L5-S1 - The Lumbosacral Joint

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L5-S1 - The Lumbosacral Joint

A skeleton of the pelvis, sacrum, lumbar spine, hip joints and femur bones.
A skeleton of the pelvis, sacrum, lumbar spine, hip joints and femur bones. sciencepics

The lumbosacral spine is comprised of the bones of the lumbar spine (L-1 through L5) and the sacrum.  A key area of transition in the spine is the place where the lumbar spine ends and the sacrum begins.  This is L5-S1, also called the lumbosacral joint.

The lumbar spine rests upon the sacrum (which means the sacrum takes the weight of the entire spine from above.)  The sacrum also connects the spine to the pelvis by means of sacroiliac joints, which are located on the sides of the sacrum.

Below the sacrum is the coccyx bone.

In an adult, the sacrum is one bone.  But in a newborn, it is five.   The sacrum fuses over time; this process begins before age 1 and completes by age 30.  (In newborns, the 5 sacral vertebrae look like lumbar vertebrae - until the sides of the sacrum harden, which takes place around the end of year one.)

Related: Normal Lumbar Lordosis

Sources:

Cheng, J., M.D., Song, J., M.D. Anatomy of the Sacrum. Medscape Multispecialty. Accessed: March 2016. http://www.medscape.com/viewarticle/461094_2

Lumbosacral spine x-ray. MedlinePlus. April 2015. Accessed: March 2016. https://www.nlm.nih.gov/medlineplus/ency/article/003807.htm

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Spondylolisthesis and the Lumbosacral Joint

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

L5 - S1 is often the site of an injury known as spondylolisthesis.  Spondylolisthesis refers to a vertebra that slips forward relative to the bone below it.  It is one type of spinal instability.

The most common type of spondylolisthesis is called isthmic spondylolisthesis, which basically means it started with a small injury (fracture) in a small area at the back of the spinal bone called the pars interarticularis.  Another name for the pars interarticularis is isthmic.  If this injury to the pars (as it's often called for short) develops - perhaps if it's not properly treated, or not treated early enough - it may develop into spondylolysis.  Spondylolysis can develop into spondylolisthesis.

Spondylolisthesis is present in up to 7% of people, depending on age and other factors.  It affects men more than women, and white people more than black.  But in women, the condition tends to progress more than it does in men.  Children and adolescents who participate in sports that require repeated flexion and extension movement of the spine are at risk for this injury.  Adults subject to degenerative changes in the spine (as most adults are) are also at risk.

Spondylolisthesis and the Lumbosacral Angle

So why does spondylolisthesis affect the L5-S1 joint in particular?  This may have to do with the angle of the top of the sacrum bone.  The top of the sacrum bone is called the sacral base, and it is not horizontal to the ground or floor.  Rather, it tips down in front and up in back.  Researchers found that the greater the tilt in the sacral base (also known as the sacral table) the more spondylolisthesis was found in patients.

Related: Lumbosacral Angle

Sources:

DeVine, J., Schenk-Kisser, J. and Skelly, A. Risk factors for degenerative spondylolisthesis: a systematic review Evid Based Spine Care J. May, 2012. Accessed March 2016.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516463/

Hosoe H., Ohmori K., Degenerative lumbosacral spondylolisthesis: possible factors which predispose the fifth lumbar vertebra to slip. J Bone Joint Surg Br. March 2008. Accessed March 2016. http://www.ncbi.nlm.nih.gov/pubmed/18310760

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