Femoral neck anteversion alters patellofemoral joint mechanics during walking: insights from musculoskeletal modeling and Monte Carlo simulations.

Femoral neck anteversion (FNA) may lead to compensatory mechanisms during gait but the isolated influence of abnormal FNA on patellofemoral joint (PFJ) mechanics is not yet well understood. We used a detailed musculoskeletal model of the knee with 12 degrees of freedom. Seven models with different FNA, ranging from -8° to 52°, were developed by rotating the proximal part of the femur and the corresponding muscle attachment points along the mechanical axis of the femur, in the transverse plane. OpenSim COMAK and Monte Carlo simulations were used to estimate muscle force and patellar kinematics while accounting for ligament force and PFJ cartilage contact force. Our analyses revealed that increasing FNA led to increased patellar lateral tilt and shift, lower PFJ cartilage pressure during the loading response, but higher cartilage pressure during mid and late stance. Also, the abduction moment arm of the gluteus minimus and medius decreased, while their extension moment arm increased with increasing FNA. Therefore, during the early and late stance phases, the hip abductor muscles in the models with increased FNA generated not only a hip abductor moment, but also a hip extensor moment. Consequently, the contribution of hip flexor muscles, including the rectus femoris, increased in these phases to counteract the increased hip extensor moment. Our findings explain why individuals with increased FNA may reduce their hip abductor moment during the second part of the stance phase, but not the first part. These findings may have clinical implications for patellar instability and joint stress.