Sacrum

The sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity. It is here that it is wedged between the two hip bones. The sacrum functions as a wedge stabilizing the bony pelvis. There are no muscles that cross the sacrum.

Anatomy of the Sacrum

The sacrum is made up of the following:

• Pelvic surface of the sacrum which is concave from above downward, and slightly so from side to side,
• Dorsal surface of the sacrum that is convex and narrower than the pelvic,
• Lateral surface of the sacrum that is broad above and narrowed into a thin edge below,
• Base of the sacrum that is broad and expanded,
• Apex which is directed downward, and presents an oval facet for articulation with the coccyx, and
• The vertebral canal which runs throughout the greater part of the bone.

The sacrum articulates four bones: the hip bone on both sides, the tailbone below and the lumbar vertebra above. Rotation of the sacrum forward a few degrees vis-a vis the ilia is sometimes called nutation. The reverse motion is called counter nutation.

The sacrum is shaped differently in males and females. In the female, it is shorter and wider. The lower half forms a greater angle with the upper; the upper half is nearly straight. The bone is also directed more obliquely backward, increasing the size of the pelvic cavity.

In males the curvature of the sacrum is more evenly distributed over the whole length and is greater than in the female.

Common Problems with the Sacrum

Common sacrum problems include:

Sacral Stress Fractures

A sacral stress fracture occurs in the young and active, commonly seen in female long-distance runners. Rather than the bone not being healthy enough, the activities simply are too much for the sacrum to resist, and a stress fracture results.

Symptoms of a sacral stress fracture most commonly include back and buttock pain, and pain with activities such as running. Treatment must include resting from activities that cause pain. Usually with a period of rest, the symptoms will resolve. Gradual resumption of activities is important to prevent a recurrence.

Sacral Insufficiency Fractures

Sacral insufficiency fractures occur in elderly patients with osteoporosis. These injuries may be the result of a fall or may be the result of no particular injury. Patients commonly complain of symptoms of low back and buttock pain.

The treatment of a sacral insufficiency fracture is usually symptomatic relief with rest and assistive devices such as a walker or cane.

Traumatic Sacral Fractures

Traumatic fractures of the sacrum can occur with pelvic fractures. These injuries commonly occur in car accidents and severe falls.

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