What Is the L5-S1 and Lumbosacral Joint?

Part of the Spine Especially Vulnerable to Injury

L5-S1 is a term used to describe a section of the spine which is comprised of the last vertebra of the lower back (called the L5) and the triangular-shaped bone immediately beneath it known the sacrum. The sacrum is composed of five bones of which the S1 is the top.

You can think of L5-S1 as the exact spot where the ​lumbar spine ends and the sacral spine begins. The joint that connects them is called the lumbosacral joint.

Structure of the Lumbar Spine and Sacrum

Coccyx - Tailbone

The human spine has a total of 33 bones of which only top 24 are fully moveable. They are broken down into sections from the top to tail, as follows:

  • Cervical spine (neck) with seven bones from C1 to C7
  • Thoracic spine (mid-back) with 12 bone from T1 to T12
  • Lumbar spine (low back) with five bones from L1 to L5
  • Sacrum with five bones from S1 to S5
  • Coccyx region (tailbone) with four bones from Co1 to Co4

While most people tend to think of the sacrum as one bone, it is actually comprised of five individual bones that start to fuse soon after birth and continue to do so until they are completely fused by around the age of 30.

The same applies to the coccyx bones which are articulated (movable) at birth but either become fused or semi-fused by adulthood.

Why the L5-S1 Is Important

A skeleton of the pelvis, sacrum, lumbar spine, hip joints and femur bones.
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The spinal column is the structure of the body which allows us to stand upright and to twist and bend to alter the position of the torso, back, and neck. The curvature of the spine is also key to its vulnerability.

In the neck and lower back, the spinal curve sweeps forward when in a neutral position, while the thoracic and sacral curves sweep in the opposite direction

It is at the L5-S1 junction that the body weight shifts direction from forward (lumbar) curve to backward (sacral) curve. It is at these junctions that are opposing stresses increase the vulnerability of the joint as the back ages or is hurt.

Because of this, the L5-S1 is one of the two most common sites for back surgery (along with the L4-L5).

The L5-S1 and Spondylolisthesis

Facet joint, pars and intervertebral joint
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The L5-S1 junction is a common site of an injury known as spondylolisthesis. Spondylolisthesis refers to a vertebra that has slipped forward relative to the bone immediately beneath it.

The most common type is called isthmic spondylolisthesis which starts with a tiny fracture in the pars interarticularis (a bone structure the connects the joints of the spine).

While the fracture tends to occur when a person is young (before the age of seven), symptoms typically do not develop until adulthood. Degeneration of the spine is later adulthood can further exacerbate the condition.

Spondylolisthesis affects the L5-S1 joint due to the angle of the sacrum itself. 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.

While spinal fusion surgery can be effective for treating spondylolisthesis, it requires a lot of recovery time and is generally not considered until a person has failed to find relief from non-surgical methods for at least six months.

View Article Sources
  • Cloete, E.; Battin, M.; Immam, F. et. al. "Ossification of sacral vertebral bodies in neonates born 24 to 38 weeks' gestational age and its relevance to spinal ultrasonography." Am J Perinatol. 2013; 30(6):519-22. DOI: 10.1055/s-0032-1329186.
  • DeVine, J.; Schenk-Kisser, J.; and Skelly, A. "Risk factors for degenerative spondylolisthesis: a systematic review. Evid Based Spine Care J. 2012; 3(2):25-34. DOI: 10.1055/s-0031-1298615.