A patient with chronic ankylosing spondylitis develops a characteristic spinal deformity. The posterior longitudinal ligament (PLL) has a specific anatomical property that makes it more protective against posterior disc herniation at the lumbar level than the anterior longitudinal ligament. What is this property?

• A The PLL is uniformly wide throughout the lumbar spine, preventing herniation at all levels equally
• B The PLL is thicker in the lumbar region than in the cervical spine, providing better protection
• C The PLL is wider over the disc than over the vertebral body, but narrows toward the posterolateral aspect — creating a zone of relative weakness where posterolateral disc herniations occur most commonly ✓
• D The PLL contains elastic fibers that allow controlled disc expansion posteriorly without herniation

Explanation

The posterior longitudinal ligament is wider over the intervertebral disc (hourglass shape: broad at disc, narrow at vertebral body) providing some central reinforcement. However, it narrows and becomes thinner laterally, creating zones of relative weakness at the posterolateral aspect of each disc. This explains why nucleus pulposus herniations occur most commonly in the posterolateral direction (not central) — the fibers of the annulus fibrosus and the attenuated PLL offer least resistance at this location. Central disc protrusions are less common precisely because the central PLL is reinforced.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.