Background and Literature Research: Fixed sagittal mal-rotation of pelvis is commonly encountered in patients with ankylosing spondylitis. The pelvis positioning for total hip arthroplasty in these patients can be a pitfall to an oblivious surgeon, and gives rise to mal-positioning of the acetabular component and subsequently leads to dislocation of the arthroplasty.

Objective: To quantify the effect of sagittal pelvic rotation on the positioning of acetabular component using three dimensional computer model.

Materials and Methods: Ten embalmed cadaveric pelvis with intact ligaments were scanned in 1 mm slices using computed tomogram (CT). The image reconstruction was done by the software “MIMICS” in microcomputer. The resulting three dimensional models can be rotated freely using “MIMICS.” Insertion of acetabular component was stimulated in different sagittal rotation of the pelvis. The ratio of the longitudinal to the transverse dimension of the obturator foramen was noted, and the uncovered area of the acetabular components was calculated.

Discussion: Pelvic rotation on the sagittal plane cannot be easily measured by radiographs. The shape of the obturator foramen on plane pelvic radiographs centered at pubic symphysis varies with the pelvic rotation on the sagittal plane and thereby serves as an indirect way to measure pelvic rotation. The shape of obturator foramen on plain radiographs therefore provided a guideline for patient positioning and the alignment of insertion of acetabular component during surgery.