Femur Rotation Increases Patella Cartilage Stress in Females with Patellofemoral Pain

Purpose This study aimed to test the hypothesis that internal rotation of the femur increases patellofemoral joint stress in persons with patellofemoral pain (PFP). Methods Patella cartilage stress profiles of nine female participants with PFP were obtained during squatting using supject-specific finite element (FE) models of the patellofemoral joint (15° and 45° of knee flexion). Input parameters for the FE model included joint geometry, quadriceps muscle forces during squatting, and weight-bearing patellofemoral joint kinematics. The femur of each model was then internally rotated 5° and 10° along its long axis beyond that of the natural degree of rotation. Using a nonlinear FE solver, quasistatic loading simulations were performed to quantify patellofemoral joint stress. Results Compared with those at the natural position of the femur, mean hydrostatic pressure and mean octahedral shear stress were significantly higher when the femur was internally rotated 5° and 10°. No significant differences in stress variables were observed when the femur was rotated from 5° to 10°. These findings were consistent across both knee flexion angles (15° and 45°). Conclusions The finding of elevated hydrostatic pressure and octahedral shear stress with internal rotation of the femur supports the premise that females with PFP who exhibit abnormal hip kinematics may be exposed to elevated patellofemoral joint stress.