What Is the L5-S1, aka the Lumbosacral Joint?

L5-S1 is a term used to describe a part of the spine that is composed of the last bone in the low back, called L5, and the triangularly shaped bone beneath, known as the sacrum. The sacrum is made of five fused bones of which the S1 is the topmost.

In fact, L5-S1 is the exact spot where the ​lumbar spine ends and the sacral spine begins. L5-S1 is the joint that connects these bones; it is also called the lumbosacral joint.

1
Structure of the Lumbar Spine and Sacrum

Coccyx - Tailbone
A.D.A.M.

The spinal column is the structure of the body that allows us to stand upright, as well as to twist, bend and otherwise alter trunk and/or neck positioning. 

There's a total of thirty three bones in the spine, but only the upper twenty four are fully movable.

The vertebrae,which is another name for spinal bones, are broken down into sections from the top to tail, as follows:

  • Cervical spine, or neck, with seven bones. These bones are labeled as C1 to C7
  • Thoracic spine, or mid-back, with twelve bones. The thoracic spine vertebrae are labeled T1 to T12.
  • Lumbar spine, which corresponds to your low back, with five bones, labeled L1 to L5.
     
  • The sacrum is one triangularly shaped bone formed of five fused bones. These five individual bones start to fuse soon after birth and continue to do so until they are completely fused by around the age of 30.

    The sacrum is usually called just that — the sacrum. But when identifying the individual fused bones that comprise the sacrum, the labeling is S1 to S5.
     
  • Like the sacrum, the coccyx, which is your tailbone, is made of individual bones, in this case, four of them, that are movable at birth but over time fuse. The coccyx is at least semi-fused, and in many cases fully fused, by adulthood.

    The coccyx as a whole is called by either name, i.e. coccyx or tailbone, but when referring to its four individual component bones, the labeling is Co1 to Co4.

2
Why the L5-S1 Is Important

A skeleton of the pelvis, sacrum, lumbar spine, hip joints and femur bones.
Sciencepics/Getty Images

Each area of the spine has a curve, and these curves go into opposing directions.

In the neck and lower back, the spinal curve sweeps forward as viewed from profile, while the thoracic and sacral curves go back.

The areas where the spinal curve directions change are called junctions. Injury risk may be higher at junctions because your body weight shifts direction as the curves shift directions.

The L5-S1 junction, located between the lumbar curve (which sweeps forward) to the sacral curve (which opposes the direction of the lumbar curve and goes backward) is particularly vulnerable to misalignment, wear and tear and injury.

This is because the top of the sacrum is positioned at an angle in most people. (Read more about this in the following section.) Aging and injury may increase the vulnerability of the L5-S1 junction even more.

Because of this, L5-S1 is one of the two most common sites for back surgery. The other is the area just above, called L4-L5.

3
The L5-S1 and Spondylolisthesis

Facet joint, pars and intervertebral joint
Dorling Kindersley/Getty Images

In the low back, the L5-S1 junction is often the site of an injury known as spondylolisthesis. Spondylolisthesis occurs when a vertebra slips forward relative to the bone immediately beneath it.

The most common variety of this condition is called isthmic spondylolisthesis. Isthmic spondylolisthesis starts as a tiny fracture in the pars interarticularis, which is an area of bone in the back that connects the adjoining parts of the facet joint.

While these types of fractures tend to occur before the age of seven, symptoms typically do not develop until adulthood. Degeneration of the spine in later adulthood can further exacerbate the condition.

The angle of the sacrum may contribute to spondylolisthesis. This is because, rather than being horizontal to the ground, the S1 tips down in the front and up in the back. By and large, individuals with a greater tilt will run a higher risk of spondylolisthesis.

Spondylolisthesis is typically treated with non-surgical interventions such as pain medications, heat and/or ice application, physical therapy, or epidural steroid injections. Spinal fusion surgery can be effective for treating spondylolisthesis, but it requires a lot of recovery time. Usually non-surgical care is tried for at least six months, but if you haven't gotten relief by then, surgery may be an option.

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