Table 2.

Definitions of the radiographic parameters to assess acetabular retroversion (See Fig. 2 for illustration): common (direct) parameters to assess acetabular retroversion require an ossified acetabulum and include the ischial spine sign [10], cross-over sign [8, 16], retroversion index [16, 23] and posterior wall sign [16, 23]. Indirect radiographic parameters allow to assess acetabular retroversion in hips without an ossified acetabulum in children and include the pelvic width index [22] and the ilioischial angle [22]

Radiographic Parameter	Description
Direct parameters to assess acetabular retroversion (requiring an ossified acetabulum)
Ischial spine sign [10] (Fig. 2A)	Positive if the ischial spine is projected medially to the pelvic brim
Cross-over sign [8, 16] (Fig. 2B)	Positive if the anterior acetabular rim crosses the course of the posterior acetabular rim
Retroversion index [16, 23] (Fig. 2C)	Allows to quantify the amount of acetabular retroversion. Ratio of ‘a’ to ‘b’ expressed as percentage. ‘a’ is defined as the distance of the cranial acetabular opening where the anterior rim is projected laterally to the posterior rim. ‘b’ is defined as entire width of the acetabular opening
Posterior wall sign [16, 23] (Fig. 2D)	Positive if the posterior acetabular wall runs medial to the femoral head center
Indirect parameters to assess acetabular retroversion (can be applied in hips without an ossified acetabulum)
Pelvic width index [22] (Fig. 2E)	Ratio of width of ischium (a) to width of ileum (b). ‘a’ is defined from symphysis to the most lateral point of the ischial tuberosity. ‘b’ is defined from center of the sacrum to the most lateral point of the iliac wing. Both distances are measured parallel to the interteardrop line
Ilioischial angle [22] (Fig. 2F)	Angle between the interteardrop line and a line connecting the intersection of the iliopectineal and iliioschial lines and a second point on the most lateral aspect on the ipsilateral obturator foramen