The sacrum is a triangular bone formed by five fused sacral vertebrae. It forms the posterior wall of the pelvis, articulates with L5 above, the coccyx below and the hip bones at the sacroiliac joints. Its foramina carry the sacral spinal nerves.
| Type | Irregular bone (fused vertebrae). |
|---|---|
| Location | Posterior pelvis, between the hip bones. |
| Landmarks | Base, promontory, ala, anterior and posterior sacral foramina, median and lateral sacral crests, sacral hiatus, cornua. |
| Articulations | Fifth lumbar vertebra, coccyx and hip bones (sacroiliac joints). |
| Muscle attachments | Gluteus maximus, piriformis, erector spinae, multifidus, iliacus. |
3D model: BodyParts3D (CC BY 4.0), adult male reference anatomy. Highlighted structure shown with nearby bones for context.
The sacral promontory is used as a landmark in obstetrics. The sacral canal is the continuation of the vertebral canal and contains the cauda equina.
Five.
The anterior and posterior rami of the sacral spinal nerves.
Test yourself
Back bone quiz
Practise origins, insertions, nerve supply and locations with instant feedback.
Explore in context
Open the sacrum in the full 3D atlas
Toggle body systems, explode the model and inspect every neighbouring structure.