groups: the first group consisted of twenty screws of each company to study the effect of incomplete seating due to residual debris, and the second group of five screws was assigned to study the effect of angulations. All screws were tightened with the torque value recommended by the manufacturer. Each group was tightened into their correspondent mounted implant (diameter 3.7mm for Zimmer and 4.0mm for Astra). A new screwdriver was used for every five screws tested, and screwdriver tips were examined under the stereo microscope for any plastic deformations. In addition, to help understand the system, microhardness and geometries of screwdrivers and screws were studied. Within the limitations of this in-vitro study, it was shown that incomplete seating of the screwdriver in the abutment screw head as well as angulations of the screwdriver in the head socket could potentially lead to stripping.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by HAYA S ALABHOOL.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Shen, Chiayi.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0042793:00001


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1 THE EFFECT OF INCOMPLETE SEATING OF THE IMPLANT SCREWDRIVER TIP IN THE ABUTMEN T SCREW HEAD, AN IN VITRO STUDY By HAYA ALABHOOL A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT O F THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2011

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2 2011 Haya Alabhool

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3 To my father, my daughter, Dalia, and all who made this possible

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4 ACKNOWLEDGMENTS I am heartily thankful to my supervisor, Dr. Chia y i Shen, whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject. I would also like to thank Dr. William C Martin for his time and effort. d Dr. Glenn turner for their insight, support, and inspiration. I offer my regards and blessings to all of those who supported me in any respect during the completion of the project. To my colleague, Dr. Aline Bowers, I express much gratitude and appreciat ion for all the help she provided me. Dr. Bowers was the first one to inspire me with the idea of this research. She conducted a pilot study to relate abutment screw head stripping to torque values and number of loosening/tightening cycles as her graduatio n project. Her research, encouragement, and personal recommendations provided me with a great foundation to start this study, which would be much harder without such information. Thank you, Aline. Last but not the least, my family and the one above all of us, God, for answering my prayers for giving me the strength and patience during this milestone.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 12 2 LITERATURE REVIEW ................................ ................................ .......................... 14 Implant Prosthesis as a Viable Treatment Option ................................ ................... 14 Prosthetic Complicatio ns In Implant Dentistry ................................ ......................... 15 External Hex Versus Internal Hex Designs ................................ ............................. 17 Mechanism of Abutment Screw System ................................ ................................ 18 Friction and Preload ................................ ................................ ......................... 18 Design of Abutment Screws to Maximize Preload ................................ ............ 19 Interaction between Drive r Tip and Retaining Screw ................................ ........ 20 Hypotheses ................................ ................................ ................................ ............. 21 3 METHODOLOGY ................................ ................................ ................................ ... 25 Design and Fa brication of a Device for Testing Hypotheses ................................ .. 25 Mounting Implants in Acrylic Blocks and Their Preparation ................................ .... 25 Effect of Incomplete Seat ing ................................ ................................ ................... 27 Preparation of Debris Loaded Screw ................................ ................................ 27 Measurement of the Debris Quantity ................................ ................................ 27 Experimental Procedure ................................ ................................ ................... 28 Effect of Improper Angulations ................................ ................................ ................ 29 Preparation of Specimen Blocks ................................ ................................ ...... 29 Experimental Procedure ................................ ................................ ................... 29 Characterization of the Latch Driver Tip and the Abutment Screw ......................... 30 Microhardn ess Measurement ................................ ................................ ........... 30 Dimension of the Implant Components ................................ ............................ 31 Appearance of the Specimens after Tests ................................ ........................ 31 Statistical Analysis ................................ ................................ ................................ .. 32 4 RESULTS ................................ ................................ ................................ ............... 39 Effect of Incomplete Seating ................................ ................................ ................... 39 Effect of Improper Angulations ................................ ................................ ................ 39

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6 Characterization of the Hex Driver Tip and the Abutment Screw ............................ 40 Microhardne ss of the Driver Tip and the Head of Abutment Screw .................. 40 Dimension of the Hex Driver and the Abutment Screw ................................ .... 40 Estimation of the Depth o f the Socket of the Abutment Screw ......................... 41 Appearance of the Tested Specimens ................................ ............................. 41 5 DISCUSSION ................................ ................................ ................................ ......... 48 Effect of Incomplete Seating ................................ ................................ ................... 48 Difference between the Two Systems ................................ .............................. 49 Stripping of Zimmer Abutment Screw ................................ ............................... 50 Stripping of Astratech Abutment Screw ................................ ............................ 51 Effect of Microhardenss on the Stripping ................................ .......................... 52 Reliability of the Experimental Data ................................ ................................ .. 53 Effect of Angulations ................................ ................................ ............................... 54 Experimental Design ................................ ................................ ........................ 54 Depth of Stripping ................................ ................................ ............................. 55 Appearance of the Driver after Tests ................................ ................................ 55 Reliability of the Experimental Data ................................ ................................ .. 56 6 SUMMARY AND CONCLUSION ................................ ................................ ............ 62 LIST OF REFERENCES ................................ ................................ ............................... 63 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 66

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7 LIST OF TABLES Table page 4 1 Results of incomplete seating of screwdriver in Astra abutment screw heads ... 43 4 2 Results of incomplete seating of screwdriver in Zimmer abutment screw heads ................................ ................................ ................................ ................ 44 4 3 Hardness values (in Kg/cm 2 ) of the latch driver and hex screw head ................. 45 4 4 Dimension (in mm) of the Hex driver tip and the outer diameter of the screw head ................................ ................................ ................................ ................ 45 4 5 Estimation of the depth (in mm) of the socket of the abutment screw ................ 45

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8 LIST OF FIGURES Figure page 2 1 Illustration of internal hex and external hex implants design. (Illustration courtesy of Haya Alabhool) ................................ ................................ ................. 23 2 2 Illustration of the implant abutment system (sourse: Jaarda MJ, Razzoog ME, Gratton DG. Geometric comparison of five interchangeable implant prosthetic retaining screws. J P rosthet Dent 1995;74(4):373 9.) ........................ 23 2 3 Typical tensile stress/strain diagram of screw placed under tensile load. P: proportional limit; E, elastic limit; Y, yield point; U, ultimate st rength (Source: Jaarda MJ, Razzoog ME, Gratton DG. Geometric comparison of five interchangeable implant prosthetic retaining screws. J Prosthet Dent 1995;74(4):373 9.) ................................ ................................ .............................. 24 3 1 Devic e for mounting implant in acrylic blo ck for testing improper seating (Photos courtesy of Haya alabhool) ................................ ................................ .... 33 3 2 Testing device for the effect of angulation on the stripping of abutment screw (Photos courtesy of Haya Alabhool) ................................ ................................ ... 34 3 3 The mounting device showing implant is being lowered into acrylic resin. The hex driver and abutment screw joint is stabilized with heavy body PVS. (Photo courtesy of Haya Alabhool) ................................ ................................ ..... 35 3 4 Implant specimens (Photo courtesy of Haya Alabhool) ................................ ..... 35 3 5 Measuring device (Photos courtesy of Haya Alabhool) ................................ ...... 36 3 6 Measure the height of the test assembly for the incomplete seating experiment. (Photo courtesy of Haya Alabhool) ................................ ................. 36 3 7 Testing of incomplete seating. (Photo courtesy of Haya Alabhool) ..................... 37 3 8 The handpiece resting on the guiding block and immobilized with a rigid PVS i mpression material (Photo courtesy of Haya Alabhool) ................................ .... 37 3 9 Astr a abutment screws embedded in epoxy resin ready for microhardness measurement. (Photo courtesy of Haya Alabhool) ................................ ............. 38 4 1 Hex driver removed from the socket filled fit check material. (Photos courtesy of Haya Alabhool) ................................ ................................ ............................... 46 4 2 Astra drivers (Photos courtesy of Haya Alabhool) ................................ .............. 46 4 3 Zimmer driver (Photo courtesy of Haya Alabhool) ................................ ............. 47

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9 5 1 Astra abutment screw a nd Zimmer abutment screw (Photo courtesy of Haya Alabhoo l) ................................ ................................ ................................ ............ 58 5 2 Illustration of how the straight Zimmer screwdriver fits inside the screw head leaving a uniform gap around it. (Illustration courtesy of Haya Alabhool) ........... 58 5 3 Illustration of how Astratech screwdriver fits inside the screw head. The screwdriver is in contact with the socket walls at the top of the socket only. (Illustration courtesy of Haya Alabhool) ................................ .............................. 59 5 4 Relationship between Astratech screwdriver and abu tment screw after multiple uses (Illustration courtesy of Haya Alabhool) ................................ ....... 59 5 5 Relationship between Zimm er screwdriver and abutment screw as the driver is raised. The contact area is decreased (Illustration courtesy of Haya Alabhool) ................................ ................................ ................................ ............ 60 5 6 Relationship between Astratech screwdriver and abutmen t screw as the driver is raised ................................ ................................ ................................ ... 60 5 7 When the screwdriver is placed at lower dep ths in the screw socket.angulations permits at least town point contact between the screwdriver and the wall socket (Illustration courtesy of Haya Alabhool) ........... 61 5 8 In depths higher than 0.93 mm, stripping occurred. (Illustration courtesy of Haya Alabhool) ................................ ................................ ................................ ... 61

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10 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirement s for the Degree of Master of Science THE EFFECT OF INCOMPLETE SEATING OF THE IMPLANT SCREWDRIVER TIP IN THE ABUTMENT SCREW HEAD, AN IN VITRO STUDY By Haya Alabhool May 2011 Chair: Chiayi Shen Major: Dental Science s In implant dentistry, the stripping of the abutment screw head socket often occurs during the restorative phase of treatment. Retrieving these deformed screws can be very time consuming and costly for both the patient and the restorative dentist. In addition, the restoration, abutment and/or t he implant can get damaged during the screw retrieval process. Such problem can be avoided if there is a good fit between the driver tip and the socket of the screw head during tightening or loosening of the abutment screw. This is often accomplished when the socket is free of debris and the driver is perpendicular to the abutment screw axis. There are several common conditions that could prevent full driver seating, such as, plaque accumulation, cotton pellet remnants, or restorative resin residue in the s crew head. The specific aims of this in vitro study are to test the hypothesis that incomplete seating of the screw driver tip in the screw head socket can lead to stripping and to determine the threshold of incomplete seating when the stripping occurs in two implant systems. Also, to test the hypothesis that angulation of the screwdriver tip in the screw head socket can lead to stripping.

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11 T wenty five Zimmer screws ( Code: MHLAS) and Twenty five Astra tech abutment screws (Code: 24449) were used in this stu dy. The screws were divided into two groups: the first group consisted of twenty screws of each company to study the effect of incomplete seating due to residual d e b r is, and the second group of five screws was assigned to study the effect of angulations. A ll screws were tightened with the torque value recommended by the manufacturer. Each group was t ightened into their correspondent mounted implant (diameter 3.7mm for Zimmer and 4.0mm for Astra). A new screwdriver was used for every five screws tested and screwdriver tips were examined under the stereo microscope for any plastic deformations. In addition, to help understand the system, microhardness and geometries of screwdrivers and screws were studied. W ithin the limitations of this in vitro study, it was shown that incomplete seating of the screwdriver in the abutment screw head as well as angulations of the screwdriver in the head socket could potentially lead to stripping.

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12 CHAPTER 1 INTRODUCTION Currently, the use of implants to replace missing teeth i s considered the most optimal treatment option because of its predictability and successful outcomes. Most restorative dentists have chosen implant therapy over conventional fixed or removable prosthesis because of the advantages offered by dental implants such as preservation of adjacent teeth, preservation of bone, provision of additional support to increase masticatory function, and resistance to diseases li ke recurrent caries 1 It was documented that implants could enhance the retention and stability of dental prosthesis, and improve occlusal function in patients with acquired or conge nital defects 2 Moreover, the literature shows the overall cumulative 5 year survival and success rate of root shaped dental implants was very excellent (98.3% and 97.3% respectively ) 3 The rehabilitation of t he edentulous maxilla with either implant supported fixed prosthesis or removable overdenture prosthesis is one of the most challenging p rocedures in implant dentistry 4 The goal of restoring the maxilla i esthetics and occlusion, and several variables should be carefully evaluated and analyzed before choosing the best final prosthesis that will meet that goal. The final decision is affected by many factors such as patient preference, phonation, oral hygiene habits, economics, facial and lip support, maxillomandibular relationship, smile line, bone quality and quantity, an d other factors 5 Also, the type of final prosthesis is used as a guide t o determine the number and position of implants before surgical insertion 1 Although implant retained or implant supported restorations have very successful outcome, one should not neglect the complications that accompany such prostheses It was concluded in many clinical studies that screw loosening is a common complication

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13 after insertion of fixed detachable hybrid prostheses. It has been shown that screw fracture ca n occur as a result of loosening of the screw joint leadi ng to failure of the prosthesis 6 Stripping o f the abutment screw h ead socket can occur if the screw is loosened and tightened more than once. Retrieving these deformed screws can be very time consuming and costly for both the patient and the resto rative dentist. In addition, the restoration, abu tment and/or the implant can be damaged during the screw retrieval process. This problem can be avoided if there is a good fit between the driver tip and the socket of the screw head during tightening or loo sening of the abutment screw. This is often accomplished when the socket is free of debris and the driver is aligned with the abutment screw axis. There are several common conditions that could prevent driver from complete seating, such as, plaque accumula tion, cotton pellet remnants, or restorative resin residue in the socket of screw head. Many studies have addressed clinical problems such as abutment/retaining screw fractures and screw loosening during service, but to date, there has been no published li terature relating screw head stripping and residual debris in the socket of screw heads. The presence of debris in the socket of screw head could lead to two scenarios of incomplete seating between the screwdriver tip and the screw head. First, the driver tip will not be fully seated in the socket reducing the area of contact with increasing stresses on the surface of contact Second, the driver may be seated with slight angulation to the axis of the abutment screw resulting in uneven contact between screw d river tip and the inner surface of the socket. Both scenarios could potentially lead to stripping of the screw head

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14 CHAPTER 2 LITERATURE REVIEW Implant Prosthesis a s a Viable Treatment Option Wearing conventional complete denture prosthesis is very diffi cult for edentulous patients especially in the mandible because of the mobility of the floor of the mouth, the thin mucosal lining of the alveolar ridge, the decreased support area, and the mobility of the mandibular jaw. Edentulous patients, who cannot fu nction using their conventional It was shown that the masticatory function of patients with implant retained prosthesis was comparable to those with natural teeth 7 Treatment options for the edentulous mandible could be no treatment, conven tional complete dentures, implant supported fixed prosthesis, implant retained and tissue supported over denture, implant retained and implant supported overdentures, and screw retained fixed detachable prosthesis with acrylic teeth which are a 8 Patients who are given a fixed prosthesis reported an increase in patient satisfaction, and they had minimal post insertion adjustments compare to patients with overdentures. However, overdentures are less expensive than fixed restorations, and still are complete denture because they lack a good muscular control. Furthermore, the amount of remaining bone is a viable factor wh en choosing between the hybrid and metal ceramic restorations. The la t ter option is more costly and may need a higher number of implants to be placed to support the prosthesis. On the other hand, the acrylic teeth in the hybrid prosthesis may need to be replaced in five to six years after insertion, which ad ds to the tota l cost 8 A dvantages and disadvantages of each option should be explained to the patient before making the final decision.

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15 Prosthetic Complications I n Implant Dentistry Complications in implant dentistry can fall into following categories: implant loss, bone loss, pe ri implant soft tissue complications, mechanical complications, and es thetic/phonetics complications 9 Since this thesis is about failure of abutment screws, the author will focus only on mechanical complications. According to the review of prosthodontic literature by Goodacre 9 the types of mechanical complications in the order of decreasing frequency were ( 1) overdenture loosing retention or in need of adjustment (30%), fracture of resin veneer of fixed partial dentures (22%), overdenture in need for reline (19%), overdenture clip/attachment fracture (17%), porcelain veneer fracture of fixed partial dentures (14%), overdenture fracture (12%), fracture of opposing prosthesis (12%), fracture of acrylic resin base (7%), prosthetic screw loosening (7%), abutment screw loosening (6%), prosthetic screw fracture (4%), metal framework fracture (3%), abutment screw fr acture (2%), and implan t fracture (1%). Complications with single tooth implants often involve the integrity of the dental implant abutment screw joint. Several published studies have discussed the general guidelines for the placement and restoration of im plants, and shown that single tooth implants complications include soft tissue complications, abutment screw fracture and most comm only, abutment screw loosening 10 The external hexed platform design was very common in the past (Fig ure 2 1) The initial design was developed by Brnemark to restore fully eden tulous patients. The coronal design was a 0.7 mm tall external hexagon that did not engage the implant as an anti rotational device. Problems occurred when the same design was used to retain single crowns because the short platform. as modified in

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16 heights of 0.9, 1.0 and 1.2 mm and flat to flat widths of 2.0, 2.4, 2.7, 3.0, 3.3 and 3.4 mm. The expanded use of the hexagonal platform led to a large number of significant complications and many studies concluded that loosening of screws i s related to the use of external hexed platform designs 11 Jemt already reported in 1991 that for implant retained and supported (fixed detachable hybrid) prostheses, 31% of the retaining screws were loose at the first follow up, and an additional 2% were loose at the second follow up 12 Another study reported 5% loosening of retaining screws placed in 91 patients and loose prosthetic retaining screws in 49% of treated maxillae and 21% of treated mandible s at the first annual follow up when Br nemark external hex implants were used 13 Screw loosening is common with fixed detachable hybrid prost heses, which can lead to serious complications such as screw fracture and failure of the prosthesis 6 The exact mechanism of retaining screw loosening in fixed detachable hybrid prosthesis is complex, because it involves fatigue cycling, oral chemical/temperature changes, and varied chewing pattern/loads Consequently, loosening of the screw will always be a concern as the restorative dentist is making attempt to prevent it. 14 It will be valuable for the restorative dentist to recognize the factors that facilitate the chance of screw deformation. In the present day, root form implants present a diversit y of internal connection designs. The connection can be further characterized as a slip fit joint, where a slight space exists between the mating parts and the connection is passive, or as a friction fit joint, where no space exists between the mating comp onents and par t s are literally forced together 15 According to the investigators of th e study, clinical experien ce shows

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17 that the internal connection designs reduced the loosening incidence but has not eliminated it. However, a later study that followed 76 edentulous patients with fixed restorations over 450 implants for 15 years, reported 37 implants and 5 fixed pr ostheses failed caused by fractures and wear of the pro sthesis, but no screw loosening 16 Purcell et al 17 investigated prosthetic complications of patients with a maxillary complete removable dental prosthesis opposing a mandibular hybrid fixed detachable prosthesis and conclu ded that common complications were prosthetic tooth fracture, tooth wear, the need for relines of removable devices and screw loosening External Hex Versus Internal Hex Designs External hex implant systems were first introduced by Brnemark, and then the y became widely used by clinicians. A new design with an internal hex was recently introduced to overcome the disadvantages of the previous design. It was claimed that internal hex systems have more efficient anti rotation resistance, offer a greater tacti le sense when abutments are seated, have permit ted less screw flexion caused by lateral forces which decreases the chance for screw loosening 18 It has been shown that in external hex implants, whenever occlusal forces go above the yield strength of the abutment screw, bendi ng stress is directly applied on the abutment implant interface. This will cause a slight deformation of the screw that could be separated at the interface and damaged 18 However, there is no convincing evidence that the implant abutment connection has an effect on screw lo o s en ing or that internal hex systems have any superiority over the external hex.

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18 Mechanism o f Abutment Screw System Friction a nd Preload Dental implant restorations are composed of three main components (Fig ure 2 2 ): 1) the implant body, which is the part t hat is integrated with the surrounding bone; 2) the abutment, the element that is fastened to the implant body by a screw to retain the restoration; and 3) the crown which replaces the clinical portion of the missing tooth 19 Abutment screws are tightened by a torque wrench w hich is a practical method to control torque value 20 When torque is applied to the screw, the screw is elongated which leads to the generation of a clamping force between the screw threads and the implant. This c lamping force increases the friction between the screw threads and the screw seat inside the implant, and is called preload. The preload will hold the joint as a unit and prevent it from separating as it works against any external load applied on the joint When functional load is applied on the implant abutment, the screw head is compressed against the seat in the implant that reduces the elongation of the screw caused by the preload. It means that the clamping force is decreased along with reduction of th e frictional forces between the threads of the implant and the screw. The functional load could be high enough to diminish the frictional force between thread s so much that the screw becomes loosened 21 Therefore, the torque applied to the abutment screw should generate enough preload that exceeds the compressive stress exerted by t he occlusal forces thus minimizes the ri sk of screw loosening and fracture of the prosthesis 22 On the other hand, extremely high values of torque can cause the screw to deform plastically followed by fracture 20, 23 To minimize screw lessening and fracture of the screw, it is necessary to keep the torque and preload delivered to the

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19 screws at optimal levels 6 Using a torque wrench to estimate preload value is common among resto rative dentist, even though it is not the most reliable method 19 Design of Abutment Screws t o Maximize Preload It has been found that 90 % of the torque applied during the first tightening of a screw system is to overcome friction between the engaging components leaving only 10% of the initial torque for producing preload 21 After multiple cycles of tightening and loosening, thread friction decreases as the contacting surface being burnished and becoming smoother. Manufacturers of implants are trying to optimize abutment screw designs in a way to maintain maximum preload and minimum input loss to friction. There are different designs of abutm ents and different geometr ies of implant/abutment interface to address joint strength, joint stability, and locational and rotational stability of the abutment retained prosthesis 15 The material properties of the implant components and their interaction with environmental conditions, such as change in surface friction due to the state of lubrication at mating screw and implant threaded surfaces 24 Figure 2 3 shows a typical stress strain curve of a material under tension. Ela stic deformation of the screw under tension takes place until elastic limit (point E) is reached. A fter that point, plastic deformation begins. For a joint assembly like the implant shown in Figure 2 2 the preload within the implant complex or an external masticatory load can result in tensile stress within the implant When there is no preload the cyclic loading exerted on the implant during mastication will also be taken up by the retaining screw. In other words, the retaining screw will be cyclic loade d th e elastically, as long as the load does not go beyond the elastic limit (point E). Even though the screw will return to its original form every time teeth are out of contact, the

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20 retaining screw can still deform and fracture due to cyclic loading. If a tensile preload that is less than the proportional limit P is applied in the course of the tightening process, the retaining screw will not be subjected to the full functional load. A significant portion of the functional load is absorbed by the joint, an d as a result there will be a small impact on the screw 19 The recommended optimal preload force applied when an implant screw is fastened should be that which produces a stress level between 60% and 75% of the yield strength of the material from which the screw is made 25 The retaining screw will fracture if the stress induced by preload equal s to the ultimate strength of the material If the stress within the retaining screw is near its yield strength, the screw will deform plastically during service and slowly loses its preload. The stress that is of most concern is the axial normal stress developed in the abutment screw shank. As described by a previous finite element analysis study 26 maximum stresses are found between the head and shank of the abutment screw and also at the root of the first threads Interaction between Driver Tip a nd Retaining Screw It is known that when surfaces in contact slide against each other wear occurs Wear means loss of material from surfa ces and is indication of damage of the surfaces. In the case of hex driver tip and hex socket screw, w ear will most likely result in unnecessary spaces between the moving components, looseness and loss of precision 27 The process of tightening and loosening of abutment screws associated with implant restorative procedure will introduce wear between the driver tip and the inner wa ll of the socket. The most severe situation is the stripping of the retaining screw during loosing phase

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21 To investigate the causes of s tripping of abutm e nt screws Dr. Aline Bowers of University of Florida hypothesized that stripping was the results of re peated action of tightening and loosening of the same pair of driver tip and abutment screw. The experimental protocol was to tighten and loosen abutment screw in an implant which had been fixed in an acrylic block. To maintain a constant torque throughout the study a calibrated implant motor ( Model DU 900 Biomet 3i Palm Beach Gardens, FL USA ) was used to deliver constant torque during tightening She used five level of torque 5, 10 15, 20 and 30 N/cm in tightening the abutment screw. At each level of torque, the screw was tightened and loosened 20 times. There was a one min break between each process. She used 20 Astra and 25 Zimmer dental abutment screws in the study. No stripping was found in the study The conclusion of the study was that abutment s crew of both systems can be retightened at least 20 times without fear of stripping The study indicated that the screw head could be contaminated with debris during tightening or loosening processes, other shapes and dimensions of the screw head, or angul ations of the driver when inserted in the head during tightening or loosening could complicated the process and leads to stripping. The author of this study showed interest in the study and focused on investigating the effect of the presence of debris in the screw head and angulations of the screwdriver during loosening and tightening as stated in the introduction Hypothes e s The presence of debris in the socket of screw head could lead to two scenarios of incomplete seating between the screwdriver tip and the screw head. First, the driver tip will not be fully seated in the socket reducing the area of contact with increasing

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22 stresses on the surface of contact. Second, the driver may be seated with slight angulation to the axis of the abutment screw resulti ng in uneven contact between screw driver tip and the inner surface of the socket. Both scenarios could lead to stripping of the screw head. Therefore, the purpose of this in vitro study is to test the following two hypotheses : 1. T o test the hypothesis that incomplete seating of the screw driver tip in the screw head socket can lead to stripping and to determine the threshold of incomplete seating resulting from debris remaining in the screw head when the stripping occurs in two implant systems. 2. T o test th e hypothesis that angulation of the screw driver tip in the screw head socket can lead to stripping.

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23 Fig ure 2 1 Illustration of (A) internal hex and (B) external hex implants design. (Illustration courtesy of Haya Alabhool) Fig ure 2 2 Illustratio n of the implant abutment system 19 (sourse: Jaarda MJ, Razzoog ME, Gratton DG. Geometric comparison of five interchangeable implant prosthetic retaining screws. J Prosthet Dent 1995;74(4):373 9. )

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24 Fig ure 2 3 Typical tensile stress/strain diagram of screw placed under ten sile load. P : proportional limit; E, elastic limit; Y yield point; U, ultimate strength 19 (Source: Jaarda MJ, Razzoog ME, Gratton DG. Geometric comparison of five interchangeable implant prosthetic retaining screws. J Prosthet Dent 1995;74(4):373 9. )

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25 CHAPTER 3 METHODOLOGY Design and Fabrication of a Device f or Testing Hypotheses To test the hypotheses, we need to immobilize the implant in a solid medium a nd keep t he screw driver in a position that aligns with the implant for driving the screw. An electrical driven handpiece (Model DU 900) was used to deliver constant torque in driving screws. Two devices were constructed. The first device (Fig ure 3 1A) was used to mount implants in acrylic blocks. The device consi s ts of two compartments: the mounting base ( Figure 3 1 A ) and the guiding block ( Figure 3 1 B ). The open space between the base and the guiding block allows visual inspection when implant is embedded in the acrylic block. The second device ( Figure 3 2 ) also consists of two components and uses the same base ( Figure 3 2 A ) shown in the first device. The top portion is to house the handpiece (Figure 3 2B) complete set up of the device (Figure 3 2C) and the open access ( Figure 3 2 D ) allows operator to make sure that the driver is properly seated in the socket of the screw head. Detailed procedures of specimen fabrication are described in respective sections. M ounting Implants in Acrylic Blocks a nd Their Preparation Implant block specimens for testing the effect of incomplete seating on stripping were fabricated using a proc edure modified from the process described by Martin and his colleagues 28 The hole in the guiding block was made to fit the diameter of the screwdriver extender. Stock abutment was fixed to the respective implant root with a screw. Each screw was tightened with recomme nded torque. A screwdriver fitted with the extender was adapted to the socket of the screw head, and the joint area was immobilized with a heavy body PVS (Aquasil R igid Ultra, Dentsply York, PA, USA ) to

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26 keep the entire assembly aligned and fit together. T he cavity in the center of the mounting base ( Figure 3 1B) was filled up to 7/8 of the volume with freshly mixed acrylic resin (Dent sply Caulk Orthodontic Resin ). The end of the extender of the implant assembly was inserted into the hole of the guiding blo ck from underside. The guiding block with the implant assembly was fitted to the base along the four posts on the base The implant assembly was then lowered slowly toward the acrylic resin filled cavity in the base till the acrylic resin reached the neck of the implant ( Figure 3 3 A ) The implant block ( Figure 3 4) was removed when the acrylic resin harden ed Implant blocks for testing the second hypothesis was prepared as those for testing the first hypothesis with one exception. When the respective screwd river was adapted to the socket of the screw, the driver was pushed off the alignment till it nudged against the rim of the stock abutment The misaligned position represents the maximum angulation that may occur in clinical situation. Heavy body PVS was u sed to maintain the misalignment during the making of the implant blocks (Figure 3 3B) Using the custom designed mounti ng block ( Figure 3 1 ), four internal hex 3.7 mm X 13 mm Zimmer implants and four internal hex 4.0 mm X 11 mm Astra implants were mounted in acrylic resin for testing the first hypothesis ( Figure 3 4 ). Each implant block was to serve as the test platform for five screws One implant block each was made from Zimmer and Astra for testing the second h ypothesis. Each implant block was expected to serve as the test platform for five screws. Additional implant blocks would be made should the implant block had worn out before completing five t e sts.

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27 Twenty hexagonal screws each from Zimmer and Astra were used to test the first hypothesis which state s that incomplete seating as a result of rem aining debris in the socket of the screw head can lead to stripping of the screw head. Five screws each from Zimmer and Astra were used to study the second part of the hypothesis which states that angulation of t he screwdriver relative to the axis of the screw can lead to stripping All abutment screws recommendations (25 N/cm for Astra screws and 30 N/cm for Zimmer screw ) The screw in e ach test specimen was tightene d into its corresponding prefabricated titanium abutment Zimmer Hex Lock C ontour and Astra TiDesign TM in the implant block. Effect o f Incomplete Seating Preparation o f Debris Loaded Screw A material called prop was used to mimic the accumulation of debr is in the socket in the screw head. Ideally a more clinically relevant material like cotton pellet or resin composites would be used, but they were h essentially is ceramic powder that is easy to carry and to be conden sed inside the very small socket in the screw head. The socket was filled in increments. After each increment the powder was carefully condensed inside the socket with the latch driver in the sock et a measuring device based on a micrometer ( Model #436, Starrett, Athol, M A USA) was constructed ( Figure 3 5). Measurement of t he Debris Quantity A metallic cylindrical cap that fits snugly to the anvil of the micrometer was machined A 1.45 mm diameter hole that fits latch driver was drill ed through the entire cylinder. The screw was tightened inside the implant in the acrylic block with the torque

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28 recommended by the manufacturers A small latch driver was inserted in the socket of the screw with the la tch end sliding inside the metallic cap ( Figure 3 6). The length registered by the micrometer represents of the baseline of the implant assembly when the screw is completely clean, and was considered the initial depth After each ength of the assembly was measured again. The difference in length with respect to the first reading was the depth of debris in mm Experimental Procedure Each screw was inserted manually into a latch driver of respective system. For screw insertion into t he specimen, a calibrated implant motor (Model DU 900) was set to the desired torque and the screw was tightened into the implant ( F igure 3 7) When t he torque was achieved the motor was turn off and waited for one minute was applied before the removal o f the screw. After 60 second s the screw was loosened and completely removed from the abutment. The procedure was repeated until the head of the screw becomes stripped or up to 10 cycles if no stripping is observed. The distance between A and B was measured changed. If the screw was tightened 10 times, and no stripping occurred, t he screw wa s filled again with debris and the previously described step was repeated again so we achieved multiple depth s for ev ery scre w. Trial was stopped if stripping occurred or if the socket is completely filled. The number of failed screw and number of cycles at failure was recorded. A new screw driver was used for every 5 screws and was studied after each trial using photogra phs and light microsc o pes The results were used to determine the mean of height of debris at which stripping occurs T he screw driver head was qualitatively evaluated before and after each trial to check if there is any obvious sign of wear on the surface.

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29 Effect o f Improper Angulations Preparation o f Specimen Blocks To test the e ffect of improper angulations the implant was fixed in the acrylic block slightly off the perpendicular axis of the mounting base. After inserting the hex driver inside the hand p iece and the screw head usi ng the handpiece guiding block the hex driver was positioned in the center of the guiding hole (Figure 3 8 ). The amount of angulation was the degree of implant tilt made during making of the implant block The body of the hand p iece was immobilized on the guiding block with a rigid PVS impression material ( Aquasil rigid Ultra, Dentsply ) There is a slot on the guiding block that is designed to house the hand piece (Figure 3 2B) All screws were tightened to the manufacturer recom mended values. Five screws of each company were used. This arrangement allowed us to measure the lowest depth of engagement when stripping occurred by raising the handpiece. Experimental P rocedure First step was to tighten and loosen each scre w with the sc rew driver placed as deep as possible in the screw head, in the predetermined angulation. If st occur within 10 cycles of tightening and loosening as described earlier in the effect of incomplete seating the guiding block along with the hand piece was raised by inserting spacers of known thickness between the mounting block and the guiding block. The same testing procedure was repeated again at the new level of height. The guiding block would be raised again and tested till stripping was obser ved. The step s of height s used in this study were 0 mm (baseline), 0.27, 0.54, 0 .93 and 1.19mm The number of cycles and the height of the guiding block when stripping occurred were recorded. Impressions of internal surface of the first screw head from eac h

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30 company were made with heavy body PVS before and after each test at each level These impressions were examined using photographs and under a stereo light microscope for degree of surface damage The results were used to establish the number of cycles be fore stripping of the screw head happens when angualtion of the screwdriver occurs. The pattern of surface damage can also be used to assess the potential of stripping. Characterization of the Latch Driver Tip and t he Abutment Screw To investigate why the two groups of abutment screws behaved differently, should it occur, it is necessary to know the difference in the design and properties of material used to manufacture abutment screws and l atch Manufacturers often provide some of the information in the sp ecification of the component but not all relevant to the study. In this study we measure d hardness of the driver tip and the head portion of the abutment screw delineate difference in design between the two systems, and record the appearance of the drive tip and abutment screws after the tests Microhardness M easurement New drivers were first stabilized on their side on a glass slide with cyanoacrylate adhesive with one flat facet set parallel to the supporting glass for indentation New abutment scre w s w ere embedded in an epoxy resin ( EpoFix, Struers A/S, Ballerup, Denmark ). The epoxy resin block was ground with silicon carbide paper to expose metal with 320, 400, 600 and 1200 grit silicon carbide paper (Mark V Laboratory, East Granby, CT USA ) with runni ng water using a lapping and polishing apparatus (Model 150;South Bay Technology, San Clemente, CA USA ). It was followed by polishing with 1 m alpha alumina powder on alpha B cloth (Mark V Laboratory) in preparation for the microhardness indentation (Fig ure 3 9)

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31 A Buhler Micro met 3 microhardness tester (Buehler Ltd, Lake Bluff, IL, USA) interfaced with a computer was used to make indentations on the metal surface. All load and 15 s dwell time was used. The near square shaped indentations were carefully observed with an optical microscope. Images of indentation were captured with a digital camera interfaced and analyzed with Omnimet 8.0 software to determine the surface hardness. Six indentations were made on each specimen. Dimension of the Implant C omponents The across flat dimension of the hex driver tip and the outer diameter of screw head were measured with a caliper To estimate the depth of the socket of both system s, we first measured t he length of the abutment screw and the hex driver of both systems individually with the same caliper. The total length of the assembly with driver inserted in the respective socket was then measured. To examine if the hex driver of o ne system adapts well with screw of different system, the total length of the assembly with Zimmer driver inserted in Astra screw and vice versa were also measured The space between the socket and driver tip can be shown by using a fit checking material ( Fit checker, GC Tokyo, Japan) The fit checking material was mixed and injected into the socket of screw head that immediately followed by insertion of the driver. The driver was held firmly with finger pressure till the material set. The driver was pulle d out and photographs of the driver tips with fit checking material were taken. Appearance of the Specimens a fter Tests Impressions were made for the inner surface of the first screw of each group during the improper angulations trial, however the change i n the inner surface of the

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32 socket was very minimal and the quality of available cameras and microscope was not high enough to show this change. H ex drivers, used drivers after each experiment were examined and photographed along with new ones. Statistical Analysis All Data were collected for both parts of this research. For the first part, results were used to determine the mean of height of de bris at which stripping occur red and the number of cycles required before stripping occurs at that height Results obtained for both systems, Astra and Zimmer, were analyzed using the two t ail S tudent t test to find if there was any statistical significant difference. For the second part, the mean of height at which stripping happens when the screwdriver is angulated i n a fixed angulation was calculated. Also, the number of cycles before stripping happened at a specific height and angulation was recorded. The results were also analyzed with two tail Student t test. For the microhardness data of the driver and the abutme nt screw, two tail Student t tests were used to determine if there were differences between manufacturers, and between driver and screw.

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33 Fig ure 3 1 Device for mounting implant in acrylic block for testing improper seating A M ounting base block (lef t) and guiding block (right). B. The guiding block incorporated into system. The screwdriver is inserted in the screw head which is tightened in the implant in the mounting base (Photos courtesy of Haya alabhool)

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34 Figure 3 2. Testing d evice for t he effect of angulation on the stripping of abutment screw A. Mounting base block B. The guiding block and rest for handpiece. C Complete set up of the testing device. D A ccess window for viewing the hex driver in contact with abutment screw (Photos c ourtesy of Haya Alabhool)

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35 Fig ure 3 3 T he mounting device showing implant is being lowered into acrylic resin. The hex driver and abutment screw joint is stabilized with heavy body PVS. (Pho to courtesy of Haya Alabhool) Fig ure 3 4 Implant speci mens : ( righ t) implant aligned with the vertical axis of the block ; ( lef t) implant seated with angulation. Hex driver was inserted and stabilized with PVS to enhance the effect of angulation. (Photo courtesy of Haya Alabhool)

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36 F ig ure 3 5 Measuring device. A. T he caliber with metal cal; B. T he arrow shows the hole that house the hex driver. (Photos courtesy of Haya Alabhool) Figure 3 6. Measure the height of the test assembly for the incomplete seating experiment. The distance between A and B is the total height of the assembly (Photo courtesy of Haya Alabhool) B A

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37 Figure 3 7 Testing of incomplete seating (Photo courtesy of Haya Alabhool) Figure 3 8 The handpiece resting on the guiding block and immobilized wi th a rigid PVS impression material. The guiding block can be raised with spacers to adjust the contact between the hex driver and the wall of the socket. (Photo courtesy of Haya Alabhool)

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38 Figure 3 9. Astra abutment screws embedded in epoxy resin rea dy for microhardness measurement. (Photo courtesy of Haya Alabhool)

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39 CHAPTER 4 RESULTS Effect o f Incomplete Seating The length measured between A and B shown in Figure 3 6 before debris w as packed into the socket of the screw served as the baseline of ea ch test. The difference between the baseline length and the length after additional debris was added reflects the height of debris added in the socket (Table s 4 1 and 4 2). The highest value of height was identified and the cycle of loosening or tightening when stripping occurred was also recorded. The results show that Astra screw heads stripped when the height of debris was between 0.79 and 1.20 mm When stripping occurred, it was often during the first loosening. The mean of debris height a t which stripp ing occurred was 0. 98 mm (SD=0.10 mm). On the other hand, Zimmer abutment screws stripped at heights b e t w een 0.4 6 and 1.05 mm Compared to Astra, Zimmer screws stripped during the first tightening procedure. Stripping occurred at a mean value of 0. 92 mm (S D=0.14 mm). During testing, we often observed metal debris in the socket of the screw or on the hex driver tip even before the height of stripping was reached. Two tail Student t test shows that there was statistical differences between the two systems at p=0.028 Effect o f Improper Angulations As described earlier, the implant body was embedded in acrylic with a slight i ncline. As shown in Figure 3 8, the handpiece rests on an extended guiding block and the hex driver is fully seated in the abutment screw (not seen in the figure). Stripping of both systems did not occur when the screwdriver was completely inserted in the screw

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40 head in spite of its angulation. Stripping finally occurred for both systems when the height of the guiding block was raised from 0. 9 3 mm to 1.19 mm, and it always happened in the first cycle of loosening If a series of thinner spacer had been used before the guiding block was raised to 1.19 mm, the results might have revealed difference in the mean height of stripping occurrence. For the data obtained in this study, no st atistics could be performed. Characterization of the Hex Driver Tip and t he Abutment Screw Microhardness of the Driver Tip and the Head o f Abutment Screw Table 4 3 shows the microhardness values of the flat facet of h ex driver and the polished surface of screw head of both systems. The har dness values of hex driver are 670 22 Kg/cm 2 and 525 12 Kg/cm 2 for Astra and Zimmer, respectively, and the difference is significant by two tail Student t test (p<0.001). The hardn ess values are 316 9 Kg/cm 2 and 32013 Kg/cm 2 for Astra and Zimmer screws respectively, and the two tail Student t test shows there is no statistically significant difference between the two systems (p=0564). The same test also shows that hardness of th e hex driver is significantly higher than that of the screw for both systems (p<0.001). Dimension of the Hex Driver and t he Abutment Screw Table 4 4 shows the dimension of the hex driver tip of both systems. The design of Zimmer driver HX 1.25D is straight with the across flat distance of 1.24 mm and diameter of the screw head is 1.96 mm Astra Hex CA Driver has a taper (~3 estimated from ph o tograph s ) and the across flat distance is 1.24 mm at the end and 1.29 mm at the mid section of the driver tip; the d iameter of the screw head is 2.17 2.32 mm.

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41 Estimation of the Depth of the Socket of t he Abutment Screw Table 4 5 shows the dimension of the driver and screw in length. Assume that the driver reached the bottom of the socket, the difference between total length and the combined length should be the depth of the socket. When the hex driver of Zimmer system was used the depth of the socket was 1.54 mm and 1.37 mm for Zimmer and Astra screw, respectively. When the Astra hex driver was used, the depth values w ere 1.19 mm and 1.34 mm for Zimmer and Astra screw, respectively. By the value obtained, it appeared that the driver by Zimmer reached the bottom of both socket, but the Astra with the taper did not reach the bottom of either socket. Therefore, the depth m easured with Zimmer driver should be considered the likely depth of the respective abutment screw. The results also implied that when Astra driver was used and without any debris in the socket, the flat surface of t he driver tip c a me in full contact with t he upper edge of the inner wall of the screw head. The appearance of fit check material left image s of the driver pull from sockets of screw head confirmed this observation (Figure 4 1) There is a thin coating of fit check material on the tip of Zimmer dr iver (Figure 4 1, left) implying that there is space between driver and the abutment screw. For the Astra system, th e driver came in full contact with the opening of the socket wall of the screw separated the portion of material inside the socket from the outer portion of the fit check material. As the driver was being pulled out, the inside portion was retained in the socket and left no coating of fit check material on the tip (Figure 4 1, right) Appearance of t he Tested Specimens Three screwdrivers from Astra and Zimmer, were examined under the microscope: one new driver, one from the incomplete seating due to debris experiment, and one

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42 from the angulations experiment. Astra screwdriver showed no obvious sign of plastic deformation after the incomplete se ating trial, while the d river u sed in the effect of angulation had a zone with dark deposit indicating rubbing between the driver and the wall of the socket (Figure 4 2) Zimmer driver, on the other hand, showed wear in each of the six edges of the driver after the debris test compared to the new driver (Figure 4 3A) Zimmer driver used in the effect of angulations also had worn edges that resulted from pressing against the wall of the socket, and wear was seen at each of the six edges angulation (Figure 4 3C)

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43 Table 4 1 R esults of incomplete seating of screwdriver in Astra abutment screw heads Astra Specimen # 1 2 3 4 Stripping observed Depth of stripping 1 0.39 0.83 0.97 SL1 0.97 2 0.43 0.57 0.83 1.05 SL1 1.05 3 0.79 ST1 0.79 4 0.40 0.66 0.93 SL1 0.93 5 0.43 0.78 0.84 0.88 ST1 0.88 6 0.20 0.89 ST1 0.89 7 0.20 0.68 0.97 SL1 0.97 8 0.84 0.93 SL1 0.93 9 1.09 1.18 SL1 1.18 10 1.03 SL1 1.03 11 0.66 0.99 SL1 0.99 12 0.54 0.62 1.20 SL1 1.20 13 0.74 0.95 ST4 0.95 14 0.90 ST2 0.90 15 0.68 0.92 SL1 0.92 16 0.43 1.05 SL1 1.05 17 0.57 0.77 1.03 1.04 SL1 1.04 18 0.81 1.05 SL1 1.05 19 0.46 0.94 SL1 0.94 20 0.87 0.98 ST1 0.98 Mean 0.98 SD 0.10 Max. depth 1.20 Min. depth 0.79 Note: 1* to 4* : number o f debris increment Stripping observed: SL indicates stripping occurred during loosening; ST indicates stripping occurred during tightening; the number indicates the cycle when stripping occurred. Depth of stripping: the highest values of the depth resulted from packing debris.

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44 Table 4 2. Results of incomplete seating of screwdriver in Zimmer abutment screw heads Zimmer Specimen # 1* 2* 3* 4* Stripping observed Depth of Stripping 1 0.41 0.92 ST1 0.92 2 0.95 ST1 0.95 3 0.46 ST1 0.46 4 0.21 0. 58 0.69 0.82 ST1 0.82 5 0.61 1.03 ST1 1.03 6 0.30 0.70 0.97 ST1 0.97 7 0.39 0.58 0.81 ST1 0.81 8 0.48 0.83 ST1 0.83 9 0.30 0.64 1.02 ST1 1.02 10 0.58 1.04 ST1 1.04 11 0.57 0.94 ST1 0.94 12 0.54 0.91 ST1 0.91 13 0.47 1.02 ST1 1.02 1 4 0.24 0.53 0.80 ST1 0.80 15 0.51 0.84 ST1 0.84 16 0.41 0.62 1.03 ST1 1.03 17 0.33 1.02 ST1 1.02 18 0.68 0.92 ST1 0.92 19 0.50 0.72 0.98 ST1 0.98 20 1.05 ST1 1.05 Mean 0.92 SD 0.14 Max. depth 1.05 Min. depth 0.46 Note: 1* to 4* : number of debris increment Stripping observed: SL indicates stripping occurred during loosening; ST indicates stripping occurred during tightening; the number indicates the cycle when stripping occurred. Depth of stripping: the highest values of the dept h resulted from packing debris.

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45 Table 4 3. Hardness values (in Kg/cm 2 ) of the latch driver and hex screw head Astra Zimmer Replication of indent Driver Hex head (polished) Driver Hex head (polished) 1 696 303 532 324 2 685 316 518 320 3 631 314 544 299 4 675 310 519 331 5 662 330 513 314 6 668 321 524 333 Mean 670 316 525 320 SD 22 9 12 13 Table 4 4 Dimension (in mm) of the Hex driver tip and the outer diameter of the screw head System Astra Z immer Across flat distance of hex driver 1.24 (end) 1.29 (at mid section) 1.24 Outer diameter of screw head 2.17 2.32 1.96 Table 4 5. Estimation of the depth (in mm) of the socket of the abutment screw System Zimmer Astra Abutment screw (length) 8.16 8.16 8.24 8.24 Drive r (length) 23.06 ( Z ) 23.94 (A ) 23. 06 ( Z ) 23. 94 ( A ) Total length (screw+driver) 31.22 32.10 31.30 32.18 Combined length 29.63 30.91 29.93 30.84 Difference 1.59 1.19 1.3 7 1.3 4 Note: Z*: hex driver for Zimmer was used; A *; hex driver of Astra was used. Combined length : length of assembly when the driver is inserted in the socket. Difference: total length minus combined length.

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46 Figure 4 1 Hex driver removed from the socket filled fit check material. Zimmer driver (left) is coated with a thin layer of fit check ma terial indicating space between the driver and the wall of the socket. Astra (right) driver is free of any material coating near the tip except chuck of material away from the tip. It implies that the flat wall of the driver came in full contact with the u pper edge of the socket wall that sever the fit checker at the contact that the rest of the material is left in the socket. (Photos courtesy of Haya Alabhool) Figure 4 2. Astra driver s : ( A ) Driver used in the effect of debris buildup, no obvious sig n of plastic deformation; ( B ) B rand new driver ; ( C ) Driver used in the effect of angulation arrows show the zone with dark deposit indicating rubbing between the driver and the wall of the socket (Photos courtesy of Haya Alabhool)

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47 Figure 4 3. Zimm er driver : (A ) The driver used in the effect of debris; area in the circle depicts the worn edge of the driver which occurred at each of the six edges. ( B ) N ew driver. ( C ) The driver used in the effect of angulation; area in the circle depicts the worn edg e of the driver resulted from pressing against the wall of the socket, it occurred at each of the six edges angulation. (Photo courtesy of Haya Alabhool)

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48 CHAPTER 5 DISCUSSION Implant retained prosthesis are more accepted now by patient s and restorative d entists. Thes e prosthesis have offered more diverse treatment op tions to restore edentulous and partially edentulous dental arches. One of the concerns in implant dentistry is the stripping of the abutment screw head when the dentist tries to remove the pr osthesis to repair or even to clean it. Stripping may occur due to residual debris in the screw head that were not carefully cleaned by the dentist, which may lead to incomplete seating of the screwdriver, hence stripping. Debris may come either from the c otton pellet or the restorative material that is used to seal the access hole of the screw. Another reason why the screwdriver is prevented from complete seating in the screw head is angulations. If the abutment walls or prosthesis walls are too long, or i n cases where implants are angulated from the long axis of the crown, direct visualization of the screw will be unattainable, and the screwdriver may be placed in the socket with an angle. It is believed that these abnormalities would lead to stripping of the screw and premature failure of the screw. This study was to use a uniformed approach that mimics both scenarios to determine the level of abnormality when the stripping occurs. Effect o f Incomplete Seating The first part of the hypothesis of this study was to show that incomplete seating of the screw driver tip in the screw head socket can lead to stripping, and also to determine the threshold of incomplete seating as the result of debris accumulation in the screw head when the stripping occurs in Astra tech and Zimmer dental implant systems.

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49 Difference between t he Two Systems Looking at the results of the effect of the incomplete seating, it was shown that Astra abutment screws stripped when the debris heights were between 0.79 and 1.20 mm with a mean va lue of 0.98 mm, while Zimmer abutment screws stripped at heights between 0.46 and 1.05 mm with a 0.92 mm mean value. The results indicated that Zimmer screws started stripping at lower hei ghts than Astra screws. There are two possible explanations : materia ls used in fabricating the components and the difference in design (Figure 5 1) Zimmer screwdriver tip was approximately 1.24 mm in diameter, and it was designed to ha ve str a ight parallel walls (Figure 4 3) In the Fit checker test, it was shown that the diamete its seat so uniform space was left between the screwdriver and the socket walls ( Figure 5 2 ) Conversely, Astra screwdriver tip had angulated wall, with a diameter of 1.24 mm at the end of the tip and 1 .29 mm at mid point between tip and base of the driver ( Figure 5 3 ). As point e d out in the r esults that the flat wall of Astra driver came in full contact with the upper edge of the socket wall and possibly not touch the floor of the socket. At zero depth, we noticed that the contact between the Astra screwdriver and the screw had become tighter after 10 loosen tighten cycles. At the beginning of the trial, the wider top of the screwdriver tip was in contact with top of the socket walls, hence the screwdriv er is not inserted all the way to the floor of the socket as was confirmed by the Fit checker test. Then, as the screw was tightened and loosened, the top of the socket wall was shaved by the screwdriver and moved laterally away from the screwdriver, and t he screwdriver may have been pushed further down in the socket, causing a tighter fit of the screwdriver in the screw socket (F igure 5 4 ) Since the hardness value of the

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50 driver is twice as much as that of the screw (Table 4 3), it is likely that pressing driver against the screw with pressure could cause screw to plastically deform especially where the area of contact is limited slightly different. One had all the six walls meeting in a sharp angle, and another had its walls meeting in a curved angle. Since either driver does not fit the sockets snugly, as the driver is turned its six sharp angles will come in contact with the socket wall and nudge into the surface. The frict ion between the driver and the socket wall, and the resistance of the socket wall to plastic deformation kept the driver and socket in contact and the torque applied was used to turn the screw. Stripping o f Zimmer Abutment Screw For the abutment screw by Z immer, a s the driver was raised by the addition of simulated debris, the area of contact decreases (Figure 5 5 ) while the torque remains the same It means the stress at the contact can increase to the level exceeding the yield strength of the screw. At th at level of stress, plastic deformation of the socket occurred and present ed no resistance to the motion of the driver which continue d to turn without touching the wall of the socket. That is when the stripping occurred. Meanwhile the amount of stress exi sted at the driver screw contact could also cause the sharp corner of the driver to deform plastically Even though the microhardness value of the Zimmer driver is higher than that of the screw (525 Kg/cm 2 vs. 320 Kg/cm 2 ), t here is evidence that the driver by Zimmer had deformed plastically at the corners (Figure 4 3A) as indicated by the flattening of the corner This surface deformation would widen the clearance between the driver and socket that prevents the driver from engaging socket and transfers the torque power to

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51 turn the screw. The result is stripping at a lower depth of debris accumulation. Based on Tables 4 2 and 4 5, we calculated that at the lowest (0.46 mm) and highest (1.05 mm) depth of stripping the actual depth of stripping at these depths would be 1.13 mm and 0.54 mm, respectively. The latter values were obtained by subtract the lowest and highest depth values from 1.59 mm (the depth of the socket of Zimmer abutment screw listed in Table 4 5). Stripping o f Astratech Abutment Screw E arlier discussion showed that the 3 taper of the Astratech driver created an intimate contact at the top of the socket with no debris present Addition of debris in the socket will raise the driver as in the case of Zimmer driver. There is, h o wever, a significan t difference between the two systems. Because of the taper, the cross section of the driver that is at the same height of the top of the socket becomes smaller as the driver is raised by the debris accumulation (Figure 5 6 ). It also means that the gap betw een the driver and the wall of the socket increase s To transfer the torque force to the shaft of the screw to turn, the six corners of the driver have to come in touch the wall of the socket At certain gap width when the driver is turned, it s six corner s may barely touch the wall of the socket and plastically deforms the area of contact overcom ing the resistance by the socket. The scenario is considered striping of the abutment screw. In our experimental design of the study, we consider ed the screw head socket had been stripped if the screw did not turn when the driver had been turned. It is entirely possible that some of the screws in the Astra group had not been physically stripped by the driver but fail to turn because of wide gap that led to non conta ct between driver and the abutment screw.

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52 Unlike driver by Zimmer, there is no sign of plastic deformation among the Astra drivers after test s (Figure 4 2A) Having no plastic deformation on the corners of the driver means that when the driver is raised hi gher by the debris these corners can still come in contact with the walls of the socket. In other words, if stripping should occur, it happens at higher depth of debris accumulation. Based on Tables 4 1 and 4 5, we calculated that at the lowest (0.79 mm) a nd highest (1.20 mm) depth of stripping, the actual depth of stripping at these depths would be 0 .58 mm and 0. 17 mm, respectively. The latter values were obtained by subtract the lowest and highest depth values from 1. 37 mm (the depth of the socket of Astr a abutm ent screw listed in Table 4 5). Effect of Microhardenss on t he Stripping Calculations of the depth of socket wall when stripping occurred show that the remaining depth of the socket at stripping was 0.17 mm to 0.58 mm for the Astra system, and was 0 .54 mm to 1.13 mm for the Zimmer system. From the design of the driver and the socket, one would expect that the Astra system to have stripped with higher values of remaining depth of the socket. The reason was that the taper of the driver would leave a gr eater gap between the driver and the wall of the socket as the driver was being raised higher by the debris. The results of our study showed the opposite. Microhardness study showed that divers of both systems were made with materials of higher hardness th an those of the respective abutment screws. Such selection w ould assure that the driver would last longer. While the hardness of the abutment screw are about the same for both systems, the hardness of driver by Astratech is harder than that of Zimmer by 27 % (670 Kg/cm 2 vs 525 Kg/cm 2 ;

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53 Table 4 3). It appears that a hardness of 670 Kg/cm 2 is strong enough to resist plastic deformation in the present experimental design. It is likely that the plastic deformation exerted on the driver by Zimmer has lowered its resistance to stripping. Reliability of t he Experimental Data It is important to point out that the reported depth of debris where stripping occurred does not necessary reflect the minimum depth of debris of stripping. In fact, the stripping should have oc curred at a lower depth, if we had add ed debris to the socket in smaller increments. A more ideal study design would be to raise the screwdriver in smaller fixed equal increment; the approach should yield more precise values. Since stripping resulted from plastic deformation of the metal, examination of the internal walls of the socket for evidence of the plastic deformation could also help establish the depth of stripping. We made impression of the worn socket using PVS impression material and examined und er light stereo microscope, but were limited by the magnification power of the scope. We also sectioned the worn screws and polished the surface for metallurgical examination but could not distinguished areas of stripping due to limitation of the microscop e. Since the debris clinicians encountered in clinical situation are often cot ton pellet or composite resins, it would be more ideal if we had used these material s instead of p luded that the size of the abutment screw was very small and using of clinical relevant material would not allow us to control the depth of debris like we could do with the p

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54 Effect of A ngulations The second part of the hypothesis was that angulation of the screw driver tip in the screw head socket can lead to stripping of the inner walls of the screw head. We have already discussed several scenarios when i nsertion of a screwdriver in the head of the abutment screw with angulation can happen Logically, even if the clinician places the screwdriver with angulations, once the driver is engaged in its seat, it should be directed into place with the proper angulation. In this study we elected to investigate the situation where the screwdriver is prevented from readjusting and complete seating due to the presence of a barrier (long prosthesis) or an undercut (remaining composite). E xperiment al D esign As described in the section of methodology, an abutment was first fixed in a respective implant wit h the respective screw, then the corresponding driver was inserted fully in the socket of the screw. With the finger pressure, the driver was forced sideway to the extreme and the joint area was stabilized with a rigid PVS impression material. An implant b lock was then made with the implanted seated with an angulation with a device described earlier (Figure 3 3). The design and the guiding block (Figure 3 8) used allowed us to maintain the same angulation even when the drive was being raised to simulate int erference of any buildup within the abutment chamber. We are aware that as the driver was being raised there would be additional space for greater angulation. To take advantage of the space to extend the degree of angulation would mean one implant block f or each extended angulation. Due to limited number of implants made available to us, we elected to study only one fixed angualtion for both systems.

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55 Depth of S tripping The results show that w hen the screwdriver is angulated and inserted all the way in the socket, One migh t think that angulations wou l d cause tripping early on Since the driver was raised by adding spacers between mounting block and the guiding block, it would b e more appropriate to state that stripping had occurred between 0.93 (the total height of the spacer before adding the last one) and 1.19 mm of depth. We made this loosening phase of the first cycle. One logical explanation of stripping at such a high depth would be that when the driver wa s angulated in our experimental design, there were at least t wo points of contact (Figure 5 7 ). One would be a corner of the driver or one edge of the driver tip nudge d against the flat wall of the socket; the other one would be the one flat surface of the driver pressed against the top of the screw. Depending upon the pressure exerted in the socket of the screw, the driver might have firmly in contact with socket wall As the screwdriver turns, the screw will also turn with it. Because of angulation and the clearance between the driver and the socket, only some of the six corners would nudge into the wall of the socket at any given moment, an d the flat surface of the driver rolled against top of the socket. Apparently the pressure exerted in the socket was high enough to keep the screw turning till the driver was raised beyond 0.93 mm from the fully seated position (Figure 5 8 ) Appearance of the Driver a fter Tests Driver by Zimmer shows obvious flattening at the corner (Figure 4 3C). It is an indication of high stress level during test. One would expect greater degree of plastic

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56 deformation inside the socket where the driver touched. There is no sign of wear on the remaining part of the corners away from the driver tip. It is an indication that either the driver did not roll against the top edge of the screw at all, or some plastic deformation (too small to be observed) had occurred due to its relative low hardness. For the Astra system, there is no sign of plastic deformation as in the study of improper seating. However, the re was a zone of dark deposits on the flat surfaces of the driver extended up to 1.2 mm from the tip of the driver. Again the hardness of the Astra driver is high enough to resist plastic deformation by the pressure occurred during testing. The driver is likely to have rolled against the top edge of the socket and being a harder metal than that of the screw, some wear debris had been generated and deposited on the driver. This zone of debris deposit is identified by a pair of arrows in Figure 4 2C. The length of the driver used in the study could have contribute d to the difference in the appearance of the driver after test. Th e length of Astra driver used was 35 mm and the length of Zimmer driver was 23 mm. Longer driver shaft might have made the Astra driver contact with the top of the socket continuously through the entire testing period. Reliability of the Experimental D ata The data indicate that a ngulations may not sound as bad we thought it could have been. However, we did observe phenomen a that did not appear in the study of the effect of incomplete seating. At the end of the study of each test, when we removed the screw f rom the implant completely, we found metal debris in the implant and noticed slight bending of the screw shaft. Both phenomena clearly indicate that the screws have been subjected to under certain level of stress by the driver.

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57 C linically when the restorat ive dentist turns a driver which has been inserted in the socket with angulation he may face two possible scenarios. First, the driver will simply fall out due to lack of engaging of the two components. Second, the restorative dentist applies pressure to keep the driver engaged and then turns the driver. The level of pressure needed would depend on individual case. Our study design fits the second scenario, and there is possibly no limit on the level of stress that may occur, because our device is rigid an d would not allow the driver to fall out of the socket. Some modification of the device is needed to address that scenario. In spite of the question raised in the last paragraph, it is clear that n egative long term effects of angulation not only on the scr ew h ead, but als o the screw threads and possibly the thread inside the implant

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58 Figure 5 1. Astra abutment screw ( left ) and Zimmer abutment screw (right). (Photo courtesy of Haya Alabhool) Figure 5 2. Illustration of how the straight Zimmer screw driver fits inside the screw head leaving a uniform gap around it. (Illustration courtesy of Haya Alabhool)

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59 Figure 5 3 Illustration of how Astratech screwdriver fits inside the screw head. The screwdriver is in contact with the socket walls at the top of the socket only. (Illustration courtesy of Haya Alabhool) Figure 5 4. Relationship between Astratech screwdriver and abutment screw a fter multiple uses T he points of contact between the screwdriver and the inner wall of the socket go through plast ic deformation and are shaped like a bevel, which allows the screwdriver to be seated further down the socket. (Illustration courtesy of Haya Alabhool )

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60 Figure 5 5. Relationship between Zimmer screwdriver and abutment screw a s the driver is raised T he contact area is decreased (arrows) leading to an increase in stress and potential stripping due to plastic deformation of the socket walls. (Illustration courtesy of Haya Alabhool ) Figure 5 6. Relationship between Astratech screwdriver and abutment scr ew as the driver is raised. After multiple uses, the top of the socket walls are beveled permitting further seating of Astratech screwdriver down the socket (A and B). As the screwdriver is raised, the gap between the driver and the wall of the socket wall will increase (C and D). At certain gap width, when the driver is turned, its six corners could barely touch the wall of the socket and (Illustration courtesy of Haya Alabhool ) plastically deforms the area of contact overcoming the resistance by the socke t (E)

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61 Figure 5 7 When the screwdriver is placed at l ower depths in the screw socket. angulations permits at least town point contact between the screwdriver and the wall socket (arrows) (Illustration courtesy of Haya Alabhool ) Figure 5 8. In depth s higher than 0.93 mm, stripping occurred. (Illustration courtesy of Haya Alabhool )

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62 CHAPTER 6 SUMMARY AND CONCLUSI ON This in vitro study showed that incomplete seating of the screw driver tip in the screw head socket can potentially lead to stripping of bo th Astra and Zimmer abutment screw heads. It also showed that angulation of the screwdriver tip in the screw head socket can potentially result in stripping. The investigators of this study realize the limitation of the experiments, and recommend further s tudies to be conducted to overcome these limitations. With more advanced equipments the screws can be examined after stripping to study the stripping pattern, and determine if what occurred was actually stripping or just lack of contacts because of the pre viously described geometries of the screwdriver tip, especially microscope to study the effect of both debris and improper angulations on the screwdriver. For the angulation s experiment, a larger number of screws can be examined, and more angulation values can be compared. In addition, the effect of angulations on the screw threads can be looked at.

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63 LIST OF REFERENCES 1. Jivraj S, Chee W, Corrado P. Treat ment planning of the edentulous maxilla. Br Dent J 2006;201(5):261 79; quiz 304. 2. Grecchi F, Zingari F, Bianco R, Zollino I, Casadio C, Carinci F. Implant rehabilitation in grafted and native bone in patients affected by ectodermal dysplasia: evaluation of 78 implants inserted in 8 patients. Implant Dent 2010;19(5):400 8. 3. Krennmair G, Seemann R, Schmidinger S, Ewers R, Piehslinger E. Clinical outcome of root shaped dental implants of various diameters: 5 year results. Int J Oral Maxillofac Implants 2 010;25(2):357 66. 4. Bosse LP, Taylor TD. Problems associated with implant rehabilitation of the edentulous maxilla. Dent Clin North Am 1998;42(1):117 27. 5. Zitzmann NU, Marinello CP. Treatment plan for restoring the edentulous maxilla with implant supp orted restorations: removable overdenture versus fixed partial denture design. J Prosthet Dent 1999;82(2):188 96. 6. Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxil lofac Implants 1992;7(3):353 9. 7. Adell R. Tissue integrated prostheses in clinical dentistry. Int Dent J 1985;35(4):259 65. 8. Chee W, Jivraj S. Treatment planning of the edentulous mandible. Br Dent J 2006;201(6):337 47. 9. Goodacre CJ, Bernal G, Run gcharassaeng K, Kan JY. Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90(2):121 32. 10. Gratton DG, Aquilino SA, Stanford CM. Micromotion and dynamic fatigue properties of the dental implant abutment interface. J Prosth et Dent 2001;85(1):47 52. 11. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9(2):149 60. 12. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Brane mark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 1991;6(3):270 6.

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64 13. Naert I, Quirynen M, van Steenberghe D, Darius P. A study of 589 consecutive impl ants supporting complete fixed prostheses. Part II: Prosthetic aspects. J Prosthet Dent 1992;68(6):949 56. 14. Al Jabbari YS, Fournelle R, Ziebert G, Toth J, Iacopino AM. Mechanical behavior and failure analysis of prosthetic retaining screws after long t erm use in vivo. Part 1: Characterization of adhesive wear and structure of retaining screws. J Prosthodont 2008;17(3):168 80. 15. Binon PP. Implants and components: entering the new millennium. Int J Oral Maxillofac Implants 2000;15(1):76 94. 16. Jemt T Johansson J. Implant treatment in the edentulous maxillae: a 15 year follow up study on 76 consecutive patients provided with fixed prostheses. Clin Implant Dent Relat Res 2006;8(2):61 9. 17. Purcell BA, McGlumphy EA, Holloway JA, Beck FM. Prosthetic co mplications in mandibular metal resin implant fixed complete dental prostheses: a 5 to 9 year analysis. Int J Oral Maxillofac Implants 2008;23(5):847 57. 18. Tsuge T, Hagiwara Y. Influence of lateral oblique cyclic loading on abutment screw loosening of internal and external hexagon implants. Dent Mater J 2009;28(4):373 81. 19. Jaarda MJ, Razzoog ME, Gratton DG. Geometric comparison of five interchangeable implant prosthetic retaining screws. J Prosthet Dent 1995;74(4):373 9. 20. Guda T, Ross TA, Lang L A, Millwater HR. Probabilistic analysis of preload in the abutment screw of a dental implant complex. J Prosthet Dent 2008;100(3):183 93. 21. Standlee JP, Caputo AA. Accuracy of an electric torque limiting device for implants. Int J Oral Maxillofac Implan ts 1999;14(2):278 81. 22. Misch CE. contemporary implant dentistry. 3 ed. St. Louis, Missouri: Mosby 2008. 23. Siamos G, Winkler S, Boberick KG. Relationship between implant preload and screw loosening on implant supported prostheses. J Oral Implantol 20 02;28(2):67 73. 24. Bickford J. Introduction to the design and behavior of bolted joints: non gasketed joints. 4 ed. Boca Raton: CRC press; 2007.

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65 25. Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated de ntal implants. Int J Oral Maxillofac Implants 1992;7(1):26 33. 26. Sakaguchi RL, Borgersen SE. Nonlinear contact analysis of preload in dental implant screws. Int J Oral Maxillofac Implants 1995;10(3):295 302. 27. Huntchings I. TRIBOLOGY. Friction and we ar of Engineering materials. : CRC press; 1992. 28. Martin WC, Woody RD, Miller BH, Miller AW. Implant abutment screw rotations and preloads for four different screw materials and surfaces. J Prosthet Dent 2001;86(1):24 32.

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66 BIOGRAPHICAL SKETCH The aut hor, Haya Alabhool, was born in 1980 in Kuwait. She grew up in Kuwait City, Kuwait. In 1998, she enrolled at Kuwait University and graduated in 2002 with a bachelor in medical sciences. In 2005, Alabhool graduated from Kuwait University School of Dentistry with a Bachelor of Dental Medicine degree. The author worked for the public health system of Kuwait from 2005 to 2008. In July 2008, she enrolled in the Graduate prosthodontics program at the University of Florida. After graduation with a Master of Scien ce in dental sciences with a specialization in prosthodontics from the University of Florida in May of 2010, she plans to return to her beloved country, Kuwait and her lovely daughter, Dalia, and work for the public health system.