The Sacrum and Coccyx

The sacrum and coccyx are unlike other bones in your vertebral spinal column and may be involved in the cause of your low back pain.

The sacrum and coccyx are two anatomical structures located near the bottom of your vertebral spinal column, below the fifth lumbar vertebra (L5). The sacrum, sometimes called the sacral spine (abbreviated S1), is a large, flat triangular-shaped bone located below L5 and in between your hip bones. Below the sacrum is the coccyx, commonly known as the tailbone.
Sacrum and SI joints labeledThe sacrum and coccyx are weight-bearing spinal structures.Individually, the sacrum and coccyx are composed of smaller bones that fuse (grow into a solid bone mass) together by age 30. The sacrum is made up of 5 fused vertebrae, and 3 to 5 small bones fuse to create the coccyx. Both structures are weight-bearing and integral to functions such as walking, standing and sitting.

A Closer Look at the Sacrum
The sacrum is located in between the right and left iliac (or hip) bones, and forms the back of the pelvis. The sacrum is where your sacral spine connects to your pelvis. The point or spinal level where L5 meets S1 is called the lumbosacral spine.

The low back (lumbar spine) with the sacrum (sacral spine) help form the lumbosacral curve, which is integral to supporting the upper body, weight-bearing, maintaining balance and functional flexibility. The lumbosacral curve is both lordotic and kyphotic, and is one of the four natural spinal curves.

The sacrum’s location—at the intersection of the spine and pelvis—means that it plays an especially important role to both your low back and hips. The sacrum’s joints are weight-bearing and help to stabilize this region of the spinal column. Like other spinal levels, ligaments, tendons and muscle help support and stabilize joint movement.

Lumbosacral joint: This joint occurs at L5 and S1—it essentially connects the lumbar spine to the sacrum.

There is a great amount of pressure at this meeting point, as the curve of your spine shifts at L5-S1 from lordotic (lumbar lordosis, forward curve) to kyphotic (sacral kyphosis, backward curve). The L5-S1 level is weight-bearing and absorbs and distributes the upper body’s weight at rest and movement. This is one reason why disc herniation and spondylolisthesis are more common at L5-S1.

Sacroiliac (SI) joints: The SI joints connect the sacrum to the left and right sides of the pelvis. Unlike other joints in the body (eg, knees), the span of movement of either SI joint is minimal. These joints are essential to walking, standing, and hip stability. Sacroiliitis and SI joint dysfunction are two spinal disorders related to the sacroiliac joints.
sacral spine superimposed on skeletal blue bodyThe sacroiliac joints connect the sacrum to the left and right sides of the pelvis. The tailbone, or coccyx is below the sacrum.A Closer Look at the Coccyx
The coccyx, or tailbone, is located just below the sacrum. Though it’s much smaller than the sacrum, it too has an important weight-bearing role. The coccyx helps support your weight while you sit. If you lean back while sitting, such as in a recliner seat, pressure on your coccyx increases.

An injury in this region can cause tailbone pain, which is known as coccydynia. Coccydynia is often characterized by inflammation of the coccyx’s connective tissue resulting in tailbone pain that worsens when sitting. Tailbone fracture that may occur from a traumatic event, such as a fall, can also cause this pain.

Talk With Your Doctor
The sacrum and coccyx both play significant roles helping to support and stabilize your vertebral spinal column, and are important to walking, standing and sitting. If you have pain in your low back, buttocks, and hips, talk with your doctor about whether your sacrum or tailbone may be the source of your symptoms.

Updated on: 02/14/18
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Spinal Muscles: A Comprehensive Guide

Muscles are named according to their shape, location, or a combination. They are further categorized according function such as flexion, extension, or rotation.
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