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Increased Conformity Offers Diminishing Returns for Total Knee Replacement Wear

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Title: Increased Conformity Offers Diminishing Returns for Total Knee Replacement Wear
Physical Description: 1 online resource (31 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: arthroplasty, biomechanics, knee, replacement, total
Biomedical Engineering -- Dissertations, Academic -- UF
Genre: Biomedical Engineering thesis, M.E.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Wear remains a significant problem limiting the lifespan of total knee replacements (TKRs). Though increased conformity between TKR components has the potential to decrease wear, the optimal amount and planes of conformity have not been investigated. This study used a computational model of a Stanmore knee simulator machine and a previously validated wear model to investigate this issue for simulated gait. TKR geometries with different amounts and planes of conformity were created and tested in two phases. The first phase utilized a wide range of sagittal and coronal conformity combinations to blanket a physically realistic design space. The second phase performed a more focused investigation of the conformity conditions from the first phase to which predicted wear volume was sensitive. For the first phase, sagittal but not coronal conformity was found to have a significant effect on predicted wear volume. For the second phase, increased sagittal conformity was found to decrease predicted wear volume in a nonlinear fashion, with wear volume reductions gradually diminishing as conformity increased. These results suggest that TKR geometric design efforts should focus on sagittal rather than coronal conformity and that increased sagittal conformity offers diminishing returns in terms of decreased wear.
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.
Thesis: Thesis (M.E.)--University of Florida, 2008.
Local: Adviser: Fregly, Benjamin J.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022169:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022169/00001

Material Information

Title: Increased Conformity Offers Diminishing Returns for Total Knee Replacement Wear
Physical Description: 1 online resource (31 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: arthroplasty, biomechanics, knee, replacement, total
Biomedical Engineering -- Dissertations, Academic -- UF
Genre: Biomedical Engineering thesis, M.E.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Wear remains a significant problem limiting the lifespan of total knee replacements (TKRs). Though increased conformity between TKR components has the potential to decrease wear, the optimal amount and planes of conformity have not been investigated. This study used a computational model of a Stanmore knee simulator machine and a previously validated wear model to investigate this issue for simulated gait. TKR geometries with different amounts and planes of conformity were created and tested in two phases. The first phase utilized a wide range of sagittal and coronal conformity combinations to blanket a physically realistic design space. The second phase performed a more focused investigation of the conformity conditions from the first phase to which predicted wear volume was sensitive. For the first phase, sagittal but not coronal conformity was found to have a significant effect on predicted wear volume. For the second phase, increased sagittal conformity was found to decrease predicted wear volume in a nonlinear fashion, with wear volume reductions gradually diminishing as conformity increased. These results suggest that TKR geometric design efforts should focus on sagittal rather than coronal conformity and that increased sagittal conformity offers diminishing returns in terms of decreased wear.
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.
Thesis: Thesis (M.E.)--University of Florida, 2008.
Local: Adviser: Fregly, Benjamin J.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022169:00001


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1 INCREASED CONFORMITY OFFERS DI MINISHING RETURN S FOR TOTAL KNEE REPLACEMENT WEAR By CARLOS MARQUEZ-BARRIENTOS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING UNIVERSITY OF FLORIDA 2008

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2 2008 Carlos Marquez-Barrientos

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3 For Jasmine

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4 ACKNOWLEDGMENTS I would like to thank Dr. Fregly and Dr. Banks with providing me w ith excellent guidance through out my time here at the University of Florida. With your help, I have developed tremendously. I would like to thank all my la b mates who have helped me along my way. I would like to thank my friends here who have helped in various point s of my life at the University. Finally, I would like to tha nk Jasmine for her tremendous contributions.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES................................................................................................................ .........7 ABSTRACT....................................................................................................................... ..............8 CHAPTER 1 INTRODUCTION................................................................................................................... .9 2 METHODS........................................................................................................................ .....11 Stanmore Simulator Machine Model......................................................................................11 Computational Wear Tests.....................................................................................................13 3 RESULTS........................................................................................................................ .......19 4 DISCUSSION..................................................................................................................... ....25 LIST OF REFERENCES............................................................................................................. ..28 BIOGRAPHICAL SKETCH.........................................................................................................31

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6 LIST OF TABLES Table page 2-1 The testing matrix for th e first phase of testing.................................................................17 2-2 The sagittal testing matrix for the second phase of testing................................................18 2-3 The coronal testing matrix for the second phase of testing...............................................18

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7 LIST OF FIGURES Figure page 2-1 Sample femoral and tibial geometry.................................................................................15 2-2 Sagittal profile of the TKR................................................................................................16 3-1 Results from the first phase of testing, moderately conformal lateral compartment.........20 3-2 Results from the first phase of testing, flat lateral compartment.......................................21 3-3 Results from tests varying the sagittal conformity in the lateral compartment.................22 3-4 Results from tests varying sagittal conformity in the medial compartment......................23 3-5 Results from tests varying the sagittal conformity in the lateral and medial compartments simultaneously............................................................................................24

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8 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the degree of Master of Engineering INCREASED CONFORMITY OFFERS DI MINISHING RETURN S FOR TOTAL KNEE REPLACMENT WEAR By Carlos Marquez-Barrientos May 2008 Chair: BJ Fregly Major: Biomedical Engineering Wear remains a significant problem limiti ng the lifespan of to tal knee replacements (TKRs). Though increased conformity between TKR components has the potential to decrease wear, the optimal amount and planes of conformity have not been investigated. This study used a computational model of a Stanmore knee simula tor machine and a previously validated wear model to investigate this issue for simulated ga it. TKR geometries with different amounts and planes of conformity were created and tested in two phases. The first phase utilized a wide range of sagittal and coronal conformity combinations to blanke t a physically realistic design space. The second phase performed a more focused inves tigation of the conformity conditions from the first phase to which predicted we ar volume was sensitive. For the first phase, sagittal but not coronal conformity was found to have a signific ant effect on predicted wear volume. For the second phase, increased sagittal conformity was found to decrease predicted wear volume in a nonlinear fashion, with wear volum e reductions gradually diminish ing as conformity increased. These results suggest that TKR geometric design efforts should fo cus on sagittal rather than coronal conformity and that increas ed sagittal conformity offers diminishing returns in terms of decreased wear.

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9 CHAPTER 1 INTRODUCTION Osteoarthritis is the most common form of arthritis in the United States and affects approximately 21 million Americans [1] Osteoarthrit is occurs when the cartilage at the end of the bones in a joint gets worn aw ay, leading to bone on bone contac t. Osteoarthritis often affects the knee joint. Knee osteoarthritis can cause a li mited range of motion, stiffness, and incredible pain. One treatment option for osteoarthritis in the knee joint is a TKR. According to the National Hospital Discharge Survey in 2003 over 475,000 patients rece ived TKRs [2]. The long term survival of TKRs is a pr oblem. A study of 11,606 TKRs found that ten years after implantation TKRs have a 91% survival rate. At fifteen years the survival rate drops to 84% and at twenty years the survival rate drops to 78% [3]. Polyethylene wear remains an important factor limiting the longevity of total knee replacements (TKRs) [4-6]. Wear particles liberated from the polyethylene tibial insert can induce osteolysis (i.e., bone cell death) which in turn can lead to component loosening [7]. Im proved wear performance is becoming increasingly important as younger, more active patients are impl anted [8]. Ideally, the implant should outlive the patient while not limiting function. Practica lly, TKR damage and survivorship have been reported to be worse in younger than in older patients [9], causing many patients to limit the physical activities in which they participate. Increased conformity between the femoral comp onent and tibial insert has been proposed as a means for reducing wear [ 4, 10-13]. Increased conformity in well-aligned implants reduces contact stresses on the polyethylene tibial insert [14-16]. Since polyethylene wear is due to the combined effect of contact stress and sliding conditions, contact stress reductions have been hypothesized to reduce wear volume as well [13]. As a side benefit, increased conformity has also been reported to improve th e stability of the implant [17] However, other studies have

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10 reported that increased conformity may have l ittle effect on polyethylene wear volume [18, 19], possibly because the decrease in contact stress is counteracted by an increase in contact area subjected to sliding. Furthermore, increased c onformity has potential disadvantages such as increased contact stress if the components are ma lalgined [20-22], increa sed wear due to easier entrapment of wear particles between the ar ticular surfaces [16], and increased component fixation forces [23]. Thus, one of the challenges of TKR design is to determine the conformity conditions that strike a balance between th ese potential advantag es and disadvantages. This study used a validated computational m odel to assess the effect of varying TKR conformity on polyethylene wear volume [24, 25]. The three-dimensional computational model, which mimicked a Stanmore knee simulator machine performing a simulated gait motion, was used to perform wear simulations in two phases. The first phase blanketed a wide range of TKR conformity conditions, while the second phase pe rformed a more focused investigation of the conditions to which wear volume was sensitive. Us e of a computational rath er than experimental approach allowed evaluation of this large range of geometric designs. The results provide general design guidelines for when increased sagittal a nd coronal conformity ma y, and may not, be valuable for reducing wear in total knee replacements

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11 CHAPTER 2 METHODS Stanmore Simulator Machine Model A computational model of a Stanmore knee simulator machine was constructed in the Pro/MECHANICA MOTION (Parametric Tec hnology Corporation, Waltham, MA) multibody dynamics simulation environment. (Figure 2-1) The tibial component in the model was allowed to translate freely in the medial -lateral (ML) and anterior-pos terior (AP) direction and was allowed to rotate freely around a superior-inferior (S I) axis. The femoral component was allowed to translate freely in the SI direction and rotate freely around an AP axis. These degrees of freedom were the same as in the real simulato r machine except for two minor modifications. In the actual machine, SI transla tion is accommodated on the tibial ra ther than the femoral side, and tibial translations are achieved vi a sagittal and coronal plane rotations about a point far below the tibial component rather than via axial plane tr anslations. Other studies have used the same modeling idealizations used here to develop com putational simulations of the Stanmore machine [17, 26, 27]. One-cycle dynamic gait simulations were perf ormed with the computational model using ISO standard motion and load inputs for th e Stanmore machine (ISO 1423-2, 2000). An AP control force and internal-external (IE) contro l torque were applied to the tibial component, while an SI control force was applied to th e femoral component. Flexion of the femoral component was prescribed about the femoral flexi on axis. Soft tissue restraints were simulated by attaching two spring bumpers to the anterior and posterior sides of the tibial component. The springs were attached at the same locations as in the actual simulator machine, and the stiffness of each spring was set to 14.28 N/mm based on pers onal communications with Dr. DesJardins at Clemson.

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12 Contact pressures between the femoral component and tibial insert were calculated using a custom elastic foundation model incorporated into the Pro/MECHANICA MOTION simulator machine model [28, 29]. Geometry evaluations required by the model were performed using the ACIS 3D Toolkit (Spatial Corpor ation, Westminster, CO). To pr event excessive interpenetration, the contact model utilized springs distributed uniformly over the ar ticulating surfaces of the tibial insert, where each spring was treated as independ ent form its neighbors and was associated with a single tibial surface element of kno wn area. The contact pressure p for each element was calculated from 1 112 E d p h (2-1) where E is Youngs modulus of the elas tic layer (= 463 MPa; [14]), is the Poissons ratio of the elastic layer (= 0.45; [30]), h is the layer thickness at the element location, and d is the elements spring deflection, defined as the in terpenetration of the undeformed surfaces in the direction of the local surface normal. The distance d for each element was computed at each time instant from the relative position and orient ation of the femoral com ponent with respect to the tibial insert. Individual element pressures were converted into element forces using the known area of each element, and these forces were treated as equal and opposite loads applied to the articulating surfaces during a dynamic simulation. Wear volume for each dynamic gait simulati on was calculated using the predicted time histories of contact pressures a nd sliding conditions for each tibia l insert surface element. Over the course of a one-cycle simulation, the tota l depth of material removed from an element Wear was predicted using Archards classic law for mild wear[31]:

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13 1 n Weariii ikpvt (2-2) where k is the material wear rate (2.59x10-7 mm3/Nm; [24]), i is a discrete time frame within the one-cycle simulation, n is the total number of time frames, i p is the element contact pressure at instant i, iv is the magnitude of the elements relative sliding velocity at instant i, and it is the time increment used in the analysis[25]. Wear volume for each surface element was calculated by multiplying element wear depth by element area, and total wear volume was calculated by summing element wear volumes over all su rface elements. One-cycle wear volume was extrapolated out to 5 million cycles, representative of the total number of cycles commonly used for testing in a simulator machine. Computational Wear Tests Computational wear testing was performe d in two phases using tibial and femoral geometries representing a wide range of sag ittal and coronal conformities. Phase one tests utilized a wide range of sagittal and cor onal conformities to blanket a broad design space representative of contemporary knee replacement ge ometries. A sagittal profile was built using the expertise of Dr. Scott Banks, who has experi ence in building TKRs. The sagittal profile had three distinct radii, during normal gait contact would be on the radii of 21.55mm (Figure 2-2). The sagittal profile was kept cons tant through all the tests. Confor mity in the sag ittal or coronal dimension was defined as the femoral radius over the tibial radius. Changes in conformity were made by changing the tibial components radii. Th e tibial component had a single radius in all cases. The first phase of testing mostly varied the conformity of the medial compartment, although the lateral compartment conformity was vari ed as well (Table 1). Results were analyzed

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14 and tests for the second phase of testing were sel ected in order to clarify questions raised by the first phase of testing. Phase 2 tests performed a more focused investig ation of the conformity conditions in phase one to which predicted wear volume was sensitive. The femoral sagittal profile used was the same one used in the first phase of testing. The femoral coronal radius used was 40mm. For this phase, sagittal conformity was varied in the late ral and medial compartments both separately and together (Table 2-1). In contrast to the first phase of testing, sa gittal conformity was varied at more points, allowing a better unde rstanding of how small increases in conformity affected wear volume. For sagittal conformity tests where onl y one compartments conformity was varied, the opposite compartments sagittal conformity was 0.50. There was a common case for all three series of sagittal tests, where both the lateral and medial compartment had a sagittal conformity of 0.50. Coronal conformity tests were conducte d to confirm first phase observations about the influence of conformal conformity (Table 2-3). The tests involved simultaneously varying the coronal conformity in both compartments.

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15 Figure 2-1. Sample femoral and tibial geometry in the computational Stanmore Simulator. The simulator was constructed in ProMECHANICA/MOTION.

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16 Figure 2-2. Sagittal profile of the TKR used in all tests There are three radii of 36.9mm, 21.55mm, and 8.56mm. During normal gait, contact occurs exclusively along the 21.55mm radius.

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17 Table 2-1. The testing matrix for the first phase of testing. Conformity is defined as the radius of the femoral component over the radius of the tibial component. The femoral components sagittal profile was constant for all the tests. The co ronal radius of the femoral component was either 20mm or 80mm. There were six different conformities in the medial compartment for both the 20mm and 80mm femoral components. The sagittal conformity of 0.91 was chosen as a data point because it corresponds to the ratio 1:1.1. For all the scenarios listed fo r the medial compartment, two tests were run. One test had a lateral compartment conformity of 0 in the sagittal and coronal dimensions. The other test had lateral comp artment conformity of 0.5 in the sagittal and coronal dimensions. In total twen ty four test cases were run. Femoral Coronal Radius (mm) Tibial Coronal Conformity Tibial Sagittal Conformity Lateral Compartment 20 0.0 0.00 20 0.5 0.00 20 0.00.50 20 0.50.50 20 0.00.91 20 0.50.91 80 0.0 0.00 80 0.5 0.00 80 0.00.50 80 0.50.50 80 0.00.91 80 0.50.91 + Coronal And Sagittal Conformity are 0.0 OR Coronal and Sagittal Conformity are 0.5

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18 Table 2-2. The sagittal testing matrix for the second phase of te sting. There were three series of tests that varied the sagittal conform ity of the tibial compartment. Medial compartment sagittal conformity was varied, lateral compartment sagittal conformity was varied, and both compartments sagittal conformity was varied simultaneously. Each series of tests had eight data points correspondi ng to the conformities listed above. Sagittal Conformity Variations Femoral Sagittal Radii (mm) 21.55 21.5521.55 21.55021.55021.55 21.55 21.55 Tibial Sagittal Radii (mm) infinite 172.4 138 86.20068.96057.47 43 28.73 Conformity 0.0 0.1250.1560.250 0.3130.375 0.501 0.750 Table 2-3. The coronal testing matrix for the se cond phase of testing. The coronal conformity was varied in both compartments simultaneously. Coronal Conformity Variations Femoral Coronal Radii(mm) 40 40 40 40 Tibial Coronal Radii (mm) infinite 160 80 53.33 Conformity 0 0.25 0.50 0.75

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19 CHAPTER 3 RESULTS The first phase tests revealed that sagittal but not coronal conform ity significantly affects predicted wear volume. Medial sagittal confor mity was found to decrease wear when going from no conformity to a moderate conformity of 0.50 if the lateral compartment was conformal in the medial and sagittal dimensions. Further increas es in sagittal conformity from a moderate conformity of 0.50 to a high conformity of 0.91 l ead to little change in wear (Figure 3-1). In contrast to the results obtained when the late ral compartment is conformal, when the lateral compartment was flat no changes in wear volume were seen when changing the medial sagittal conformity from either 0 to 0.50 or from 0.50 to 0.91 (Figure 3-2). The second phase results indicate d that increases in sagittal conformity decreased wear if the opposite lateral compartment had some sa gittal conformity. Increases in sagittal conformity were found to decrease wear in the tibial compartm ent (medial or lateral) when conformity was increased (Figures 3-3, 3-4, 3-5) The largest effect from increases in sagittal conformity was observed when conformity was incr eased from a very low conformity to a higher conformity. Increases in conformity over 0.50 re sulted in very small decreases in wear. The common case for all three sagittal tests had a ma ximum pressure of 48MPa, well above the yield pressure of 35MPa. The second phase coronal tests found that coronal conformity had little impact on wear, which confirme d the first phase findings. :

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20 Figure 3-1. Results from the first phase of testing, moderately conformal lateral compartment. This graph shows all the tests where the lateral compartment has a sagittal and a coronal conformity of 0.5. Initially ther e is a sharp drop in wear volume produced with the increase in sagittal conformity from 0 to 0.5. The increase in sagittal conformity from 0.5 to 0.91 causes little change in wear volume

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21 Figure 3-2. Results from the first phase of testing, flat lateral compartment. This graph shows all the tests where the lateral compartment has a sagittal and a coronal conformity of 0. In contrast to the tests where the lateral compartment had some conformity, the increase in sagittal conformity from 0 to 0. 5 leads to very small increases or decreases in wear volume. The increase in sagittal conformity from 0.5 to 0.91 caused a slight drop in wear volume

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22 Figure 3-3. Results from tests varying the sagittal conformity in the lateral compartment. The sagittal conformity in the medial compar tment was constant at 0.50. Wear in the lateral compartment decreases with increasing conformity while wear in the medial compartment holds relatively constant. The largest decrease in w ear comes with the initial increase in conformity and after a certain point, increases in sagittal conformity provide diminishing returns.

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23 Figure 3-4. Results from tests varying sagittal conformity in the medial compartment. The sagittal conformity in the lateral compartment was constant at 0.50. Wear in the medial compartment decreases with increa sing conformity while the wear in the lateral compartment stays relatively constant. The largest decrease in wear comes with the initial increase in conformity and after a certain point, increases in sagittal conformity provide diminishing returns. The results agree with the trends in Figure 1 that showed that increasing conformity in a compartment decreases wear in the compartment, while wear in the compartment that does not have the conformity being varied stays relatively constant. The initial wear in the medial compartment for the medial sagittal conformity te sts is larger than the wear in the lateral compartment for the lateral sagittal conformity tests. This can be explained by the internal-external torque that is applied duri ng the Stanmore simulations which causes the loads on the different compartments to be asymmetric.

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24 Figure 3-5. Results from tests varying the sa gittal conformity in the lateral and medial compartments simultaneously. As in the tests where the medial and lateral compartment sagittal conformity were varied separately, the largest decrease in wear is seen with the initial incr ease in sagittal conformity. Also similar to the other tests there are diminishing returns on the decrea se in wear with increasing sagittal conformity. The initial wear is greater than in the tests where the medial and lateral compartment sagittal conformity were varied separately, which is probably due to the fact that, in contrast to th e other two series of tests, neither compartment has sagittal conformity initially.

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25 CHAPTER 4 DISCUSSION The computational tests conducted here offer a broad overview on the effects of increased conformity in the coronal and sagittal dimensions The computational simulator, which is based on the Stanmore simulator, took TKR geometries of different conformities and predicted wear using a previously validated wear model. A wide range of TKR conformities were tested in two phases. The first phase of testing raised quest ions, which the second phase of testing attempted to answer. Results from the two phases of test ing indicate that increase d sagittal conformity decreases the wear volume if the opposite comp artment has some sagittal conformity. The largest decreases in wear volume occur when increasing sagittal conformity in a compartment from a low conformity. Coronal conformity was found to be unimportant. The wear results for the tests where the medial compartments sagitta l conformity was varied and the wear results from the tests where the sagittal conformity was varied in the lateral compartment are not identical due to the non-symmetry of the applie d loads (Figures 3-3 and 3-4). The IE torque applied to the tibia resulted in the fe mur pivoting on the lateral compartment. The current studies results are in contrast to a previous in vitro simulator study [18]. The study looked at an existing TKR design and modified the design so the tibial insert had a larger sagittal radius. The wear results from the two designs were not sta tistically different. A lack of a drop in wear volume could possibly be explained by the difference in femoral sagittal profile and that the tibial insert had several radii compar ed to the current studies inserts which had one. Even though idealized geometries were used in this study, they still yield valuable insight into the effect of sagittal and coronal conformity changes on wear volume. Retrieval studies have attempted to analyze the effect of conformity on wear in retrieved TKRs, but few have looked at conformity in the sagittal and coronal planes ex plicitly [4, 11, 32]. The results from the current

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26 study suggest that increased conformity can re duce wear, something which has been suggested by previous studies [4, 10-13, 15]. Several retr ieval studies have shown that more conformal inserts tend to reduce wear [4, 11]. Quantitative conclusions on the influence of conformity are hard to draw from retrieval studies due to confounding factors such as unknown patients activity levels, differing UHMWPE manufacturing techniques, and a lack of detailed information given on the conformity of the TKRs retrieved. Finite Element (FE) studies have shown that increased conformity can lead to decrea sed stresses if the component is properly aligned [14-16]. Decreased contact stress has been linked to decreased wear factors [33]. The current study is based on certain limitations and assumptions. During a simulation, the surfaces of the TKR component are not updated, but during an in vitro simulation geometry can change considerably. A previous study usin g the wear model used in the current study to predict wear in a generated AMTI machine found that wear volum e prediction was insensitive to surface updating, although other factors, such as wear depth, were sensitive to surface updating [24]. Similarly wear volume was found to be insensitive to creep or to modeling the UHMWPE as a non-linear material, things not accounted for in the current st udy [24]. The wear factor was taken as a constant, although wear factors can increase considerably with cross shear [34]. A study which used fluoroscopy to track the motion of a TKR patient found that there was little cross shear motion in the TKR during gait and stair climbing [35], so the assumption of a constant wear factor should not be a problem. The simulations r un in this study were exclusively gait simulations. A previous study using the contact code found that more realistic damage areas were found if stair as well as gait loads were simulated, alt hough gait was assumed to be the predominate factor [25]. The current studys fo cus on gait simulations should not be a problem since we are just looking at overall wear trends.

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27 Overall the current study offers detailed info rmation on the effect of conformity in the sagittal and coronal dimensions has on wear volume in TKRs. Sa gittal conformity was found to reduce wear under certain conditions and a point of diminishing re turns on wear reduction was identified. The information in this study can help future TKR designers identify where the advantages of TKR conformity st art to be out weighed by the di sadvantages of TKR conformity.

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28 LIST OF REFERENCES [1] NIH, 2006, "Osteoarthritis," U. S. D. o. H. a. H. Services, ed. [2] Kozak, L. J., Lees, K. A., and DeFrances C. J., 2006, "National Hospital Discharge Survey: 2003 annual summary with detailed diagnos is and procedure data," Vital Health Stat 13(160), pp. 1-206. [3] Rand, J. A., Trousdale, R. T., Ilstrup, D. M., and Harmsen, W. S., 2003, "Factors affecting the durability of primary total knee prostheses," J Bone Joint Surg Am, 85-A(2), pp. 259-265. [4] Benjamin, J., Szivek, J., Dersam, G., Per sselin, S., and Johnson, R., 2001, "Linear and volumetric wear of tibial inserts in posterior cruc iate-retaining kn ee arthroplasties," Clin Orthop Relat Res(392), pp. 131-138. [5] Naudie, D. D., Ammeen, D. J., Engh, G. A., and Rorabeck, C. H., 2007, "Wear and osteolysis around total knee arthroplasty," J Am Acad Orthop Surg, 15(1), pp. 53-64. [6] Sharkey, P. F., Hozack, W. J., Rothman, R. H., Shastri, S., and Jacoby, S. M., 2002, "Insall Award paper. Why are total knee arthr oplasties failing today?," Clin Orthop Relat Res(404), pp. 7-13. [7] Fisher, J., Bell, J., Barbour, P. S., Tipper, J. L., Matthews, J. B., Besong, A. A., Stone, M. H., and Ingham, E., 2001, "A novel method for the prediction of functional biological activity of polyethylene wear debris," Proc In st Mech Eng [H], 215(2), pp. 127-132. [8] 2003, "NIH Consensus Statement on tota l knee replacement," NIH Consens State Sci Statements, 20(1), pp. 1-34. [9] Roberts, V. I., Esler, C. N., and Ha rper, W. M., 2007, "What impact have NICE guidelines had on the trends of hip arthroplasty since their publication? The results from the Trent Regional Arthroplasty Study between 1990 and 2005," J Bone Joint Surg Br, 89(7), pp. 864-867. [10] Bartel, D. L., Bicknell, V. L., and Wri ght, T. M., 1986, "The effect of conformity, thickness, and material on stresses in ultra-hi gh molecular weight components for total joint replacement," J Bone Joint Surg Am, 68(7), pp. 1041-1051. [11] Collier, J. P., Mayor, M. B., McNamara, J. L., Surprenant, V. A., and Jensen, R. E., 1991, "Analysis of the failure of 122 polyethylene in serts from uncemented tibial knee components," Clin Orthop Relat Res(273), pp. 232-242. [12] Dennis, D. A., 2006, "Trends in total knee arthroplasty," Orthopedics, 29(9 Suppl), pp. S13-16. [13] Kuster, M. S., and Stachowiak, G. W., 2002, "Factors affecting polye thylene wear in total knee arthroplasty," Orthopedics, 25(2 Suppl), pp. s235-242.

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29 [14] Bartel, D. L., Rawlinson, J. J., Burstein A. H., Ranawat, C. S., and Flynn, W. F., Jr., 1995, "Stresses in polyethylene components of contemporary total knee replacements," Clin Orthop Relat Res(317), pp. 76-82. [15] Kuster, M. S., Horz, S., Spalinger, E., Stachowiak, G. W., and Gachter, A., 2000, "The effects of conformity and load in total knee replacement," Clin Orthop Relat Res(375), pp. 302312. [16] Sathasivam, S., and Walker, P. S., 1994, "Optimization of the b earing surface geometry of total knees," J Biomech, 27(3), pp. 255-264. [17] Luger, E., Sathasivam, S., and Walker, P. S., 1997, "Inherent differen ces in the laxity and stability between the intact knee and tota l knee replacements," Knee, 4(1), pp. 7-14. [18] Essner, A., Klein, R., Bushelow, M., Wang, A. G., Kvitnitsky, M., and Mahoney, O., 2003, "The effect of sagittal conform ity on knee wear," Wear, 255, pp. 1085-1092. [19] Bei, Y., Fregly, B. J., Sawyer, W. G. Banks, S. A., and Kim, N. H., 2004, "The relationship between contact pressure, insert thickness, and mild wear in total knee replacements," Cmes-Computer Modeling in Engineering & Sciences, 6(2), pp. 145-152. [20] D'Lima, D. D., Chen, P. C., and Colwell, C. W., Jr., 2001, "Polyethylene contact stresses, articular congruity, and knee alignment, Clin Orthop Relat Res(392), pp. 232-238. [21] Liau, J. J., Cheng, C. K., Huang, C. H., Lee, Y. M., Chueh, S. C., and Lo, W. H., 1999, "The influence of contact alignment of the tibio femoral joint of the prostheses in in vitro biomechanical testing," Clin Biomec h (Bristol, Avon), 14(10), pp. 717-721. [22] Liau, J. J., Cheng, C. K., Huang, C. H., and Lo, W. H., 2002, "The effect of malalignment on stresses in polyethylene component of total kn ee prostheses--a finite el ement analysis," Clin Biomech (Bristol, Avon), 17(2), pp. 140-146. [23] Morra, E. A., Postak, P. D., Plaxton, N. A., and Greenwald, A. S., 2003, "The effects of external torque on polyethylene ti bial insert damage patterns," Clin Orthop Relat Res(410), pp. 90-100. [24] Zhao, D., Sakoda, H., Sawyer, W. G., Banks S. A., and Fregly, B. J., 2008, "Predicting knee replacement damage in a simulator machine using a computational model with a consistent wear factor," J Biomech Eng, 130(1), p. 011004. [25] Fregly, B. J., Sawyer, W. G., Harman, M. K., and Banks, S. A., 2005, "Computational wear prediction of a total knee replacement fr om in vivo kinematics," J Biomech, 38(2), pp. 305314. [26] Godest, A. C., Beaugonin, M., Haug, E., Taylor, M., and Gregson, P. J., 2002, "Simulation of a knee joint replacement during a gait cycle using explicit finite element analysis," J Biomech, 35(2), pp. 267-275.

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30 [27] Knight, L. A., Pal, S., Coleman, J. C., Br onson, F., Haider, H., Levine, D. L., Taylor, M., and Rullkoetter, P. J., 2006, "Comparison of l ong-term numerical and ex perimental total knee replacement wear during simulated gait loading," J Biomech. [28] Bei, Y., and Fregly, B. J., 2004, "Multibody dynamic simulation of knee contact mechanics," Med Eng Phys, 26(9), pp. 777-789. [29] Fregly, B. J., Bei, Y., and Sylvester, M. E., 2003, "Experimental ev aluation of an elastic foundation model to predict contact pressures in knee replacements," J Biomech, 36(11), pp. 1659-1668. [30] Kurtz, S. M., Jewett, C. W., Bergstrom, J. S., Foulds, J. R., and Edidin, A. A., 2002, "Miniature specimen shear punch test for UHMWPE used in total joint replacements," Biomaterials, 23(9), pp. 1907-1919. [31] Archard, J. F., and Hirst, W ., 1956, "THE WEAR OF METALS UNDER UNLUBRICATED CONDITIONS," Proceedings of the Royal Society of London Series aMathematical and Physical Sciences, 236(1206), pp. 397-&. [32] Blunn, G. W., Joshi, A. B., Minns, R. J., Li dgren, L., Lilley, P., Ryd, L., Engelbrecht, E., and Walker, P. S., 1997, "Wear in retrieved cond ylar knee arthroplasties. A comparison of wear in different designs of 280 retr ieved condylar knee prostheses," J Arthroplasty, 12(3), pp. 281290. [33] Rose, R. M., Goldfarb, H. V., Ellis, E., and Crugnola, A. M., 1983, "ON THE PRESSURE-DEPENDENCE OF THE WEAR OF ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE," Wear, 92(1), pp. 99-111. [34] Kang, L., Galvin, A. L., Brown, T. D., Jin, Z., and Fisher, J., 2008, "Quantification of the effect of cross-shear on the wear of c onventional and highly cross-linked UHMWPE," J Biomech, 41(2), pp. 340-346. [35] Hamilton, M. A., Sucec, M. C., Fregly, B. J., Banks, S. A., and Sawyer, W. G., 2005, "Quantifying multidirectional sliding motions in total knee replacements," Journal of TribologyTransactions of the Asme, 127(2), pp. 280-286.

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31 BIOGRAPHICAL SKETCH Carlos graduated from the University of Wi sconsinMadison with a BS in engineering mechanics. In order to pursue his interests in developing new medical technologies he came to the University of Florida to earn a ME in biomedi cal engineering. In th e future, Carlos hopes to become involved in engineering projects that matter.