Figure of 8 sign

The crossover sign, also known as the 'figure of 8' sign, is a plain film sign that indicates acetabular retroversion . Acetabular retroversion is a form of pincer morphology and predisposing factor for femoroacetabular impingement and is thought to promote osteoarthritis of the hip .

Radiographic features

Plain radiograph

The crossover sign describes the appearance of the anterior acetabular wall lying anterior to the posterior acetabular wall in the superior portion of the joint, on a frontal pelvic radiograph . A line drawn down the anterior wall will intersect with a line drawn along the posterior wall.

Practical points

• the presence of the crossover sign may not correlate well with the degree of acetabular retroversion
• must be interpreted on a centered pelvic radiograph (an AP radiograph centered over the hip will overestimate the acetabular version)  and may be associated with a number of false positives when CT is subsequently performed to accurately measure retroversion
• it is crucial to pay attention to the pelvic tilt on a pelvic radiograph: an increased pelvic tilt or a rotation to the ipsilateral hip cause a more marked crossover sign, mimicking acetabular retroversion (and vice versa) 
• presence of the crossover sign has been associated with a higher likelihood of progression of osteoarthritis over a minimum 10-year follow-up period