Trunk (thorax, abdomen, back)
Sacrum
Os sacrum
- Origin
- Triangular bone formed by the fusion of 5 sacral vertebrae; articulates with L5 and both iliac bones
- Insertion
- Attachment site for gluteus maximus, piriformis, iliacus, coccygeus, and strong sacroiliac ligaments
- Function
- Keystone of the pelvic ring; transmits axial spinal load through SI joints to the lower limbs
- Innervation
- Sacral plexus (S1–S4)
- Movement & chain
- Nutation and counternutation at the sacroiliac joint during walking and childbirth
- Palpation
- Median sacral crest and sacral cornua flanking the sacral hiatus are palpable posteriorly
- Clinical
- Sacral insufficiency fractures in osteoporotic patients; sacroiliitis in ankylosing spondylitis; cauda equina compression
- Segmental innervation
- S1–S5
- In clinical practice
- Sacral hiatus landmark for caudal epidural block; primary pressure ulcer (decubitus) risk site
- Region in the model
- Trunk (thorax, abdomen, back)