ArticlesAnatomySpine and backSpineSacrum

ArticlesAnatomySpine and backSpineSacrum

Sacrum

Author: Ed Madeley • Reviewer: Dimitrios Mytilinaios, MD, PhD
Last reviewed: October 30, 2023
Reading time: 10 minutes

The sacrum is an irregularly shaped bone, made up of a group of five fused vertebrae in the area of what is commonly known as the base of the spine. Regarded as the keystone of the human body, the sacrum is important because it forms a link between the spine and the iliac bones, and also has an important part to play in hip stability.

Sacrum is sexually dimorphic. It is shorter and wider in females than in males. In females, the sacrum is also distributed more obliquely backwards which increases the size of the pelvic cavity, making it more convenient for enduring pregnancy and offering more space for the developing fetus.

Key facts
Landmarks	Dorsal surface: - 3 crests: median crest (due to fusion of spinous processes S-S3), intermediate crest (due to fusion of ariticular processes L5-S4), lateral crest (due to fusion of transverse processes) - 4 pairs of posterior sacral foramina Pelvic surface: - transverse lines (fused intervertebral discs of sacral vertebrae) - sacral promontory (posterior margin of the pelvic inlet) - 4 pairs of anterior sacral foramina Lateral surface - articulates with ilium Base - articulates with L5 Apex - articulates with coccyx
Joints	Lumbosacral joint - between the superior auricular process of sacrum and inferior articular facets of L5 Sacrococcygeal joint - between the apex of the sacrum and the base of the coccyx Sacroiliac joint - between lateral sacral surface and iliac articulation surface
Muscle attachments	Pelvic surface: piriformis, iliacus, coccygeus muscles Dorsal surface: erector spinae aponeurosis, gluteus maximus muscles
Clinical relations	Spondylolisthesis, disc herniation, disc degeneration, sciatica

This article will discuss the anatomy of the sacrum.

Contents

Articulations
Bony landmarks
Ligaments of the sacrum
Muscle attachments
Clinical aspects
Sources

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Articulations

Lumbosacral joint

The lumbosacral (L5/S1) joint comprises the superior articular process of the sacrum which articulates with the inferior articular facets of L5. There is a great deal of pressure occurring at this joint, as the weight of all of the vertebrae above bears down on it, relying on it for their stability.

To bear this weight and to achieve this stability, the orientation of the L5/S1 facet joint is mostly in the coronal plane, providing more support to the joint. This articulation is united by an intervertebral disc, which is deeper anteriorly along with the vertebral body of L5. This contributes to the lumbosacral angle.

Sacrococcygeal joint

The sacrococcygeal joint (S5/C1) is an amphiarthrodial joint that lies between the sacral apex and the base of the coccyx. The disc in this joint is much thinner owing to the reduction in pressure as the majority of the pressure at this point has been distributed superior to the S5/C1 joint, across the appendages.

Sacroiliac joint

The sacroiliac joint (SIJ) is a synovial joint occurring between the sacral and iliac articular surfaces. There is a lot of debate about the range of movement that occurs at this joint, with research suggesting it could be between 2 and 18 degrees. One firm belief is that the movement that does occur is a secondary movement, important for the mechanical movements of the hip.

The SIJ allows for simultaneous rotational and translational movement associated with walking gait. The slight movements of the SIJ relieve pressure on the hips, as the natural swinging of the hips that occurs during walking would increase the risk of pelvic fracture.

There is a delicate balance between the right and left SIJs, because the origin point of the axis of motion is precisely in the middle of the posterior iliac spines. A dysfunctional posture can cause the axis of motion to become displaced, which could compromise the soft tissue holding the two articular surfaces in place. As a result, SIJ pain is relatively common.

Bony landmarks

At the base of the sacrum is located the first of the five fused sacral vertebrae. The S1 vertebra is the biggest in the sacrum, whose concave superior articular facets project posteromedially to communicate with L5. Although it is fused it still possesses transverse processes and pedicles.

Pelvic surface

The pelvic surface, located inside the curve of the sacrum contains four pairs of foramina. These foramina allow the anterior rami of the first four sacral spinal nerves to pass through.

Dorsal surface

The midline of the dorsal surface of the sacrum bears a projection of bone known as the median sacral crest. This crest is palpable and is formed by the fusion of sacral spines of S1 to S4. The landmark of the sacral hiatus, which is caused by the lamina of S5 failing to meet in the median plane resulting in an exposed dorsal surface, is located just below the S4 tubercle. Just lateral to this are the four pairs of sacral foramina.

Lateral surface

The sacral lateral surface consists of the fusion of the transverse processes which narrows as it travels inferiorly. There is an auricular surface on the lateral part of the sacrum that articulates with the ilium. Just posterior to the auricular surface is a rough prominence that contains three depressions intended for the attachment of strong ligaments.

Sacral canal

The triangular sacral canal is formed by the sacral vertebral foramina, and descends from the opening on the basal surface to the sacral hiatus. Since the spinal cord finishes at around L2 the canal does not carry the spinal cord. It instead carries the filum terminale, long threadlike bands of connective tissue and the cauda equina, long sacral roots of spinal nerves.

Apex

Finally, the apex of the sacrum is the inferior segment. It is the fifth sacral vertebral body and has an oval shaped facet for articulation with the coccyx. The apex projects posteriorly to increase the size of the pelvic cavity.

Photograph of the human sacrum with labeled bony landmarks of its dorsal surface.

Test yourself on the sacrum and coccyx in this quiz.

Ligaments of the sacrum

A strong ligamentous architecture enables mechanical stabilization of the sacrococcygeal and sacroiliac joints.
The sacrococcygeal joint is strengthened by the anterior and posterior sacrococcygeal ligaments. The superficial posterior sacrococcygeal ligament arises from the margin of the sacral hiatus and attaches to the dorsum of the coccyx. The deep posterior sacrococcygeal ligament extends from the dorsal surface of the fifth sacral vertebral body to the dorsal surface of the coccyx. Bilaterally, the lateral posterior sacrococcygeal ligament spans from the inferolateral angles of the sacrum to the transverse processes of the coccyx.
The weight-bearing sacroiliac joint complex is supported and secured by three ligaments, namely the anterior, interosseous and posterior sacroiliac ligaments. The interosseous sacroiliac ligament constitutes the major bond between the ilium and sacrum, filling the gaps between these two bones at the posterosuperior aspect of the joint. The posterior sacroiliac ligament lies superficial to the interosseous sacroiliac ligament and consists of several fibers connecting the posterior superior iliac spine (PSIS), iliac crest as well as the lateral and intermediate sacral crests.

Muscle attachments

The piriformis muscle, attaching onto the pelvic surface (S2-S4), is one of the major muscles on the sacrum. The iliacus and coccygeus also attach to the pelvic surface superolaterally and inferolaterally respectively. The erector spinae aponeurosis and multifidus cover the dorsal surface while the gluteus maximus attaches to the lateral borders of the sacrum.

Clinical aspects

Spondylolisthesis is a slippage of one vertebra on another, usually L5 on S1 as a result of the extreme pressure upon that joint. There are five different causes of the slippage, these being:

Dysplastic spondylolisthesis: congenital defect.
Isthmic spondylolisthesis: a fracture of the isthmus (or pars interarticularis), reducing the strength of the facet joint in the sagittal plane causing the anteriorization of the vertebrae.
Degenerative spondylolisthesis: due to degeneration of the discs and the facet joints.
Traumatic spondylolisthesis: caused by acute fractures in the neural arch. Separate to fractures in the isthmus.
Pathologic spondylolisthesis: bone disease weakening the bone. Possibly due to metastatic disease, osteoporosis or tumours.

Along with slippage, there are other more typical dysfunctions that occur around the sacrum due to the load placed on it, such as disc herniation and degeneration, facet joint pain, and sciatica due to L5 nerve root (part of long sciatic nerve) impingement.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

References:

Sandstring, S. (ed.) (2008) Grey’s Anatomy, 40th edition, London: Churchill Livingstone, Chapter 24 The Back, page 724.
Anon. Facet Joint Injections. The Pain clinic. (accessed 5th of October 2014).
Goode A,Hegedus E, Sizer P, Brismee J-M, Linberg A, Cook C:Three-Dimensional Movements of the Sacroiliac Joint: A Systematic Review of the Literature and Assessment of Clinical Utility. J Man Manip Ther. 2008; 16(1): 25–38. (accessed 5th of October 2014)
Vleeming A, Schuenke MD, Masi AT, Carreiro JE, Danneels L, Willard FH (2012) The sacroiliac joint: an overview of its anatomy, function and potential clinical implications (page537-68)
Sandstring, S. (ed.) (2008) Grey’s Anatomy, 40th edition, London: Churchill Livingstone, Chapter 80, Pelvic Girdle, Gluteal Region and Thigh, (page 1366)
Sandstring, S. (ed.) (2008) Grey’s Anatomy, 40th edition, London: Churchill Livingstone, Chapter 24 The Back, (page 725)
Hyde, T: Spondylolysis and spondylolisthesis. Spine Health. (accessed 5th of October 2014).

Illustrators:

Sacrum (cadaveric dissection) - Prof. Carlos Suárez-Quian

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