Bilateral Symmetry and Sex Differences in Hip Joint Helical Axes of Motion During Gait

To quantify helical axis of motion (HAM) orientation, center of rotation (COR), and the bilateral symmetry, sex differences, and differences in HAM COR and dispersion between periods of hip flexion/extension during gait in healthy young adult hips.

Synchronized biplane radiographs of 46 hips were collected at 50 images/sec during treadmill gait. Each subject received a CT scan, which was segmented to create subject-specific pelvis and proximal femur bone models. Bone motion was determined with submillimeter accuracy by matching digitally reconstructed radiographs created from the bone models to the distortion-corrected radiographs using a validated volumetric model-based tracking technique. The femoroacetabular HAM was calculated for every three-degree interval of hip flexion/extension and described by the COR location and the HAM orientation relative to the sagittal, transverse, and coronal planes of the pelvis. Symmetry was measured by absolute side-to-side differences (SSD_A).

The SSD_A in HAM orientation was 31.3° or less, and the SSD_A in COR was 2.3 mm. The COR was located at neither the center of the acetabulum nor the femur head center; the COR in women was located more anteriorly and superiorly during extension compared with men; the instantaneous axis of rotation was more dispersed throughout extension than flexion.

The hip COR moves throughout gait and is not located at the center of the acetabulum in healthy young adults. The observed SSD_A, sex differences, and flexion/extension differences in COR location and HAM dispersion have implications for patient evaluation, computational modeling, and hip joint replacement design.