Anterior mechanical impingement of the prosthetic femoral neck against the anterior acetabular rim during total hip arthroplasty (THA) is most likely to act as a fulcrum, resulting in which specific direction of joint instability?

Explanation:
The correct option is b. Component impingement creates a dangerous mechanical fulcrum. When a patient flexes, adducts, and internally rotates the hip (the classic position of instability), the anterior aspect of the prosthetic femoral neck strikes the anterior rim of the acetabular cup. Because the femur continues to be driven forcefully by the patient's body weight and musculature, the head is violently levered out of the back of the socket, resulting in a posterior dislocation. Correctly optimizing combined anteversion prevents this anterior impingement and subsequent posterior failure. Reference: Lewinnek GE. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am.