Sacrum

The wedged-shaped sacrum (Latin sacred bone) is usually composed of five fused sacral vertebrae in adults. It is located between the hip bones and forms the roof and posterosuperior wall of the posterior half of the pelvic cavity. The triangular shape of the sacrum results from the rapid decrease in the size of the inferior lateral masses of the sacral vertebrae during development. The inferior half of the sacrum is not weight-bearing; therefore, its bulk is diminished considerably. The sacrum provides strength and stability to the pelvis and transmits the weight of the body to the pelvic girdle, the bony ring formed by the hip bones and sacrum, to which the lower limbs are attached.

The sacral canal is the continuation of the vertebral canal in the sacrum. It contains the bundle of spinal nerve roots arising inferior to the L₁ vertebra, known as the cauda equina (Latin horse tail), that descend past the termination of the spinal cord. On the pelvic and posterior surfaces of the sacrum between its vertebral components are typically four pairs of sacral foramina for the exit of the posterior and anterior rami of the spinal nerves. The anterior (pelvic) sacral foramina are larger than the posterior (dorsal) ones.

The base of the sacrum is formed by the superior surface of the S₁ vertebra. Its superior articular processes articulate with the inferior articular processes of the L₅ vertebra. The anterior projecting edge of the body of the S₁ vertebra is the sacral promontory (Latin mountain ridge), an important obstetrical landmark. The apex of the sacrum, its tapering inferior end, has an oval facet for articulation with the coccyx.

The sacrum supports the vertebral column and forms the posterior part of the bony pelvis. The sacrum is tilted so that it articulates with the L₅ vertebra at the lumbosacral angle, which varies from 130° to 160°. The sacrum is often wider in proportion to length in the female than in the male, but the body of the S₁ vertebra is usually larger in males.

The pelvic surface of the sacrum is smooth and concave. Four transverse lines on this surface of sacra from adults indicate where fusion of the sacral vertebrae occurred. During childhood, the individual sacral vertebrae are connected by hyaline cartilage and separated by IV discs. Fusion of the sacral vertebrae starts after age 20; however, most of the IV discs remain unossified up to or beyond middle life.

The dorsal surface of the sacrum is rough, convex, and marked by five prominent longitudinal ridges. The central ridge, the median sacral crest, represents the fused rudimentary spinous processes of the superior three or four sacral vertebra; S₅ has no spinous process. The intermediate sacral crests represent the fused articular processes, and the lateral sacral crests are the tips of the transverse processes of the fused sacral vertebrae.

The clinically important features of the dorsal surface of the sacrum are the inverted U-shaped sacral hiatus and the sacral cornua (Latin horns). The sacral hiatus results from the absence of the laminae and spinous process of S₅ and sometimes S₄. The sacral hiatus leads into the sacral canal. Its depth varies, depending on how much of the spinous process and laminae of S₄ are present. The sacral cornua, representing the inferior articular processes of S₅ vertebra, project inferiorly on each side of the sacral hiatus and are a helpful guide to its location.

The superior part of the lateral surface of the sacrum looks somewhat like an auricle (Latin external ear); because of its shape, this area is called the auricular surface. It is the site of the synovial part of the sacro-iliac joint between the sacrum and ilium. During life, the auricular surface is covered with hyaline cartilage.