Where it is and and what it does

By Anne Asher, CPT 

 Medically reviewed by Oluseun Olufade, MD

The lumbosacral joint, also called L5-S1, is a term used to describe a part of the spine. L5-S1 is the exact spot where the lumbar spine ends and the sacral spine begins. The lumbosacral joint is the joint that connects these bones.

L5-S1 is composed of the last bone in the low back, called L5, and the triangle-shaped bone beneath, known as the sacrum. The sacrum is made of five fused bones, of which the S1 is the topmost.

This article goes over the anatomy and function of the L5-S1. It also covers some of the medical problems that can affect this part of the spine.

Anatomy

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Science Picture Co / Getty Images

The spinal column is the structure that allows you to stand upright. It also helps you twist, bend, and otherwise alter your trunk and neck position.

There are typically 24 movable bones in the spine that connect to the sacrum, a bony structure located below the lumbar vertebrae, and the coccyx, also called the tailbone. The sacrum and the coccyx each consist of multiple bones that fuse over time.

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

  • Cervical spine: Located in the neck, it has seven bones, labeled as C1 to C7

  • Thoracic spine: Located in the mid-back, it has 12 bones. The thoracic spine vertebrae are labeled T1 to T12.

  • Lumbar spine: Corresponding to your low back, it has five bones. These bones are labeled L1 to L5.

  • Sacrum: This triangularly shaped bone is formed of five bones. These bones start to fuse soon after birth and continue to do so until they are completely fused by around the age of 30. The individual fused bones are labeled S1 to S5.

  • Coccyx: The tailbone is likewise made of individual bones that are movable at birth but fuse over time. The coccyx is semi-fused, and in many cases fully fused, by adulthood. The component bones are labeled as Co1 to Co4. Most people have four segments, but some have three or five.

Function

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Jan-Otto / Getty Images

Each area of the spine has a curve, and these curves go in opposing directions. In the neck and lower back, the spinal curve points forward, as viewed in profile. The thoracic and sacral curves go back.

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

The L5-S1 junction is located between the lumbar curve and the sacral curve. The lumbar curve sweeps forward. The sacral curve opposes the direction of the lumbar curve and goes backward.

The L5-S1 junction 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. Aging and injury may increase the vulnerability of the L5-S1 junction even more.

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

Spondylolisthesis

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1Photodiva / Getty Images

In the low back, the L5-S1 junction is often the site of a problem 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 degenerative spondylolisthesis. It generally occurs when the spine starts to wear down with age. 

Isthmic spondylolisthesis is another common variant. Isthmic spondylolisthesis starts as a tiny fracture in the pars interarticularis. This is an area of bone in the back that connects the adjoining parts of the facet joint.1

While these types of fractures tend to occur before the age of 15, symptoms often do not develop until adulthood. Degeneration of the spine in later adulthood can further worsen the condition.

The angle of the sacrum may contribute to spondylolisthesis. This is because the S1 tips down in the front and up in the back rather than being horizontal to the ground. Individuals with a greater tilt will usually have a higher risk of spondylolisthesis.2

Spondylolisthesis is typically treated with non-surgical interventions. These could include:

  • Pain medications

  • Heat and/or ice application

  • Physical therapy

  • Epidural steroid injections

Spinal fusion surgery can be effective for treating symptoms related to spondylolisthesis. However, it requires a lot of recovery time and can have additional risks. Usually, non-surgical care is tried for at least six months. If you haven't gotten relief by then, surgery may be an option.

Summary

The L5-S1 is also called the lumbosacral joint. It is the part of the spine where the lumbar spine ends and the sacral spine begins. It helps you twist, bend, and stand upright.

Because of its location, the L5-S1 is vulnerable to wear and tear and injury. One of the more common problems with the L5-S1 is spondylolisthesis, which occurs when a vertebra slips forward. This condition is usually treated non-surgically.

Sources

1.American Academy of Orthopaedic Surgeons. Spondylolysis and spondylolisthesis.

2.Gong S, Hou Q, Chu Y, Huang X, Yang W, Wang Z. Anatomical factors and pathological parts of isthmic fissure and degenerative lumbar spondylolisthesis. Chronic Dis Prev Rev. 2019;9:1-6

3.American Association of Neurological Surgeons. Herniate disc.

4.Bertilson BC, Heidermakr A, Stockhaus M. Irritable bowel syndrome–a neurological spine problem. JAMMR. 2015;4(24):4154–68. doi:10.9734/BJMMR/2014/9746

5.Akca N, Ozdemir B, Kanat A, Batcik OE, Yazar U, Zorba OU. Describing a new syndrome in L5-S1 disc herniation: Sexual and sphincter dysfunction without pain and muscle weakness. J Craniovertebr Junction Spine. 2014;5(4):146–50. doi:10.4103/0974-8237.147076

By Anne Asher, CPT
Anne Asher, ACE-certified personal trainer, health coach, and orthopedic exercise specialist, is a back and neck pain expert.