To investigate the relationship between relative location of the sacral base and spinal alignment in standing healthy adult volunteers. One hundred seventy-two volunteers (men = 83, mean age = 39.3years [20 70], women = 89, mean age = 39.6years [20 62]) with no history of spinal disease were imaged using a low-dose biplanar slot-scanning 3D X-ray imaging system. A circle was drawn around three points: cranial vertex of the iliac crest (A), caudal vertex of the ischium (B), and anterior vertex of the pubis. Pelvic height (PH) was defined as the diameter (A B). A tangent line perpendicular to PH (C) was drawn by passing through (A). Sacral height (SH) was defined as the distance between (C) and the center of the sacral base parallel to PH. Relative SH (rSH) was calculated as SH/PH 100. Mean (SD) rSH was 18.3 3.2 (men 20.0 2.9, women 16.7 2.6). rSH significantly positively correlated with thoracic kyphosis (r = 0.20, p 0.05), lumbar lordosis (r = 0.28, p 0.05), pelvic incidence (r = 0.28, p 0.05), and sacral slope (r = 0.32, p 0.0001), and significantly negatively correlated with pelvic thickness (r = 0.66, p 0.0001). rSH did not correlate with pelvic tilt. The center of the sacral base is normally located 3.8 0.8cm caudal to the cranial vertex of the iliac crest. The sacral base was located more caudally in men than in women, regardless of age. The more caudal the sacral base, the angle of the spino-pelvic parameters (TK, LL, PI, SS) progressively increases along with a decrease in the sacro-acetabular distance (Pth). Pelvic tilt did not correlate with the location of the sacrum.
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