Fifth lumbar vertebra (L5)

The fifth lumbar vertebra (L5) is the largest of the five lumbar vertebrae and is considered an atypical vertebra due to its shape. 

Gross anatomy

L5 is the largest, most inferior lumbar discovertebral unit in the vertebral column, and participates in forming the lumbar lordosis (from L1 to L5, ranging from 20° to 40°). 

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

Unlike other lumbar vertebrae, L5 has a distinct transverse process described below.

Anterior components of L5 include:

• body: large, transverse width > AP diameter
  • axial view: oval/kidney-shaped
  • sagittal view: wedge-shaped, taller anteriorly, shorter posteriorly

Posterior components of L5 include:

• pedicles: project directly posteriorly
• transverse processes (features below unique to L5):
  • short, massive, pyramidal
  • base is attached from pedicle to the lateral side of the vertebral body
• lamina: broad, thick, overlap minimally
• articular process: superior and inferior (with pars interarticularis between)
• facet joints: obliquely oriented, cylindrically shaped articular surfaces
  • superior articular facets are concave and face posteromedially
  • inferior articular facets are convex and face anteriorly
• spinous process: single, short, thick, roughly horizontal

Articulations

• superior articular facet with the inferior articular facet of L4
• inferior articular facet with the superior articular facet of sacrum: the L5 inferior articular facets face well forward and are received by backward-facing facets of the sacrum
• intervertebral discs: superior and inferior

Relations

• anteriorly: anterior longitudinal ligament
• posteriorly
  • posterior longitudinal ligament
  • ligamentum flavum
  • interspinous ligament
  • supraspinous ligament
  • erector spinae muscle
  • external vertebral venous plexus (lying within the muscle)
• laterally
  • iliolumbar ligament
  • lumbar fascia
  • psoas major muscle
  • intertransverse ligaments
  • regional/segmental lumbar arteries and veins
• superiorly: L4/5 intervertebral disc
• inferiorly: L5/S1 intervertebral disc
• centrally
  • vertebral foramen
  • cauda equina
  • basivertebral veins and internal vertebral venous plexus

Variant anatomy

See main article: lumbosacral transitional vertebrae.

Radiographic features 

Plain radiograph

AP view of the lumbosacral junction with ~25° cranial angulation (Ferguson view) demonstrates the L5/S1 disc space tangentially and the L5 pars en face.

Related pathology 

• spondylolisthesis