bio mechanics of nerve 03

8/9/2019 Bio Mechanics of Nerve 03

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IntroductionyAnatomy And physiology peripheral

nerve

yAnatomy And physiology spinal nerve

yBiomechanics of peripheral nerve

yBiomechanics of spinal nerve


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Nervous systemInput

(sensory)

Processor

(CNS)

Out put

(motor/ glan ular)


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Nervous Syste

NS

Brain, Spinal cord

branches

PNS

Spinal Nerve (31 pairs)

branches

ranial Nerve (12 pairs)

branches


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Anatomy and physiology of

peripheral nerve

Structure of peripheral nerveNerve fibers

Connective tissues

Blood vessels


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Structure and function of nerve

fiber


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AXONy This the anato ical connection between the cell body

and its end organs.

y The connection is aintained by the axonaltransport. This helps in the transporting the variousche ical synthesized (protein) in the cell body to theperiphery in both the directions.

y

The speed of the axonal transport ay vary fro 1-400/ day

Most of the axon of the PNS are yelinated one.


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Features of myelinated nerve fibery The axon is surrounded by the yelin sheath for ed

by the Schwann cells. The Schwann cell encircles theaxon in concentric layers of protein and lipids

(cholesterol, lecithin, spingo yelin)alternatively. Itpushes the cytoplas and nucleus to periphery.

y Node of Ranvier: this is the gap in the axon where theyelinated sheath is absent. The gap is about 1-2

y This node of Ranvier helps in the saltatoryconduction of i pulse. Saltatory conduction isdirectly proportional to the dia eter of the fiber.


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Sequence of the conduction of

impulse in nerve fiber

S all dia eter sensory nerve fiber

Dull diffuse pain

Sensory nerve fibers

touch Pressure heat Cold Kinesthetic sensation

Large dia eter nerve fibers

skeletal uscles


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y

Myelin sheath high insulating capacity. So it restrictsthe conduction of i pulse within the nerve fiber andprevent to sti ulate the neighbor fibers. e.g. like ainsulated copper wire winded in a otor.

y The for ation of yelin sheath around the axon is

calledmyelinogenesis. In the peripheral nerve itstarts at 4th onth of intrauterine life and co pletesonly in the 2ndyear after birth.

y The covering of the yelin sheath is called

neurilemma.


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Properties of the nerve fiberExcitability

Conductivity

Refractory period

Su ation

Adaptation

InfatigabilityAll or none law


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Intraneural connective tissues

Epineuriu

Covering ofthe nerve orbundle

Perineuriu

Covering ofeach fascicle

Endoneuriu

Covering of eachnerve fiber


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Epineuriumy This is a connective layer.

yActs as cushion during the ove ent of the fibers,

protecting fascicles fro the external trau a andaintain the local oxygen supply syste via epineural

blood vessels.

yA ount of epineural connective tissue varies a ong

the nerves and at different level within the sa e nerve.yWhere the nerve lies close to the bone or passes the

joints, the epineuriu is ore abundant as the need ofprotection is greater in that locations.


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Perineurium This a la ellar sheath that enco passes each fascicle.

It has great echanical strength as well as

bio echanical barrier. Strength: fascicle can be inflated with fluid up to

pressure of about 1000 of Hg before the Perineuriuruptures.

Biochemical barrier: che ically Perineuriu isolatesnerve fro their surrounding and preserves the ionicenviron ent of the interior of fascicle.


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Endoneuriumy Co posed of fibroblast and collagen.

y Interstitial pressure in the fascicle esp. endoneural

fluid pressure (1.50.7 of Hg) is slightly elevatedco pared to subcutaneous (-4.70.8 of Hg) and

uscle tissue (-22 of Hg).

y This pressure is counteract by the perineuriu . When

there is incision or injury to the perineuriu the nervefiber herniates affect icrocirculation & function


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Microvascular system


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y Peripheral nerve is well vascularized along with all

three layers.y Both i pulse propagation and axonal transport

depends up on the local oxygen supply. So theicroascular syste has a large reserve capacity.

y The large blood vessels approach the nerve at differentseg ent along its course. Then they divide intoascending and descending branches.

y These branches run longitudinally and frequentlyanasta ose with the vessels of perineuriu and

endoneuriu .

y This longitudinal syste consists of large arteriolesand venules of dia eter 50-100 .


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yWithin each fascicle lies a longitudinally orientedcapillary plexus with loop for ation at various levels.The vessels penetrate the perineuriu obliquelythough the perineuriu . Due to the structural

peculiarity, they are easily closed like valves.yWhen the endoneurial f luid pressure increases the

intrafascicular blood flow decreases.

y The built-in safety syste of longitudinal anasta oses

provide wide range of safety.y E.g. if the scitica-tibial nerve co plex (15c ) is surgically separated

fro the surrounding structure and regional nutrient vessels, thereis no detectable reduction in blood circulation. ( for 7-8c distal tocut end.)


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ANATOMYAND PHYSIOLOGYOF

SPINAL NERVE

In the early e ryological develop ental stages, the length ofthe spinal cord is sa e as the length of spinal colou n.

In the fully grown individual the spinal cord ends at conusedullaris. (L1 vertebra level)

The spinal nerve leaves the spinal canal through intervertebralfora en in the lu bar and sacral spine therefore has to pass frothe point where it leaves spinal cord.( lower thoracic to L1).

At lu bosacral level the spinal nerve roots co e out as abundle, which rese bles the tail of horse. So its called as caudaequina.


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y Two types of nerve roots are found in the spinalcord level. Ventral/anterior root and dorsal/posterior root.

yAnterior nerve root carries otor fibers and

posterior nerve root carries the sensory fibers.y The cell body of the otor axon lies in the anterior

horn cells spinal cord.

y The cell body of the sensory axon lies in the dorsal

ganglion which lies close to the intervertebralfora en. Unlike the nerve roots, the dorsalganglion are not enclosed with the CSF and

eninges.


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y Both otor and sensory nerve roots co bine to for

the spinal nerve which passes though intervertebralfora en.

yAfter the spinal nerve leaves the intervertebralfora en, they divide in to two ain branches called

dorsal ra i and ventral ra i.y The arachnoid space and CSF surrounding each nerve

root pair reduces caudally. Co pression injury ofnerve root ay induce an increase in the per eability

of the endoneurial capillaries, resulting in the ede a.y The location where the nerve roots are tightly enclosed

by the connective tissues, ore prone for co pressioninjury: entrap ent syndro e.


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Membranous covering of spinal nervey The axons in the endoneuriu are separated fro the CSF

by a thin layer of connective tissues called root sheath.

y Root sheathis the structural analogue to pia ater that

covers spinal cord. It is 2-5 layers.y The cells of the proxi al part of the outer layers of root

sheath are si ilar to that of pia ater and the distal partare si ilar to the dura ater.

y The cells of inner layer of the root sheath is si ilar to the

cells perineuriu of peripheral nerve.

y The inner layers of the root sheath constitue a barrierbetween the endoneuriu and CSF. This is a weak barrier which prevent the acro olecule only.


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Microvascular system of spinal nerve

y The seg ental arteries divide into three brancheswhen they approaches intervertebral fora en.

y Anterior branch: posterior abdo inal wall and lu ber plexus

y Posterior branch: paraspinal uscles and facet joints.

y Inter ediate branch: content of spinal canal

y The inter ediate branch joins the nerve at dorsalganglion and divide in to three branches

y To dorsal root

y To ventral roty To vasa corona of spinal cord ( edullay artery)


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y 7 8 re ain of the 128 fro the e bryological period of

life, and each supplies ore than one seg ent of spinalcord. The ain edullary artery in the thoracic regionwas discovered in 1881. they run parallel to the nerveroots.

yIn hu an being there is no connection between thesevessels and vascular network of nerve roots. ccasionallythey supply blood to the nerve roots. So they are called asextrinsic vascular syste

y

The vasculature of the nerve root is for ed by branchesfro the inter ediate branch of the seg ental arterydistally and by branches fro the vasa corona of spinalcord proxi ally. This is called as intrinsic vascularsyste .


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y There are vessels co ing fro distal and proxi al

seg ent of nerve roots anasto oses at the 2/3rd totallength of nerve root fro spinal cord. At this locationthe nerve is ore vulnerable .

y The arteries of intrinsic syste send branches to the

deeper part of the nerve tissue in a T anner. Toco pensate the elongation of the nerve roots, thearteries are coiled longitudinally and steep running

anner between fascicles.

y

There is barrier of the endoneurial capillaries inperipheral nerve called as blood nerve barrier, whichis si ilar to blood brain barrier of CNS. But thisbarrier is not so well developed as in spinal nerve root.


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Biomechanical behavior of peripheral

nerves

External trau a to the extre ities and nerveentrap ent ay produce echanical defor ation of

the peripheral nerves. That result in the detoriation ofthe nerve function and structure. Where the built insafety echanis of nerves fail to protect.

The co on ode of injury to nerve is as follows:

y tensile or stretch during extension

y co pressive during crushing


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Stretching (tensile) injury of peripheral

nerve

y Nerves are stronger structure with considerable tensilestrength.

y E.g. axi u load that can be sustained byedian nerve: 70-220 N

ulnar nerve: 60-150 N

y Severe intraneural tissue da age occurs by tension

before the nerve breaks.y The elsasticity and bio echanical properties of a

nerve are co plicated by the fact that the nerve is notisotropic aterial. Each co ponent has ownbio echanical property.


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y

When tension is applied to a nerve, initial elongationof a nerve under very s all load is followed by aninterval in which stress and elongation shows linearrelationship of elastic aterial.

yAs the li it of the linear region is approached, thenerve fiber starts to rupture inside the endoneurialtube and inside intact perineuriu . Perineurial sheathruptures at approxi ately 25-30% of elongation abovethe length.

yAfter this the elastic property disintegrate with plasticproperty i.e. no response to the release of load.


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yAlthough there is variation in the tensile strength of

various nerve, the axi u elongation at elastic li itis 20%, and co plete structural failure at 25-30%elongation. This ay change the echanicalproperties of a nerve i.e. increased stiffness anddecreased elasticity of a nerve.

y Tensile injuries are associated with severe accidentssuch as when high energy tension is applied tobrachial plexus in birth related injury, high speedvehicular collision, fall fro a height.

y It ay lead to partial or co plete loss of function i.e.otor/ sensory in the upper extre ity depending

upon the structure involved and extent of injury.


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y It is observed that there is considerable structural

injury to perineuriu though it is not visible on thesurface of the nerve.

y In so e high energy plexus injuries, an extre estretching lesion is caused by sudden violent trau a.

In this situation there is a suturing of the two cut endsunder oderate tension, when there is substantial gapin the nerve root. In this condition, oderate gradualtension is applied to bring the nerve fibers together to

aintain the continuity of the nerve trunk.

y It ay stretch the blood vessels and angulate theresulting in decrease in fascicular cross-sectional area.


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y Under this stretched sutured nerve, the perineuriu

tightens and the endoneurial f luid pressure isincreased and the intrafascicular capillaries ay beobliterated. The flow is i paired and at certain stagethe icrovascular circulation ceases.

y E.g. tibial nerev of a rabbity I paired blood flow at 8% elongation

y Ceassation of blood flow at 15% elongation

y E ven ore graduated stretching, for long durationshows the growth of intraneural tu or called as

schwannomas.


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Compression injury

y It is known that co pression of a nerve can inducesy pto s such as nu bness, pain and uscleweakness.

y

The biological basis for the change in the function andstructure has been investigated. This investigationshows that a ild co pression induce structural andfunctional changes. Mainly two echanical factors areconcerned.

y Pressure level

y Mode of co pression


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Critical Pressure level of compression injury:

y This is the level of pressure, were there is changes inintraneural blood flow, axonal transport and function ofa nerve. Even the duration of co pression alsoinfluences the develop ent of these changes.

y At 30 Hg of local co pression, functional changesay occur in the nerve, but the viability ay in

jeopardize during prolonged co pression (4-6hrs). Thisis due to i paired blood supply.

y e.g. 32 Hg of pressure was recorded around theedian nerve in the carpal tunnel in the patient with

carpal tunnel syndro e. (2 Hg in control group)


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y So the long standing and inter ittent low pressurelevels (30-80 Hg) ay induce ede a, which inturn ay beco e organized into fibrotic scar in nerve.

y Doublecrush syndrome:long standing co pression

at approxi ately 30 Hg also brings changes in theaxonal transport syste which lead to depletion of theprotein distal to the co pression site. So the blockageof axonal transport induced by co pression ay cause

the axon to be ore susceptible to additionalco pression distally. This called Doublecrushsyndrome.


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y At 80 Hg pressure:y Co plete cessation of intraneural blood flow

resulting in ischae ia.

y Blood f low is recovered if the pressure is withdrawn

after 2 hrs.

y At 200 400 Hg pressure:y Direct da age to the nerve structure

y Rapid detoriation of nerve

y Inco plete recovery


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Mechanical aspect ofthe nerve:y Electron icroscopic analysis of defor ation of nerve

fiber of peroneal nerve of the baboon hind li b inducedby tourniquet co pression (edge effect) is as follows:

y A specific lesion was induced in the nerve fibers at both

edges of the co pressed nerve seg enty The node of ranvier were displaced towards the non

co pressed seg ent.

y The nerve fibers in the centre of the co pressed nerve

seg ent, where the hydrostatic pressure is highest is notaffected.

y The large dia eter fibers are affected but the thinner fibersare spared.


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y Clinically it shows that the large fibers ( otor) are

affected and the thin fibers (pain) are preserved.y So the net effects of the pressure depends upon

y Magnitude

y Duration

y Mode

y There is two basic type of pressure application.

y Unifor pressure: due to pneu atic tourniquet aroundthe longitudinal nerve, controlled co pression isproduced. E.g. edian nerve ion carpal tunnel

y Lateral pressure: the nerve is co pressed by two parallelflat rigid surfaces oving towards each other. E.g. Spinalnerve co pression by herniated disc.


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y Defor ation in unifor circu ferential pressure dueto pneu atic tourniquet, the cross section of the nervefor extre ity re ain circular but dia eter is reduceddue to co pression.

yAs the tissue aterials are inco pressible, it has toove fro centre line towards each of the free edges.

y The displace ent of the tissues builds up fro zero atcentre line to axi u at the edge of the tourniquet.

y The shear stress cause this displace ent. This regionsustain axi u pressure gradient and displace nt.

Deformation


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y Lateral co pression defor s the nerve fro circularshape to elliptical. Displace ent is perpendicular tothe direction of force in centre and along with the linetowards edges.

y The degree of co pression is easured by axi uextension ratio which is defined as the axi u

dia eter divided by the initial dia eter of the nerve.

Deformation


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Triggers pain

Defor ation affect pacinian corpuscle which senses pressure on skin

Affect the per eability and electrical properties

Stretching of the e brane of nerve fibers

ateral co pression of nerve


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Duration pressure Vs pressure levelsy Mechanical factor depends upon the pressure along

with duration which shows visco-elasticity ( ti edependent) property.

y High pressure (400 Hg) for 2 hrs cause severe injurythan that for 15 inutes.

y Low pressure (30 Hg) for 2-4 hrs produce reversiblechanges

y Long duration low pressure causes ische ia of nerve.


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Biomechanical behavior of spinal nerve

roots:

y The rots in the thecal sac lack perineuriu and

epineuriu , but under tensile loading they exhibitboth elasticity and tensile strength. So the ulti ateload for ventral root is about 2-22 N and for dorsal rootabout 5-33 N.

y Length of the spinal nerve rot fro spinal cord tillfora en is 60 at L1 level and 170 at S1 level


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y The echanical properties of hu an spinal nerve roots

are different at its location in spinal canal andintervertebral fora en.

y E.g.. S1 nerve root at S1 level

y Intrathecal portion: 13 N

y

Intervertebral fora en: 73Ny So the value for ulti ate load are approxi ately 5 ti es

higher the fora inal portion than for intrathecal portion.

y Cross-sectional area of the nerve rot in theintervertebral fora en is significantly larger than thatat the sa e nerve root in the thecal sac. The ulti atestrain under tensile stress is 13-19% for hu an nerve rotat L5-S1 level


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y Spinal nerve roots are not static structure. They ove

along with the surrounding structures as per require ent.So the nerve roots in the intervertebral fora en shouldhave capacity to glide.

further irritation to nerve root

Microstretching injury during spinal ove ent

I pair the gliding capacity

Chronic irritation with subsequent fibrosis around nerve root

due to disc herniation and/or fora inal stenosis.


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y E.g. During SLR nerve rots at level of intervertebral

fora en undergoes lengthening of approxi ately2-5 .

y Different bio echanical factors are involved indifferent pathogenesis of various sy pto s included

by disc herniation and spinal stenosis causing pain.

y Central spinal stenosis

y The pressure is applied circu ferentially around the

nerve roots in the cauda equina at a slow and gradualrate which results in different sy pto s that discherniation.


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D

ische

rniationy Only one nerve is co pressed. Individual nerve roots are

adhered to the surrounding structure. So the co pressionresults in increased intraneural tension.

y In an study done by Spencer et al (1984) in a cadaver, they

easured the contact force between the herniated disc anddefor ed nerve roots, which was about 400 Hg withreduced disc height, the contact force and pressure b/wherniated disc and defor ed nerve roots is reduced.

y

So they suggested why sciatica pain is relieved afterche onucleolysis, and as disc degeneration progress overti e, the disc height reduces.


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Experimental compression of spinal nerve

root:

y In 1991 Ol arker did an experi ental study to evaluatethe effects of co pression of cauda equina in pig atknown pressure level.

y Cauda equina was co pressed by inflatable balloon fixedto the spinal cord. The cauda equina was observed frotranslucent balloon. A vital icroscope was fixed in theballoon.

y In this study, showed the intrinsic nerve roots bloodvessels at various pressure levels. Pressure was increasedby 5 Hg every 20 seconds. The blood flow and blood vessels dia eter was easured by the vital icroscope

si ultaneousl .


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y The average occlusion pressure at the arterioles was found to

be slightly below and directly related to the systolic pressureBP and the blood flow was dependent on the blood flow in theadjacent venules.

y So this corroborates the assu ption that venular stasis ayinduce capillary stasis and thus changes in the

icrocirculation of nerve tissues.y The pressure of 5-10 Hg is sufficient to induce an

occlusion.

y The effects of gradual deco pression, after initial co pressionwas aintained for only a short period. The average pressurefor starting the blood flow was slightly lower at deco pressionthan co pression for arterioles, capillaries and venules.

y So it shows that vascular i pair ent persist at lower pressurelevels.


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yAcompression induced impairment of the

vasculature ay be the one echanis of nerve rootdysfunction. The nerve roots ay derive a considerablenutrition via diffusion fro CSF. To assess theco pression induced effects on nerve rots, an

experi ent was designed in which H-labeled methylglucosewas injected to be transported to the nervetissues in the co pressed seg ent through blood flowand CSF.

y

The result showed that no echanis fro CSFdiffusion could be expected at low pressure. 10 Hgco pression was sufficient to induce 20-30% reductionin the transport of ethyl glucose to nerve roots.


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y Compressionmay induce increase in vascular

pe

rme

abilityleading to endoneural ede a. Thisincrease in the endoneural fluid pressure i pair theblood flow and jeopardize the nutrition to the nerveroots. The ede a persist after re oval of theco pression. The presence of endoneural ede a is

related to the for ation of intraneural fibrosis andreduce the recovery in people with disc co pressiondisorders.

y To assess the nutritional ede a due to co pression was

studied by injecting the Evans blue-labeled albuminin the nerve tissue. And co pression was at various levelfor different duration. The study proved that the ede a isproduced even at low pressure and location was at the

edges of co pression


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y The function of the nerve root has been studied bydirect electrical sti ulation and recording either onthe nerve itself or corresponding uscular syste

y During 2 hrs co pression period, a critical pressurelevel for inducing a reduction of MAP a plitudesee s to be b/w 50-75 Hg. Higher pressure (100-

20 Hg) ay induce total conduction block withvarying degrees of recovery after co pression releases.

y To study the effect of co pression on sensory nervefibers, electrodes were placed over sacru to record

co pound nerve AP after sti ulating the tail, i.e.distal to the co pression zone. So it showed thatsensory nerve roots are ore prone for co pressioninjury than otor fibers. Blood flow aintains the

function of the nerve roots.


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Multiple levels of spinal nerve root

compression:yWith double or ultiple level of nerve co pression or

spinal stenosis shows ore pronounced sy pto s.

y The blood supply to the nerve root seg ent b/w the

two co pressed sites is severely i paired though itre ians unco pressed.

y More the distance b/w the two co pression sites,ore functional i pair ent of the nerve

y E.g. in single balloon 50 Hg: no reduction in APy 10 Hg pressure at each balloon: 60% reduction in

AP


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y Constrictor odel of a dog

y Outer etal shell

y Inner a aroid

Chronic nerve root compression level:


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Long standing co pression

Decreased conduction

Increase in secretion ofsu stance

Dordsal root ganglion

roduces pain


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Summary


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Thank you

bio mechanics of nerve 03

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