Are contemporary femoral components sizing and design likely to affect functional results in TKA? A mathematical model of an implanted knee to predict knee forces.

PURPOSE: This study is aimed to investigate the effects of the choice of femoral and tibial components on several mechanical outputs that might be associated with total knee replacement surgery outcomes using a validated computational model: the Kansas knee simulator.

METHODS: Two models from the same range of implants were taken into account: Model 1, the femoral component fitted the femoral epiphysis, with physiological positioning of the articulating surface using a 10-mm-thick tibial component, and in Model 2, the femoral component was 4 mm smaller than in Model 1, and a 14-mm-thick tibial component was used with a similar tibial resection and the tibio-femoral joint line was 4 mm more proximal to compensate the increased posterior bone resection and maintain proper soft-tissue tension in flexion. Changes in reaction forces and contact pressures between the components, changes in extensor muscle forces and changes in patello-femoral joint kinematics during walking gait have been studied.

RESULTS: While the computational model predicted that most kinematic and kinetic outputs, including tibio-femoral and patello-femoral joint motions, contact forces, pressures and areas, were similar for Model 1 and Model 2, and a dramatic difference has been found in the extensor muscle forces necessary to flex and extend the knee. To reproduce the same knee motion with a knee reconstructed as in Model 2, a patient would need to generate approximately 40% greater extensor muscle force throughout the gait cycle in order to do so.

CONCLUSION: As a consequence of such a large increase in the extensor muscle force, the knee motions would probably be compromised and, subsequently, a patient with a knee reconstructed as in Model 2 would be less likely to be able to reproduce normal knee function and therefore more likely to report poor outcome.