The Anatomy of the Sacrum

A fusion of vertebrae that helps humans stand and walk

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The sacrum is a single bone made from (usually) five separate vertebrae fused together during adulthood. It is a concaved sphenoid bone sitting at the bottom of the spinal column and looks like an inverted triangle with the base (wide portion) at the top and the apex (the pointy end) at the bottom.

The sides of the sacrum are connected to the right and left iliac bones. The apex is connected to the coccyx (tailbone) and the base is connected to the largest and lowest of the lumbar vertebrae, L5. The sacrum is the foundation of the lower back and the pelvis.

Anatomy

The human sacrum is a robust bone that can handle quite a bit of pressure and motion. It is the anchor point that holds the spinal column together with the pelvis. The sacrum, along with the coccyx, also provides a stable platform for humans to sit upright.

The sacrum is larger and more robust in humans than in other mammals because we walk upright and need extra stability for balance and mobility. The size and orientation of the sacrum also impact the birthing process.

Humans are born with four to six sacral vertebrae rather than a single bone. As people mature, the overall shape of the sacrum solidifies and the sacral vertebrae fuse together into a single structure. This is documented to happen beginning in the mid-teens and completing sometime in the early to mid-twenties.

Fusion does not happen between all sacral vertebrae at the same time. It starts with the fusion of S1 and S2.

This process is thought to begin earlier in females than in males. As we learn more about the timing of the fusion of the sacral vertebrae, it becomes a useful tool with which to estimate the age and sex of skeletal remains.

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

Much of our understanding of the function of the sacrum is still evolving. While the sacrum supports the majority of the weight of the upper body in humans when sitting or standing, this is not the same in mammals that walk on four legs. Other primates have similarities to both humans and quadripedal mammals.

All of this variability is still not well understood. Even in humans, there is relatively common variability in the number of vertebrae that make up the sacrum and in the process of fusion.

Structure

The sacrum is a sphenoid (irregular) bone that makes up the posterior third of the pelvic girdle.

There is a ridge across the anterior (front) portion of the S1 vertebra that is known as the sacral promontory. Along both sides of the sacrum are small holes that are left over after the individual vertebrae have fused together. In most cases, there are four on each side. Depending on the number of sacral vertebrae, there can be three to five on each side. These are known as the sacral foramen.

Each anterior foramen is usually wider than the corresponding posterior or dorsal (back side) foramen. Each of the sacral foramina (plural of foramen) channels 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 a ridge formed from the spinous processes of the sacral vertebrae called the median sacral crest.

There is a hollow space that runs from the top (base) of the sacrum to the bottom (apex) and is known as the sacral canal. The sacral canal provides a channel for the very end of the spinal cord.

The auricular surface is the attachment point that articulates with the iliac bone on either side of the sacrum. Just behind the auricular surface is the sacral tuberosity, a rough area serving as an insertion point (attachment area) for ligaments. There is a complex web of ligaments that binds the entire pelvic girdle together.

At the lowest part (inferior aspect) of the sacrum is the most narrow point known as the apex. It is connected to the coccyx (tailbone).

Location

The sacrum is at the level of the lower back, just above the intergluteal cleft (butt crack), which starts at about the level of the coccyx. The sacrum is curved forward (concaved) and terminates at the coccyx. The curvature is more pronounced in females than in males.

The base of the sacrum is the widest part and is connected to the L5 lumbar vertebra via the lumbosacral joint. Even though it's called the base, it is actually at the top (superior aspect) of the sacrum rather than at the bottom. The encapsulated disc between these two vertebrae is a common cause of lower back pain.

On either side of the lumbosacral joint are winglike structures known as the sacral ala, which connect with the iliac bones and form the top of the SI joint. Attached to either side of the sacrum are the iliac bones. These are the wings of the pelvis that provide stability and strength for walking and standing.

Anatomical Variations of the Sacrum

The most common anatomical variation of the sacrum has to do with the number of sacral vertebrae. There are documented anomalies between the number of sacral vertebrae. The most common is five, however, there are examples of four and six sacral vertebrae among humans.

Other variations have to do with surfaces and curvature. The curvature of the sacrum varies widely between individuals.

In some cases, the first and second sacral vertebrae do not fuse and could remain separately articulated.

Failure of the vertebral canal to completely close during formation is a condition known as spina bifida. One of the most common failure points for this congenital defect is the sacral canal.

Function

In humans, the sacrum provides stability and a foundation for the formation of the pelvis. Because we are bipedal, humans need a strong point for muscles of the legs and core to attach. The pelvis needs to be wide enough to provide leverage for motion and balance, as well as to facilitate birthing.

The sacrum articulates with the surrounding bones and allows for motion and birthing by providing flexibility in the pelvic girdle.

If the entire pelvis was fused and rigid, nuances of motion needed for balance would be much harder and take significantly more energy. Indeed, 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 for the spinal column to attach to the pelvis, which provides stability for the body core. It also acts as a platform to rest the spinal column when sitting.

Associated Conditions

The sacrum is often implicated as a focal point for lower back pain. Forces applied to the sacrum and to the sacroiliac (SI) joint that 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 an inflammation of the SI joint, known as sacroiliitis. This is a diagnosis of exclusion, which means it is the default diagnosis when nothing else can be found to be causing pain in the region.

Spina bifida is a congenital condition that can be caused by a malformation of the sacral canal.

Chordoma is a primary bone cancer that could appear in the sacrum. About half of all chordomas develop there. Chordoma tumors can also form in other parts of the vertebral column or at the base of the skull.

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