Components
The intervertebral disc is composed of:
- nucleus pulposus
- annulus fibrosus
- cartilaginous endplates
These components work together to transmit load while allowing controlled spinal motion.
Nucleus pulposus
The nucleus is rich in water and proteoglycans. Its matrix attracts water and develops hydrostatic pressure under compression.
Functions:
- distributes compressive load
- converts axial compression into radial stress on the annulus
- contributes to disc height and flexibility
Annulus fibrosus
The annulus consists of concentric lamellae. Collagen fibres in adjacent lamellae run in alternating oblique directions.
This arrangement:
- resists circumferential tension
- restrains the pressurised nucleus
- resists torsional loading
Outer annulus contains more type I collagen; the inner disc has relatively more type II collagen and proteoglycan-rich matrix.
Endplates
The cartilaginous endplates:
- separate disc from vertebral body
- distribute pressure
- provide a major route for diffusion of nutrients and metabolic waste
The adult disc has very limited direct vascularity.
Motion segment
A spinal motion segment consists of two adjacent vertebrae and the structures connecting them, including:
- intervertebral disc
- paired facet joints
- ligaments
- associated muscles and neural elements
The disc primarily manages compression and motion; facets contribute to guidance and restraint.
Degeneration
Disc degeneration involves a progressive shift in matrix homeostasis:
- proteoglycan loss
- reduced water content
- fissuring of the annulus
- altered load transfer
- reduction in disc height
- changes in endplates and adjacent vertebral bone
As nucleus pressurisation decreases, a greater proportion of load is transferred to the annulus and posterior elements.
Degeneration involves more than dehydration. Features include:
- loss and fragmentation of proteoglycans
- reduced nucleus pressurisation
- annular fissuring
- cell senescence and altered matrix turnover
- endplate change
- altered segmental load sharing
- osteophyte formation and facet overload
As disc height decreases, foraminal dimensions may reduce and load transfer to posterior elements can increase. Segmental instability may occur in some phases of degeneration, while advanced degeneration can later become relatively stiff.
Ageing versus degeneration
Ageing produces gradual cellular and matrix changes in most discs. Degeneration implies structural and functional deterioration beyond simple chronological ageing and is influenced by genetics, loading, smoking, injury and other factors.
Clinical links
- annular fissures may be associated with disc herniation
- loss of disc height alters facet loading
- altered segmental mechanics may contribute to foraminal narrowing and degenerative deformity
- imaging abnormalities do not always correlate with symptoms
Viva framework
Describe the three components, then explain how a hydrated nucleus and tensioned annulus create a load-bearing composite. Finish with nutrition and the mechanical consequences of degeneration.
Disc biomechanics
The disc behaves as a composite structure. A hydrated nucleus generates pressure under axial load, while the annulus contains that pressure through circumferential tension. The endplates transmit load between disc and vertebral bodies and permit diffusion of nutrients.
Loading is time-dependent. Under sustained compression, fluid gradually leaves the disc and disc height falls. During unloading, water is reabsorbed. This explains diurnal variation in disc height and illustrates viscoelastic behaviour.
The annulus also resists torsion and bending. Because collagen fibres in adjacent lamellae run in opposite oblique directions, different fibre groups become tensioned with opposite directions of rotation.
Nutrition and cell environment
The adult disc is largely avascular. Most nutrient transport occurs by diffusion through the cartilaginous endplates and, to a lesser extent, from vessels at the outer annulus. Endplate calcification, smoking and other factors that impair diffusion can adversely affect the disc cell environment.
Disc cells live in relatively low oxygen tension and must maintain a large extracellular matrix despite limited nutrient supply. The balance between proteoglycan synthesis and degradation is therefore vulnerable to ageing, genetic predisposition and altered mechanical conditions.
Disc herniation
A disc herniation requires failure or fissuring of the annulus with displacement of nucleus material. Terminology should distinguish broad-based bulging from focal protrusion or extrusion. Clinical significance depends on the relationship to neural structures and the inflammatory response, not merely the presence of an imaging abnormality.
A useful concept is that radiculopathy can reflect both mechanical compression and chemical inflammation around a nerve root.
Endplate failure
The vertebral endplate can fail under excessive load, particularly when bone quality is poor. Endplate defects can influence disc nutrition and may alter disc mechanics. Schmorl nodes represent herniation of disc material through the endplate into the vertebral body and are not necessarily symptomatic.
Clinical interpretation
MRI demonstrates morphology but must be correlated with symptoms and examination. Degenerative signal change, bulging and even protrusions can occur in asymptomatic individuals. Avoid treating an image in isolation.
When discussing a painful degenerative segment, consider:
- disc
- facet joints
- endplates
- neural compression
- muscle and sagittal balance
- psychosocial and systemic contributors
FRCS synthesis
The central viva relationship is: water-rich proteoglycan nucleus generates pressure; annular collagen contains that pressure; endplates transmit load and support diffusion. Degeneration disrupts all three parts, changing both biology and mechanics of the motion segment.