Design optimization of a total knee replacement for improved constraint and flexion kinematics.

Total knee replacement (TKR) constraint and flexion range of motion can be limiting factors in terms of kinematics performance and cause for revision. These characteristics are closely related to the shape of the implant components. No previous studies have used a rigorous and systematic design optimization method to determine the optimal shape of TKR components. Previous studies have failed to define a quantifiable objective function for optimization, have not used any optimization algorithms, and have only considered a limited design space (4 or less design variables). This study addresses these limitations and determines the optimum shape of the femoral component and ultra high molecular weight polyethylene (UHMWPE) insert in terms of kinematics. The constraint characteristics with respect to those of the natural knee, the importance of the posterior cruciate ligament, and the flexion range of motion were all considered. The kinematics optimized design featured small femoral radii of curvature in the frontal and sagittal planes, but asymmetric with slightly larger radii of curvature for the lateral condyle. This condyle was also less conforming than the medial side. Compared to a commercially available TKR design, the kinematics performance (based on constraint and flexion range of motion) was improved by 81%, with constraint characteristics generally closer to those of the natural knee and a 12.6% increase in the flexion range of motion (up to 143°). The results yielded a new TKR design while demonstrating the feasibility of design optimization in TKR design.