Overview
The lumbosacral joint is a set of two articulations between the fifth lumbar (L5) and first sacral (S1) vertebrae. The sacrum is connected with the fifth lumbar vertebra at an angle of approximately 140° (lumbosacral angle). The movements in this joint are limited to mostly flexion and extension, with a minimal amount of lateral flexion.
Articulations
Anterior intervertebral joint
- A symphysis joint, formed between the vertebral bodies of L5 and S1
- Separated by an intervertebral disc, which is in contact with the superior surface of the S1 vertebral body and the inferior surface of L5 vertebral body.
- As the sacrum is inclined anteriorly, the S1 and L5 vertebral bodies articulate at an angle called lumbosacral angle, which averages 140°
Facet (zygapophysial) joints
- A synovial plane joint, formed between the inferior articular processes of the L5 vertebra and superior articular processes of S1 vertebra.
- The facets of S1 are positioned posteromedially and interlock with the anterolaterally placed inferior facet of the articular processes of L5 vertebrae.
- The facet joints prevent the dislocation of the lumbar vertebra along the incline of the sacrum.
Ligamentous Attachments
Common ligaments of the vertebral column:
- Anterior longitudinal ligament – runs along the anterior surface of the vertebral bodies (firmly united to the periosteum) and intervertebral discs (attaching to the anterior annulus). Becomes the anterior atlantooccipital membrane at atlas (C1)
- Posterior longitudinal ligament – posterior to the vertebral bodies (to which it attaches loosely) and intervertebral discs (to which it is firmly attached). Becomes the tectorial membrane at axis (C2)
- Ligamentum flavum – paired ligaments which run between adjacent laminae of the vertebral bodies and are present from C2/3 to the sacrum. Becomes the posterior atlantooccipital membrane at atlas (C1)
- Interspinous ligament – connect adjoining spinous processes of the vertebra
- Supraspinous ligament – connects together the apices of the spinous processes of the vertebra
The lumbosacral joint also specifically has:
Iliolumbar ligament
- Arises from the apex of the transverse process of L5 vertebra and radiates inferolaterally across the sacroiliac ligament to attach on the inner lip of the iliac crest of the pelvis.
- An accessory band of the ligament sometimes arises from the L4 vertebra that also attaches on the iliac crest posterior to the main band.
- Restricts flexion, extension and lateral flexion of the lumbosacral joint.
Lateral lumbosacral ligament
- Arises from the lower margin of the transverse process of L5 vertebra and passes obliquely inferiorly to attach on the ala of the sacrum.
Innervation
The lumbosacral joint is innervated by branches of the anterior and posterior rami of L5 and S1 spinal nerves.
Arterial Supply
Bony portion – Iliolumbar and superior lateral sacral arteries, that arises from the posterior division of the internal iliac artery. Also receives contributions from the median sacral artery, that arises from the abdominal aorta.
Intervertebral disc – avascular structure that receives its nutrients via diffusion from the adjacent vertebral bodies.
Movements
- The lumbosacral joint is the site of most movements of the lumbar spine, the degree of motion may depend on age.
- Primary movements: flexion and extension, and a minimal degree of lateral flexion.
- Flexion – limited by the posterior part of the intervertebral disc and the ligaments of the vertebral column, namely the posterior longitudinal ligament, ligamenta flava, interspinous and supraspinous ligaments.
- Extension – limited by the anterior longitudinal ligament, anterior part of the intervertebral disc and the close-packing of the facet joints.
Muscles
- Flexion: Psoas major, rectus abdominis, external abdominal oblique, internal abdominal oblique, transversus abdominis
- Extension: Interspinales, multifidus, longissimus lumborum, iliocostalis lumborum
Anatomical Variants
- Lumbarization of S1 – assimilation of S1 to lumbar spine
- Sacralization of L5 – assimilation of L5 to the sacrum