Estimation of Transverse Plane Pelvic Rotation Using a Posterior-Anterior Radiograph

STUDY DESIGN: Experimental and analytical study of transverse plane pelvic rotation. OBJECTIVES: To determine how pelvic rotation projected onto the transverse plane relates to coronal plane anatomic landmark location. SUMMARY OF BACKGROUND DATA: Current spine deformity instrumentation may be used to apply transverse plane loads to the spine that may be transmitted to regions not included in the instrumentation, including the pelvis. METHODS: An anatomically correct, sawbones model of an adult female pelvis was marked with lead shot at prominent radiographic landmarks, rotated at different angles in the sagittal, transverse, and coronal planes, and left/right (L/R) ratios of the medial-lateral distances between similar landmarks determined. An analytical equation was also derived to determine the degree of rotation in the transverse plane, using medial-lateral and anterior-posterior distances between same landmarks. RESULTS: The L/R ratio for the coronal plane distance between the inferior ilium at the sacro iliac joint (SI) and anterior superior iliac spine (ASIS), the SI-ASIS measurement, proved the most reliable of the four ratios studied to determine the extent of pelvic rotation in the transverse plane. Assuming the pelvis is symmetric, the most important factor is location of the compared landmarks. A long distance between the landmarks in both the coronal and transverse plane and a large angle between the line joining the two landmarks and the coronal plane of the pelvis, as viewed in the transverse plane, are best. Transverse plane pelvic rotation up to 20 degrees is accurately reflected nearly linearly by L/R SI-ASIS ratios. CONCLUSIONS: Bony pelvis landmarks visible on coronal plane radiographs can be used to estimate transverse plane pelvic rotation, but precise conversion to degrees rotation requires additional information on specific patient pelvic morphology.