FP5.9 Effect of Pelvic Tilt on Common Radiographic Markers of Acetabular Morphology: A 3-Dimensional Digitally Reconstructed Radiograph Analysis

Purpose: Radiographic markers including the cross-over sign (COS), ischial spine sign (ISS), and the posterior wall sign (PWS), are frequently used to qualitatively assess acetabular morphology and help guide clinical decision-making. The purpose of this study was to analyze the extent of induced vertical tilt and horizontal rotation needed to elicit a change in findings from positive-to-negative, and vice versa, of acetabular measurement techniques using digitally-reconstructed AP radiographs (DRRs).

Methods: Computed tomography (CT) scans of the pelvis were retrospectively collected, and imported into 3-dimensional imaging software, producing transformed AP DRRs of the pelvis. Once rendered, pelvises were standardized to 0° of tilt and rotation, defined as a distance of 2 centimeters from the superior margin of the pubic symphysis to the tip of the coccyx. Pelvises were initially assessed for positive or negative findings of COS, ISS, and PWS in this state. Then, at one-degree intervals, pelvises were tilted (along the vertical axis) and then rotated (along the horizontal axis) until a change in findings was appreciated. The amount of tilt and rotation needed to change the findings in each hip was recorded, and standard descriptive statistics were calculated.

Results: Twenty hips (ten DRRs pelvises) were assessed. On normalized AP pelvis, 5% had a positive COS (95% were negative), 20% had a positive ISS (80% negative), and 80% had a positive PWS (20% negative). In those with an initially positive COS, an average of 6° of tilt towards an outlet view, and 7° of rotation towards the contralateral hip changed the findings to negative. Amongst the negative hips, an average of 6.1±4.0° of tilt towards the inlet view, and 7.9±4.5° of rotation towards the ipsilateral hip, turned the COS sign positive. For initially positive ISS, 5.25±1.9° of tilt towards the outlet view, and 5.5±3.3° towards the contralateral hip negated the finding. For those negative, a mean of 7.8±3.7° inlet tilt and a mean of 5.25±2.45° ipsilateral rotation to turn positive. For PWS, those initially positive required a mean of 8.8±5.5° of tilt towards the outlet view, and 5.45±4.5° of rotation contralaterally to negate. Those with negative PWS required 11.25±4.1° of inlet tilt and 11.75±4.0° of rotation ipsilaterally to turn positive.

Conclusion: Findings for COS, ISS, and PWS were noted to change with small degrees of tilt and rotation. As such, close consideration should be given to pelvic position when assessing for positive findings of COS, ISS, and PWS.