Fig. 15.

The stages of the nucleus pulposus displacement. The migrated intradiscal nucleous pulposus fragment displaces posteriorly. The arrows indicate the separation of the intradiscal fragment from the remaining nucleous pulposus material. (a) Acute herniation. It occurs at the early stages of degeneration when the intradiscal pressure is still relatively high. It causes the annulus fibrosus fibres to rupture and lead to acute local inflammation. (b) Subacute herniation. This usually arises only when the disc material migrates peripherally with increasing intradiscal pressure increases and improves when the intradiscal pressure drops. The remaining intact fibres of the annulus fibrosus recoil to bring the extruded material back into the disc space. (a) Chronic herniation. Chronic protrusions persist because of high intradiscal pressure pushing the nucleous pulposus material out of the disc