The sacrum (os sacrum) is the triangular shaped bone (irregular bone), which is composed of five sacral vertebrae (S1 to S5, fused in adults). The fused sacral vertebrae form a central sacral body, and paired sacral alae, arising laterally from S1.

Base of sacrum consists of the:

Promontory (promontorium) represents the sharp osseous prominence that extends anteriorly from the superior margin of the base of S1. It is an important obstetrical landmark.

Sacral ala (ala sacralis) or sacral wing represents part of the base of the sacrum situated laterally to the vertebral body of the S1.

Superior articular process with the posteromedial articular surface of S1 vertebra. It articulates with the L5 inferior articular facet.

The rest of superior and inferior articular processes are known as intermediate sacral crests (crista sacralis intermedia). The articular processes are located bilaterally to the median sacral crest.

Lateral part (pars lateralis) is derived from the transverse processes and rudimentary ribs and includes the following properties:

Auricular surface (facies auricularis) which is an ear-shaped articular surface for articulation with the ilium to make sacroiliac joint.

Sacral tuberosity (tuberositas sacralis) which is a rough area behind the auricular surface for the attachment of ligaments coming from the ilium.

Pelvic or anterior surface (facies anterior)is concave and it has four pairs of the:

Anterior sacral foramina (foramina sacralia anteriora) for exit of the ventral branches of the sacral plexus.

Transverse ridges or lines (lineae transversae) which are sites of fusion of the bodies of the sacral vertebrae.

Intervertebral foramina.

Dorsal or posterior surface (facies posterior) is convex and contains the:

Median sacral crest (crista sacralis mediana)  is unpaired, central longitudinal ridge and represents  the remnant four upper sacral spinous processes. It  is visible on the dorsal surface as a fused vertical midline of the reduced spinous processes of S1 to S4 vertebrae (S5 has no spinous process).

Intermediate sacral crest (crista sacralis intermedia)  is paired and represents the fused articular processes.

Lateral sacral crest (crista sacralis lateralis) is paired and represents the fused tips of the transverse processes.

Sacral hiatus (hiatus sacralis).

Posterior sacral foramina (foramina sacralia posteriora) for exit of the dorsal branches of the sacral nerves.

Sacral horn (cornu sacralis) represents the inferior articular processes of S5 vertebra, project inferiorly on each side of the sacral hiatus.

The sacral canal, formed by vertebral foramina of sacral vertebrae (before fusion) is the continuation of the vertebral canal, and it ends inferiorly at the sacral hiatus. Inferiorly, the narrow sacral apex articulates with the coccyx.

Sacral hiatus (hiatus sacralis) which is inverted U-shaped aperture results from the absence of vertebral laminae and the spinous process of S5 vertebra. It leads into the the inferior end of the vertebral canal.

Apex of the sacrum (apex ossis sacri) which articulates with the coccyx.

Clinical comments

The fifth lumbar vertebra is occasionally fused with the top of the sacrum. This is called the sacralization of the fifth lumbar vertebra (Fig. 2-22). The first sacral vertebra could be separated from the remaining sacral bone. On X-ray this is seen as 6th lumbar vertebra. This is called the lumbalization of the first sacral vertebra. This is also known as a transition vertebra or assimilation vertebra.