The sacrum is a triangular bone at the base of the spine, which is formed by five sacral vertebrae that fuse with each other between ages 18 and 30.

The sacrum articulates with four bones - with the last (fifth) lumbar vertebra above, the coccyx below, and the ilium part of the hip bone on each side. It contributes to the formation of the pelvic skeleton.

The sacrum has a triangular-shape with the apex directed downwards and the base directed upwards. The apex is the lower tip of the sacrum that connects with the coccyx, while the base is the broad upper part of the sacrum and it features the sacral promontorium and the superior articular processes.

The sacral bone has four surfaces: pelvic, dorsal and two lateral surfaces.

The pelvic surface of the sacrum is the anterior side that faces the pelvic cavity. The pelvic surface is concave and presents the following landmarks:

Sacral promontory (promontorium) - a prominent upper anterior margin of the body of the first sacral vertebra, projecting quite far into the pelvic inlet.
Transverse lines (4) - four fusion lines between the bodies of the five sacral vertebrae situated on the anterior aspect of the sacrum.
Anterior sacral foramina (4 pairs) - anterior openings in the sacrum for the passage of nerves and blood vessels.

The dorsal surface is the posterior convex side of the sacrum and it features the following landmarks:

Superior articular process (2) - two large processes on the posterior aspect of the base of the sacrum that articulate with the corresponding inferior articular processes of the fifth lumbar vertebra.
Median sacral crest - a median ridge formed by the remnants of the spinous processes of the sacral vertebrae.
Posterior sacral foramina (4 pairs) - four pairs of posterior openings in the sacrum for the passage of nerves and blood vessels.
Intermediate sacral crests - remnants of articular processes of the sacral vertebrae located on each side of the median sacral crest.
Lateral sacral crests - several posterior bilateral remnants of transverse processes of the sacral vertebrae.
Sacral cornu (2) - "sacral horns", hook-shaped processes that extend downward on either side of the sacral hiatus; the sacral horns are remnants of articular processes of the fifth sacral vertebra.
Sacral canal and sacral hiatus - the sacral canal is the inferior end of the vertebral canal running through most of the sacrum.The inferior opening of the sacral canal is called the sacral hiatus, located in the posterior wall of the sacral canal usually at the level of the third to fourth sacral vertebrae (S3 - S4). The sacral hiatus is where the filum terminale emerges and it serves as the injection site of the lower epidural anesthesia.

The lateral surfaces (or lateral parts, masses) are the two lateral aspects of the sacrum that derived from the fusion sites of rudimentary ribs and transverse processes of the sacral vertebrae.

Each lateral surface presents the following landmarks:

Auricular surface - an ear-shaped articular surface situated on each lateral part of the sacrum for articulating with the ilium.
Sacral tuberosity - a rough area on each lateral surface of the sacrum behind the auricular surface for attachment of the posterior sacroiliac ligaments.

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Sacral vertebrae