Ischial spine projection into the pelvis - a new sign for acetabular retroversion

Description: Femoroacetabular impingement may occur in patients with so-called acetabular retroversion, which is seen as the crossover sign on standard radiographs. We noticed when a crossover sign was present the ischial spine commonly projected into the pelvic cavity on an anteroposterior pelvic radiograph. To confirm this finding, we reviewed the anteroposterior pelvic radiographs of 1010 patients.
Ischial spine projection into the pelvis : a new sign for acetabular retroversion. Kalberer F, Sierra RJ, Madan SS, Ganz R, Leunig M. Clin Orthop Relat Res. 2008 Mar;466(3):677-83 Full text