Siebenrock et al. ask whether pelvic tilt alone can produce or eliminate the radiographic signs of acetabular retroversion on a standard AP pelvis film. Using 86 clinical radiographs to establish gender-specific reference values and 4 cadaver pelves across a 21° tilt range, they show that both the cross-over and posterior wall signs are highly sensitive to patient positioning.
Every time you see a cross-over sign, the first question is not "is this acetabulum retroverted?" but "where is the sacrococcygeal joint?"
Measure the symphysis-to-sacrococcygeal joint distance and compare it to the gender-specific mean (32 mm men, 47 mm women). A value above the mean means the pelvis is anteriorly tilted, and the retroversion signs you're seeing may be an artifact of positioning. A repeat film in a corrected position can eliminate apparent retroversion entirely.
Also confirm the sacrococcygeal joint sits on the midline of the symphysis. As little as 6° of rotation (16 mm of offset) is enough to produce ipsilateral retroversion signs in every pelvis tested — a subtlety that is easy to overlook on a routine film.
This paper is foundational for FAI and dysplasia workup: before recommending a periacetabular osteotomy or labral surgery based on retroversion signs, the AP pelvis must be taken in a standardized, reproducible position. It also explains why CT version measurements at the mid-hip level may not reflect the anterosuperior roof, the zone that matters most for impingement.
Siebenrock et al. ask whether pelvic tilt alone can produce or eliminate the radiographic signs of acetabular retroversion on a standard AP pelvis film. Using 86 clinical radiographs to establish gender-specific reference values and 4 cadaver pelves across a 21° tilt range, they show that both the cross-over and posterior wall signs are highly sensitive to patient positioning.
Every time you see a cross-over sign, the first question is not "is this acetabulum retroverted?" but "where is the sacrococcygeal joint?"
Measure the symphysis-to-sacrococcygeal joint distance and compare it to the gender-specific mean (32 mm men, 47 mm women). A value above the mean means the pelvis is anteriorly tilted, and the retroversion signs you're seeing may be an artifact of positioning. A repeat film in a corrected position can eliminate apparent retroversion entirely.
Also confirm the sacrococcygeal joint sits on the midline of the symphysis. As little as 6° of rotation (16 mm of offset) is enough to produce ipsilateral retroversion signs in every pelvis tested — a subtlety that is easy to overlook on a routine film.
This paper is foundational for FAI and dysplasia workup: before recommending a periacetabular osteotomy or labral surgery based on retroversion signs, the AP pelvis must be taken in a standardized, reproducible position. It also explains why CT version measurements at the mid-hip level may not reflect the anterosuperior roof, the zone that matters most for impingement.