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BASIC SCIENCE OF SPINAL DISCS

BASIC SCIENCE OF SPINAL DISCS. Mr O M Stokes. Orthopaedic SPR Core Curriculum Teaching. EMBRYOLOGY. 4 / 40 gestation Vertebrae formed by 2 adjacent sclerotomes (mesoderm) Mesenchymal cells Located between VBs Contribute to disc formation Notochord (endothelial germ layer)

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BASIC SCIENCE OF SPINAL DISCS

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  1. BASIC SCIENCE OF SPINAL DISCS Mr O M Stokes Orthopaedic SPR Core Curriculum Teaching

  2. EMBRYOLOGY • 4 / 40 gestation • Vertebrae formed by 2 adjacent sclerotomes (mesoderm) • Mesenchymal cells • Located between VBs • Contribute to disc formation • Notochord (endothelial germ layer) • Forms nucleus pulposus

  3. STRUCTURE - Macro • 2º cartilaginous joint • Outer annulus fibrosus • fibrocartilaginous • Inner nucleus pulposus • ↑ size caudally • L4/5 largest + most avascular • Cervical discs – thicker anteriorly • Thoracic discs – uniform in height • Lumbar discs – thicker anteriorly

  4. ANNULUS - Micro • Attachments • ALL + PLL • VBs (+ ribs) • Outer annulus • Lamellea • Densely packed layers type I collagen • Fibroblasts • 30º to horizontal • Alternate in each layer • Resist tension + shear • Inner annulus • Less dense type II collagenous matrix • Lacks lamellar arrangement • Chondrocytes

  5. NUCLEUS - Micro • Cellular component • @ birth • Mass of interconnected notochordal cells • Chorda reticulum • In adults • Chondrocyte-like cells • Acellular component • Type II collagen • Mucoprotein gel (polysaccharides) • Proteoglycan matrix • Hyaluronan filaments, aggrecan molecules + link proteins • Interaction with water gives • Viscoelasticity, stiffness + R to compression

  6. Visco-Elastic Definitions Viscous - Time dependant and non recoverable Elastic - Time independent and recoverable Creep - Time dependant deformation in response to a constant load Stress Relaxation - Time dependant decrease in load required to maintain a constant strain Time dependant strain behaviour - the faster the rate of strain, the higher the given stress for a given level of strain Hysteresis - Strain energy is lost due to friction

  7. DISC ANCHORING • Outer 1/3 • Sharpey’s fibres • Ring apophysis • Inner 2/3 • Curve into + form fibrocartilaginous component of vertebral end plate • No fibrillar connection to subchondral bone • Susceptible to shear • End plate • Young – hyaline cartilage • Elderly – calcified cartilage + bone

  8. COLLAGEN • ↑ type II towards NP • Other fibril-forming collagens • V + XI • Non-fibril-forming collagens • VI (unique to discs), IX + XII • Much of I + II is X-linked with pyridinoline residues • Imp in tissues bearing compressive load

  9. NERVES • Outer ring of annulus only • Dorsally • Sinuvertebral n • Ventrally • Sympathetic chain VASCULARITY • Surface of annulus only • Disc sustenance • Diffusion + convection • Porous central cavity of end plate • Porous-permeable solid matrix

  10. NORMAL AGEING - From 3rd decade • NP – loss of cells + PGs (↓water content) • ↑ proportion of non-aggregated PGs and non-collagenous proteins (↑ keratin sulfate) • ↓ size aggrecan + ↓ conc. link proteins • AF – fibrotic • Fissures + cracks • Myxomatous degeneration – loss microstructure • Complete by 6th – 7th decade • Stiff fibrocartilage

  11. Involved Neuropeptides • Substance P • CGRP • VIP • C-flanking peptide of neuropeptide Y (CPON)

  12. FUNCTION • Facilitate motion • Resist • Tension • Rotation • Shear • Redistribute • Compression

  13. Fibre arrangement in AF • Resist • Tension • Rotation • Shear • Viscoelastic properties of NP – allow load absorption + maintenance of height • Hydrostatic p. • Donnan osmotic p. • Loose framework of porous-permeable collagen-PG matrix

  14. Donnan Osmotic Pressure • Repulsive forces • Fixed –vely charged PGs • Freely mobile interstitial counter-ions

  15. BIPHASIC THEORY • The inner AF also has viscoelastic properties • Permits large deformations in response to load • Creates intradiscal fluid-flows that dissipate • Energy • Creep • Compression generates hoop stresses in outer layers • Inner layers deform + act as shock absorbers

  16. Prolonged axial loading • Water squeezed out of discs • Load removed (sleep) • In-flow of water Explains why most disc herniations occur in the morning

  17. ANNULAR TEAR • Due to sudden ↑ in intradiscal p. • Symptoms due to NP contents irritating innervated outer AF layers

  18. HERNIATION • Occurs at insertion of outer AF into VB • C / L spine • Greatest stresses • Bulging of AF / herniation NP material • Nat Hx – 90 % Pts pain free @ 3 / 12 • But self-repair of AF is limited

  19. DEGENERATION • Multifactorial causes • ↓ nutritional supply • ↓ r/o waste products • ↓blood supply • Calcification of end plates • Accumulation of degraded matrix macromolecules • ↓ water concentration

  20. Predisposing Factors • Physical activity (axial loading) • Immobilization • Vibration • Deformity (scoliosis etc.) • ↓ blood flow • DM • Smoking • Arterial disease

  21. Degeneration Distinct From Ageing? • Degeneration • Disruption of collagen fibrils • Impaired formation • ↑ X-linking • ↑ denaturation • Annulus delamination • Through interlamellar GS • ↑ solid behaviour • End plate • Thinning • Micro-fractures • ↑ discal fluid exudation upon loading

  22. Degenerative Pain Theories • Nerve + bv in-growth • Altered biomechanical loading • Surrounding innervated structures • Cell necrosis • Stimulating inflammatory cascade • Cytokine + free radical release • Sensitise nerve endings

  23. THANK YOU

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