Correlation of the cross-over ratio of the cross-over sign on conventional pelvic radiographs with computed tomography retroversion measurements

Objective To find a correlation between the cross-over ratio of the cross-over sign on conventional anteroposterior (AP) pelvic radiographs and retroversion measurements ('roofedge angle' and 'equatorial-edge angle) on computed tomography (CT) scans. This would facilitate the interpretation of the cross-over sign regarding the amount of acetabular retroversion. Materials and methods Correctly projected AP pelvic radiographs (2,925 hips) were examined for the presence of the cross-over sign (COS), and the overlap ratio of the COS was measured. On CT scans of the same patients the 'roof-edge angle' (RE angle) and the 'equatorial-edge angle' (EE angle) were also calculated. Results A statistically significant but only weak relationship could be found between the overlap ratio of the COS and the 'roof-edge angle' (P<0.0001; correlation coefficient-0.486) and between this ratio and the 'equatorial-edge angle' (P<0.0001; correlation coefficient -0.395). Conclusion A relationship between the overlap ratio and orientation measurements on CT scans could be found, but it was less strong than expected.