All about L5-S1 (Lumbosacral Joint)

The L5-S1 spinal motion segment, also called the lumbosacral joint, is the transition region between the lumbar spine and sacral spine in the lower back. In this region, the curvature of the spine changes from lumbar lordosis (forward curve) to sacral kyphosis (backward curve). L5-S1 helps transfer loads from the spine into the pelvis and legs.

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The L5-S1 motion segment has distinctive anatomy and receives a higher degree of mechanical stress and loads compared to the segments above. Cramer GD. The Lumbar Region. In: Clinical Anatomy of the Spine, Spinal Cord, and Ans. Elsevier; 2014:246-311. doi:10.1016/b978-0-323-07954-9.00007-4 These characteristics may make L5-S1 susceptible to traumatic injuries, degeneration, disc herniation, and/or nerve pain.

Anatomy of the L5-S1 Spinal Motion Segment

This motion segment typically includes the following structures:

L5 and S1 vertebrae

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The L5-S1 segment spans two different portions of the spine, the lumbar and the sacral.

The L5 and S1 vertebrae have different features:

• L5 consists of a vertebral body in front and an arch in the back that has 3 bony protrusions: a prominent spinous process in the middle and two transverse processes on the sides. These protrusions serve as attachment points for ligaments.
• S1, also called the sacral base, is the upper and wider end of the triangular-shaped sacrum. S1 consists of a body on the top with wing-shaped bones on either side, called the alae. At the back, the S1 vertebra contains a long bony prominence called the median ridge. There are bony openings (foramina) on the right and left sides of this ridge.
• L5 and S1 are joined by the lumbosacral facet joints lined with articular cartilage.

L5-S1 intervertebral disc

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The L5-S1 disc protects and structurally supports the vertebrae.

A disc made of a gel-like material (nucleus pulposus) surrounded by a thick fibrous ring (annulus fibrosus) is situated between the vertebral bodies of L5 and S1. This disc provides cushioning and shock-absorbing functions to protect the vertebrae during spinal movements.

L5 spinal nerve

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Nerve roots branch out from the spinal cord at each spinal segment.

The L5 spinal nerve roots exit the spinal cord through small bony openings (intervertebral foramina) on the left and right sides of the spinal canal. These nerve roots join with other nerves to form bigger nerves that extend down the spine and travel down each leg.

• The L5 dermatome is an area of skin that receives sensations through the L5 spinal nerve and includes parts of the knee, leg, and foot.

  Kayalioglu G. The Spinal Nerves. In: The Spinal Cord. Elsevier; 2009:37-56. doi:10.1016/b978-0-12-374247-6.50008-0

• The L5 myotome is a group of muscles controlled by the L5 spinal nerve and includes specific muscles in the pelvis and legs, which are responsible for leg and foot movements.

  Kayalioglu G. The Spinal Nerves. In: The Spinal Cord. Elsevier; 2009:37-56. doi:10.1016/b978-0-12-374247-6.50008-0

The L5-S1 motion segment provides a bony enclosure for the cauda equina (nerves that continue down from the spinal cord) and other delicate structures.

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Common Problems at L5-S1

The L5-S1 situated at the bottom of the vertebral column is typically subject to excessive biomechanical stress, leading to more loads and an increased risk of injury. Common problems include:

Disc problems

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The L5-S1 disc is vulnerable to herniation.

Lower back disc herniation typically occurs at the L5-S1 level. Donnally III CJ, Butler AJ, Varacallo M. Lumbosacral Disc Injuries. [Updated 2019 Apr 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448072/ A steeper inclination of this disc leads to a higher degree of shear stresses and increases the risk disc of injury and degeneration.

Facet joint pain

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The facet joints can be damaged by inflammation or osteoarthritis.

The lumbosacral joint may develop wear-and-tear arthritis (osteoarthritis) over time due to its high load-bearing function. Alexander CE, Varacallo M. Lumbosacral Facet Syndrome. [Updated 2019 Mar 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK441906/ Facet joint pain may also occur due to due to inflammatory conditions, such as rheumatoid arthritis or degenerative spondylolisthesis. Perolat R, Kastler A, Nicot B, et al. Facet joint syndrome: from diagnosis to interventional management. Insights Imaging. 2018;9(5):773–789. doi:10.1007/s13244-018-0638-x

Spondylolysis

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The L5 vertebra is susceptible to spondylolysis, which is the fracture of the pars interarticularis (a small segment of bone from the vertebral arch joining the facet joints) most commonly due to repetitive stress on the bone. Cramer GD. The Lumbar Region. In: Clinical Anatomy of the Spine, Spinal Cord, and Ans. Elsevier; 2014:246-311. doi:10.1016/b978-0-323-07954-9.00007-4 It can occur on one or both sides. Frequently, the bone does not break, but becomes stressed and this condition is called a pars stress reaction.

Spondylolisthesis

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In spondylolisthesis of the L5-S1 level, the L5 vertebra slips over the S1 vertebra.

If the pars of L5 fractures on both sides, the vertebra may slip over S1, a condition called spondylolisthesis. Most commonly, spondylolisthesis of L5 is caused by repetitive stress to the pars interarticularis and is most has commonly seen in children and adolescents. Cramer GD. The Lumbar Region. In: Clinical Anatomy of the Spine, Spinal Cord, and Ans. Elsevier; 2014:246-311. doi:10.1016/b978-0-323-07954-9.00007-4 A pars fracture due to trauma is rather uncommon.

While rare, malignant tumors, such as sacral chondromas may occur in the L5-S1 level. Pillai S, Govender S. Sacral chordoma : A review of literature. Journal of Orthopaedics. 2018;15(2):679-684. doi:10.1016/j.jor.2018.04.001 This level is also subject to other problems arising from developmental variations in the shape of S1, a fusion of L5 with S1, and the presence of extra facet joints between L5-S1.

Common Symptoms and Signs Stemming from L5-S1

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Issues in the L5-S1 segment can cause pain throughout the lower body.

Vertebral and disc pain from L5-S1 may occur suddenly following an injury or gradually develop over a period of time. Typically, a dull ache or sharp pain may be felt in the lower back. Discogenic pain is typically worsened by prolonged sitting, standing in one place, and repetitive lifting and bending activities.

Compression or inflammation of the L5 and/or S1 spinal nerve root may cause radiculopathy symptoms or sciatica, characterized by:

• Pain, generally felt as a sharp, shooting, and/or searing feeling in the buttock, thigh, leg, foot, and/or toes
• Numbness in the foot and/or toes
• Weakness in the leg and/or foot muscles and an inability to lift the foot off the floor (foot drop)

It is also possible for a stabbing pain or ache to be isolated to any of these (dermatomal) areas. While these symptoms typically affect one leg at a time, sometimes, both legs may be affected together.

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Cauda equina syndrome may occur at L5-S1 due to an injury to the cauda equina nerves that descend from the spinal cord. This syndrome is a medical emergency and typically causes severe pain, weakness, numbness, and/or tingling in the groin, genital region, and/or both legs. There may also be loss of bowel and/or bladder control. The condition must be treated on an urgent basis to preserve leg function and restore bowel and/or bladder function.

See Cauda Equina Syndrome Symptoms

Nonsurgical treatments are often tried first for symptoms that stem from L5-S1. In rare cases, surgery may be considered.

See Non-Surgical Treatments for Lower Back Pain

Goto Lingchuang Yihui to know more.

Dr. David DeWitt is an orthopedic surgeon practicing at the NeuroSpine Center of Wisconsin, where he specializes in spine surgery. He has more than 15 years of experience evaluating and treating spine diseases and trauma.

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The spinal cord starts at the base of the brain, runs throughout the cervical and thoracic spine, and typically ends at the lower part of the thoracic spine.

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The spinal cord is an extension of the body's central nervous system.

The spinal cord does not run through the lumbar spine (lower back). After the spinal cord stops in the lower thoracic spine, the nerve roots from the lumbar and sacral levels come off the bottom of the cord like a "horse's tail" (named the cauda equina) and exit the spine.

Because the lumbar spine has no spinal cord and has a large amount of space for the nerve roots, even serious conditions—such as a large disc herniation—do not typically cause paraplegia (loss of motor function in the legs).

Spinal Cord and Spinal Nerve Root Anatomy

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Spinal nerve roots branch off from either side of the spinal cord. 

The spinal cord can be divided into segments according to the nerve roots that branch off from it. There are a total of:

• 8 cervical spinal nerves
• 12 thoracic spinal nerves
• 5 lumbar spinal nerves
• 5 sacral spinal nerves
• 1 coccygeal spinal nerve

For most spinal segments, the nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.

• Cervical spine nerve roots. In the neck, the nerve root is named for the lower segment that it runs between (e.g. C6 nerve root at C5-C6 segment).
  • Watch Cervical Nerve Anatomy Animation
• Lumbar spine nerve roots. In the lower back, the nerve is named for the upper segment that it runs between (e.g. L4 nerve root at L4-L5 segment).
  • Watch Lumbar Nerve Anatomy Animation

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Spinal Nerve Irritation from a Herniated Disc

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A herniated spinal disc can compress the spinal cord or spinal nerve root. 

The nerve passing to the next level runs over a weak spot in the disc space, which is the reason discs tend to herniate right under the nerve root.

• Cervical disc herniations (in the neck) tend to irritate the C6 spinal nerve exiting at the C5-C6 spinal segment.
• Lumbar disc herniations (in the low back) tend to irritate the L4 spinal nerve exiting at the L4-L5 spinal segment.
• Thoracic disc herniations (in the upper back) are less common than in the neck or lower spine, but they do occur.

Sometimes, a herniated disc will cause only leg pain or arm pain and not lower back pain or neck pain, and may initially be thought to be a problem with the patient's leg or arm.

• Arm pain from a cervical disc herniation is usually accompanied by numbness/tingling and runs to the fingers.

  See Understanding Hand Pain and Numbness

• Leg pain from a lumbar disc herniation will usually run below the knee, and possibly to the foot, and may be accompanied by numbness or other neurological symptoms.

  See Leg Pain and Numbness: What Might These Symptoms Mean?

The Essentials of a Pinched Lumbar Spinal Nerve

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The L5 and S1 spinal nerve roots are at a higher risk of irritation or compression.

The two nerves most commonly pinched in the lower back are L5 and S1.

• Pinched nerve at L5-S1 spinal segment. The L5 nerve supplies the nerves to the muscles that raise the foot and big toe, and consequently, impingement of this nerve may lead to weakness in these muscles. Numbness for L5 runs over the top of the foot.
• Pinched nerve at S1-S2 spinal segment. Impingement of the S1 nerve can lead to weakness with the large gastronemius muscle in the back of the calf, causing difficulty with foot push, along with numbness on the outer side of the foot. The S1 nerve root is also responsible for the ankle jerk reflex (tapping on the achilles tendon causes the foot to go down), therefore a loss of this reflex indicates S1 impingement.

See All about L5-S1 (Lumbosacral Joint)

The Essentials of a Pinched Cervical Spinal Nerve

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Different symptoms are caused by the impingement of various nerve roots.

Most cervical pathology will lead to pinching of either C6 or C7 nerve roots in the neck, although sometimes the C5 or C8 nerves may be pinched.

Depending on which nerve root is pinched, the following symptoms are likely:

• Pinched nerve at C5. This can cause shoulder pain, deltoid weakness, and possibly a small area of numbness in the shoulder. On physical exam, a patient’s biceps reflex may be diminished.
• Pinched nerve at C6. This can cause weakness in the biceps and wrist extensors, and pain/numbness that runs down the arm to the thumb. On physical exam, the brachioradialis reflex (mid-forearm) may be diminished.

  Watch Spinal Motion Segment: C5-C6 Video

• Pinched nerve at C7. This can cause pain/numbness that runs down the arm to the middle finger. On physical exam, the triceps reflex may be diminished.
• Pinched nerve at C8. This can cause hand dysfunction (this nerve supplies innervation to the small muscles of the hand). Pain/numbness can run to the outside of the hand (little finger) and impair its reflex.

Read more about Cervical Spinal Nerves

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Healing from a Pinched Nerve

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Nerves can takes weeks to months to heal.

The nerve consists of one long cell from the low back or neck down to the foot or hand, so the nerves tend to heal slowly. The nerves heal from the top down, and depending on how much damage is done at the time the nerve becomes impinged (pinched), it may take weeks to months for the nerve to fully to heal.

Treatment of neural impingement is directed at relieving the pain and then allowing the nerve to heal on its own. Nerves need both inflammation and pressure to be painful, so either relieving the inflammation or the pressure can relieve the pain.

Dr. Thomas Scioscia is an orthopedic surgeon with 15 years of experience treating conditions of the back and neck. He specializes in spine surgery, minimally invasive spine surgery, and cervical disc replacement.

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