Effect of femoral component design and quadriceps load on patellofemoral kinematics after total knee arthroplasty: an in vitro cadaveric study

Purpose Patellofemoral (PF) complications are a common cause of dissatisfaction and revision following total knee arthroplasty (TKA), often linked to altered kinematics and implant design. "Patella-friendly" femoral components with a wider, funnel-shaped trochlear groove may better restore native patellar motion. This study evaluated PF kinematics both before and after TKA performed using kinematic alignment, investigating the role of implant design and quadriceps loading. Methods In total, 12 paired fresh-frozen cadaveric lower limbs were tested before and after TKA. Within each pair, one limb received a traditional medial pivot femoral component, while the contralateral limb received a "patella-friendly" medial pivot femoral component. Native and implanted knees were tested by flexing the knee under the action of an external load applied through the quadriceps tendon, varying its magnitudes (20, 160, 280N) and directions in the frontal (neutral,+/- 6 degrees, +/- 12 degrees) and sagittal plane (neutral, +5 degrees anterior). Motion was captured using an eight-camera optoelectronic system. Results In the reference condition (20N, neutral direction), neither design showed statistical differences versus native (p > 0.05). However, the patella excursion in varus-valgus rotation was much higher in the specimens implanted with the traditional femoral component design (35.1 degrees versus 14.6 degrees native) than with the patella-friendly (20.5 degrees). Differences between the designs emerged mainly with quadriceps load variations, especially frontal direction changes, which significantly affected patellar motion in both native and implanted knees (p < 0.05). Overall, the patella-friendly design better reproduced native kinematics under most conditions. However, with extreme medial loading (12 degrees), three out of six specimens implanted with the patella-friendly femoral component were untestable owing to instability, and others exhibited high lateral displacement and trochlear dysplasia. In contrast, all traditional design implants remained stable, though with greater deviation from native kinematics. Conclusions This study provides foundational insights into PF biomechanics before and after TKA with kinematic alignment. By analyzing the interplay between implant geometry and quadriceps loading direction, it emphasizes the importance of selecting femoral components on the basis of individual patient anatomy. Our findings suggest that patella-friendly femoral components-although capable of better reproducing native motion in some cases-may not be suitable for patients with medially directed quadriceps forces or severely varus morphotypes.