Fig. 1.

a Anteroposterior (AP) radiograph of a patient with acetabular dysplasia and acetabular anteversion. The anterior line always runs medial to the posterior line. b AP radiograph of a patient with acetabular dysplasia and acetabular retroversion. The superior aspect of the anterior line is lateral to the posterior line, and the anterior and posterior lines cross each other at the proximal part of the acetabulum. The posterior line is medial to the centre of the femoral head, representing positive posterior wall sign