Sacral Spine

The sacral spine, or the sacrum, is located at the bottom of the vertebral column, below the lumbar vertebrae and above the coccyx (tailbone). It is a large, triangular bone composed of five fused sacral vertebrae, labeled S1 through S5. Unlike the rest of the vertebral column, the sacral vertebrae do not have intervertebral discs between them and are immobile.

Here are the key anatomical features of the sacral spine:

1. Sacral Promontory: This is the anterior lip of the body of S1 and is an important anatomical landmark.
2. Ala (Wings): These are the wide, lateral parts of the sacrum, which are the sites of muscle attachment and articulate with the iliac bones to form the sacroiliac joints.
3. Median Sacral Crest: This is found on the posterior side of the sacrum, formed by the fused spinous processes of the sacral vertebrae.
4. Sacral Canal: This is the continuation of the vertebral canal, running through the center of the sacrum. It terminates as the sacral hiatus near the coccyx. The sacral canal houses the sacral nerves.
5. Sacral Foramina: There are four pairs of these openings, which allow for the passage of the sacral nerves and blood vessels. The anterior (ventral) sacral foramina transmit the ventral rami of the sacral spinal nerves, while the posterior (dorsal) sacral foramina transmit the dorsal rami.
6. Sacral Hiatus: This is the inferior opening of the sacral canal, found at the lower end of the sacrum.
7. Apex of the Sacrum: This is the narrow, inferior part of the sacrum, which articulates with the coccyx.

The sacrum, by articulating with the pelvic bones, plays a crucial role in transferring the weight of the body from the spinal column to the lower limbs during standing and walking. The sacrum’s unique shape also contributes to the curvature of the lower back and forms the back part of the pelvic cavity. Despite being largely immobile, the sacrum and its articulations can sometimes be implicated in lower back pain and sacroiliac joint dysfunction.