Intervertebral Disc¶
Definition¶
The intervertebral disc is a fibrocartilaginous structure situated between adjacent vertebral bodies from C2–C3 to L5–S1 (23 discs total). Each disc consists of a central nucleus pulposus surrounded by a peripheral annulus fibrosus, bounded superiorly and inferiorly by cartilaginous endplates. The discs account for approximately 25% of the total height of the vertebral column and serve as the primary shock absorbers of the spine.
Anatomy¶
Nucleus Pulposus¶
- Centrally located, gelatinous structure composed primarily of water (80–90% in youth), type II collagen, and proteoglycans (aggrecan)
- Functions as a hydraulic cushion, distributing compressive forces
- Avascular — receives nutrition via diffusion through the endplates
- Positioned slightly posterior to the center of the disc
- Progressively loses water content and height with aging (desiccation)
Annulus Fibrosus¶
- Concentric rings (lamellae) of type I collagen fibers arranged in alternating oblique orientations
- Stronger anteriorly and laterally; thinnest posterolaterally — the most common site of disc herniation
- Contains the nucleus pulposus and resists tensile, rotational, and shearing forces
- Outer fibers are innervated (sinuvertebral nerve) and may be a source of discogenic pain when torn
Vertebral Endplates¶
- Thin layers of hyaline cartilage covering the superior and inferior surfaces of each vertebral body
- Critical pathway for disc nutrition via diffusion from vertebral body vasculature
- Endplate damage disrupts nutrition and accelerates disc degeneration
Clinical Pearl
The annulus fibrosus is thinnest posterolaterally, explaining why the vast majority of disc herniations occur in a posterolateral direction, compressing the traversing nerve root. The posterior longitudinal ligament provides some reinforcement in the midline but is weaker laterally.
Disc Nomenclature¶
The American Society of Spine Radiology (ASSR) and American Society of Neuroradiology (ASNR) standardized disc terminology:
| Term | Definition |
|---|---|
| Bulge | Circumferential, symmetric extension of disc beyond the vertebral body margins (> 50% of circumference); NOT considered a herniation |
| Protrusion | Focal extension with a base wider than the apex; the displaced material maintains continuity with the disc |
| Extrusion | Focal extension with a base narrower than the apex in any plane; may extend superiorly or inferiorly |
| Sequestration | Extruded fragment that has lost continuity with the parent disc (free fragment) |
See Disc Bulge vs Herniation Nomenclature for detailed classification.
Imaging Findings¶
Radiography¶
Radiographs cannot directly visualize the disc but demonstrate indirect signs:
- Disc space narrowing — loss of disc height indicating degeneration
- Vacuum phenomenon — gas (nitrogen) within the disc, pathognomonic of degeneration
- Endplate sclerosis — reactive bony changes adjacent to degenerated discs
CT¶
- Disc material appears as soft tissue density
- Calcification within the disc is readily identified
- Vacuum phenomenon is highly conspicuous
- CT myelography can demonstrate disc herniations indenting the contrast-filled thecal sac
MRI¶
MRI is the gold standard for disc evaluation:
| Disc Status | T1 Signal | T2 Signal | Description |
|---|---|---|---|
| Normal (young) | Intermediate | Bright (white) | Well-hydrated nucleus with distinct intranuclear cleft |
| Early degeneration | Intermediate | Decreased (gray) | Loss of hydration beginning |
| Advanced degeneration | Intermediate–low | Dark (black) | Complete desiccation, often with height loss |
| Annular fissure | — | Bright spot in annulus | High-signal zone (HIZ) at the posterior annulus on T2 |
| Herniation | Same as disc | Same as disc | Focal extension beyond the vertebral margins |
Pfirrmann Classification
The Pfirrmann grading system classifies lumbar disc degeneration on sagittal T2 MRI from Grade I (bright, homogeneous signal, normal height) to Grade V (dark signal, collapsed disc space). It is widely used in clinical practice and research.
Key Points¶
- The intervertebral disc consists of the nucleus pulposus, annulus fibrosus, and cartilaginous endplates
- The annulus is thinnest posterolaterally, predisposing to posterolateral herniations
- Disc nutrition occurs by diffusion through the endplates — endplate damage accelerates degeneration
- MRI T2 signal in the disc reflects hydration status — bright is normal, dark is degenerated
- Standardized nomenclature distinguishes bulge, protrusion, extrusion, and sequestration
References¶
- Fardon DF, Williams AL, Dohring EJ, Murtagh FR, Gabriel Rothman SL, Sze GK. Lumbar disc nomenclature: version 2.0: Recommendations of the combined task forces of the North American Spine Society, the American Society of Spine Radiology and the American Society of Neuroradiology. Spine J. 2014;14(11):2525-45. Available from: https://pubmed.ncbi.nlm.nih.gov/24768732/
- Waxenbaum JA, Reddy V, Williams C, Futterman B. Anatomy, Back, Intervertebral Discs. StatPearls Publishing; 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470583/
- Nedresky D, Reddy V, Singh G. Anatomy, Back, Nucleus Pulposus. StatPearls Publishing; 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK535373/
- Pfirrmann CW, Metzdorf A, Zanetti M, Hodler J, Boos N. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine. 2001;26(17):1873-8. Available from: https://pubmed.ncbi.nlm.nih.gov/11568697/
- Wu Q, Huang JH. Intervertebral disc aging, degeneration, and associated potential molecular mechanisms. J Head Neck Spine Surg. 2017;1(4):555569. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6003710/
- Brinjikji W, Diehn FE, Jarvik JG, et al. MRI Findings of Disc Degeneration are More Prevalent in Adults with Low Back Pain than in Asymptomatic Controls: A Systematic Review and Meta-Analysis. AJNR Am J Neuroradiol. 2015;36(12):2394-9. Available from: https://www.ajnr.org/content/36/12/2394
- Intervertebral disc. Radiopaedia.org. Available from: https://radiopaedia.org/articles/intervertebral-disc