The sacrum (or sacral bone, latin: os sacrum) is a triangular bone at the base of the spine, which is formed by five sacral vertebrae (S1 - S5) 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 on each side. The sacrum contributes to the formation of the pelvic skeleton.

The shape of the sacrum is triangular with the apex directed downwards and the base of the sacrum directed upwards. The apex is the lower tip of the sacrum that gives attachment to the coccyx, while the base is the broad upper part of the sacrum and it features the promontorium and the superior articular processes.

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

Pelvic surface of sacrum

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

  • promontorium, 
  • transverse lines (4), 
  • anterior sacral foramina (4 pairs).

The promontorium (also known as sacral promontory, or promontory of sacrum) is a prominent upper anterior margin of the body of the first sacral vertebra, projecting quite far into the pelvic inlet.

The transverse lines are four fusion lines between the bodies of the five sacral vertebrae situated on the anterior aspect of the sacrum.

The anterior sacral foramina are anterior openings in the sacrum for the passage of nerves and blood vessels.

Dorsal surface of sacrum

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

  • superior articular process (2),
  • median sacral crest, 
  • posterior sacral foramina (4 pairs), 
  • intermediate sacral crests, 
  • lateral sacral crests, 
  • sacral cornu (2), 
  • sacral canal, 
  • sacral hiatus.

The superior articular processes are 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.

The median sacral crest is a median ridge on the dorsal surface of the sacrum, which is formed by the remnants of the spinous processes of the sacral vertebrae.

The posterior sacral foramina are four pairs of posterior openings in the sacrum for the passage of nerves and blood vessels.

The intermediate sacral crests are remnants of articular processes of the sacral vertebrae located on each side of the median sacral crest.

The lateral sacral crests are several posterior bilateral remnants of transverse processes of the sacral vertebrae.

The two sacral cornu (or sacral horns) are hook-shaped processes that extend downward on each side of the sacral hiatus. The sacral horns are remnants of articular processes of the fifth sacral vertebra.

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 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.

Lateral surfaces of sacrum

The lateral surfaces (or lateral parts, masses) of the sacrum 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, 
  • sacral tuberosity.

The auricular surface is an ear-shaped articular surface situated on each lateral part of the sacrum for articulating with the ilium.

The sacral tuberosity is a rough area on each lateral surface of the sacrum behind the auricular surface for attachment of the posterior sacroiliac ligaments.