sacrum
Meaning of sacrum: Five vertebrae that stop being separate bones, forming the wedge through which the entire upper body transfers its weight to the legs.
Definition of sacrum
The sacrum is the large triangular bone at the base of the spine, wedged between the two hip bones to complete the back of the pelvis. It forms from separate vertebral segments, usually five, which fuse into one solid bone by early adulthood. Above it sits the lowest lumbar vertebra; below it the small tailbone.
What does the sacrum do?
It is the keystone of the pelvis. Weight from the head, arms and trunk travels down the spine, arrives at the sacrum, and is passed sideways through the two sacroiliac joints into the hip bones and then down the legs. Those joints are built for strength rather than movement and are held by some of the thickest ligaments in the body, allowing only a small amount of give. The sacrum also anchors muscles of the back, buttock and pelvic floor, and its curved shape helps form the space the pelvic organs occupy. In the female pelvis that curve and the width between the hip bones affect the shape of the birth canal.
What passes through it?
The sacrum is pierced by paired openings on its front and back surfaces. Sacral nerves emerge through these to supply the buttocks, the backs of the legs, the pelvic floor, the bladder and the bowel. An important detail is often stated loosely: the spinal cord itself ends higher up, around the top of the lumbar spine, so what runs down through the sacral canal is a bundle of nerve roots rather than cord. Injury or compression at this level therefore affects those roots, and can disturb sensation in the saddle area and control of the bladder and bowel.
How does it relate to the bones around it?
| Bone | Position | Nature |
|---|---|---|
| Lumbar vertebrae | Above the sacrum | Five separate mobile bones |
| Sacrum | Base of the spine | Fused segments forming one bone |
| Coccyx | Below the sacrum | Several small fused segments, the tailbone |
| Hip bones | Either side | Joined to the sacrum at the sacroiliac joints |
Fusion is what separates the sacrum from the lumbar spine above it, and the number of segments involved varies between people, with a lowest lumbar vertebra sometimes partly joined to the sacrum. The skin over the sacrum lies directly on bone, which is why it is a common site for pressure injury in anyone confined to bed.
Used in a sentence
Imaging of the lower spine included evaluation of the sacrum for evidence of fracture.
Good to know
The sacrum is formed from vertebral segments that fuse into a single bone during development, unlike the separate mobile vertebrae above it, and the spinal cord ends above this level so the sacral canal carries nerve roots rather than cord.
Word origin
Where “sacrum” comes from
From Latin os sacrum, meaning sacred bone, a name whose origin is disputed and often traced to the bone's use in ancient ritual or to its size.