Anatomy of Intervertebral Disc

Structure of the Disc

• The disc lies between two vertebral end plates, separating it from the vertebral body above and below.
• The disc consists of two main parts: an outer fibrous coat (annulus fibrosus) and an inner gelatinous core (nucleus pulposus).
• The annulus fibrosus is formed of concentric lamellae of yellow elastic fibers rich in collagen.
• The nucleus pulposus is a mucopolysaccharide-protein complex that is rich in water (80% of its content).

Direction of Herniation

• Herniation can occur in four directions: anterior, posterior, upward, or downward.
• Upward or downward herniation occurs through the cancellous bone of superjacent and interjacent bodies, resulting in Shmorl's nodules.
• Anterior herniation occurs in the retroperitoneal space, having a negligible compressing effect due to the elastic structure.
• Posterior direction is the most important direction of lumbar disc herniation, which can be either direct posterior or posteriolateral.

Posterior Herniation

• Direct posterior herniation: compresses the thecal sac (neural tube), which may affect all cauda equina roots if severely compressed (cauda equina syndrome).
• Posterolateral herniation: affects only the offending nerve root, resulting in most cases a single affected nerve root.