Posterior wall sign (femoroacetabular impingement)

Posterior wall sign is a radiographic sign used to assess the morphology of posterior acetabular wall. A deficient posterior acetabular wall is often associated with acetabular retroversion or acetabular dysplasia, while a too-prominent posterior acetabular wall is a predisposing factor for posterior impingement.

Radiographic features

Plain radiograph

In a normal hip, the outline of the posterior acetabular wall projects over the center of the femoral head. In case of a deficient posterior acetabular wall, its outline is visible medial to the femoral head center. Conversely, in case of a prominent posterior acetabular wall, its outline can be seen lateral to the femoral head center.

Practical points

• too-prominent posterior acetabular wall can be associated with coxa profunda or protrusio acetabuli but it can also occur as an isolated entity
• posterior wall sign has to be assessed on a centered pelvic radiograph, as a radiograph centered over the hip will give false results