Introduction: Given the fact that some spinal disorders, such as idiopathic scoliosis but also Scheuermann's disease have sex‐related prevalence rates, it is surprising that only a few studies have analyzed differences in the normal spino‐pelvic alignment between the sexes. Moreover, no study has ever analyzed the differences in sagittal spinopelvic alignment and sagittal inclination of each individual vertebra between the sexes with an accurate 3D reconstruction technique. This is the primary goal. Additionally, in this age of instrumented spinal fusion and an emphasis on restoration of sagittal spinal balance, it is of great importance to have a set of reference values of normal spinal alignment in men and women.
Methods: Simultaneous biplanar radiographs from head to the in a freestanding upright position were obtained of 30 asymptomatic women (mean age of 26 yrs, range 20–42) and 30 asymptomatic men (mean age of 27 yrs, range 21–49) using the EOS Simaging device (Biospace Med, Paris). Subsequently, a 3D reconstruction of the bony shape of the spine, sacrum and pelvis was made by two observers using accurate reconstruction software. Independent samples t‐tests were used to analyze differences in the spinal and pelvic parameters between the genders.
Results: Age and BMI was equally distributed between the sexes. The female spine was significantly more dorsally inclined (11° vs 8°; P<0.01). High thoracic and thoracolumbar vertebrae were significantly more dorsally inclined in women (Figure). The inter and intra observer reliability were both excellent.
Conclusion: An important biomechanical consequence of our results is that the female spinel is more subjected to dorsally directed shear loads (DDSLs). DDSLs make spinal segments less rotationally stable as was shown by Kouwenhoven et al. in a previous biomechanical studie (Spine 2007). This signifies that certain areas may be less rotationally stable already in the normal female population, and may explain why progressive idiopathic scoliosis (under certain still undetermined circumstances during growth) occurs more in girls than in boys.
Significance: This study sheds new light on the well known predominance of girls with idiopathic scoliosis and the etiology of idiopathic scoliosis in general.