Lumbosacral joint

The lumbosacral joint (Latin: articulatio lumbosacralis) is a complex of articulations between the fifth lumbar (L5) and the first sacral (S1) vertebrae. It is composed of three joints that belong to two different joint types:

• Anterior intervertebral joint - cartilaginous junction;
• Facet (zygapophyseal) joints (2) - synovial joints.

Classification of lumbosacral joint

The anterior intervertebral joint is a secondary cartilaginous joint (also called symphysis) that provides limited mobility. Therefore, functionally it is classified as amphiarthrosis.

Both facet joints are plane-type synovial joints. This articulation type provides the broadest range of movements, and the facet joints are classified as diarthroses.

Note: To read more in detail about the joint types and their functions, please visit our article on the classification of joints.

Articulating structures

The anterior intervertebral joint is an articulation formed by three structures: vertebral bodies of the fifth lumbar (L5) and first sacral (S1) vertebrae and an intervertebral disc that lies between both. The following structures articulate within the joint:

• Superior articular surface of the sacrum (sacral plateau)
• Superior and inferior surfaces of the intervertebral disc
• Inferior articular surface of the body of the fifth lumbar vertebra (L5)

Both facet joints are formed between the articular processes of the last lumbar (L5) and first sacral (S1) vertebrae. All mentioned processes contain surfaces called facets that articulate with each other. They are reciprocally curved and complement each other.

The lumbar facet joints are positioned in the sagittal plane. The articular facets are directed at the right angle to the transverse plane and at a 45-degree angle to the frontal plane. However, a wide variety in shape and alignment is possible.

Ligaments

As mentioned previously, the anterior intervertebral joint is classified as a symphysis, meaning it lacks not only a joint cavity but also a fibrous joint capsule that is present in the facet joints. Therefore, the lumbosacral joint complex is primarily stabilized and strengthened by several ligaments, and they include the following:

• Iliolumbar ligament - stabilizes the anterior intervertebral joint;
• Anterior longitudinal ligament - supports the anterior intervertebral joint;
• Posterior longitudinal ligament - provides stability to the anterior intervertebral joint and facet joints;
• Ligamenta flava - strengthen the facet joints;
• Interspinous ligament - supports the facet joints;
• Supraspinous ligament - provides stability to the facet joints.

The iliolumbar ligament originates from the anteroinferior aspect of the tip of the transverse process of the fifth lumbar vertebra (L5). The ligament has two bands (superior and inferior) that radiate laterally and downward.

The superior (upper) band goes in the lateral direction, crosses over the anterior sacroiliac ligament and attaches to the inner lip of the iliac crest anterior to the sacroiliac joint. The superior band of the iliolumbar ligament serves as an attachment site for the quadratus lumborum muscle. Moreover, it blends superiorly with the anterior layer of the thoracolumbar fascia.

The inferior (lower) band is also known as the lumbosacral portion of the iliolumbar ligament, and it is sometimes referred to as a separate lumbosacral ligament. It attaches to the anterosuperior lateral surface of the sacrum and blends with the anterior sacroiliac ligament.

The anterior and posterior longitudinal ligaments are strong fibrous bands that run along the entire length of the spine. The anterior longitudinal ligament attaches to the anterior surfaces of the vertebral bodies and their respective intervertebral discs. The posterior longitudinal ligament is located inside the vertebral canal and is attached to the posterior surfaces of the vertebral bodies and their respective intervertebral discs.

The ligamenta flava are paired bands that connect the adjacent spinal laminae at the posterior wall of the vertebral canal. Laterally, the ligamenta flava merge with the fibrous capsules of the facet joints. The interspinous ligaments are weak and membranous bands that stretch between the bases of the adjacent spinous processes.

The supraspinous ligament is a strong cord-like band that connects the apices of adjacent spinous processes. In some people, this band extends till the sacral median crest, while in others, it terminates at the L4 level and, in that case, does not provide stability to the lumbosacral joint.

Movements

The lumbosacral joint allows such movements as flexion, extension and lateral flexion of the lumbar spine. All these motions are limited at facet joints. At the anterior intervertebral joint, on the other hand, flexion and extension are not limited, while lateral flexion can be performed only to a minimal degree. Also, the anterior intervertebral joint transfers loads from the spine to the pelvis and legs.

Related structures

The most notable structures found on the anterior aspect of the lumbosacral joint are the common iliac arteries and veins, median sacral vessels, and the superior hypogastric plexus. The sympathetic trunks, obturator nerves and lumbosacral trunks are located near the junction on either lateral side.

Neurovascular supply

The lumbosacral junction receives arterial blood supply mainly from the iliolumbar and upper lateral sacral arteries of the internal iliac artery and the median sacral artery of the abdominal aorta. However, these arteries provide arterial supply only for bony structures of the joint (both vertebrae). The intervertebral disc receives arterial blood from the external capillaries that branch off the segmental arteries. Nutrients and oxygen are also received through the endplates from the endplate capillaries that come from the vertebral body.

The venous drainage of the facet joints occurs via the internal and external vertebral venous plexuses. The internal vertebral venous plexus drains into the external vertebral venous plexus via the intervertebral veins. The external venous plexus empties into the median and/or lateral sacral veins. Venous drainage of the anterior intervertebral joint happens through the subchondral venous plexus, or venous blood is carried to the veins of the narrow spaces between two adjacent vertebrae.

The lumbosacral joint receives innervation from branches of the fifth lumbar (L5) and first sacral (S1) spinal nerves.

References:

1. Kayalioglu, G., Paxinos, G., Watson, C. (2009). The Spinal Cord (1st Edition). Elsevier
2. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.