Sacrum: Surfaces, Foramina, Hiatus and Relations

By Dr Richard Miller, MBChB FRCS · Reviewed 1 October 2026

The sacrum is the large triangular bone at the base of the spine, formed from five fused sacral vertebrae and wedged between the two hip bones. It joins L5 above, the coccyx below and the ilia at the sacroiliac joints, transmitting body weight to the pelvis. Its canal holds the lower cauda equina and its foramina transmit the sacral nerves.

Sacrum · key facts

Type: Irregular bone formed by fusion of five sacral vertebrae
Articulations: L5 (lumbosacral joint), ilia (sacroiliac joints), coccyx (sacrococcygeal joint)
Key landmarks: Promontory, ala, anterior and posterior sacral foramina, median, intermediate and lateral sacral crests, auricular surface, sacral hiatus and cornua
Muscle attachments: Piriformis, gluteus maximus, erector spinae, multifidus, iliacus (small part of ala), coccygeus near the apex
Ossification: Primary centres for each sacral vertebra plus costal elements forming the alae; fusion from adolescence into early adulthood

3D model of the sacrum: sacrum — 3D model showing the sacrum.BodyParts3D, © The Database Center for Life Science licensed under CC Attribution 4.0 International

Structure

The sacrum is a curved, wedge-shaped bone with a base above, an apex below, a concave pelvic surface in front and a rough dorsal surface behind.

Base

The base is the upper surface of S1. Its body articulates with L5 through the lumbosacral intervertebral disc, and its superior articular processes articulate with the inferior articular processes of L5 at the facet joints. The projecting anterior edge of the S1 body is the sacral promontory, which forms the back of the pelvic inlet. On each side of the body the broad ala (wing) represents fused transverse processes and costal elements.

Pelvic surface

The pelvic surface is smooth and concave. Four transverse ridges mark the fused discs, and at their ends lie four pairs of anterior sacral foramina for the anterior rami of S1 to S4.

Dorsal surface

The dorsal surface is rough and convex, with three pairs of ridges and a midline crest:

Median sacral crest: fused spinous processes.
Intermediate sacral crests: fused articular processes.
Lateral sacral crests: fused transverse processes.

Four pairs of posterior sacral foramina, lying between the intermediate and lateral crests, transmit the posterior rami of S1 to S4.

Sacral canal and hiatus

The sacral canal continues the vertebral canal. It contains the cauda equina, the filum terminale and the lower end of the dural sac, which usually ends at about S2. The laminae of S5 fail to meet in the midline, leaving the sacral hiatus, bordered on each side by the sacral cornua. The hiatus is covered by the superficial posterior sacrococcygeal ligament.

Lateral surface

The upper lateral surface carries the ear-shaped auricular surface for the synovial part of the sacroiliac joint, with a rough area behind it for the interosseous sacroiliac ligament.

Relations

The pelvic surface of the sacrum lies behind the rectum and sacral plexus, while its dorsal surface lies beneath the thick muscles of the lower back and buttock.

In front: the rectum, separated by the mesorectum and presacral (rectosacral) fascia; the median sacral vessels in the midline; the sympathetic trunks converging towards the ganglion impar in front of the coccyx; the presacral venous plexus.
Laterally on the pelvic surface: piriformis arising between the anterior foramina, with the sacral plexus on its front.
Crossing the ala: the lumbosacral trunk (L4, L5), descending to join the sacral plexus.
Behind: erector spinae and multifidus in the midline; gluteus maximus arising from the lower lateral dorsal surface.
Ligaments: the sacrotuberous and sacrospinous ligaments attach to its lateral margin and convert the sciatic notches into the greater and lesser sciatic foramina.

Sacroiliac joint and sex differences

The sacroiliac joint is a strong, only slightly mobile joint that transfers the weight of the trunk from the sacrum to the hip bones.

Its front part is a synovial joint between the auricular surfaces of sacrum and ilium; its back part is a syndesmosis held by the very strong interosseous and posterior sacroiliac ligaments. The sacrum is set like a keystone, and body weight tends to push its upper end forward and down, which these ligaments resist, helped by the sacrotuberous and sacrospinous ligaments.

The female sacrum is usually shorter, wider and less curved, with the S1 body forming a smaller proportion of the base than the alae. This widens the pelvic inlet.

Clinical relevance

The sacrum is a route for regional anaesthesia, a site of fractures and variants that confuse spinal level counting, and a hazard in rectal surgery.

Caudal epidural block: the needle passes through the sacral hiatus, found between the sacral cornua, into the epidural space of the sacral canal. It is widely used in children.
Sacral fractures: from high-energy pelvic trauma, or low-energy insufficiency fractures in osteoporotic bone. Fractures through the foramina or canal can injure sacral nerves and affect bladder and bowel control.
Transitional vertebrae: sacralisation of L5 or lumbarisation of S1 changes the vertebral count and can cause wrong-level spinal surgery.
Presacral bleeding: the presacral venous plexus can bleed heavily during mobilisation of the rectum if the presacral fascia is breached.
Spina bifida occulta: failure of the laminae to fuse is common at S1 and usually harmless.
Sacroiliitis: an early feature of axial spondyloarthritis such as ankylosing spondylitis.

How it is examined

On a cadaveric spotter, in an OSPE and in MRCS Part B anatomy.

Orient a sacrum by its surfaces: the smooth concave surface with the larger anterior foramina faces the pelvis; the rough convex surface with crests faces backwards; the base with the S1 body faces up.
A pin in the notch at the lower end of the dorsal surface is the sacral hiatus; the two small projections either side are the sacral cornua, used to locate it for a caudal block.
Know which rami pass through which foramina: anterior rami through the anterior (pelvic) foramina, posterior rami through the posterior foramina.
Expect a follow-up on where the dural sac ends (about S2) and why that matters for caudal anaesthesia.

Key points

The sacrum is formed from five fused vertebrae and forms the back of the pelvis.
The promontory is the anterior edge of S1 and marks the back of the pelvic inlet.
Four pairs of anterior and posterior foramina transmit sacral anterior and posterior rami.
The sacral hiatus between the cornua gives access to the sacral epidural space.
The dural sac usually ends at about S2.
The auricular surface forms the synovial part of the sacroiliac joint.

Common questions

What is the sacral hiatus?

The sacral hiatus is a gap at the lower end of the back of the sacrum, where the laminae of the fifth sacral vertebra fail to meet. It is bordered by the sacral cornua and covered by ligament. It opens into the sacral canal below the end of the dural sac, so anaesthetists use it to inject local anaesthetic into the epidural space as a caudal block.

How many vertebrae make up the sacrum?

The sacrum is made of five sacral vertebrae, S1 to S5, which fuse together during adolescence and early adulthood. The lines of fusion remain visible as transverse ridges on the pelvic surface. Some people have a transitional vertebra, where L5 is partly fused to the sacrum or S1 is partly separated from it, which changes the apparent number of lumbar and sacral segments.

What passes through the sacral foramina?

The four pairs of anterior sacral foramina on the pelvic surface carry the anterior rami of S1 to S4, which join the sacral plexus on the front of piriformis, together with small lateral sacral vessels. The four pairs of posterior sacral foramina on the dorsal surface carry the posterior rami of S1 to S4 to the muscles and skin of the lower back and buttock.

References

Gray's Anatomy: The Anatomical Basis of Clinical Practice. Standring S (ed). Elsevier. 42nd edition, 2020.
Moore's Clinically Oriented Anatomy. Moore KL, Dalley AF, Agur AMR. Wolters Kluwer. 9th edition, 2022.
Last's Anatomy: Regional and Applied. Elsevier.