Summary

In TKA, lengthening the lateral femoral condyle by 4.5 mm significantly increases patella joint contact pressures, medializes patella tracking and increases lateral tilt during knee flexion.

Abstract

Introduction

The position of the femoral component in total knee arthroplasty (TKA) is influenced by various surgeon-controlled factors such as alignment choice, joint line obliquity, tibiofemoral compartment balancing, and the patellofemoral joint (PFJ). Alterations from native anatomy, particularly changes in the lateral femoral condyle length, can significantly impact PFJ biomechanics. This study aims to evaluate the effect of lateral femoral condyle lengthening on PFJ kinematics and kinetics.

Methods

This biomechanical study utilized cadaveric knees, maintaining native muscle attachments from the pelvis to the knee. TKA was performed using the CORI robotic system (Smith & Nephew) using Legion II components and a posterior-stabilized tibial insert via medial parapatellar approach. Femoral and tibial resection depths included cartilage thickness, and patella resurfacing was conducted using a custom dome-shaped prosthesis to measure pressures across three patellar regions. Kinematics were tracked with the OptiTrack motion capture system.

The SimVITRO (Cleveland clinic, OH) musculoskeletal simulator was used to test loading conditions in the knee joint through coordinated control of an external loading device (rotopod). Controlled loading conditions were simulated utilising a hybrid of knee flexion position control and real-time proportional-integral-derivative (PID) force feedback control to simulate passive knee flexion and descending stairs. The forces trajectories were adapted from previously published in vivo testing data. The simulated conditions tested the knee through a range of motion of 0°- 100° flexion.

Femoral components were positioned to lengthen the lateral condyle by a maximum amount (4.5m), medium amount (2.5 (mm) and a neutral amount (0mm) from the native thickness. The joint reaction forces in each state was recorded and compared between groups.

Results

Six specimens were analyzed to assess the effects of lateral femoral condyle lengthening.
• Kinetics: Lengthening the lateral femoral condyle by 4.5 mm resulted in a 50% increase in mean patella pressures at 45° and 100° flexion. A 2.5 mm lengthening caused a 10% increase in patella pressure at 100° flexion but did not significantly affect pressure at 45° flexion compared to the neutral state.
• Kinematics: A 4.5 mm lengthening of the lateral femoral condyle led to a more medialized patella track. While native patella excursion ranged from 0–9 mm lateral translation, the 4.5 mm lengthening reduced this range to 0–2.5 mm. A 2.5 mm lengthening produced a lateral translation range of 0–5 mm. A 4.5 mm lengthening of the lateral femoral condyle led to a more laterally tilted patella during flexion (20°) compared to the native state (12°). There were no significant differences to the patella flexion/extension angle between the 3 states tested.

Conclusion

In TKA, lengthening the lateral femoral condyle by 4.5 mm significantly increases patella joint contact pressures, medializes patella tracking and increases lateral tilt during knee flexion. These findings underscore the importance of precise femoral component placement to optimize PFJ mechanics. These findings have implications for surgeons performing TKA.