Tradeoffs between motion and stability in posterior substituting knee arthroplasty design.

The purpose of this study was to examine how changes in component geometry of posterior substituting knees affect tibiofemoral kinematics and prosthesis stability. Most posterior cruciate ligament substituting prostheses rely on an articulation between a femoral cam and tibial spine to provide anterior-posterior stability of the knee. Failure of this ligament substitution mechanism has resulted in knee dislocations with several different posterior substituting designs. A computer model of a generic posterior substituting prosthesis was altered to analyze the effects of five design parameters (tibial spine height, spine anterior-posterior position, femoral component posterior radius, and femoral cam anterior-posterior and distal-proximal position) on prosthesis stability, tibiofemoral kinematics, and maximum obtainable knee flexion. Prosthesis stability was characterized by a 'dislocation safety factor', defined as the vertical distance from the bottom of the femoral cam to the top of the tibial spine. Computer simulations revealed that posterior substituting knees are most likely to dislocate at maximum knee flexion. Prosthesis stability can be improved by increasing the tibial spine height and moving the femoral cam posteriorly. Our results suggest there is a tradeoff between maximum knee flexion and prosthesis stability. We found that relatively small gains in maximum knee flexion, made through design changes, may cause substantial decreases in prosthesis stability.