Where is sacrum

The areas of the sacrum and coccyx are the focus for this article on the spinal anatomy. In our series on the anatomy of the spine we started at the top, focusing on the cervical , thoracic and then lumbar region of the spine. Directly below the last lumbar vertebrae, L5, is the sacrum and coccyx.

The pelvic cavit y is the area of the body inside the pelvic bones. The reproductive organs, urinary bladder, colon and rectum all reside within the pelvic cavity. The sacral region of the spine is composed of 5 vertebrae in youth, S1-S5. Somewhere around the age of these five vertebrae fuse together to form one bone, referred to as the sacrum. While other mammals may also have a sacrum, they are different than those of humans. For example, the sacrum of a dog is formed from the fusion of three sacral vertebrae, not five as in humans.

You may have heard of the sacroiliac joint SI joint as it can often be the cause of lower back pain. There are several strong ligaments that keep the bones together in the SI joint. This relaxation over time may allow for excess movement in the SI joint. This is thought to be the source of the lower back pain some women experience during their menstrual cycle and childbirth. Many nerves of the cauda equina at the inferior end of the spinal cord pass through the sacrum.

These nerves enter the sacrum from the vertebral foramen of the lumbar vertebrae through the tunnel-like sacral canal. From the sacral canal these nerves branch out and exit the sacrum through four pairs of holes on the sides of the canal called the sacral foramina or through the sacral hiatus at the inferior end of the canal. The sacrum serves several important functions in the skeletal, muscular, nervous, and female reproductive systems. Acting as the keystone of the pelvis, the sacrum locks the hip bones together on the posterior side and supports the base of the spinal column as it intersects with the pelvis.

Several key muscles of the hip joint, including the gluteus maximus, iliacus and piriformis, have their origins on the surface of the sacrum and pull on the sacrum to move the leg.

The sacrum also surrounds and protects the spinal nerves of the lower back as they wind their way inferiorly toward the end of the trunk and into the legs. 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. 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.

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. The sacrum is often implicated as a focal point for lower back pain.

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.

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