Sacrum and Coccyx

The broader base of the sacrum faces anterosuperiorly toward the abdomen; its elevated central third is the upper part of the first sacral vertebral body and bears a smooth oval area for the attachment of the lumbosacral intervertebral disk. Its projecting anterior border is the sacral promontory. On each side, the costotransverse elements of the first vertebra are fused to form a wing-shaped lateral mass (sacral ala), separated from the pelvic surface by a curved line, which is the sacral portion of the arcuate pelvic brim. The articular processes are fused, like most of the other components of the sacral vertebrae, but the superior articular processes of the first vertebra remain and project upward for articulation with the inferior articular processes of the fifth lumbar vertebra. They are flattened and face almost directly backward to assist in preventing subluxation (spondylolisthesis) of the last lumbar vertebra at the angulated lumbosacral junction.

The narrow apex is the lower end of the sacrum and articulates with the coccyx. The pelvic surface is concave both vertically and horizontally and shows four transverse ridges indicating the lines of fusion between the bodies of the original five vertebrae. On either side of the ridges, four pelvic sacral foramina permit the passage of the anterior rami of the first four sacral nerves and their associated vessels.

The convex posterior surface shows irregular median, intermediate, and lateral sacral crests representing, respectively, the fused spinous, articular, and transverse processes. The areas between the median and intermediate crests are the fused laminae, and there are four pairs of posterior sacral foramina for the passage of the posterior rami of the upper four sacral nerves. The laminae of the fifth and, occasionally, the fourth vertebra fail to unite and thus leave a hiatus, which is exploited for the injection of epidural anesthetics. The hiatus is bounded on each side by a cornu, a relic of the inferior articular process, and transmits the small fifth sacral and coccygeal nerves.

The parts of the sacrum lateral to the sacral foramina are produced by the fusion of the costal, transverse, and pedicular elements of the five vertebrae. The upper, broader parts of their lateral surfaces bear uneven auricular, or ear-shaped, surfaces for articulation with similar surfaces on the iliac parts of the hip bones. This canal surrounds and protects the terminations of the dural and arachnoid sheaths and the subarachnoid space, which end at about the level of the second sacral vertebra and enclose the sacral and coccygeal roots of the cauda equina and the lower intrathecal portion of the filum terminale. The dura mater is separated from the walls of the canal by fibrofatty tissue, fine arteries, and nerves and sacral internal vertebral venous plexuses.

Coccyx. The small, triangular coccyx is formed by the fusion of four (occasionally, three or five) rudimentary tail vertebrae. Its base articulates with the sacral apex, and its apex is a mere button of bone. Most of the features of a typical vertebra are lacking, but the first coccygeal vertebra has small transverse processes and a cornu on each side, which is sometimes large enough to articulate with the corresponding sacral cornu. Transverse sections of the sacrum reveal the triangular sacral end of the vertebral canal.

Only gold members can continue reading. Log In or Register to continue