BASIC SCIENCE OF BASIC SCIENCE OF SPINAL DISCSSPINAL DISCS

Mr O M StokesMr O M Stokes

Orthopaedic SPR Core Curriculum Teaching

EMBRYOLOGYEMBRYOLOGY

4 / 40 gestation4 / 40 gestation

Vertebrae formed by 2 adjacent Vertebrae formed by 2 adjacent sclerotomes (mesoderm)sclerotomes (mesoderm)

Mesenchymal cellsMesenchymal cells Located between VBsLocated between VBs Contribute to disc formationContribute to disc formation

Notochord (endothelial germ layer)Notochord (endothelial germ layer) Forms nucleus pulposusForms nucleus pulposus

STRUCTURE - MacroSTRUCTURE - Macro

22º cartilaginous jointº cartilaginous joint

Outer annulus fibrosusOuter annulus fibrosus fibrocartilaginousfibrocartilaginous

Inner nucleus pulposusInner nucleus pulposus

↑ ↑ size caudallysize caudally L4/5 largest + most avascularL4/5 largest + most avascular

Cervical discs – thicker anteriorlyCervical discs – thicker anteriorlyThoracic discs – uniform in heightThoracic discs – uniform in heightLumbar discs – thicker anteriorlyLumbar discs – thicker anteriorly

ANNULUS - MicroANNULUS - Micro

AttachmentsAttachments ALL + PLLALL + PLL VBs (+ ribs)VBs (+ ribs)

Outer annulusOuter annulus LamelleaLamellea

Densely packed layers Densely packed layers type I collagentype I collagen

FibroblastsFibroblasts

3030º to horizontalº to horizontal

Alternate in each layerAlternate in each layer

Resist tension + shearResist tension + shear

Inner annulusInner annulus Less dense type II collagenous Less dense type II collagenous

matrixmatrix Lacks lamellar arrangementLacks lamellar arrangement ChondrocytesChondrocytes

NUCLEUS - MicroNUCLEUS - MicroCellular componentCellular component @ birth@ birth

Mass of interconnected notochordal cellsMass of interconnected notochordal cells Chorda reticulumChorda reticulum

In adultsIn adultsChondrocyte-like cellsChondrocyte-like cells

Acellular componentAcellular component Type II collagenType II collagen Mucoprotein gel (polysaccharides)Mucoprotein gel (polysaccharides) Proteoglycan matrixProteoglycan matrix

Hyaluronan filaments, aggrecan molecules + link proteinsHyaluronan filaments, aggrecan molecules + link proteinsInteraction with water givesInteraction with water gives

Viscoelasticity, stiffness + R to compressionViscoelasticity, stiffness + R to compression

Visco-Elastic Definitions

Viscous - Time dependant and non recoverableElastic - 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

DISC ANCHORINGDISC ANCHORING

Outer 1/3Outer 1/3 Sharpey’s fibresSharpey’s fibres Ring apophysisRing apophysis

Inner 2/3Inner 2/3 Curve into + form fibrocartilaginous component of vertebral end Curve into + form fibrocartilaginous component of vertebral end

plateplate No fibrillar connection to subchondral boneNo fibrillar connection to subchondral bone

Susceptible to shearSusceptible to shear

End plateEnd plate Young – hyaline cartilageYoung – hyaline cartilage Elderly – calcified cartilage + boneElderly – calcified cartilage + bone

COLLAGENCOLLAGEN

↑ ↑ type II towards NPtype II towards NPOther fibril-forming collagensOther fibril-forming collagens V + XIV + XI

Non-fibril-forming collagensNon-fibril-forming collagens VI (unique to discs), IX + XIIVI (unique to discs), IX + XII

Much of I + II is X-linked with pyridinoline Much of I + II is X-linked with pyridinoline residuesresidues Imp in tissues bearing compressive loadImp in tissues bearing compressive load

NERVESNERVESOuter ring of annulus onlyOuter ring of annulus only DorsallyDorsally

Sinuvertebral nSinuvertebral n VentrallyVentrally

Sympathetic chainSympathetic chain

VASCULARITYVASCULARITYSurface of annulus onlySurface of annulus only

Disc sustenanceDisc sustenance Diffusion + convectionDiffusion + convection

Porous central cavity of end platePorous central cavity of end plate

Porous-permeable solid matrixPorous-permeable solid matrix

NORMAL AGEING - From 3NORMAL AGEING - From 3rdrd decade decade

NP – loss of cells + PGs (↓water content)NP – loss of cells + PGs (↓water content) ↑ ↑ proportion of non-aggregated PGs and non-proportion of non-aggregated PGs and non-

collagenous proteins (↑ keratin sulfate)collagenous proteins (↑ keratin sulfate) ↓ ↓ size aggrecan + ↓ conc. link proteinssize aggrecan + ↓ conc. link proteins

AF – fibroticAF – fibrotic Fissures + cracksFissures + cracks Myxomatous degeneration – loss microstructureMyxomatous degeneration – loss microstructure

Complete by 6Complete by 6thth – 7 – 7thth decade decade Stiff fibrocartilageStiff fibrocartilage

Involved NeuropeptidesInvolved Neuropeptides

Substance PSubstance P

CGRPCGRP

VIPVIP

C-flanking peptide of neuropeptide Y (CPON)C-flanking peptide of neuropeptide Y (CPON)

FUNCTIONFUNCTION

Facilitate motionFacilitate motion

ResistResist TensionTension RotationRotation ShearShear

RedistributeRedistribute CompressionCompression

Fibre arrangement in AFFibre arrangement in AF ResistResist

TensionTensionRotationRotationShearShear

Viscoelastic properties of NP – allow load Viscoelastic properties of NP – allow load absorption + maintenance of heightabsorption + maintenance of height Hydrostatic p.Hydrostatic p. Donnan osmotic p.Donnan osmotic p. Loose framework of porous-permeable Loose framework of porous-permeable

collagen-PG matrixcollagen-PG matrix

Donnan Osmotic PressureDonnan Osmotic Pressure

Repulsive forcesRepulsive forces Fixed –vely charged PGsFixed –vely charged PGs Freely mobile interstitial counter-ionsFreely mobile interstitial counter-ions

BIPHASIC THEORYBIPHASIC THEORY

The inner AF also has viscoelastic propertiesThe inner AF also has viscoelastic properties

Permits large deformations in response to loadPermits large deformations in response to load

Creates intradiscal fluid-flows that dissipateCreates intradiscal fluid-flows that dissipate EnergyEnergy CreepCreep

Compression generates hoop stresses in outer layersCompression generates hoop stresses in outer layers

Inner layers deform + act as shock absorbersInner layers deform + act as shock absorbers

Prolonged axial loadingProlonged axial loading Water squeezed out of discsWater squeezed out of discs

Load removed (sleep)Load removed (sleep) In-flow of waterIn-flow of water

Explains why most disc herniations occur in Explains why most disc herniations occur in the morningthe morning

ANNULAR TEARANNULAR TEAR

Due to sudden ↑ in intradiscal p.Due to sudden ↑ in intradiscal p.

Symptoms due to NP contents irritating Symptoms due to NP contents irritating innervated outer AF layersinnervated outer AF layers

HERNIATIONHERNIATION

Occurs at insertion of outer Occurs at insertion of outer AF into VBAF into VBC / L spineC / L spine Greatest stressesGreatest stresses

Bulging of AF / herniation Bulging of AF / herniation NP materialNP material

Nat Hx – 90 % Pts pain free Nat Hx – 90 % Pts pain free @ 3 / 12@ 3 / 12 But self-repair of AF is limitedBut self-repair of AF is limited

DEGENERATIONDEGENERATION

Multifactorial causesMultifactorial causes ↓ ↓ nutritional supplynutritional supply ↓ ↓ r/o waste productsr/o waste products

↓↓blood supplyblood supplyCalcification of end platesCalcification of end platesAccumulation of degraded matrix Accumulation of degraded matrix macromoleculesmacromolecules↓ ↓ water concentrationwater concentration

Predisposing FactorsPredisposing Factors

Physical activity (axial loading)Physical activity (axial loading)

ImmobilizationImmobilization

VibrationVibration

Deformity (scoliosis etc.)Deformity (scoliosis etc.)

↓ ↓ blood flowblood flow DMDM SmokingSmoking Arterial diseaseArterial disease

Degeneration Distinct From Ageing?Degeneration Distinct From Ageing?DegenerationDegeneration

Disruption of collagen fibrilsDisruption of collagen fibrilsImpaired formationImpaired formation↑ ↑ X-linkingX-linking↑ ↑ denaturationdenaturation

Annulus delaminationAnnulus delaminationThrough interlamellar GSThrough interlamellar GS↑ ↑ solid behavioursolid behaviour

End plateEnd plateThinningThinningMicro-fracturesMicro-fractures

↑ ↑ discal fluid exudation upon loadingdiscal fluid exudation upon loading

Degenerative Pain TheoriesDegenerative Pain Theories

Nerve + bv in-growthNerve + bv in-growth

Altered biomechanical loadingAltered biomechanical loading Surrounding innervated structuresSurrounding innervated structures

Cell necrosisCell necrosis Stimulating inflammatory cascadeStimulating inflammatory cascade

Cytokine + free radical releaseCytokine + free radical releaseSensitise nerve endingsSensitise nerve endings

THANK YOUTHANK YOU