Sacrum

The sacrum is a large, triangular bone at the base of the spine and the posterior part of the pelvis. In adults it typically consists of five fused sacral vertebrae (S1–S5) that unite during adolescence or early adulthood. The sacrum transmits weight from the axial skeleton to the pelvic girdle and forms the posterior portion of the pelvic ring, helping to stabilize the pelvis and protect the pelvic organs.

Anatomy and surfaces: The anterior (pelvic) surface is smooth and concave, facing the pelvic cavity, while the posterior surface is rough and contains the dorsal sacral foramina. The median sacral crest runs down the midline and consists of fused spinous processes; the two lateral crests are formed by fused transverse processes, and the intermediate crests lie between them. There are four pairs of anterior sacral foramina and four pairs of posterior sacral foramina for the exit of ventral and dorsal rami, respectively. The sacral promontory is the projecting edge of S1 at the anterior border. At the inferior end, the sacrum bears the sacral cornua and ends in the sacral hiatus, where the sacral canal ends. The sacrum articulates with the ilia at the sacroiliac joints (via the auricular surfaces) and with the coccyx at the sacrococcygeal joint. The superior articular facets of S1 articulate with the fifth lumbar vertebra.

Function and clinical relevance: The sacrum helps transfer weight to the pelvis and serves as an attachment for ligaments and muscles of the pelvic floor and lower back. Variations include sacralization of L5 or lumbarization of S1. Clinical considerations include sacral fractures and sacroiliac joint disorders.