The posterior annulus is the critical zone for disc herniation — posterolateral fibres lack PLL reinforcement and experience the highest tensile forces during lumbar flexion with rotation. Sequential fibre failure produces the radial fissure through which the nucleus pulposus herniates to compress adjacent nerve roots. The posterolateral herniation at L4-L5 and L5-S1 compresses the L5 and S1 roots respectively.
| Origin | Posterior vertebral body endplate (inferior) |
|---|---|
| Insertion | Posterior vertebral body endplate (superior of adjacent vertebra) — reinforced by the PLL centrally but bare posterolaterally (the herniation zone) |
| Actions | Resists posterior disc herniation; the posterolateral zone is the weakest — no PLL reinforcement and the highest compressive forces during flexion-rotation |
|---|
L4-L5 posterolateral disc herniation compresses the traversing L5 root (not the exiting L4) producing foot dorsiflexion weakness, first toe weakness, and lateral leg numbness. L5-S1 herniation compresses S1 producing ankle plantarflexion weakness, reduced Achilles reflex, and lateral foot numbness. Discectomy removes the herniated fragment — the remaining annulus heals over 6-8 weeks.
Posterior annulus radial tear allowing nucleus herniation to compress traversing nerve root managed with conservative physiotherapy or microdiscectomy for refractory radiculopathy.
