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Increased internal femoral torsion is regarded as a risk factor for patellar instability. Biomechanical investigations confirming this hypothesis are missing.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.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 < 0.001). Patellar shift did not change significantly with increased internal femoral torsion (P ≥ 0.30).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.Femoral torsion has a significant impact on the patellofemoral joint.Increased internal femoral torsion produces a lateralizing force on the patella.20° internal torsion and an intact medial patellofemoral ligament can be a risk factor for patellar instability.10° internal torsion and a torn medial patellofemoral ligament can be a risk factor for patellar instability.