1Anthropology, University of Washington, Seattle, WA, 2Orthopaedics & Sports Medicine, University of Washington, Seattle, WA, 3Orthopaedic Surgery, University of California, San Francisco, CA, 4Mechatronic Systems Engineering, Simon Fraser University, Surrey, British Columbia, Canada, 5Physical Therapy Department, Faculty of Health Professions, Ono Academic College, Kiryat Ono, Israel, 6Anatomy and Anthropology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
March 27, 2015 , Archview Ballroom
Sacral orientation is strongly linked to lumbar lordosis and recent studies have demonstrated that the lumbar spine is sexually dimorphic with women having greater lordosis than men. Given that the pelvis is well-established as sexually dimorphic, we hypothesized that differences in sacral orientation between men and women, similar to those seen in lumbar lordosis, exist. In addition, we assessed the effects of postural orientation on sex-differences in the lumbosacral spine.
We measured lumbar lordosis using lumbar angle (between L1 and S1 cranial endplates) and sacral orientation using sacral tilt (orientation of the sacrum relative to the horizontal plane; ‘posture-dependent’) and pelvic incidence (orientation of the sacrum relative to the acetabula; ‘posture-independent’). We retrospectively collected measures of sacral tilt, pelvic incidence, and lumbar angle from radiographs of 200 adult subjects: 121 standing (48 women and 73 men), 75 supine (39 women and 36 men), and 4 unknown. We found that sex differences in lumbar lordosis and sacral orientation are dependent on postural orientation. Sacral tilt and lumbar angle are significantly greater in women than men when standing (p=0.027 and p=0.001, respectively), but not when supine. As expected, pelvic incidence was not different between standing and supine individuals; however, surprisingly, it was also not different between women and men.
Sexual dimorphism in sacral orientation, then, appears to be primarily postural and this result, combined with the postural component inherent in lumbar lordosis, suggests that sexual dimorphism in lumbar lordosis is driven by differences that occur when individuals are in an upright posture.
Funding for this project was provided by the Jerome Debs Chair in Orthopaedic Research at the University of Washington.