disc nomenclature

1. COMPONENTS AND STRUCTURE OF THE IVD

The intervertebral disc (IVD) is located between vertebral bodies and consists of three primary components: the nucleus pulposus, annulus fibrosis, and cartilage end-plates. It is innervated by the sinuvertebral nerve.

  • Nucleus Pulposus (NP): Composed of randomly organized collagen and radially arranged elastin in a hydrated gel featuring low-density chondrocyte-like cells.

    • Functions: Acts as a hydraulic shock absorber, maintains spinal flexibility, and withstands compression.

    • Aggrecan: A proteoglycan within the NP that binds water to allow for hydration and reversible deformation under load.

  • Annulus Fibrosis (AF): Composed of 15−2515 - 25 concentric rings called lamellae.

    • Functions: Contains the NP, anchors the disc to the vertebral body (VB), resists rotation, and provides strength and stability.

  • Cartilage End Plate (CEP): A thin layer (< 1\text{ mm}) of hyaline cartilage anchoring the disc to the vertebral body.

    • Functions: Serves as a mechanical barrier and a medium for nutrient transport.

2. CLASSIFICATION OF A NORMAL DISC

A normal disc is defined by its morphological integrity. It maintains its position within the boundaries of the ring apophyses, shows no signs of desiccation or displacement, and has an intact annulus fibrosis and well-hydrated nucleus pulposus.

3. SUBCATEGORIES OF DISC DEGENERATION

Disc degeneration is categorized by specific pathological changes:

  • Annular Fissure: A separation between annular fibers or a separation of fibers from their attachment to the VB.

  • Physiological Signs: Includes disc desiccation (loss of hydration), fibrosis, narrowing of the disc space, and the presence of intradiscal gas.

  • Structural Changes: Osteophyte formation (bony overgrowths) and end plate sclerosis.

4. PATHOANATOMIC CLASSIFICATIONS OF DISC HERNIATION

Disc herniation is the displacement of disc material beyond the intervertebral disc space.

  • Spatial Classifications:

    • Focal: Localized herniation.

    • Broad-based: Displacement involving a larger portion of the disc circumference.

    • Regions: Can occur in central, paramedian, foraminal, or extraforaminal regions.

  • Morphological Classifications:

    • Protrusion: The displaced material remains at the level of the disc space with a base wider than the displacement itself.

    • Extrusion: The displaced material has a base narrower than the displacement or loses continuity.

    • Migration: Displacement of the material away from the original site of extrusion.

    • Sequestration: The displaced fragment loses all continuity with the parent disc.

  • Schmorl Node: A localized intravertebral body herniation where disc material is displaced through the cartilage end plate into the vertebral body.

5. PROFILE OF A DISC BULGE

A disc bulge is defined by specific geometric criteria that distinguish it from herniation:

  • Extent: Disc tissue extends > 25\% beyond the ring apophyses.

  • Integrity: The annulus remains intact.

  • Context: Often considered a finding associated with generalized degeneration rather than an acute focal injury.