Crossover sign | Radiology Signs

Answer:

The crossover or figure-of-eight sign is seen on the anteroposterior pelvic radiograph when the projected line of the anterior acetabular wall crosses the line of the posterior wall before reaching the lateral edge of the acetabulum, instead of remaining medial to it. It indicates apparent acetabular retroversion and is used in the assessment of pincer-type femoroacetabular impingement. Two cautions are essential. First, the sign is exquisitely sensitive to pelvic tilt and rotation, so a technically adequate film is required, judged by the coccyx being centred over the symphysis and by a symphysis-to-sacrococcygeal distance of about 1 to 3 cm; a tilted or rotated pelvis creates or abolishes the sign. Second, comparative studies with three-dimensional CT have shown that the crossover sign overestimates true retroversion, since it reflects the shape of the acetabular rim rather than the orientation of the acetabulum as a whole. It should therefore be interpreted with the posterior wall sign, the ischial spine sign, the lateral centre-edge angle, the alpha angle for cam morphology and, where surgery is contemplated, cross-sectional imaging. Clinically the relevance is that retroversion focally over-covers the anterosuperior rim, causing impingement, labral injury and chondral delamination in young active adults.

Why is it called so?

The two acetabular wall lines cross over one another, producing a figure-of-eight configuration on the radiograph.

Pathophysiology

In a normally anteverted acetabulum the anterior wall is medial to the posterior wall throughout its course, so the two projected lines converge only at the lateral rim. When the cranial part of the acetabulum is retroverted, the anterior wall is displaced laterally in its upper portion and its line crosses that of the posterior wall. The underlying anatomy is usually a focal cranial deformity of the rim, sometimes with a deficient posterior wall or a prominent ischial spine projecting into the pelvis, rather than rotation of the whole acetabulum. During hip flexion and internal rotation the over-covering anterosuperior rim abuts the femoral head-neck junction, crushing the labrum against the rim and shearing the adjacent cartilage, which is the mechanism of pincer impingement and of the contrecoup posteroinferior cartilage lesion.

Alternative names: Figure-of-eight sign; acetabular retroversion sign

Other associated named signs: Posterior wall sign, the ischial spine sign and the pistol grip deformity of cam impingement

References

1. Zaltz I, Kelly BT, Hetsroni I, Bedi A. The crossover sign overestimates acetabular retroversion. Clin Orthop Relat Res. 2013;471(8):2463-70. PMID: 23136058.