Acetabular retroversion as a rare cause of chronic hip pain: recognition of the "figure-eight" sign.

While well-recognized in the orthopedic literature as a cause of chronic hip pain, acetabular retroversion has not been specifically described in the radiologic literature. Acetabular retroversion represents a particular form of hip dysplasia characterized by abnormal posterolateral orientation of the acetabulum. This pathophysiology predisposes the individual to subsequent anterior impingement of the femoral neck upon the anterior acetabular margin and fibrous labrum. Without treatment, cases may progress to damage of the anterior labrum and cartilage, with eventual early onset of osteoarthritic disease. This impinging condition has been described as occurring in isolation or as part of a complex dysplasia. We describe two cases of acetabular retroversion diagnosed by conventional radiographic evaluation of the pelvis, one in isolation and one occurring in the setting of a larger congenital syndrome. These cases illustrate the utility of the "figure-eight" sign in identifying abnormalities of acetabular version and thus assisting clinicians in properly identifying these individuals so that appropriate therapy may be instituted.