The sacrum is the wedge-shaped fused bone forming the posterior pelvis, articulating with L5 above, the coccyx below, and the ilium bilaterally at the sacroiliac joints. The sacral foramina transmit the anterior (ventral) rami of S1-S4 anteriorly and the posterior (dorsal) rami posteriorly. The sacral canal contains the cauda equina terminating at S2 and the filum terminale.
An expanded entry for Sacrum.
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| Landmarks |
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| Ossification | Five sacral vertebrae fuse progressively from S1 to S5 between 18-30 years. The SI joint surface becomes increasingly irregular with age. |
Sacral insufficiency fractures from osteoporosis produce the H-shaped fracture pattern visible on bone scan and MRI. Sacroplasty (cement injection into the fracture) provides pain relief. Sacral neoplasms (chordoma, arising from notochordal remnants) require wide en-bloc resection with sacrectomy. Sacral nerve stimulation electrodes are placed through the S3 foramen to treat overactive bladder and faecal incontinence.
Osteoporotic fractures of the sacral ala in older adults, often missed on plain radiographs.
Part of high-energy pelvic ring injuries, with risk of sacral nerve root damage.
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