Explanation:
The correct option is a. The posterior approach requires transection of the short external rotators and the posterior capsule, inherently destabilizing the posterior soft-tissue envelope. To prevent the femoral head from levering out posteriorly during flexion, adduction, and internal rotation, the surgeon must achieve optimal 'combined anteversion' (the sum of acetabular cup anteversion and femoral stem anteversion, typically aiming for 25 to 50 degrees). While using larger femoral heads (e.g., 32mm or 36mm) increases jump distance and helps stability, accurate component positioning to prevent mechanical impingement remains the primary dynamic safeguard. Reference: Lewinnek GE et al. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am.