Lumbosacral Joint
Lumbosacral joint is the synovial articulation between L5 and the sacral base, permitting flexion-extension.
The lumbosacral joint is the articulation between the fifth lumbar vertebra (L5) and the base of the sacrum (S1). It represents the transitional zone between the mobile lumbar spine and the rigid sacrum, which is anchored to the pelvis. This joint bears a significant load and is subject to high mechanical stress due to its anatomical position and function. It plays a crucial role in posture, spinal flexibility, and weight transmission from the trunk to the pelvis and lower limbs.
Structure
The lumbosacral joint is a complex articulation involving both intervertebral and zygapophyseal (facet) joints. It includes a robust intervertebral disc, paired synovial joints, and strong supporting ligaments.
1. Intervertebral Joint (L5–S1 Disc)
- Formed by the inferior surface of the L5 vertebral body and the superior surface of the S1 body
- Separated by the L5–S1 intervertebral disc composed of nucleus pulposus and annulus fibrosus
- Wider anteriorly than posteriorly, contributing to the lumbosacral angle
2. Zygapophyseal Joints
- Paired synovial joints between the inferior articular processes of L5 and the superior articular processes of S1
- Oriented more coronally than in other lumbar levels, limiting rotation and providing stability
3. Lumbosacral Angle
- Angle formed by the intersection of the long axis of the lumbar spine and sacrum
- Typically ranges from 120° to 130°
- Increased angle may contribute to lumbar lordosis and mechanical stress
Location
The lumbosacral joint is located at the junction of the lumbar spine and the sacrum. It lies just above the sacral promontory and directly inferior to the iliac crest level. This joint is deep to the posterior abdominal wall and can be visualized radiographically on lateral lumbar spine imaging.
Function
- Weight Transmission: Transfers the load of the upper body from the lumbar spine to the pelvis via the sacrum
- Mobility and Flexibility: Permits flexion, extension, and limited lateral bending; minimal rotation due to facet joint orientation
- Postural Support: Maintains lumbar lordosis and helps align the center of gravity
- Shock Absorption: The intervertebral disc cushions compressive forces during movement
Ligaments Supporting the Lumbosacral Joint
| Ligament | Function |
|---|---|
| Anterior Longitudinal Ligament | Runs along anterior vertebral bodies; prevents hyperextension |
| Posterior Longitudinal Ligament | Runs within the vertebral canal; limits hyperflexion |
| Ligamentum Flavum | Connects adjacent laminae; preserves upright posture |
| Interspinous and Supraspinous Ligaments | Restrict spinal flexion |
| Iliolumbar Ligament | Extends from L5 transverse process to iliac crest; stabilizes L5–S1 junction |
Clinical Significance
- Lumbosacral Strain: Common site of mechanical low back pain due to muscle or ligament strain at this transition point
- Disc Herniation (L5–S1): The most frequent site of lumbar disc prolapse, potentially compressing the S1 nerve root and causing sciatica
- Spondylolisthesis: Forward displacement of L5 over S1, often due to defect or fracture in the pars interarticularis
- Degenerative Changes: Facet joint arthritis and disc degeneration can lead to stenosis and nerve impingement
- Surgical Relevance: The lumbosacral junction is a key site in spinal fusion surgeries and instrumentation
Last updated on May 9, 2025
