The triangular bone formed by the fusion of five vertebrae, wedged between the two hip bones at the back of the pelvis
Sacrum — fusion of S1 to S5.
Anatomy · Bones
Sacrum
Five fused vertebrae
Pelvis · spine
The sacrum is a large triangular bone at the base of the spine, formed by the fusion of five sacral vertebrae. It sits between the two hip bones, forming the back wall of the pelvis, with the fifth lumbar vertebra above and the coccyx below. Its concave front surface increases the volume of the pelvic cavity.
How to spot them
- ▸A single triangular (inverted) wedge of bone wedged between the two hip bones, forming the back of the pelvis
- ▸Concave front (pelvic) surface with four pairs of anterior sacral foramina and transverse ridges marking fused vertebrae
- ▸A forward-projecting lip at the top of the front surface, the sacral promontory
- ▸Roughened lateral auricular surfaces join the ilium at the sacroiliac joints
What to know
The forward-projecting lip at the top of its front surface, a landmark in obstetrics, forms the back of the pelvic inlet
Sacrum — the sacral promontory, the posterior margin of the pelvic inlet.
Four pairs of openings on the front and back of this bone let the sacral nerves leave the canal within it
Sacrum — the anterior and posterior sacral foramina.
Its rough ear-shaped lateral surfaces form the immobile joints with the hip bones
Sacrum — the auricular surface and sacroiliac joints.
A gap at its lower end, where the laminae of the last vertebra are missing, is the site for caudal epidural injections
Sacrum — the sacral hiatus at the lower end of the sacral canal.
Its base articulates with the last lumbar vertebra, and its apex with the coccyx
Sacrum — the lumbosacral and sacrococcygeal joints.
Key ideas
- The fusion of the sacral vertebrae does not start until about age 20 and is not complete until middle age.
- The sacrum is generally wider and shorter in females, one of the features used to estimate sex from a skeleton.
Quick-fire facts
- •Formed by five fused sacral vertebrae, S1 to S5
- •It forms the posterior wall of the bony pelvis
- •The sacral canal is the continuation of the vertebral canal, ending at the sacral hiatus
- •The median sacral crest is the fused spinous processes; the lateral sacral crest is the fused transverse processes
- •Its base articulates with L5 (lumbosacral joint), its sides with the ilium, and its apex with the coccyx
- •The anterior surface is concave and helps increase the volume of the pelvic cavity
- •The sacrum and coccyx are called the 'false' vertebrae, because they are fused
- •The sacrum is weight-bearing at the broad base and tapers to a non-weight-bearing apex
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Sources
- Kenhub — Sacrum: Anatomy and clinical aspects
- TeachMeAnatomy — The Vertebral Column
- OpenStax — Anatomy and Physiology 2e, 7.3 The Vertebral Column