Lumbosacral joint

Articular surfaces

The lumbosacral joint is a set of two articulations; an anterior intervertebral joint and two zygapophyseal (facet) joints:

• Anterior intervertebral joint: formed between the vertebral bodies of L5 and S1. They are 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°
• Zygopophysial joints: those are two articulations 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 zygapophyseal joints prevent the dislocation of the lumbar vertebra along the incline of the sacrum.

Ligaments

The lumbosacral joint is stabilized by the iliolumbar and the lateral lumbosacral ligaments. In addition, like all vertebral articulations, the joint is also stabilized by the ligaments of the vertebral column such as anterior and posterior longitudinal ligaments, ligamenta flava, interspinous and supraspinous ligaments.

• 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. The iliolumbar ligament is superiorly continuous with the anterior and middle layers of the thoracolumbar fascia. An accessory band of the ligament sometimes arises from the L4 vertebra that also attaches on the iliac crest posterior to the main band. The iliolumbar ligament is essentially a much stronger equivalent for the intertransverse ligaments of other vertebrae. It is important for restricting 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 inferolaterally to attach on the ala of the sacrum. This ligament is partially continuous with the iliolumbar ligament at its origin point, and blends with the anterior sacroiliac ligament at its insertion on the sacrum.

Innervation

The lumbosacral joint is innervated by branches of the anterior and posterior rami of L5 and S1 spinal nerves.

Blood supply

The bony portion of the lumbosacral joint receives arterial blood supply from the iliolumbar and superior lateral sacral arteries, that branch off the internal iliac artery. The joint also receives contributions from the median sacral artery, that arises from the abdominal aorta. On the other hand, the intervertebral disc is an avascular structure that receives its nutrients via diffusion from the adjacent vertebral bodies.

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Movements

The lumbosacral joint is the site of most movements of the lumbar spine. The movements occurring in this joint are mostly flexion and extension, and a minimal degree of lateral flexion. The degree of motion in the lumbosacral joint varies depending on age and individual characteristics. Before puberty, the lumbosacral joint accounts for about 75% of the total range of flexion and extension of the lumbar spine, which amounts to approximately 18°. The degree of lateral flexion is minimal and amounts to about 7° before puberty. The degree of these movements significantly decreases with age, especially after 35 years of age.

Flexion is 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 of the joint is limited by the anterior longitudinal ligament, anterior part of the intervertebral disc and the close-packing of the zygapophyseal joints.

Muscles acting on the lumbosacral joint

The main flexors of the lumbosacral joint are psoas major and rectus abdominis, with contributions from other anterior abdominal muscles, namely the external abdominal oblique, internal abdominal oblique and transversus abdominis.

The main extensors of the lumbosacral joint are interspinales, multifidus, longissimus lumborum, and iliocostalis lumborum.