Overview of the Lumbosacral Joint (L5-S1)

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 triangularly shaped bone beneath, known as the sacrum. The sacrum is made of five fused bones of which the S1 is the topmost.

Anatomy

Pelvis Joints showing the coccyx
Science Picture Co / Getty Images

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 are typically 24 movable bones in the spine that connect to sacrum and coccyx, which each consist of multiple bones that fuse over time.

The vertebrae, which is another name for spinal bones, are broken down into sections from the 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, labeled L1 to L5.
  • Sacrum: This triangularly shaped bone is formed of five bones that start to fuse soon after birth and continue to do so until they are completely fused by around the age of 30. When identifying the individual fused bones, the labeling is S1 to S5.
  • Coccyx: The tailbone is likewise made of individual bones that are movable at birth but fuse over time. The coccyx is at least semi-fused, and in many cases fully fused, by adulthood. The component bones are labeled is Co1 to Co4. Most people have four segments, but some have three or five.

Function

Discus L5/S1
Jan-Otto / 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 points 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. 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

Human Spine, Pelvis, Chiropractic, Orthopedic, Medical Model, Heathcare, Isolated
1Photodiva / 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 7, 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 symptoms related to spondylolisthesis, but it requires a lot of recovery time and can have additional risks. 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 in some cases.

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Article Sources
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  1. American Academy of Orthopaedic Surgeons. Spondylolysis and spondylolisthesis. Updated September 2016.

  2. Antoniades SB, Hammerberg KW, Dewald RL. Sagittal plane configuration of the sacrum in spondylolisthesis. Spine. 2000;25(9):1085-1091. doi:10.1097/00007632-200005010-00008

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