The sacrum is formed by the fusion of five sacral vertebrae, forming the posterior wall of the pelvis and transmitting all upper body loads to the ilia via the sacroiliac joints. The sacral foramina (S1-S4 anterior and posterior) transmit the sacral nerve roots forming the sacral plexus. The sacral hiatus (unfused S4-S5 arch) is the entry point for caudal epidural injections. The median sacral crest represents the fused spinous processes.
An expanded entry for Sacrum.
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| Ossification | The five sacral vertebrae fuse progressively from below upward during the teenage years, with complete fusion usually by age 25-30. |
Sacral stress fractures (insufficiency fractures in osteoporotic patients and fatigue fractures in runners) present as bilateral or unilateral sacral pain reproduced by the seated flexion test. MRI confirms the H-pattern (Honda sign) of bilateral vertical sacral alar fractures with a horizontal connecting fracture. Treatment ranges from protected weight bearing to sacroplasty cement injection for refractory osteoporotic fractures.
Osteoporotic fractures of the sacral ala in older adults causing low back and buttock pain after little or no trauma.
Part of high-energy pelvic ring injuries; fractures through the foramina risk sacral nerve root damage.
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