Lumbar spine

The lumbar spine (often shortened to L-spine) consists of five adjacent vertebrae of the lower vertebral column. They participate in the lumbar lordosis, a natural curve in the spine, that is convex anteriorly. 

Articulations of the facet (zygapophyseal) joints permit flexion/extension and abduction movements. Rotation is greatly limited, and may occur only slightly at the lumbosacral joints.

For a basic description of the anatomy of a generic vertebra, see vertebrae.

Gross anatomy

Similar to other vertebrae, the osteology of each lumbar vertebrae can be subdivided anteriorly to posteriorly:

• body: kidney shaped, convex anteriorly, flattened posteriorly; resulting vertebral canal is triangular in cross-section
• pedicles: project directly posteriorly, attached to the upper half of the body
• transverse processes: spatulate, project laterally on both sides; L3 most often has the longest transverse processes of the lumbar spine, a fact that can be used to number the vertebrae
• lamina: broad, thick, overlap minimally
• articular processes (superior and inferior): lie at the lateral angle of the laminae (junction with pedicle), connected via the pars interarticularis
• spinous process: single, short, thick, roughly horizontal, hatchet-shaped (upper border is straight, lower border curves down)

L1 to L4 are considered typical lumbar vertebrae, whereas due to various and distinctive differences the fifth lumbar vertebra is considered atypical.

Articulations

Vertebrae articulate with one another via:

• intervertebral discs (superior and inferior): wedge shaped (taller anterior, shorter posterior), and contribute most to the lumbar lordosis
• facet (zygapophyseal) joints: obliquely oriented, cylindrically-shaped articular surfaces
  • superior articular facets are concave and face posteromedially
  • inferior articular facets are convex, and face anteriorly

Inferior articular process of the vertebrae above always lies posterior to the superior articular process of the vertebrae below.

Attachments

A large number of attachments occur at the transverse process. On its anterior surface, a vertical ridge serves as an important landmark.

• medial to ridge: psoas muscle
• at the ridge: psoas fascia, anterior layer of the lumbar fascia, medial and lateral arcuate ligaments (at L1)
• lateral to ridge: quadratus lumborum partially inserts
• transverse process tip: middle layer of lumbar fascia

Posterior surface receives attachments of erector spinae. Back muscles (multifidus, longissimus) attach to the mammillary process and accessory tubercle:

• mammillary process: located on the superior articular process, behind the margin of the articular facet, projects posteriorly
• accessory tubercle: located at the root of the transverse process, projects posteriorly

Relations and/or boundaries

Can be superimposed onto the three-column concept.

Anterior column

• anterior longitudinal ligament (ALL): anterior to vertebral body

Middle column

• posterior longitudinal ligament (PLL): lies posterior to vertebral body
• centrally
  • vertebral foramen
  • cauda equina
  • basivertebral veins and internal vertebral venous plexus
• laterally
  • iliolumbar ligament
  • lumbar fascia
  • quadratus lumborum muscle
  • intertransverse ligaments
  • regional/segmental lumbar arteries and veins

​Posterior column

• ligamentum flavum
• interspinous ligament
• supraspinous ligament
• erector spinae muscles
• external vertebral venous plexus (lying within muscle)

Arterial supply

Arterial supply comes from regional/segmental lumbar arteries.

Venous drainage

Venous drainage is complex. Lumbar vertebral bodies are posteriorly perforated by a pair of basivertebral veins that drain into the internal vertebral venous plexus. Regional/segmental lumbar veins also contribute.

Variant anatomy

• transitional vertebrae of the lumbar spine are possible at multiple levels in the lumbar spine
• limbus vertebrae represent herniation of the disc nucleus pulposus through the superior endplate and can simulate a fracture

Development

Transverse processes are actually fused ribs. Mammillary process and the accessory tubercle are the remnants of the true transverse process, typically seen on thoracic vertebrae.