Objective: Compared to sagittal pelvic tilt, only a few studies have examined axial rotation on anteroposterior radiographs. We therefore quantified 3-D pelvic rotation using the width and height ratio of the obturator foramina under the various pelvic tilts. Methods: Using CT reconstructions of 10 healthy pelvises, anterior pelvic planes (APPs) were rotated by 20° in 5° increments on the axial plane with various degrees of sagittal pelvic tilt. The correlation between the pelvic rotation angle and the width ratio (WR) in the axial plane and the height/width ratio (H/W) in the sagittal plane were examined. Results: Axial pelvic rotation and WR showed a high linear correlation regardless of the sagittal tilt, with the correlation coefficient ranging from 0.93 to 0.98 in males and 0.87 to 0.95 in females. The angle that resulted in a WR of 1: 2 was approximately 13° in males and 18° in females. H/W also showed a linear regression with sagittal tilt. Axial rotation was determined by the following equation incorporating pelvic tilts; axial rotation (male: female) = (19.9: 24.2) + (2.1: 3.6) × Hright/Wright + (0.9: 1.5) × Hleft/Wleft - (23.2: 25.1) × WR. Conclusions: Pelvic tilt and rotation could be quantified by the equation using width and height ratios of the obturator foramina on a plain anteroposterior radiograph. Width and height ratios of the obturator foramina proved to be useful parameters in clinical practice for understanding pelvic rotation.
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