Effect of Pelvic Tilt and Rotation on Common Qualitative Acetabular Radiographic Markers: A Three-Dimensional...

Radiographic markers including the crossover sign (COS), ischial spine sign (ISS), and posterior wall sign (PWS) are commonly used to assess acetabular morphology. The aim of this study was to determine the vertical tilt or horizontal rotation needed to elicit sign changes on anterior-posterior (AP) digitally reconstructed radiographs (DRRs).

CT scans of the pelvis were retrospectively collected and imported into three-dimensional (3D) imaging software to create 3D AP digitally reconstructed radiographs (DRRs). Rendered pelvises were standardized to 0° of tilt and rotation and assessed for positive or negative findings of COS, ISS, and PWS. Then, the DRR was manipulated at 1° intervals (from 0° to 20° in each direction), progressively inducing pelvic tilt in the vertical axis, and the process was then repeated in the horizontal axis. The degree at which the sign changed from positive to negative or vice versa in each hip was recorded.

A total of 20 hips were assessed (five men, five women). On the standardized AP pelvis radiographs, positive COS, ISS, and PWS were 45%, 25%, and 75% respectively. The mean rotation required to change COS, ISS, and PWS from positive to negative or vice versa was 7.92° ± 4.25°, 5.55° ± 2.63°, and 6.37° ± 3.89°, respectively. The mean tilt required to flip COS, ISS, and PWS was 8.80° ± 5.73°, 7.73° ± 3.65°, and 9.55° ± 4.98°.

Close consideration should be given to pelvic position when assessing positive findings of COS, ISS, and PWS because all signs were noted to flip with small degrees of rotation and tilt.