What Does the Sacrum Do?

A fusion of vertebrae that helps humans stand and walk

Medically reviewed by Michael Menna, DO

The sacrum is a single bone comprised of five separate vertebrae. It is shaped like an upside-down triangle and sits at the bottom of the spinal column, connecting it to the pelvis.

This robust bone can endure a lot of pressure and motion. Along with the coccyx (tailbone), the sacrum provides a stable platform to help you sit upright.

This article discusses the anatomy and function of the sacrum bone. It also looks at some health conditions related to the sacrum bone.

Formation of the Sacrum

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. It 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 sacrum in a female is wider, shorter, and has a more curved top, called the pelvic inlet. The male sacrum is longer, more narrow, and flatter than the female sacrum.

There is other 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.

Sacrum Anatomy and Structure

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

There are small holes called 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.

Related: The Intervertebral Foramen

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. This is a ridge formed from the spinous processes of the sacral vertebrae.

The top of the sacrum is called the base. It is connected to the largest and lowest of the lumbar vertebrae, L5.

The bottom, which is connected to the tailbone (coccyx), is referred to as the apex. The sacral canal is a hollow space that runs from the base to the apex. The sacral canal serves as a channel at the end of the spinal cord.

The sides of the sacrum connect to the right and left hip (iliac) bones. The attachment point is called the auricular surface.

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

Explainer

The sacrum is not the same as the tailbone. The bottom of the sacrum connects to the tailbone and is called the apex.

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 and ends at the coccyx. The curvature is more pronounced in females than in males.

Takeaway

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 called sacral ala, which connect to the iliac bones and form the top of the sacroiliac 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.

What the Sacrum Does

Understanding of what the sacrum does is still evolving, but here are some of its proven functions:

• Serves as an anchor point where the spinal column can attach to the pelvis and provide stability for the body's core

• Acts as a platform for the spinal column to rest on when sitting

• Facilitates childbirth: The human body can move and give birth to offspring because the sacrum connects with surrounding bones and gives the pelvic girdle flexibility.

The sacrum supports upper body weight when you are sitting or standing. Humans need a larger sacrum than other mammals because we walk upright and need extra stability for balance and mobility.

Explainer

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.

Associated Conditions

The sacrum is often implicated as a focal point for lower back pain. It is estimated that 28.0% of men and 31.6% of women aged 18 years or older had lower back pain in the past 3 months. There are several reasons why the sacrum might hurt.

• Sacroiliitis: This common condition is inflammation of the sacroiliac (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. Sacroiliac joint dysfunction is thought to account for between 15% and 30% of all lower back pain complaints.

• Chordoma: This 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.

• Spina bifida: 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.

Related: An Overview of Spina Bifida

Summary

The sacrum is a single bone located at the base of your spine. It consists of five separate vertebrae that fuse during adulthood.

The sacrum helps support your upper body when you sit or stand. It also gives the pelvic girdle flexibility during childbirth.

Problems with the sacrum account for a significant portion of lower back problems. One of the most common of these is sacroiliitis.

Read the original article on Verywell Health.