A three-dimensional geometric model reveals posture-dependent effects of femoral anteversion on hip structural alignment

Femoral anteversion is a key morphological parameter of the human hip joint and varies widely across individuals. It is often considered mechanically destabilizing, based mainly on analyses of upright stance and walking. However, the hip is a three-dimensional load-bearing joint, and joint reaction force direction systematically changes with hip flexion. How anteversion interacts with this flexion-dependent load direction remains unclear. In this study, we develop a minimal three-dimensional geometric model to isolate this interaction. The femoral neck is modeled as a load-bearing axis defined by anteversion and neck–shaft (CCD) angle, while joint reaction force rotates posteriorly with increasing flexion. Alignment is quantified as the absolute dot product between neck axis and load direction. Results show that anteversion has no effect at neutral posture, decreases alignment in low-to-mid flexion, and partially restores alignment during deep flexion. These findings indicate that the mechanical role of femoral anteversion is inherently posture dependent.