Increased internal femoral torsion can be regarded as a risk factor for patellar instability — A biomechanical study

Abstract Background Increased internal femoral torsion is regarded as a risk factor for patellar instability. Biomechanical investigations confirming this hypothesis are missing. Methods Eight fresh-frozen cadaver knees were tested on a specially designed simulator. Patellar motion and patellofemoral pressure were evaluated for 0°, 10°, and 20° of increased internal and external femoral torsion with native and with transected medial patellofemoral ligaments used to simulate patellar instability. A regression analysis was used for statistical analysis. Findings In native medial patellofemoral ligaments, there were no significant changes in mean or peak pressures for any torsional states ( P  ≥ 0.07). At 20° increased internal femoral torsion, there was a significant center of force shift towards the lateral side ( P  = 0.01). Patellar shift was directed laterally at low knee flexion angles up to 30°. Lateral patellar tilt increased significantly at 10° and 20° of increased internal femoral torsion ( P  ≤ 0.004). In transected medial patellofemoral ligaments, mean pressure (P ≤ 0.005) and peak pressure ( P  ≤ 0.02) decreased significantly for all torsional states. There was a significantly greater lateral center of force shift with increased internal femoral torsion ( P  ≤ 0.04). Lateral patellar tilt increased significantly ( P P  ≥ 0.30). Interpretation In a native medial patellofemoral ligament, 20° of increased internal femoral torsion can be regarded as a significant risk factor for patellar instability. With an insufficient medial patellofemoral ligament, 10° of increased internal femoral torsion already represents a significant risk factor.