Sacral bone: What is it? Structure Function and Anatomy

It is a large, triangular bone located at the base of your spine.

It usually consists of five initially separate vertebrae that begin to fuse at the age of 16-18 years, and are usually fully fused by the mid-30s.

The sacrum sits like a wedge between the two hip bones (ilium), forming the sacroiliac joint (SI joint). Under the sacrum is the coccyx.

The sacrum and coccyx are two anatomical structures located near the lower part of the spine, below the fifth lumbar vertebra (L5).

The sacrum, sometimes called the sacral column, is a large, flat, triangular bone.

Structure of the sacrum

The sacrum is formed by the fusion of five sacral vertebrae that has three surfaces, a base and an apex.

The body of the first segment is large and is similar to the lumbar vertebra, while the bodies of the next bones become progressively smaller, flattened from the back and curved to give them shape.

The sacrum articulates with four other iliac bones on each side. It is inclined forward and curved with anterior concavity and posterior convexity that allows more space for the pelvic cavity.

The curvature of the sacrum varies in individuals.


Individually, the sacrum and coccyx are made up of smaller bones that fuse (grow into a solid bone mass).

The sacrum consists of 5 fused vertebrae and 3 to 5 small bones fuse together to create the coccyx. Both structures support weight and integrate functions such as walking, standing and sitting.


The sacrum is located 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 column .

The lower part of the back (lumbar spine) with the sacrum (sacral column) helps form the lumbosacral curve, which is essential for supporting the upper body, supporting weight, maintaining balance and functional flexibility.

The lumbosacral curve is both lordotic and kyphotic, and is one of the four natural spinal curves.

The location of the sacrum, at the intersection of the spine and the pelvis, means that it plays an especially important role in both the lower back and the hip. The joints of the sacrum support the weight and help to stabilize this region of the spine.

Like other spinal levels, ligaments, tendons and muscles help support and stabilize joint movement.

Lumbosacral joint:

This joint occurs in L5 and S1, essentially connecting the lumbar spine with the sacrum.

There is a lot of pressure at this meeting point, since the curve of the spine changes in L5-S1 of lordotic (lumbar lordosis, curve forward) to kyphosis (kyphosis sacral, backward curve).

Level L5-S1 supports the weight and absorbs and distributes the weight of the upper body at rest and movement. This is one reason why disc herniation and spondylolisthesis are more common in L5-S1.

Sacroiliac joints:

These joints connect the sacrum to the left and right sides of the pelvis. Unlike other joints in the body (eg, knees), the lapse of movement of any of these joints is minimal.

These joints are essential for walking, standing and for hip stability. Sacroiliitis and dysfunction of the sacroiliac joint are two vertebral disorders related to the sacroiliac joints.

The sacroiliac joints connect the sacrum to the left and right sides of the pelvis.

The coccyx is just below the sacrum, although it is much smaller, it also has an important weight-bearing role. The coccyx helps support your weight while you sit. If you lean back while sitting, as in a reclining seat, the pressure on your coccyx increases.

An injury in this region can cause coccyx pain, which is known as coccydynia.

The coccygeia is often characterized by inflammation of the connective tissue of the coccyx, which produces a coccyx pain that worsens when sitting. The spinal fracture that can occur from a traumatic event, such as a fall, can also cause this pain.

Talk to your doctor

The sacrum and coccyx play important roles that help support and stabilize the spine and are important for walking, standing and sitting.

If you have pain in your lower back, buttocks and hips, talk to your doctor about whether your sacrum or coccyx may be the source of your symptoms.