The Intervertebral Disc

OPBSMusculoskeletalBiomechanics

Must Know

• The disc is a fibrocartilaginous joint allowing multiaxial motion while bearing spinal compression.
• Discs sit between all vertebral bodies except C1 and C2.
• Spine has 33 vertebrae. 7 cervical lordotic, 12 thoracic kyphotic, 5 lumbar lordotic, 5 fused sacral, 4 fused coccygeal.
• Nucleus pulposus (NP) acts as an osmotic pump, drawing water and generating hydraulic pressure.
• Anulus fibrosus (AF) contains NP pressure and limits intervertebral rotation.
• End plate (EP) is a cartilage and bone bilayer. A thin hyaline cartilage plate covers the upper and lower surfaces of each vertebral body. It must be strong yet permeable.
• The disc is the largest avascular structure in the body. Cells lie up to 6 to 8 mm from vessels.
• Nutrition diffuses from capillaries in the outer AF and bony EP. EP permeability is highest centrally.
• Low oxygen (0.5 to 5 percent) forces glycolysis, producing lactic acid.

More on spinal anatomy

• Erector spinae extend the spine. Psoas flexes the hip bilaterally and bends the trunk unilaterally.
• Multifidus links segments and acts as a bowstring maintaining lordosis.
• Passive supports include ALL, PLL, ligamentum flavum, facet capsule, interspinous and supraspinous ligaments.

Composition

Percent	Nucleus pulposus	Anulus fibrosus	Cartilage end plate
Water	77	60	55
Proteoglycan	14	5	8
Collagen	4	15	25
Elastin	Under 1	Under 10	Under 1

Region	Cell	Main products	Collagen fibrils
NP	Chondrocyte like	Aggrecan, type II collagen	Type II, 10 to 100 nm, irregular
AF	Fibrocyte like	Types I, II, III and VI collagen, chondroitin and keratan sulfate	Type I, 100 to 200 nm, rising peripherally. Inner AF mostly type II
Cartilage EP	Chondrocyte like	Proteoglycan, type II collagen	Type II

• Aggrecan (3000 to 7000 kDa) carries negatively charged sulfated GAGs that bind sodium and draw water.
• Swelling pressure is resisted by circumferential tension in AF collagen.
• Types I and II make up about 90 percent of disc collagen.
• AF has 10 to 25 concentric lamellae with fibres alternating at plus and minus 30 degrees.
• Elastin crisscrosses between AF lamellae, controlling interlamellar sliding and radial NP bulge.
• Cartilage EP is 0.1 to 2.0 mm thick, thinner centrally and at upper spinal levels.
• Advanced glycation end products form irreversible cross-links, stiffening and dehydrating the disc. Diabetes speeds this.
• Most abundant nonstructural proteins are zinc dependent MMPs, TIMPs and ADAMTS.
• Notochordal cells at birth are replaced by chondrocyte like cells over the first two decades.
• NP is semi-liquid and gelatinous, derived from notochord, with water falling from about 90 percent in childhood.
• NP lies in the posterior disc, especially in the lumbar spine, so it herniates posteriorly more easily.

More on structure, mechanics and imaging

• Outer AF anchors to the vertebral rim through a fibrocartilage enthesis with a calcified tidemark.
• Collagen of the cartilaginous and bony EP layers is completely separate.
• Paraspinal cross-sectional area is smaller in low back pain.
• Bending stiffness rises with bending angle. Compression gives immediate elastic then viscoelastic height loss.
• High pressure and proteoglycan normally repel nerve ingrowth.
• Advanced MRI such as T1 rho, T2 mapping, sodium MRI and spectroscopy detect chemical change.
• NP has the most water and proteoglycan and least collagen. Outer AF is the reverse. EP has most collagen.
• MMP cleavage of link protein fragments proteoglycan in degeneration.
• Collagen triple helix is 300 nm long and 1.5 nm wide.
• Peripheral AF fibres run on into the EP. Central NP to EP integration is convoluted.
• Notochordal cell apoptosis follows rising gravity load and falling vertebral perfusion. EP or inner AF cells then fill the NP.
• Nutrient competition caps cell density near 1600 cells per cubic mm.
• The NP makes up about 15 percent of the disc.

Biomechanics

• The bare spinal column buckles under axial load. Stability needs passive osteoligamentous and active muscle constraint.
• NP is pressurised at rest by pre-stress from the longitudinal ligaments.
• Under load the NP acts hydrostatically, spreading pressure through the disc.
• Load gives compressive stress in the NP and tensile hoop stress in the AF.
• The AF resists bending and torsion and limits excessive movement.
• NP cells sense hydrostatic pressure. AF cells sense stretch under biaxial load.
• EP sees biaxial tension. Bone is weaker in tension, so the EP is the weak link in compression.
• The semipermeable EP lets water leave when load exceeds swelling pressure and return when unloaded.
• The hydrophilic NP imbibes water overnight and loses it through the day.
• Static load shrinks the NP so the AF behaves as a compressive strut, raising MMPs and apoptosis.
• Low cyclic loads are homeostatic. High loads at low frequency or long duration turn catabolic.
• Both too little and too much load are catabolic, matching a U shaped link between exposure and back pain.

Stimulus	Anabolic range	Catabolic range
Hydrostatic pressure in vitro	0.2 to 1.0 MPa	1 to 4 MPa
AF cyclic tension	1 percent strain at 1 Hz	Over 5 percent strain

Degeneration

• Begins in the second decade as notochordal cells disappear.
• GAG falls 3 to 4 percent a year, fastest at L5/S1.
• Almost everyone over 60 has at least one degenerate level.
• Earliest sign is NP water loss from falling swelling pressure.
• Dehydration brings height loss, inner AF stress shielding, EP stress redistribution, hypermobility and facet overload.
• NP fibroses and blurs into the AF. Fissures, EP erosion or avulsion and tidemark separation follow.
• End stage stiffening restabilises the segment.
• Degeneration occurs when matrix damage outpaces repair. NP cells become senescent and apoptotic.
• Schmorl node is NP herniation into vertebral body trabecular bone. Common in asymptomatic people.

Innervation and Pain

• The disc generates pain in about half of chronic low back pain.
• Healthy innervation reaches only the outer three annular layers.
• Fibres are group III A-delta (myelinated, sharp pain) and group IV C (unmyelinated, dull ache).
• Cell bodies sit in the dorsal root ganglia.
• Routes to the DRG are the sinuvertebral nerve and the paravertebral sympathetic trunk.
• Sinuvertebral nerve is a recurrent branch of the ventral primary ramus.
• Basivertebral nerve supplies the EP through the posterior neurovascular foramen. EP nerve density can exceed peripheral AF.
• IL-1 drives NGF and BDNF, which with GAG loss allow neoinnervation and neovascularisation.

Modic type	T1	T2	Histology
I	Low	High	Fibrovascular, active inflammation, EP fissuring
II	High	Iso or slightly high	Fatty marrow
III	Low	Low	Dense woven bone, sclerosis

• Types I and II can convert to each other. Type I can return to normal.
• Modic I and II are highly specific but poorly sensitive (15 to 65 percent) for discogram pain.
• Modic I is especially painful and predicts persistent symptoms.

More on pain chemistry and end plate defects

• bFGF, IGF-1 and PDGF from damaged areas promote granulation tissue.
• Disc derived IL-1, IL-6, IL-8, TNF-α, PGE2, MCP-1 and NGF sensitise and recruit nerves.
• Lactic acid from anaerobic metabolism irritates EP nerves.
• Larger EP defects in cadavers link to worse degeneration and more back pain, odds ratio 17.88.
• Painful Schmorl nodes show marrow lesions, suggesting a traumatic cause.

Diagnostics

• Thompson grade. Radiographic, detects only gross deformity.
• Provocation discography. Benchmark for discogenic pain. Low pressure technique gives specificity 0.94, false positive 6 percent.
• MRI. Benchmark for disc anatomy. Healthy NP is bright on T2. Cartilage EP is invisible from short T2.
• Pfirrmann grade. I to V on T2 MRI, V most degenerate.
• Routine MRI. Not advised unless structural pathology is suspected and conservative care fails.

Numbers to Remember

• Lumbar disc pressure. Sleep 0.1 MPa (10 N per square cm), quiet standing 0.5 MPa, routine 2 MPa, lifting in flexion 3 MPa.
• Daily change. Stature falls 15 to 25 mm. Disc volume varies about 10 percent.
• Nerve fibres. A-delta 1 to 5 µm, C fibres 0.5 to 2 µm.

Memory Aids

• Modic 1 wet, 2 fat, 3 hard. Oedema, fat, then sclerosis.

Exam Traps

• Elastin is concentrated in the AF, under 10 percent against under 1 percent in the NP.
• Disc cell density does not reliably fall in advanced degeneration, as height loss shortens diffusion.

Related pages. Meniscus and Labrum, Vertebral Joints & Ligaments, Lumbar Disc Prolapse, Lumbar Spondylosis, Degenerative Disc Disease, and Radiculopathy

Practise Type X questions on this topic

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.