Posterior Longitudinal Ligament

1. Overview

The posterior longitudinal ligament (PLL) is a major stabilizing ligament of the vertebral column. It runs along the posterior surface of the vertebral bodies, within the vertebral canal, and helps maintain spinal alignment while limiting hyperflexion. The PLL is narrower and weaker than its anterior counterpart (the anterior longitudinal ligament) but is critically positioned to protect neural elements, particularly the spinal cord and nerve roots.

2. Location

The PLL is located inside the vertebral canal, along the posterior aspects of the vertebral bodies:

• Extends from: The body of the axis (C2 vertebra) to the sacrum.

• Positioned posterior to: The vertebral bodies and intervertebral discs.

• Anterior to: The spinal cord and meninges.

• Spans: The entire length of the vertebral column except the atlas (C1), with a continuation into the tectorial membrane superiorly.

3. Structure

The PLL is a narrow, ribbon-like ligament composed of dense regular connective tissue:

• Width: Narrow over vertebral bodies and slightly wider over intervertebral discs.

• Fibers: Longitudinally oriented collagen fibers arranged in superficial (long) and deep (shorter, segmental) layers.

• Attachment: Firmly adheres to intervertebral discs and vertebral margins, but less so to the vertebral bodies themselves.

• Histology: Dense collagen with some elastic fibers; provides strength with limited flexibility.

4. Function

The posterior longitudinal ligament plays several important mechanical roles:

• Limits hyperflexion: Resists excessive forward bending of the spine, especially in the cervical and thoracic regions.

• Supports intervertebral discs: Provides posterior reinforcement of the annulus fibrosus.

• Maintains alignment: Helps keep vertebral bodies in a straight column and contributes to overall spinal stability.

5. Physiological role(s)

Although passive, the PLL supports various physiological processes by maintaining the integrity of the spinal column:

• Protects neural elements: Forms part of the anterior wall of the vertebral canal, safeguarding the spinal cord and nerve roots.

• Limits disc protrusion: Acts as a barrier to posterior migration of intervertebral disc material (e.g., in disc herniation).

• Contributes to proprioception: Contains mechanoreceptors that provide feedback to the central nervous system regarding spinal position and motion.

6. Clinical Significance

The PLL is involved in several spinal pathologies and clinical scenarios:

• Disc herniation:

  • Posterolateral disc herniation is more common than posterior due to the PLL’s reinforcement in the midline but relative weakness laterally.

• Spinal canal stenosis:

  • Thickening or ossification of the PLL (as seen in OPLL) can compress the spinal cord or cauda equina, leading to neurological deficits.

• Ossification of the posterior longitudinal ligament (OPLL):

  • More common in East Asian populations; can cause progressive myelopathy due to spinal cord compression.

• Trauma and instability:

  • Disruption of the PLL (e.g., in spinal fractures or dislocations) is considered a marker of spinal instability and may warrant surgical intervention.

• Surgical relevance:

  • The PLL is often removed during spinal decompression surgeries like laminectomy or discectomy to relieve pressure on the spinal cord or nerves.