The embryology and development of the hip joint are complex. In few areas of the body can so many morphological variations be seen. Minor anatomical abnormalities in the acetabular and femoral head shape, femoral–neck angle, and joint congruency, are frequent. Controversies still exist on the importance of these variations and on whether or not they put the patient at risk of developing early degenerative changes (Wedge et al. 1990). Even in clinically asymptomatic hips, the acetabulum is not always of the same shape or same depth. Minor degrees of ‘dysplasia’ have no more effect than to create a shallow acetabulum or possibly predispose to a labral tear (Dorrel & Caterall, 1986). Greater degrees may cause congenital subluxation or dislocation (Wiberg, 1939; Ponseti, 1978a).