AP radiographs of a female volleyball player with right hip pain, and a history of right hip posterior dislocation. The small circle is in the center of the femoral head. Clearly, the posterior wall of the acetabulum is medial to the center of the femoral head—this is called the posterior wall sign. This demonstrates posterior wall insufficiency. The black line outlines the ischial spines with the arrows pointing to the prominent ischial spines. This would suggest the patient has acetabular retroversion, which is the reason for her posterior wall insufficiency.