Lumbar disc prolapse occurs most commonly in a posterolateral direction rather than straight posteriorly. The anatomical reason is:

• A The anterior longitudinal ligament is thicker than the posterior longitudinal ligament
• B The nucleus pulposus is displaced forward by the strong posterior longitudinal ligament
• C The posterior longitudinal ligament is narrow and weak over the posterolateral aspect of each disc body ✓
• D The ligamenta flava push the disc material laterally during flexion

Explanation

The posterior longitudinal ligament is well developed opposite the vertebral bodies but narrows and becomes thin over the back of each disc, offering least resistance posterolaterally where the nucleus herniates and compresses the traversing nerve root. The anterior longitudinal ligament is indeed strong, but that fact supports option A's premise without explaining the direction, and the key restraint is the weakness of the PLL laterally.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.