Explanation:
The correct option is d. Femoral offset is defined as the horizontal distance from the center of rotation of the femoral head to a line bisecting the long axis of the femoral shaft. Restoring this native offset is mechanically critical because it dictates the lever arm of the abductor musculature (primarily the gluteus medius). If a surgeon uses a stem with insufficient offset, the abductor lever arm is shortened. This means the muscles must generate vastly more force to stabilize the pelvis during the single-leg stance phase of gait. This inevitably leads to profound abductor fatigue, weakness, lateral hip pain, and a classic compensatory Trendelenburg lurch. Reference: Bourne RB et al. The role of femoral offset in total hip arthroplasty. Clin Orthop Relat Res.