Anatomy of the sacrum

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Anatomy of the sacrum

The sacrum is anatomically classified as part of the vertebral column articulating superiorly with the fifth lumbar vertebra at the lumbosacral junction. The sacrum and coccyx are the distal portions of the vertebral column and are also structurally part of the pelvis. The sacrum and coccyx are classified as irregular bones and are also atypical vertebrae. A typical vertebra is a single bone having distinct parts like the spinous process, transverse processes, vertebral body and foramen, pedicles and lamina. The sacrum is actually a composite bone formed by the fusion of its 5 sacral vertebrae into a single bone at about age 20 years. This fusion presents distinct anterior and posterior sacral landscapes. On the whole, these fused sacral segments can still be identified on the adult sacrum allowing for accurate localization numbering. The main functions of the sacrum are to strengthen the pelvis, which stabilizes it, and transmitting the weight of the body to the pelvic girdle through the sacroiliac joints (SI joints). To perform these functions the sacrum forms five main articulations: 1-2) two with the inferior articular processes of the fifth lumbar vertebra, 3-4) bilaterally with the ilium to form right and left sacroiliac joints, and 5) inferiorly with the coccyx.

The anterior or pelvic surface of the sacrum is concave, which is a shape that adds depth to the pelvic cavity. This surface forms the posterior ring of the pelvis. Four pairs of foramina are seen on the anterior surface. They are the remnants of what would be intervertebral foramen of a typical vertebra seen in other parts of the spine. Structures seen on the anterior surface include: prominent sacral wings (called ala), superior articular processes, sacral promontory, transverse ridges (lines), vertebral bodies, and anterior (pelvic) sacral foramina. Sacral vertebrae are numbered from top to bottom so that the superior vertebra is the first sacral vertebra. It articulates with the 5th lumbar vertebra. Distally, the 5th sacral vertebra articulates with the 1st coccygeal segment. The 1st sacral vertebra articulates with the 5th lumbar vertebra through their articular processes forming bilateral zygoapophyseal joints at the lumbosacral junction. Thus the lumbosacral junction occurs at the L5/S1 articulations.