The acetabular opening angle in Perthes' disease. Radiographic study of 62 unilateral cases.

Pelvis AP radiographs in 62 patients with unilateral Perthes' disease were studied in search of changes of the acetabulum throughout the active stages of this disease. The length of the acetabular roof, the diameter of the acetabulum, and the opening angle of the acetabulum, were compared with the contralateral hip. The length of the acetabular roof did not change, but the opening angle and the diameter of the acetabulum increased. This increase began in the earliest stage of the disease in half of the cases; it reached a maximum during the resorptive and the reconstructive stages, then it decreased to reach normal values at skeletal maturity. These changes were related to the extent of the epiphyseal necrosis. Probable causes were local hyperemia and a mechanical adaptation of the acetabulum to a cartilaginous coxa magna. The opening angle has a prognostic value and influences our choice of therapy.