Anatomy of Lumbar Vertebrae

Typical Lumbar Vertebrae

• The body of a typical lumbar vertebra is massive, with a transverse diameter greater than its anterior diameter and height.
• The size and shape of the vertebra reflect the need to support great compressive loads caused by body weight, ground reaction forces, and muscle contraction.
• Laminae and pedicles are short and thick, forming the posterior and lateral walls of the nearly triangular vertebral canal.
• Pedicles are short and thick, projecting posterolaterally.
• Laminae are short and broad.
• Superior articular facets are moderately concave, facing medial to posterior-medial.
• Inferior articular facets are vertical and convex, facing slightly anteriorly and laterally.
• Mamillary processes, small bumps, are located on the posterior edge of each superior articular facet and serve as attachment for the multifidus and medial intertransverse muscles.
• Transverse process is long and slender, extending horizontally.
• Accessory processes, small and irregular bony prominences, are located on the posterior surface of the transverse process near its attachment to the pedicle, and serve as attachment sites for the multifidus and medial intertransverse muscles.
• Spinous process is broad and thick, extending horizontally.
• Vertebral foramen is triangular and larger than the thoracic vertebral foramen but smaller than the cervical vertebral foramen.

5th Lumbar Vertebra (L5)

• The fifth lumbar vertebra is a transitional vertebra, differing from the rest of the lumbar vertebrae in that it has a wedge-shaped body.
• The anterior portion of the body is of greater height than the posterior portion.
• The L5/S1 lumbosacral disk is also wedge-shaped.
• The inferior articular facets of L5 articulate with the superior articular facets of the sacrum.
• Apophyseal joints are typically oriented much closer to the frontal plane than the other lumbar articulations.

Lumbosacral Joint

• The lumbosacral joint is formed by two separate articulations: the anterior intervertebral joint and the facet joints.
• The anterior intervertebral joint is formed between the vertebral bodies of L5 and S1, separated by a large intervertebral disc and connected by vertebral ligaments.
• The facet joints are two zygapophysial joints formed between the articular processes of L5 and S1 vertebrae.
• The facets of S1 are positioned posteromedially and interlock with the anterolaterally placed inferior facet of the articular processes of L5 vertebrae, preventing the dislocation of the lumbar vertebra along the incline of the sacrum.
• The lumbosacral joint is the site of most movements of the lumbar spine, including flexion, extension, and a minimal degree of lateral flexion.

Lumbosacral Angle (Lumbo)sacral Angle or Sacral Angle

• The first sacral segment, which is inclined slightly anteriorly and inferiorly, forms an angle with the horizontal called the lumbosacral angle.
• The size of the angle varies with the position of the pelvis and affects the superimposed lumbar curvature.
• An increase in this angle will result in an increase in lordosis of the lumbar curve and will increase the amount of shearing stress at the lumbosacral joint.