Date of Award
Master of Science (MS)
DesJardins, John D
Total knee replacements (TKR) are one of the most frequently implanted medical devices, with over 600,000 procedures performed in the United States in 2012. In order to ensure TKR longevity, wear tests are frequently conducted on these implants prior to patient implantation. Variations in implant geometry, material, and surface treatments are all tested, however, TKR alignment may also play a role in the long-term success of the knee implant. When testing knee designs with complex tibial and femoral geometries it is essential that the implant be aligned as the implant manufacturers intended so as to best represent the function of the implant system. Although critical, a key alignment variable that is largely overlooked is femoral axis selection. Currently, femoral axis alignment is simply selected so as to minimize its effect on implant mechanics during walking simulation; a result that might completely misrepresent the implant designer's intent. The purpose of this study was to create a computational model to determine the effect of femoral axis selection on contact-point bearing migration prior to simulator fixation and examine trends in femoral axis selection based on implant geometry. Using 3D optical scans of seven femurs, 3Matic STL for model remeshing, and COMSOL Multiphysics for simulation this study recreated the single-axis rotation of each femoral component in a wear simulator. The lowest femoral contact point was then tracked between 0º and 120º flexion over four hundred possible femoral axes alignment options. The computational model was verified statistically and calculated the location of the ideal axes of rotation for all seven femurs. Reduction of P/D lowest contact-point translation during simulator flexion was found to be dependent on the range of flexion. Single-axis knee designs were found to exhibit a lower tolerance to varied femoral axes of rotation, but still maintained lower mean P/D displacements. Anterior/posterior translation patterns during simulator flexion were found to vary significantly with femoral axis selection. Interestingly, A/P translation patterns were more consistent between varying flexion axes in implants with multiple axes of curvature compared to single-axis designs. TKR alignment in single-axis simulators clearly affects proximal/distal and anterior/posterior lowest contact-point migration and thus possibly implant mechanics during functional testing. An implant that incorporates a geometry that is minimally affected by malalignment should enhance clinical outcomes and provide more consistent functional measures during simulation and use.
Marais, Nicholas, "PRIMARY FLEXION AXIS SELECTION IN TOTAL KNEE REPLACEMENTS USING COMPUTATIONAL ANALYSIS" (2015). All Theses. 2215.