The crossover sign, also known as the 'figure of 8' sign, is a plain film sign that indicates acetabular retroversion 2. Acetabular retroversion is a form of pincer morphology and predisposing factor for femoroacetabular impingement and is thought to promote osteoarthritis of the hip 3.
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 1,2. A line drawn down the anterior wall will intersect with a line drawn along the posterior wall.
- 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) 1 and may be associated with a number of false positives when CT is subsequently performed to accurately measure retroversion 4
- 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) 1
- presence of the crossover sign has been associated with a higher likelihood of progression of osteoarthritis over a minimum 10-year follow-up period 3
- 1. Moritz Tannast, Klaus A. Siebenrock, Suzanne E. Anderson. Femoroacetabular Impingement: Radiographic Diagnosis—What the Radiologist Should Know. (2012) American Journal of Roentgenology. 188 (6): 1540-52. doi:10.2214/AJR.06.0921 - Pubmed
- 2. Bardakos NV, Villar RN. Predictors of progression of osteoarthritis in femoroacetabular impingement: a radiological study with a minimum of ten years follow-up. (2009) The Journal of bone and joint surgery. British volume. 91 (2): 162-9. doi:10.1302/0301-620X.91B2.21137 - Pubmed
- 3. Reto Sutter, Marco Zanetti, Christian W. A. Pfirrmann. New Developments in Hip Imaging. (2012) Radiology. 264 (3): 651-67. doi:10.1148/radiol.12110357 - Pubmed
- 4. Zaltz I, Kelly BT, Hetsroni I, Bedi A. The crossover sign overestimates acetabular retroversion. (2013) Clinical orthopaedics and related research. 471 (8): 2463-70. doi:10.1007/s11999-012-2689-5 - Pubmed