Date of Award


Document Type


Degree Name

Master of Science (MS)



Committee Member

Dr. Melinda Harman, Committee Chair

Committee Member

Dr. John DesJardins

Committee Member

Dr. Hai Yao


Total knee replacement (TKR) survival rates depend heavily on the kinematic performance of the knee. Various factors contribute to implant failure, with the most common being aseptic loosening, infection, and instability. Instability is defined as abnormal or excessive displacement and can have many surgical or technical causes, and surgeons adjust for concerns of instability by choosing certain designs over others based on preference and experience. The designs of interest for this study include cruciate retaining and posterior stabilized, fixed bearing and mobile bearing, and symmetric and asymmetric. These designs vary in conformity, which is defined by the radii of the articular surfaces and exists on a spectrum, unlike the other categorical TKR design classifications. Conformity index (CI) is defined as the ratio of the femoral component radii and the polyethylene insert radii in the sagittal plane, with more similar radii predicted to provide more articular constraint. The broad objective of this thesis was to understand the role of TKR conformity for providing articular constraint to resist mechanical loads and ultimately affect joint stability. The specific aim of this thesis was to measure CI and articular constraint for a broad spectrum of TKR designs to test the hypothesis that increased CI increases overall articular constraint in AP translation and IE rotation. The results from mechanical testing were used to develop a predictive model of TKR behavior. Twenty total TKRs were selected from an IRB approved registry and tested under ASTM F1223 standard for anterior-posterior translation and internal-external rotation at 0°, 30°, 60°, and 90° of flexion. After testing, the hysteresis loops produced were used to classify four movement characteristics to describe the overall behavior of each implant at each angle. CI was measured using reverse engineering software at the approximate center of the medial and lateral condyle on the femoral component at all four angles, and the polyethylene insert radii was measured in the medial and lateral plateau. CI broadly decreased within individual TKR designs, contradicting the perception of a device as overall high or low conforming. Due to differences in movement characteristics of CI > 0.7 and CI < 0.7, the observations were divided into high and low conforming groups. The conclusions of this study indicate the importance of selecting design parameters to induce constraint in measurable ways. TKR conformity varied widely among the contemporary TKR designs, and provided noticeable articular constraint in certain designs. Increased CI was not found to increase overall articular constraint for all design groups, but was found to increase articular constraint in AP translation and IE rotation for some design groups. The predictive model was found to correlate certain design parameters with increased articular constraint, but further work includes improving the specificity of the model. Overall, CI added articular constraint in AP translation and IE rotation for many common TKR designs of the last twenty years.



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