The sacrum is formed by five fused sacral vertebrae forming a triangular bone that wedges between the two iliac bones of the pelvis at the sacroiliac joints, transmitting weight from the spine to the lower limbs.
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| Landmarks |
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| Ossification | Five sacral vertebrae fuse progressively from S1 downward, with complete fusion by age 25. The sacroiliac joints show progressive ossification after age 40. |
The sacrum transmits the body weight from the spine to the pelvis through the sacroiliac joints and is the posterior boundary of the true pelvis. Sacral fractures in high-energy pelvic ring disruptions can injure the sacral nerve roots producing neurogenic bladder, bowel dysfunction, and sexual dysfunction. Sacral insufficiency fractures in osteoporotic elderly patients produce acute back and buttock pain that is easily missed on plain radiographs but clearly visible on MRI or bone scan.
Osteoporotic stress fractures of the sacral ala in older adults cause low back and buttock pain with little or no trauma and are often missed on plain radiographs.
High-energy pelvic ring injuries frequently involve the sacrum; fractures through the sacral foramina can injure the sacral nerve roots.
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