Sacrum

Os sacrum

The sacrum is the large wedge of fused vertebrae below the lumbar spine, set between the two hip bones at the back of the pelvis.

At a glance

English: Sacrum
Latin (TA2): Os sacrum
System: Bone
Region: Abdomen
Side: Single, on the midline

About the sacrum

The sacrum is the wedge of bone at the lower end of the spine, wedged between the two hip bones at the back of the pelvis. It begins as five separate sacral vertebrae, S1 to S5, which fuse into a single triangular bone in early adult life, usually between about 18 and 30 years. Its broad upper end, the base, faces upward and forward; its narrow lower end, the apex, points down and carries an oval facet for the coccyx.

Seen from the front, the pelvic surface is hollowed and crossed by four transverse ridges that mark where the vertebral bodies joined. At the ends of each ridge an anterior sacral foramen lets out the front branch of a sacral nerve. The front edge of the first body juts forward as the sacral promontory, part of the rim of the pelvic inlet. On each side of the base a wide wing, the ala, carries the psoas major and lumbosacral trunk and forms part of the pelvic brim. The back is rougher and narrower: a median crest of small tubercles marks the old spinous processes, flanked by grooves for the multifidus, and lower down the laminae of S5 (sometimes S4) fail to meet, leaving the sacral hiatus. Smaller posterior foramina carry the back branches of the nerves.

The sacral canal continues the vertebral canal through most of the bone. Laterally, the ear-shaped auricular surface on the upper half forms the sacroiliac joint with the ilium, and the rough sacral tuberosity behind it holds the strong posterior sacroiliac ligaments. The thinner lower half anchors the sacrotuberous and sacrospinous ligaments, some fibres of gluteus maximus and the coccygeus. In all, the sacrum meets the fifth lumbar vertebra above, the coccyx below and an ilium on each side. It can tilt a little in the sagittal plane, a nodding movement called nutation, with the reverse called counter-nutation.

Clinically, the lumbosacral region is a frequent site of spina bifida, and failure of the lower spine to form is called sacral agenesis or caudal regression. Sacral fractures are not common but often injure the sacral nerves. The sacrum is also a classic site for chordoma, a tumour arising from remnants of the notochord.

Function and supply

Function: Passes the weight of the upper body to the pelvis through the sacroiliac joints, closes the back of the pelvic ring and protects the sacral nerve roots.
Nerve supply: Its canal and foramina carry the sacral spinal nerves, whose front branches leave through the anterior foramina and back branches through the posterior foramina.
Blood supply: The lateral sacral arteries enter through the anterior sacral foramina.

Parts of the sacrum

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Ala of sacrumAla ossis sacriThe broad wing on each side of the base, formed from fused transverse elements. Its upper surface carries psoas major and the lumbosacral trunk, and it forms part of the rim of the pelvic inlet.
Anterior sacral foraminaForamina sacralia anterioraThe paired openings on the pelvic surface at the ends of the transverse ridges, shrinking in size from top to bottom. The front branches of the sacral nerves leave through them to form the sacral plexus.
Apex of sacrumApex ossis sacriThe narrow lower tip of the bone, formed by the small body of the last sacral vertebra. It carries an oval surface that joins the coccyx.
Auricular surface of sacrumFacies auricularis ossis sacriThe ear-shaped joint surface on the upper half of each side of the sacrum. It is covered in cartilage and meets a matching surface on the ilium at the sacroiliac joint.
Base of sacrumBasis ossis sacriThe wide upper end, formed by the first sacral vertebra and tilted upwards and forwards. Its body joins the fifth lumbar vertebra through an intervertebral disc.
Dorsal surface of sacrumFacies dorsalis ossis sacriThe rough, convex back of the bone, narrower than the front. It carries three rows of crests and the posterior foramina, and gives origin to multifidus and erector spinae.
Intermediate sacral crestCrista sacralis medialisA row of low bumps on each side of the median crest, lying just inside the posterior foramina. They are what remains of the joint processes of the sacral segments after fusion.
Intervertebral foraminaForamina intervertebraliaShort channels inside the sacrum that branch out from the sacral canal on each side. Each one splits to open forwards through an anterior foramen and backwards through a posterior foramen, so each sacral nerve divides within the bone.
Lateral part of sacrumPars lateralis ossis sacriThe solid mass of bone on each side of the foramina, formed from fused transverse elements and ribs. It carries the ala above, the auricular surface and the sacral tuberosity.
Lateral sacral crestCrista sacralis lateralisA row of rough tubercles outside the posterior foramina, made by fused transverse processes. The posterior sacroiliac and sacrotuberous ligaments attach along it.
Median sacral crestCrista sacralis medianaA ridge of small bumps down the middle of the back of the sacrum, the fused remnants of the spinous processes of the upper sacral vertebrae. It is felt in the cleft above the buttocks.
Pelvic surface of sacrumFacies pelvica ossis sacriThe smooth, concave front of the sacrum, which faces into the pelvis. It carries the transverse ridges and anterior foramina, gives origin to piriformis, and lies behind the rectum.
Posterior sacral foraminaForamina sacralia posterioraThe paired openings on the back of the sacrum, smaller and less regular than those in front. The back branches of the sacral nerves leave through them to supply the skin and muscles over the sacrum.
PromontoryPromontoriumThe front edge of the top of the first sacral body, jutting forwards into the pelvis. It forms the back point of the pelvic inlet and is used to measure the inlet in obstetric practice.
Sacral canalCanalis sacralisThe tunnel running down through the sacrum, continuing the vertebral canal of the spine. It holds the lower nerve roots of the cauda equina and ends below at the sacral hiatus.
Sacral hiatusHiatus sacralisA gap at the lower end of the back of the sacrum where the laminae of the last segment, and sometimes the one above, do not meet. It opens into the sacral canal and is the route for a caudal epidural injection.
Sacral hornCornu sacraleA short downward projection flanking the sacral hiatus on either side, which represent the lower joint processes of the last sacral segment. They can be felt and guide a needle into the hiatus, and they link to the coccygeal horns.
Sacral tuberosityTuberositas ossis sacriA rough area behind the auricular surface on each side, pitted by deep marks. The strong interosseous and posterior sacroiliac ligaments attach here and hold the sacrum against the ilium.
Superior articular process of sacrumProcessus articularis superior ossis sacriA pair of upward projections at the back of the base, with concave facets facing backwards and inwards. They form facet joints with the downward-facing processes of L5.
Transverse ridgesLineae transversaeFour horizontal ridges across the middle of the pelvic surface. They mark where the bodies of the five sacral vertebrae fused together.

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At a glance

About the sacrum

Function and supply

Related structures

See the sacrum where it really sits, with every layer of the body around it.