Posterior Longitudinal Ligament (Lumbar) Detail

ligamentum longitudinale posterius lumbale detail

The lumbar posterior longitudinal ligament lines the posterior vertebral bodies within the spinal canal, narrowing over the vertebral bodies (8mm wide) and expanding over the disc spaces (12-15mm wide). The PLL is weaker at the posterolateral disc margin — the preferential site of posterolateral disc herniation.

Region: Lower Back

Biomechanics

Function & Actions

Resists posterior disc herniation centrally, limits lumbar hyperflexion, reinforces the posterior annulus fibrosus centrally, and is thinner and less well-developed at the posterolateral disc margins where most herniations occur.

Clinical Relevance

Clinical Notes

The posterolateral PLL weakness explains why lumbar disc herniations occur most commonly at the posterolateral margin rather than centrally — the central PLL is stronger. In discectomy, the PLL may be preserved or removed depending on the extent of decompression required. OPLL at the lumbar level is rarer than cervical but may produce cauda equina syndrome.

Pathology

Common Injuries & Conditions

Posterolateral Disc Herniation at PLL Weakness

Lumbar disc material herniated through the posterolateral PLL weakness producing nerve root compression — the most common pattern in lumbar disc disease, managed by microdiscectomy when conservative treatment fails.

Lumbar PLL Disruption in Burst Fracture

PLL tear allowing retropulsion of bone fragments from the posterior vertebral body wall into the spinal canal in lumbar burst fractures, requiring surgical decompression when causing neurological deficit.