Intervertebral disc

Embryology

• Vertebral column and discs develop at 4 weeks.
• Two adjacent sclerotomes combine to form pre-cartilaginous vertebral body.
• Mesenchymal cells lie between the vertebrae.
• Notochord forms the nucleus pulposus.
• Circular fibers form around this to create the annulus fibrosus.
• The disc is a secondary cartilaginous joint.

Structure

• Discs get larger more caudally, with the largest being L4/5.
• Thicker anteriorly in cervical and lumbar spine, contributing to lordosis.
• L5/S1 is the most avascular disc and thickest anteriorly.

Annulus

Outer Annulus

• Smaller in size.
• Made of Type 1 collagen in lamellae.
• Adjacent lamellae are obliquely oriented with respect to each other.
• Resists tensile and shear stresses.
• Contains fibroblasts and nerves.
• Attached to anterior longitudinal ligament (ALL) and posterior longitudinal ligament (PLL).

Inner Annulus

• Larger in size.
• More Type 2 collagen.
• No lamellar organization.
• Contains chondrocytes.

Nucleus Pulposus

• Made of Type 2 collagen.
• Rich in mucoid proteoglycan matrix.
• Proteoglycans (PGs) attract water, accounting for 80% content.
• PGs provide compressive strength.
• The matrix gives visco-elastic and anisotropic properties.
• Greater stiffness with rapid loading.
• Elasticity and resistance to compression.

Attachment to Vertebrae

• Outer 1/3 of the disc connects directly via Sharpey’s fibers, forming a ring apophysis.
• Inner 2/3 folds in to contribute to the vertebral end plate.

End Plate

• Covered with hyaline cartilage in young individuals.
• Changes to calcified cartilage with age.
• No fibrillar connections between the end plate and vertebra.
• Vulnerable to shear stresses.

Collagens Present in the Disc

• Type 5, Type 6 (unique to intervertebral disc), Type 9-12.
  • Type 1 in outer annulus.
  • Transition to Type 2 more centrally.
  • Type 6 is unique to discs.

Nerve Supply

• Only the outer annulus has a nerve supply.
• Dorsal: Sinuvertebral nerves (branches of spinal nerves).
• Ventral: Sympathetic chain.

Blood Supply and Nutrition

• Capillary plexus on the surface of the disc.
• Nutrition is via diffusion through the end plates.

Age-Related Changes

• Disc becomes dehydrated and fibrotic over time.
• Nucleus pulposus is replaced by fibrocartilage.
• PG content decreases.
• Chondroitin sulfate decreases.
• Chondroitin-keratin sulfate ratio decreases.
• Water content drops.
• Relative collagen content increases.
• Disc becomes unable to resist load.
• Nucleus starts behaving more like a solid.
• Fissures and cracks develop in the annulus.
• Alters mechanical properties of the spine.
• Destabilizes the spinal motion segment.

Consequences of Age-Related Changes

• Facet joint degeneration.
• Risk of disc herniation.
• Ligamentum flavum buckling.
• Further loss of disc height.
  • Narrowing of neural foraminae, leading to foraminal/recess stenosis.
  • Further facet arthrosis.
  • Flavum hypertrophy.
  • Canal stenosis.
  • Degenerative spondylolisthesis.

Disc Function

• Primary source of spinal mobility.
• Resists tensile, compressive, and shear stresses while maintaining movement.
• Anatomic Properties:
  • Outer annulus resists shear and torsion due to alternating lamellar structure and Type 1 collagen.
  • Nucleus pulposus and inner annulus better resist compression, converting compression to radial forces.
  • These forces generate hoop stresses in the annulus.
• The disc functions as a biphasic material.
  • Outer annulus undergoes hoop stresses in compression.
  • Inner layers act as shock absorbers, dissipating stresses.

Disc Injury and Healing

Herniation

• Annular tear starts in the inner annulus due to sudden increases in intradiscal pressure.
• Pain arises from the nuclear part bulging through the tear and irritating nerves in the outer annulus.
• If the tear propagates to the outer annulus, bulging or herniation occurs, causing pain from the disc and nerve root impingement.
• 90% of herniations are asymptomatic at 3 months due to:
  • Resorption of nucleus material.
  • Annular tear does not repair itself.

Disc Degeneration

• Occurs with age due to chronic dehydration of the disc and impaired nutrition.
• PG content decreases, reducing water content.
• Relative collagen content increases.
• Decreased vascularity and progressive calcification of endplates impair diffusion.
• End plate microfractures occur, permanently deforming the disc.
  • The disc loses its elastic properties and behaves like a solid.
  • Alters mechanics and load distribution, leading to osteophyte formation, facet joint arthrosis, and pain.