The Anatomy of the Sacrum

A fusion of vertebrae that helps humans stand and walk

In This Article
Table of Contents

The sacrum is a single bone comprised of five separate vertebrae that fuse during adulthood. It forms the foundation of the lower back and the pelvis.

The sacrum is a concave sphenoid bone that sits at the bottom of the spinal column. It looks like an inverted triangle: the widest part (base) is at the top, and the pointy end (apex) is at the bottom.

The sides of the sacrum connect to the right and left hip (iliac) bones. The apex is connected to the tailbone (coccyx). The base is connected to the largest and lowest of the lumbar vertebrae, L5.

Anatomy

The human sacrum is a robust bone that can endure a lot of pressure and motion. It serves as an anchor point that holds the spinal column together with the pelvis. The sacrum and coccyx provide a stable platform for humans to sit upright.

Humans need larger sacrums than other mammals because we walk upright and need extra stability for balance and mobility. The size and orientation of the sacrum also affect the human birthing process.

Humans are born with four to six sacral vertebrae rather than a single bone. Fusion does not occur in all sacral vertebrae at the same time: it starts with the fusion of S1 and S2.

As a person gets older, the overall shape of the sacrum solidifies, and the sacral vertebrae fuse into a single structure. The process typically begins in the mid-teens and finishes sometime in the early to mid-twenties and is thought to start earlier in females than males.

The timing of the fusion of the sacral vertebrae can be a useful tool for estimating the age and sex of skeletal remains. For example, the female sacrum is wider, shorter, and has a more curved (concave) top, called the pelvic inlet. The male sacrum is longer, more narrow, and flatter than the female sacrum.

Our understanding of what the sacrum does is still evolving. In humans, one of the main functions of the sacrum is to support upper body weight when we are sitting or standing. However, it doesn't perform this function in mammals that walk on four legs (quadripedal).

There is also variability in the human sacrum, though it is not well understood. For example, the number of bones that make up the sacrum and the progression of the fusion process can vary from one person to the next.

Structure

The sacrum is an irregular (sphenoid) bone that makes up the back (posterior) third of the pelvic girdle. A ridge across the front (anterior) portion of the S1 vertebra is called the sacral promontory.

There are small holes (foramen) along both sides of the sacrum that are left over when individual vertebrae fuse together. Depending on how many sacral vertebrae there are, there can be three to five sacral foramen on each side (though there are usually four).

Each anterior foramen is usually wider than the corresponding posterior or dorsal (on the backside) foramen. Each sacral foramina (plural of foramen) is a channel for the sacral nerves and blood vessels.

There are small ridges that develop between each of the fused sacral vertebrae called transverse ridges or transverse lines.

Along the dorsal midline of the sacrum is the median sacral crest, a ridge formed from the spinous processes of the sacral vertebrae.

The sacral canal is a hollow space that runs from the top (base) of the sacrum to the bottom (apex). The sacral canal serves as a channel at the end of the spinal cord.

The sacrum connects (articulates) to the iliac bone on either side at an attachment point called the auricular surface.

Just behind the auricular surface is a rough area called the sacral tuberosity, which serves as an attachment area (insertion point) for the complex web of ligaments that holds the pelvic girdle together.

The lowest part (inferior aspect) of the sacrum is the narrowest point, known as the apex. The apex of the sacrum is connected to the tailbone (coccyx).

Location

The sacrum is at the level of the lower back, just above the intergluteal cleft (more commonly known as the crack of the butt). The cleft starts at about the level of the tailbone or coccyx.

The sacrum is curved forward (concaved) and ends (terminates) at the coccyx. The curvature is more pronounced in females than in males.

The base of the sacrum is the widest part. Even though it's called the base, it is actually at the top (superior aspect) of the sacrum rather than the bottom.

Here, it connects to the L5 lumbar vertebra via the lumbosacral joint. The disc that is between these two lumbar vertebrae is a common source of lower back pain.

On either side of the lumbosacral joint are winglike structures (sacral ala) which connect to the iliac bones and form the top of the sacroiliac (SI) joint.

Attached to either side of the sacrum are the iliac bones. These wings of the pelvis provide stability and strength for walking and standing.

Anatomical Variations of the Sacrum

The most common anatomical variation of the sacrum applies to the number of sacral vertebrae. While the most common is five, anomalies documented in humans have included having four or six sacral vertebrae.

Other variations are related to the sacrum's surface and curvature. The curvature of the sacrum varies widely between individuals. In some cases, the first and second sacral vertebrae do not fuse and instead remain separately articulated. 

Failure of the vertebral canal to completely close during formation is a condition known as spina bifida, which may arise from the sacral canal.

Function

The human sacrum provides a strong foundation for the formation of the pelvis. Since humans walk on two legs (bipedal), the body needs a stable point where the muscles of the legs and core can attach.

The human pelvis also needs to be wide enough to provide leverage for motion and balance, as well as to facilitate childbirth. The human body can move and give birth to offspring because the sacrum articulates with surrounding bones and gives the pelvic girdle flexibility.

If the entire pelvis were fused and rigid, the nuances of motion needed for balance would be much harder and take significantly more energy. By contrast, the swaying seen when other primates walk upright is an example of the energy cost associated with a smaller and less flexible pelvis.

The sacrum creates an anchor point where the spinal column can attach to the pelvis and provide stability for the body's core. It also acts as a platform for the spinal column to rest on when sitting.

Associated Conditions

The sacrum is often implicated as a focal point for lower back pain. Forces applied to the sacrum and SI joint (which connects the sacrum and iliac bones) may account for as much as 27% of all lower back pain complaints.

One of the most common is sacroiliitis, which is inflammation of the SI joint. This is a diagnosis of exclusion, which means a doctor only makes the diagnosis when all other possible causes of pain have been ruled out.

Chordoma is a type of primary bone cancer. About half of all chordomas form in the sacrum, but the tumors can also develop elsewhere in the vertebral column or at the base of the skull.

People can also be born with conditions affecting the sacrum. For example, spina bifida is a congenital condition that can arise from the malformation of the sacral canal.

Was this page helpful?

Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Gruss LT, Schmitt D. The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation. Philos Trans R Soc Lond, B, Biol Sci. 2015;370(1663):20140063. doi:10.1098/rstb.2014.0063

  2. Sheng S-R, Wang X-Y, Xu H-Z, Zhu G-Q, Zhou Y-F. Anatomy of large animal spines and its comparison to the human spine: a systematic reviewEur Spine J. 2010;19(1):46-56. doi:10.1007/s00586-009-1192-5

  3. Stranding S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice, International Edition. 41st ed. (Anand N, ed.). Elsevier Health Sciences; 2015:729.

  4. Bergman RA, Afif AK, Miyauchi R. Opus V: Skeletal System: Sacrum And Coccyx. Anatomy Atlases: Illustrated Encyclopedia Of Human Anatomic Variation. Published January 1, 2019.

  5. Mahato NK. Morphological traits in sacra associated with complete and partial lumbarization of first sacral segment. Spine J. 2010;10(10):910-5. doi:10.1016/j.spinee.2010.07.392

  6. Wong M, Kiel J. Anatomy, Abdomen and Pelvis, Sacroiliac Joint. [Updated 2019 Feb 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan.

  7. U.S. Department of Health & Human Services, National Institutes of Health National Library of Medicine, Lister Hill National Center for Biomedical Communications. Chordoma. Genetics Home Reference. Updated May 2015.

Additional Reading