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Biomechanic of SpineStructure of the Spine a curved stack of 33 vertebrae structurally divided into five regions: cervical region - 7 vertebrae thoracic region - 12 vertebrae lumbar region - 5 vertebrae sacrum - 5 fused vertebrae coccyx - 4 fused vertebrae

Structure of the SpineVertebraeIntervertebral DiscsAnnulus fibrosusNucleus pulposus

LigamentsAnterior longitudinal ligamentPosterior longitudinal ligamentSupraspinous ligamentInterspinous ligamentIntertransverse ligamentsLigamentum FlavumFacet Capsular Ligament

4Structure of the Spine

Spinal CurvesInfluenced by heredity, pathological conditions, individuals mental state, and forces to which the spine is habitually subjected.Primary Spinal CurveSecondary Spinal CurveLordosisKyphosisScoliosis6Loads on the SpineForces acting on spine:Body weightTension in the spinal ligamentsTension in surrounding musclesIntraabdominal pressureAny applied external loadsFlexion Relaxation PhenomenonBody Movement Speed7Cervical SpineSeven vertebrae C 1-7More flexibleSupports the headWide range of motionRotation to left and rightFlexionPeripheral nervesArmsShoulder, Chest and diaphragmThoracic SpineMid-back or dorsal regionTwelve vertebrae T 1-12Ribs attached to vertebrae Relatively immobilePeripheral nervesIntercostalLumbar SpineLower backFive vertebrae L 1-5Carries the the weight of the upper body Larger, broader Peripheral nervesLegsPelvisSacral and Coccygeal regionSacrum Triangular structure Base of the spineConnects spine to pelvisNerves to pelvic organsCoccyxFew small bonesRemnant of tail

Motion SegmentTwo adjacent vertebraeIntervertebral discSix degrees of freedomFlexion-extensionLateral flexionAxial rotation

Types of motion

Motion Segment

Motion of Entire Spine

Compressive Strength of Spine

Stress-Strain Curve

Failure Strength of Spinal Ligaments

Weight bearing properties of motion segment unit

Intervertebral DiscSoft fibro-cartilaginous cushionsBetween two vertebraAllows some motionServe as shock absorbersTotal 23 discs th of the spinal column's lengthAvascular Nutrients diffuse through end platesIntervertebral Disc FunctionsMovement of fluid within the nucleusAllows vertebrae to rock back and forthFlexibilityAct to pad and maintain the space between the twenty-four movable vertebraeAct as shock absorbersAllow extension and flexion Mechanically:Annulus fibrous acts like coiled springNucleus pulposus acts like ball bearing

Intervertebral Disc AnatomySpongy center Nucleus pulposusSurrounded by a tougher outer fibrous ring Anulus fibrosus

Annulus FibrosisCollagen arranged in sheets called lamellae (outer layers).These lamellae are arranged in concentric rings -10-12 layers that lessen in number with age and thicken (fibrose). Enclose the nucleus and oriented in opposite directions at an angle of 120 degrees (or 45-65 degrees).Controls the tensile loading from shear, accessory motions in the anterior compartment and disc forces which can be up to 5x the external compression force.

27Annulus FibrosisMostly avascular and lacking innervation but the outermost layers are probably innervated (sinovertebral nerve).Thickest anteriorly.Outermost 1/3 connects to vertebral body via Sharpies fibers.Outer 2/3 connect to the end plate.

28AnnulusIn Bending Increased tensile force posteriorly Increased compressive force anteriorlyIn Rotation Reorientation of collagenous fibersTightening of fibers traveling in one directionLoosening of fibers traveling in opposite direction

Anulus FibrosusStrong radial tirelike structure Series of lamellaeConcentric sheets of collagen fibers Connected to end platesOrientated at various anglesUnder compressionBecome horizontalEncloses nucleus pulposus

Disc StructureNucleus Pulposus (NP) is located in the center except in lumbar lies slightly posterior. Gelatinous mass rich in water binding PG (proteoglycan) AKA (glycoaminoglycos) GAG-protein molecule.Chondrotin-4 sulfate in PG molecule gives the disc a fluid maintaining capacity (hydrophyllic) - decreases with age.Hydration of the disc will also decrease with compressive loading - this loss of hydration decreases its mechanical function.

3180-90% is H2O decreases with age.Disc volume will reduce 20% daily (reversible) which causes a loss of 15-25 mm of height in the spinal column.Acts as a hydrostatic unit allowing for uniform distribution of pressure throughout the disc.

Disc Structure32Compressive stresses on the disc translate into tensile stresses in the annulus fibrosis This makes the disc stiffer which adds stability and support to the spine.Bears weight and guides motion.Avascular - nutrition diffusion through end-plate.

Disc Structure33Theory of weight bearingNucleus pulpous imbibes waterDevelops internal pressurePressure exerted in all directionsLateral forces Against annulusSuperiorly and inferiorly directed forces Against end platesIncreases stiffness Of end plate and annulus fibrosusTheory of weight bearing (contd)

Mechanical Characteristics

Tensile stiffness of the disc annulus in different directionsHighest along 150Lowest along the disc axis Strength

Highest Along normal direction of annulus fibers( 3 times stronger than that along horizontal direction)Shear & Tensile CharacteristicsIn direct shear testsShear stiffness in horizontal direction260 N/mm2Spine rarely fails in pure shearSimilarly under normal physiologic activitiesPure tensile loading doesnt occurBut annulus undergoes tensile loading duringBending Axial rotationExtensionLoads on the Spine

In normal standing position, body weight acts anterior to the spine, creating a forward bending load (moment) on the spine.Loads on the SpineBecause the spine is curved, body weight, acting vertically, has components of both compression (Fc) and shear (Fs) at most motion segments.

FcFswt

Loads on the SpineDuring lifting, both compression and anterior shear act on the spine. Tension in the spinal ligaments and muscles contributes to compression.

Muscle tensionShear reaction forceCompression reaction forceJoint centerLoads on the SpineLumbar hyperextension can create a bending load (moment) in the posterior direction.

compressiontensionLoads on the Spine

hyperextensionLumbar hyperextension produces compressive loads at the facet joints. Loads on the SpineSpinal rotation generates shear stress in the intervertebral discs.

Superior viewLateral viewTypes of Segmental LoadingAxial CompressionBendingTorsionShear45Axial CompressionCaused by gravity, ground reaction forces, muscle contraction and ligaments reaction to tensile forces.Intradiscal loads can range from 294N to 3332N depending upon position.Most load in anterior segment, posterior can load from 0-30% depending upon segments position.Compression at the disk causes tension at the annulus, changing the angle of the fibers and increasing the stability.Creep will occur in the disc, will be larger with increased force and aging.5-11% of H2O is lost through creep.Creep is rapid 1.5-2mm in 10 min.Plateaus at 90 minutes.46BendingCombination of compression, shear and tensile forces on the segment from translation.Bending into flexion will be resisted by posterior annulus, PLL and the facet capsule and anterior compressive forces on the anterior structures causing disc displacement.For extension posterior compressive forces in anterior segment and there is a tensile load in facet capsule and ALL. 47TorsionCaused by axial rotation and coupled motions.Stiffness may increase due to facet compression with certain motions i.e., flexion increases torsional stiffness at L3-4.Annulus fibrosus resists, 1/2 fibers CW other 1/2 CCW facets may help depending upon the orientation (resists in a tensile manner).When combined with flexion the amount of force required for tissue failure is decreased.48ShearFacet joint resists especially in the lumbar area.Annulus will undergo some tensile forces depending upon direction and the fiber orientation or angle.Discs also resist but if creep occurs - the facet may undergo more loading.49MobilityAmount and direct of motion in a segment is determined by:Vertebral body/disc size.Facet orientation frontal vs. sagittal.50FlexionSuperior vertebra will anterior tilt and forward gliding will occur: Widening the intervertebral foramina 24%. Adds compressive forces on the anterior aspect of the anterior segment moving the nucleus pulposus posteriorly.Tensile forces placed on posterior annulus, flavum, capsule and PLL.Central canal is widened51ExtensionSuperior vertebra will tilt and glide posteriorly and the intervertebral foramina narrowed up to 20%.The central canal is also narrowed.Nucleus pulposus moves anteriorly52Lateral FlexionSuperior vertebra will translate, tilt and rotate over inferior - direction will differ.Concavity towards, convexity oppositeTensile forces on convexity, compressive forces on concavityExtension in ipsilateral facet.Flexion in contralateral facet.

53RotationAccessory motions are like lateral flexion due to same coupling in cervical and upper thoracic spine.Exception with lower T/S and L/S in neutral coupling then opposite.If the motion segment is flexed or extended spine,the coupling will be the same.54

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