Art Hr Ology

Arthrology

Introduction

Acompletestudyofhumanjointsincludessynchondroses,syndesmoses,symphyses,gomphoses,sutures,andsynovialjointsEachoftheseclassificationsincludesjointscapableofmovement.

Movementdefinitelytakesplaceatmostsyndesmoses,suchasthedistaltibiofibularjoint Thesymphysispubismoves,especiallyduringpregnancy Thereissomemovementoftheteethintheirsockets(gomphoses).

Infact,eventhesuturesoftheskullaremovable,atleastthroughthethirddecadeoflife,andsomeinvestigatorshaveclaimedthattheymovespontaneously,witharhythmindependentofheartrateorrespiratoryrate.

Introduction

Basedonthisclaim,afewpractitionersactuallyapplytherapeuticmobilizationtothesuturesofthecranium However,forthesakeofsimplicity,andbecausetheemphasisofclinicalapplicationisonjointmobilizationtechniques,thediscussioninthischapterisrestrictedtothesynovialjoints,whicharethemostnumerousandmostfreelymovableofthevarioustypesofhumanjoints.

Introduction

Themutualinfluencesofstructureandfunctionareemphasizedbecausethetwocannotbedealtwithadequatelyorunderstoodwhenconsideredindependently.

Thetraditionalanatomicconceptofsynovialjointsmustbeexpandedtoaphysiologicconcept;inadditiontothosestructuresthatanatomicallydefineajoint,thosestructuresresponsiblefornormalmovementatthejointmustalsobeincluded(Fig.31).

Withthisapproachthesynovialjointisconsideredthebasicunitofthemusculoskeletalsystemandusedasareferencefordiscussingnormalfunctionanddisordersofthissystem.

Introduction

KINEMATICS

ClassificationofJointSurfacesandMovements

Thenatureofmovementatanyjointislargelydeterminedbythejointstructure,especiallytheshapesofthejointsurfaces Thetraditionalclassificationofsynovialjointsbystructureincludesthecategoriesofspheroid,trochoid,condyloid,ginglymoid,ellipsoid,andplanarjoints.

Itshouldbeapparenteventosomeonewithonlyabasicknowledgeofhumananatomyandkinesiologythatthisclassificationdoesnotaccuratelydefinetheshapesofjointsurfacesorthemovementsthatoccurateachtypeofjoint Theheadsofthefemurandhumerusdonotformtruespheresorevenpartsoftruespheres.

ClassificationofJointSurfacesandMovementsVariouskindsofjoints.Fibrous:A,syndesmosis(tibiofibular);B,suture(skull).Cartilaginous:C,symphysis(vertebralbodies);D,synchondrosis(firstribandsternum).Synovial:E,condyloid(wrist);F,gliding(radioulnar);G,hingeorginglymus(elbow);H,ballandsocket(hip);I,saddle(carpometacarpalofthumb);J,pivot(atlantoaxial).


Aginglymus,suchasthehumeroulnarjoint,doesnotallowatruehingemotiononflexionandextensionbutratherahelicalmovementinvolvingconsiderablerotation.

Thehumeroradialjoint,whichisatrochoidjoint,doesnotmoveaboutasingleaxis,orpivot,becausetheheadoftheradiusisoval,havingalongerdiameteranteroposteriorlythanmediolaterally.

Theinterphalangealjoints,thecarpometacarpaljointofthethumb,thehumeroulnarjoint,andthecalcaneocuboidjointscanbeconsideredassellar However,themovementsoccurringatthesejointsvarygreatly,asdotheshapesofthejointsurfaces.


Therefore,althoughthisclassificationofjointstructuremayserveapurposefortheanatomist,initselfitisnotadequatefortheclinician,suchasthephysicaltherapist,whomustbeconcernedwiththefinerdetailsofjointmechanics.


Asimilarproblemexistswithclassifyingjointmovement.Thetraditionalclassificationofjointmovementincludesthefollowing:

AngularIndicatinganincreaseordecreaseintheangleformedbetweentwobones(e.g.,flexionextensionattheelbow)

CircumductionMovementofabonecircumscribingacone(e.g.,circumductionatthehiporshoulder)

RotationMovementoccurringaboutthelongitudinalaxisofabone(e.g.,internalexternalrotationattheshoulder)

SlidingOneboneslidesoveranotherwithlittleornoappreciablerotationorangularmovement(e.g.,movementbetweencarpals)


Therearetwoproblemswiththisclassificationsystemthatmakeitinadequateforthosecliniciansconcernedwithjointmechanics.

First,itdescribesmovementoccurringbetweenbonesbutignoresmovementoccurringbetweenjointsurfacesWhenmovementisdefined,whathappensatthejointisoftenignored Ananalogyistoconsiderthemovementofadoorbuttoignorethehinge.

Second,angularmovementsalmostneveroccurwithoutsomerotation;rotationnearlyalwaysoccurswithsomeangularmovement;glidingusuallyinvolvesangularandrotarymovement,andsoon.

Again,theclassificationneedstobeexpandedtotakeintoconsiderationthespecificsofjointmovement.


Movementsoccurringbetweenbonesmustbedefinedinsuchawaythattheycaneasilyberelatedtomovementsoccurringbetweenrespectivejointsurfaces Therefore,itishelpfultodefinethemechanicalaxisofanyjointasalinethatpassesthroughthemovingbone,touchingthecenteroftherelativelystationaryjointsurfaceandlyingperpendiculartoit(Fig.32).

Osteokinematicmovement(movementoccurringbetweentwobones)cannowbedefinedaccordingtothemechanicalaxisratherthanaccordingtothelongaxisofthemovingbone,ashasbeendoneinthepast.


Byrelatingosteokinematicmovementtoarthrokinematicmovement(movementoccurringbetweenjointsurfaces),themovementofthemechanicalaxisofthemovingbonerelativetothestationaryjointsurfacecanbeconsidered.Inotherwords,onejointsurfacecanbeconsideredasstationaryanditsopposingjointsurfaceasmovingrelativetoit.

Thisrelativemovementisdefinedaccordingtothepathtracedbythelinerepresentingthemechanicalaxisofthejointonthestationarysurface Themechanicalaxisisdeterminedatthestartingpointofamovement;oncemovementhasbegun,itmaintainsthesamerelationshiptothemovingbonewhilemovingrelativetothestationarybone.

JOINTSURFACES

Beforediscussingtypesofmovement,itisnecessarytodefinetheshapesofjointsurfacesbecausetheylargelydeterminethetypesofmovementsthatmayoccuratthejoint

Nojointsurfaceresemblesatruegeometricform;jointsurfacesareneitherspheres,ovals,orellipses,noraretheytruepartsofthese.

JOINTSURFACES

However,anyjointsurfacecanbethoughtofasbeingpartofanovoidsurface,thatis,resemblingthesurfaceofanegg(Fig.33A).

AtypicalexampleisasagittalsectionofafemoralcondyleSomejointsurfaces,ratherthanrepresentingpartofasimpleovoid,mightbeconsideredacomplexovoid,orsellar,surface(Fig.33B).

JOINTSURFACES Ifacrosssectionofanovoidsurfaceisexamined,itisclearthattheradiusofthejointsurfacechangesconstantly,formingacardioidcurve(Fig.34).

Asellarsurfaceisconvexinonecrosssectionalplaneandconcaveintheplaneperpendiculartoit,althoughthesurfacesofeachofthesecrosssectionsmayberepresentedbyacardioidcurve.

JOINTSURFACES Referringagaintoasimpleovoid(Fig.35),theshortestdistancebetweenanytwopointsonthesurfaceistermedachord,andanyotherlineofcontinuousconcavitytowardthechordisanarc.

Athreesidedfiguremadeupofthreechordsisatriangle.

Athreesidedfigureinwhichatleastonesideisformedbyanarcisatrigone.

JOINTMOVEMENTS

Anymovementinwhichthebonemovesbutthemechanicalaxisremainsstationaryistermedaspin (Fig.32A) Truespinofthehumerus,then,wouldbeamovementofflexioncombinedwithsomeabduction,becausetheglenoidcavityfacesslightlyforward Thebone,duringtruespin,rotatesaboutitsmechanicalaxis.

Whenthemechanicalaxisofthejointandthelongaxisofthemovingbonecoincide,suchasatthemetacarpophalangealandthefemorotibialjoints,thespiniswhatistraditionallytermedrotation atthejoint.

JOINTMOVEMENTS

Whereaxesdonotnecessarilycoincide,suchasatthehipandshoulder,spindoesnotalwaysoccurwhenthejointrotates. Forexample,spinoccursduringinternalandexternalrotationattheshoulderwhenthehumerusisin90 ofabduction;themechanicalaxis(definedaccordingtothestartingpositionofmovement)coincideswiththelongaxisofthehumerus.

Amovementinwhichthemechanicalaxisfollowsthepathofachordistermedachordate,orpure,swing.

Iftheendofthemechanicalaxistracedthepathofanarcduringmovementofthebone,thebonehasundergoneanarcuate,orimpure,swing(Fig.32B).

JOINTMOVEMENTS Animpureswingcanbethoughtofasapureswingwithanelementofspin,orrotation,aboutthemechanicalaxis.Thiselementofrotation,whichaccompanieseveryimpureswing,istermedconjunctrotation.

Habitualmovementsatanyjointareusuallyimpureswings Itfollowsthatmosthabitualmovements,orthosemovementsthatoccurmostfrequentlyatanyjoint,involvesomeconjunctrotation.

Itiswellknown,forexample,thatthetibiarotatesduringflexionextensionattheknee Ifonecarefullywatchestheulnaflexingandextendingonthehumerus,asimilarrotationisseen;theulnapronatesatthelimitsofextensionandsupinatesattheextremesofflexion.

JOINTMOVEMENTS Althoughsuchconjunctrotationoccursateveryjoint,itoccurstoamuchgreaterdegreeatjointswithsellarsurfacesthanatthosewithsimpleovoidsurfaces.

Bynowitshouldbeevidentthatmostofwhathavebeentraditionallyconsideredangularmovementsareactuallyhelicalmovementsbecauseofthiselementofconjunctrotationthataccompaniesthem Thisrotarycomponentisanessentialfeatureofnormaljointmechanics.

Arthrokinematics

ThestudyofwhathappensbetweenjointsurfacesonjointmovementisknownasarthrokinematicsWhenaboneswingsrelativetoanotherbone,oneoftwotypesofmovementmayoccurbetweenjointsurfaces.

Ifpointsatcertainintervalsonthemovingsurfacecontactpointsatthesameintervalsontheopposingsurface,onesurfaceissaidtorollontheopposingsurface(Fig.36A).

Thisisanalogoustoatireonacarcontactingtheroadsurfaceasthecarrollsdownthestreet;variouspointsonthetirecontactvariouspointsontheroad,thedistancebetweencontactpointsonthetireandroadbeingthesame.

Arthrokinematics

If,however,onlyonepointonthemovingjointsurfacecontactsvariouspointsontheopposingsurface,slide istakingplace(Fig.36B) Thisisanalogoustoatireonacarthatisskiddingonice;thetireisnotturningbutismovingrelativetotheroadsurfaceitissliding.

Arthrokinematics

Inmostmovementsathumansynovialjoints,bothslideandrolltakeplacesimultaneously Ifonlyrolltookplace,themovingbonewouldtendtodislocatebeforemuchmovementcouldoccur;ifonlyslideoccurred,impingementofjointsurfaceswouldpreventfullmovement(Fig.37).

Statedinanotherway,themovingbonemustrotateaboutaparticularcenterofmotion(orcentersofmotion)fornormalglidingtooccuratthejointsurfaces Iftheboneshouldmoveaboutanyothercentroidofmovementthanwhatisnormalforthatjoint,abnormalmovementwilloccurbetweenjointsurfaces.

Arthrokinematics Conversely,ifnormalmovementdoesnotorcannotoccurbetweenjointsurfaces,themovingbonecannotmoveaboutitsnormalcentroidofmovement Thiswillbediscussedlater,inthesectiononanalysisofaccessoryjointmotions.

Clinically,ameniscustear,whichcausesabnormalmovementbetweenjointsurfaces,altersthenormalcentroidofmovementatajoint.

Arthrokinematics

Thisoftenresultsinabnormalstressestothejointcapsule,whicharemanifestedaspainandmuscleguarding Atightjointcapsule,whichcausesalterationinthenormalcentroidofmovement,resultsinabnormalmovementbetweenjointsurfaces,usuallywithprematurecartilaginouscompressionbeforethemovementiscompleted.

SummaryofJointFunction

Theterminologyofjointfunctionhasbeenexpandedtoaccommodatethemorespecificfeaturesofjointkinematics,includingtherelationshipsbetweenjointstructureandfunctionandthetypesofmovementsoccurringbetweenjointsurfaces.

Indoingsothefollowingosteokinematicterms,orthosetermsdefiningmovementbetweentwobones,havebeendefined:

MechanicalaxisLinedrawnthroughthemovingbone,atthestartingpositionofamovement,thatpassesthroughthecenteroftheopposingjointsurfaceandisperpendiculartoit

SpinMovementofaboneaboutthemechanicalaxis


PureswingMovementofaboneinwhichanendofthemechanicalaxistracesthepathofachordwithrespecttotheovoidformedbytheopposingjointsurface;alsocalledchordateswing

ImpureswingMovementinwhichthemechanicalaxisfollowsthepathofanarcwithrespecttotheopposingovoidsurface

ConjunctrotationElementofspinthataccompaniesimpureswing;alsotherotationthatmayoccurwithasuccessionofswings


Thefollowingarthrokinematicterms,orthosethatdefinethetypesofmovementoccurringbetweenjointsurfaces,havealsobeendefined:

RollMovementinwhichpointsatintervalsonthemovingjointsurfacecontactpointsatthesameintervalsontheopposingsurface

SlideMovementinwhichasinglecontactpointonthemovingsurfacecontactsvariouspointsontheopposingsurface

SpinTypeofslidethataccompaniesspinofabone;onehalfofthejointsurfaceslidesinonedirectionwhiletheotherhalfslidesintheoppositedirection(i.e.,themovingjointsurfacerotatesaboutsomepointontheopposingjointsurface)


DistractionSeparationofjointsurfaces CompressionApproximationofjointsurfaces;alwaysoccurswhenmovingtowardtheclosepackedposition

Theclosepackedpositionwasdefinedforajointinwhichthefollowingthreeconditionsexist:

1.Thejointsurfacesbecomemaximallycongruent.2.Thejointcapsuleandmajorligamentsbecometwisted,causingjointsurfacestoapproximate.3.Thejointbecomeslockedsothatnofurthermovementispossibleinthatdirection.

ClinicalApplications:TERMINOLOGY

Accessoryjointmovements aresimplythosearthrokinematicmovementsthatmustoccurfornormalosteokinematicmovementtotakeplace Theseincludeslides,rolls,distractions,compressions,orconjunctrotations.

Considertheosteokinematicmovementofthehumerusmovingfromtherestingpositionwiththearmatthesidetotheclosepackedposition.

Thejointisconvexonconcave Theheadofthehumerusmustrollinthesamedirectioninwhichtheboneswings.


Itmustslideoppositethisdirectionorsomewhatinferiorlyandinward Becausetheclosepackedpositionisbeingapproached,thejointsurfacesarebecomingapproximated.

Itisamovementofimpureswing,soaconjunctrotation(inthiscasealateralrotation)mustoccur Ifanyoneoftheseaccessorymovementsdoesnotorcannotoccur,thenthisparticularswingofthehumeruscannotbeperformedpainlesslyorharmlesslythroughthefullrange.

Iffullosteokinematicmovementdoesoccur,itdoessoattheexpenseofthecapsuleorligaments,whichmustbeabnormallystretched,orofthearticularcartilage,whichmustbeabnormallycompressed.


Thetermcomponentmotions canbeusedsynonymouslywithaccessorymovements.

Forexample,lateralrotationofthetibiaisreferredtoasacomponentofkneeextension Likewise,spreadingofthedistaltibiaandfibulaisacomponentofdorsiflexionattheankle.

TheclinicianmustbeawareofthecomponentmotionsnecessaryforeachosteokinematicmovementatajointManyofthesearelistedinAppendix.


Jointplaymovements arethoseaccessorymovementsthatcanbeproducedpassivelyatajointbutcannotbeisolatedactively Theyincludedistractions,compressions,slides,rolls,orspinsatajointinaparticularposition.

Jointplaymovementsareusedwhenapplyingspecificmobilizationtechniquestorestoreaccessorymovementssothatfullandpainlessosteokinematicmovementmayberestored Forexample,inferiorglideoccursattheshoulderduringactiveelevation.

Itcanbeperformedpassively,butinitselfcannotbeperformedactivelybyvoluntarymusclecontraction;inferiorglideisajointplaymovementattheglenohumeraljoint.


Jointmobilization isaverygeneraltermthatmaybeappliedtoanyactiveorpassiveattempttoincreasemovementatajoint.

Inadditiontotraditionalmethodsofincreasingjointmovement,suchasactive,passive,andactiveassistedrangeofmotiontechniques,jointmobilizationincludesspecificpassivemobilizationtechniques.

Thesetechniquesareaimedatrestoringthosecomponentmovementsthatpermitpainfreeorharmlessosteokinematicmovement Theyareusedespeciallytorestorethosejointplaymovementsthatcannotbeisolatedactively.


Specificpassivemobilizationtechniquesaregraded(Fig.310).Grades1through4areoftenreferredtoasarticulation techniques,whicharepassiverhythmicoscillations.

Grade5isamanipulation techniquethatisahighvelocity,lowamplitude,passivethrust.


Thesegradesarerelativetothepathologicamplitudeofjointplaymovementthatexistsatthejointandnottothenormalamplitudethatshouldexist.

Therearetwomaincriteriafortheselectionoftheparticulargradetobeused:(1)thedegreeofpainorprotectivemusclespasmduringpassivejointplaymovement(irritability)and(2)thedegreeofrestrictionofjointplaymovement.

Thegreatertheirritability,thelowerthenumericgradeofmovementused Painandspasmmustbeavoided.

Manipulationisusedprimarilywhenaveryslight,minimallypainfulrestrictionexists.


Athirdcriterionofselectionmightapplyhere,namelytheskillandexperienceoftheoperator,becausemanipulativemaneuversshouldonlybeattemptedafterarticulationtechniqueshavebeenmasteredandaftermuchpractice.

ThetermsofjointmovementareshowninFigure311.


Specificaccessoryjointmotionsthatarelimitedmayberestoredbymanualoscillationsorthrusts Theprimarygoalofusingspecificjointmobilizationtechniquesisrestorationofnormal,painfreeuseofthejoint.

Theemphasisisnotonforcingaparticularanatomic(osteokinematic)movementatajoint,ashasbeendoneinthepastwithtraditionalmethodsofmobilization;rather,itisonrestoringnormaljointmechanicstoallowfull,painfreeosteokinematicmovementtooccur.


Inthisway,rangeofmotionisrestoredtothejointwithlessriskofdamagingthejointbycompressingisolatedportionsofarticularcartilage,andwithlesspainandmuscleguardingfromoverstretchingisolatedcapsuloligamentousstructures,asmaywelloccurifanosteokinematicmovementisforcedtheabsenceofnecessarycomponentmovements.

Thisistosaythatspecificpassivemobilization,correctlyapplied,isainsafer,moreefficient,andlesspainfulmethodofincreasingrangeofmotionatajoint.


Mennell,inhislectures,oftenusestheanalogyofadoorhavinglostmovementbecauseofafaultyhinge Effortstorestoremotionbypushinghardonthedoorarelikelytoresultinfurtherdamagetothehinge

Thelogicalmethodtoremedythesituationistodirectonesattentiontothehingetorestorenormalmechanicstothehinge,therebyrestoringnormalmovementofthedoor.

Thisdiscussionignoresthephysiologicconceptofthejoint Thisisdonesolelyforthesakeofsimplicity.


Obviously,whenrestoringnormaljointmechanicsisconsidered,attentionmustbegiventotheanatomicjointalongwiththosestructuresresponsibleforactivemovementofthejoint

Forexample,activeabductionattheshoulderisoftenlostbecauseoftheabsenceofinferiorglide Relativetotheanatomicjoint,thejointplaymovementofinferiorglidemaybelimited However,theproblemmayalsobephysiologic,inthatinferiorglidemaynotbeoccurringowingtoweaknessofthesupraspinatusmuscle.

Theseareverydifferentproblemsleadingtosimilarresults Thenatureoftheproblemmustbebroughtoutbyathoroughevaluation.

ANALYSISOFACCESSORYMOTIONS

ClinicalAssessment.Manyofthemeansofdeterminingwhichaccessorymovementsarecomponentsofspecificosteokinematicmovementshavealreadybeendiscussed

Forinstance,thedirectionofrollisalwaysthesameasthatfortheswingofthebone Ifaconvexsurfacemovesonaconcavesurface,slidewilloccurinthedirectionoppositetotheroll;ifaconcavesurfacemovesonaconvexsurface,slideoccursinthesamedirectionastheroll Distractionoccurswhenmovingoutoftheclosepackedposition;compressionoccurswhenmovingintotheclosepackedposition Thesecomponentscanallbedeterminedforanyjointmovinginanydirection.


Someoftheothercomponentmotions,aslistedforeachjointintheAppendix,mustbememorizedordeducedanatomically.

Onewayofassessingthestateofaparticularaccessorymovement(itsamplitudeandirritability)isclinically,byevaluatingthejointplaymovements Theseexaminationmaneuversareessentiallythesameasthespecificmobilizationtechniques.

Ratherthanbeingperformedasagraded,therapeutictechnique,theyareusedtodeterminetheamplitudeofajointplaymovementandwhetherthemovementcausespainorspasm.


Theamplitudeofmovementandpossiblerestrictionmustbecomparedwiththeoperatorsconceptofnormalforthatmovement,atthatjoint,forthatbodytype.

ThisrequiresexperienceinevaluatingnormalaswellaspathologicjointsWheneverpossible,thepathologicjointmustbecomparedwithahealthycontralateraljoint.

Thedegreeofirritabilityisdeterminedbythepatientssubjectiveresponseandbythepresenceofprotectivemusclespasmwhenperformingtheexaminationmovement.


Proficiencyinclinicallyevaluatingjointplaymovementsandcorrelatingfindingstoknowledgeofaccessorymovementsatthejoint,aswellasothersymptomsandsignspresentingonexamination,isessentialtotheeffectiveapplicationofjointmobilizationtechniquesandmanagementofmusculoskeletaldisorders.

NEUROLOGY JOINTNUTRITION LUBRICATION ResolvingProblemsofJointSurfaceWear

NEUROLOGY

Theneuroanatomyand,especially,theneurophysiologyofjointsaresubjectsnotwellcoveredinthecurrentliterature FollowingtheoriginalworksofSherringtononneuromuscularphysiology,avastamountofinformationwascollectedconcerningtheroleofmuscleandtendonreceptorsininfluencingposture,movement,muscletone,andvariousreflexphenomena.

Littlehasbeendoneuntilrecentlytoidentifyspecificjointreceptors,andevenlesshasbeendonetodeterminetheirclinicalsignificance.

Becausethesubjectofjointneurologyisdirectlyrelevanttothemanagementofcommonmusculoskeletaldisorders,anoverviewispresentedhere.

Innervation

Jointstendtoreceiveinnervationfromtwosources:(1)articularnervesthatarebranchesofadjacentperipheralnervesand(2)branchesfromnervesthatsupplymusclescontrollingthejointEachjointisusuallysuppliedbyseveralnerves,andtheirdistributionstendtooverlapconsiderably.

Ingeneral,aparticularaspectofajointcapsuleisinnervatedbybranchesofthenervesupplyingthemuscleormusclesthatwould,whencontracting,preventoverstretchingofthatpartofthecapsule.

Innervation

Onenotableexceptionistheanteroinferioraspectoftheglenohumeralcapsule,whichisinnervatedbyabranchfromtheaxillarynerve.

Thenervefibersofanparticularnervearepurelyafferent,withtheexceptionofsmallvasomotorefferentstothebloodvessels.

Thefibersizesrangefromlargemyelinatedfiberstosmallmyelinatedandunmyelinatedfibers.

Receptors

Jointreceptorstransmitinformationaboutthestatusofthejointtothecentralnervoussystem Thecentralnervoussysteminterpretstheinformationsentbythejointreceptorsandrespondsbycoordinatingmuscleactivityaroundthejointtomeetjointmobilityandstabilityrequirements.

Jointreceptorsfunctiontoprotectthejointfromdamageincurredbygoingintothepathologicrangeofmotion Theyarealsopartlyresponsiblefordeterminingtheappropriatebalancebetweensynergisticandantagonisticmuscularforcesandforgeneratinganimageofbodypositioningandmovementwithinthecentralnervoussystem.

Receptors

Fourtypesofjointreceptorshavebeenidentified,eachservingarelativelyspecificroleinthesensorimotorintegrationofjointfunction.

TypeI:Postural DescriptionEncapsulatedendings,similartoRuffinicorpuscle LocationNumerousinthesuperficialjointcapsule;usuallyfoundinclustersofsix;locatedprimarilyintheneck,hip,andshoulder

RelatedfiberSmall(6to9m)myelinated(relativelyslowconduction) StimulusChangingmechanicalstressesinthejointcapsule;maybeactivatedbythepresenceofpositionalfaults;maybemoreactivewithtractiontechniquesthanwithoscillations

Receptors

TypeII:Dynamic DescriptionThicklyencapsulated,similartopaciniancorpuscle LocationSparse(relativetotypeI);foundinjointcapsuleandligaments(deeperlayersandfatpads);primarilylocatedinthelumbarspine,hand,foot,andjaw

RelatedfiberMedium(9to12m)myelinatedStimulusSuddenchangesinjointmotion;maybemoreactivewithoscillationtechniquesthanwithtraction

ActionRapidlyadapng(actsfor12secondfollowingeachmoon),lowthreshold;dynamicmechanoreceptor

Receptors

FunctionFiresonlyonquickchangesinmovement;providesinformationconcerningaccelerationanddecelerationofjointmovement;actsatinitiationofmovementasaboostertohelpovercomeinertiaofbodyparts;producesincreasedtoneinthemusclebeingstretchedandrelaxationinthemuscleantagonistictotheonebeingstretchedwhenthejointisatendrange,notactiveinmidrangeofmotion;inhibitspain

TypeIII:Inhibitive DescriptionThinlyencapsulated,similartoGolgiendorgan LocationPrimarilylocatedinintrinsicandextrinsicjointligaments,superficiallayersofthecapsule;inthelumbarspinenotdetectedinthelongitudinalposteriorligament,longitudinalanteriorligament,oriliolumbarligament

Receptors

RelatedfiberLarge(13to17m)myelinated(fastconduction) StimulusStretchatendrange;moreactivewithfastmanipulationtechniques

ActionVeryslowlyadapting(actsforseveralminutesfollowingtheinitialstimulation),highthreshold;dynamicmechanoreceptor

FunctionMonitorsdirectionofmovement;hasreflexeffectonmuscletonetoprovideabrakingmechanismagainstmovementtendingtooverdisplacethejoint(movementtoofastortoofar);inhibitsmuscletone;respondstostretchatendofrange

TypeIV:Nociceptive DescriptionFreenerveendingsandplexus

Receptors

LocationLocatedinmosttissues:fibrouscapsule,intrinsicandextrinsicligaments,fatpads,periosteum(absentinarticularcartilage,intraarticularfibrocartilage,andsynovium)RelatedfiberSmall(2to5m)myelinatedandunmyelinated(2m)(slowconduction)

StimulusSignificantmechanicaldeformationortension;directmechanicalorchemicalirritation

ActionNonadapting,highthreshold;painreceptors FunctionInactiveundernormalconditions;activewhenrelatedtissueissubjecttosignificantdeformationorothernoxiousmechanicalorchemicalstimulation;producestonicmusclecontraction

ClinicalConsiderations

Itisapparentfromthepreviousdescriptionsthatstimulationofjointreceptorscontributestosenseofstaticposition(typeI),senseofspeedofmovement(typeI),senseofchangeinspeedofmovement(typeII),senseofdirectionofmovement(typesIandIII),regulationofposturalmuscletone(typeI),regulationofmuscletoneattheinitiationofmovement(typeII),regulationofmuscletoneduringmovement(coordination)(typeII),andregulationofmuscletoneduringpotentiallyharmfulmovements(typeIII).

Ofcourse,skinreceptors,connectivetissuereceptors,andmusclereceptorsalsocontributetomanyofthesesamefunctions.


Thefollowingaresomeoftheclinicalproblemsthatremainunresolved,oronlypartiallyresolved:1. Howimportantarethesejointreceptors,relativetomuscleandskin

receptors(e.g.,intheregulationofmuscletone,posture,andmovement)?2. Aresomeofthepersistentproblems,suchaschroniclimp,residual

incoordination,chronicinstability(givingway),andchronicmuscleatrophy,thatareencounteredinpatientsfollowingsomejointinjuriestheresultofdamagetothesereceptors?

3. Howmighttreatmenttechniques,suchasjointmobilization,neuromuscularfacilitation,andinhibition,berefinedtoaccommodatethefunctionsofthesejointreceptors?


Oneparticularlyinterestingstudydemonstratesacaseinwhichmalocclusionofdentures,causingabnormalafferentdischargefromthetemporomandibularjointcapsules,resultedinanalmosttotalreflexinhibitionofthetemporalmusclesduringactiveocclusionbythepatient.

Restorationofnormaljointmechanics,byremodelingofthedentures,restorednormalmuscularactivity.AstudybyWykeshowedsignificantposturalchangesinaboywithapparentalterationofafferentimpulsesfromtheanklecapsuleafterinjurytothelateralaspectofthecapsule.


Theposturaldeficitpersisteddespiteanotherwisecompleterecovery,withrestorationofnormalstrengthandrangeofmotionandwithnoresidualpain Theboysonlycomplaintwasthatofoccasionalgivingwayoftheankle.

Freemanadvocatestheuseofcoordinationexercisesonabalanceboardforpatientswithchronicankleinstabilityintheabsenceofdemonstrablestructuralinstability Hereportsgoodresultswithsuchaprogram,attributingsuchgivingwayattheankletoalterationofnormaljointafferentflowfollowinginjurytothejoint,suchasfromananklesprain.


Mostphysicaltherapistshaveencounteredthecommonphenomenonofgrossquadricepsatrophyfollowingkneeinjurydespitepreventiveeffortstomaintainmusclefunction.

Althoughthereislittlecurrentliteratureonthesubject,itseemsreasonabletoattributethisproblemtoreflexmuscleinhibitionbyabnormaljointreceptorstimulation.

Asfarastechniquesoftreatmentareconcerned,itisinterestingtorelatewhatisknownaboutthefunctionofthesejointreceptors,andwhatisknownofarthrokinematics,totechniquesthathavealreadyevolved.


Considerthediagonalpatterncommonlyusedinproprioceptiveneuromuscularfacilitationtechniquesmovingthearmthroughflexion,abduction,externalrotationtoextension,adduction,andinternalrotation.

Partoftheexplanationofthispatternreferstomovingfromapositionofmaximumelongationandunspiralingoffunctionallyrelatedmusclestoapositionofspiralingandshorteningofthesesamemuscles

Inaddition,thispatterninvolvesmovingalljointssimultaneouslyfromaclosepackedtoaloosepackedposition


Indoingso,thejointcapsuleofeachjointmovesfromapositionofmaximumshorteningandspiralingtoapositionoflengtheningandunspiraling.

Studiesthusfaronanimalshaveindicatedthatmaximumafferentstimulationoccurswhenapproachingtheclosepackedpositionofajoint;thisistobeexpectedsinceitisthepositionofmaximumtighteningofthecapsuleandligamentsinwhichthereceptorslie.

Thetechniquesofproprioceptiveneuromuscularfacilitationevolvedwithprimaryconsiderationoftheneurophysiologyofmuscles,usingmovementpatternsthatcombineactionsoffunctionallyrelatedmusclestobringaboutamutualfacilitationofeachmuscleinthechain.


Itisnowsuggestedthatthesepatternsalsocombinefunctionallyrelatedjointmovementsthataddtothefacilitativeeffectofthepatternsonthemusclesinvolvedthroughjointreceptorstimulation.

Italsoseemsprobablethatthejointreceptorsplayasignificantroleinothertechniquesoffacilitation,suchasquickstretch,thattendtostimulatethetypeIIreceptor.

Itisimportanttoconsiderthefunctionofjointreceptorswhenusingjointmobilizationorothertreatmenttechniquesinvolvingjointmovement Theeffectivenessofeffortstoincreasemovementatajointwillnaturallybecompromisedbyanymusclecontractiontendingtorestrictjointmovement.


Emphasis,then,mustbemadeonavoidingreflexmusclecontractionsthatwouldtendtopreventorrestrictadesiredjointmovementForthisreasonandforotherobviousreasonspainmustbeavoidedduringjointmobilization,becauseitiswellknownthatpainatajointtendstoelicitareflexmuscleresponsetorestrictmovementatthejoint.

SuddenjointmovementtendstostimulatefiringofthetypeIIIreceptors,whichsetsupareflexmusclecontractiontorestrictfurthermovement.


GradualinitiationofmovementtendstostimulatethetypeIIreceptor,whicheffectsasmallfacilitativemuscularresponse.

Passiveandactivemobilizationtechniquesarebestperformedrhythmically,withoutsuddenchangesinspeedordirectionofmovement.

AmanipulationmustbeperformedsoquicklythatitiscompletedbeforethereflexmuscularresponseproducedbystimulationofthetypeIIIreceptorcanacttointerferewiththemovement Similarly,itmustbeperformedthroughaverysmallamplitudetominimizethenumberoftypeIIIreceptorsstimulated.


WithrespecttothetypeIVpainreceptors,itisworthemphasizingthatarticularcartilage,fibrocartilage(e.g.,menisci),synovium,andcompactboneareessentiallyaneural.Thisiswelldocumentedinanatomicstudiesandclinically.

Intheanatomicjointsthemajorpainsensitivestructuresarethefibrouscapsule,ligaments,andperiosteum.Thiscarriessomeimportantclinicalimplications Itsuggeststhatpathologicconditionsthatmightalterjointmechanics,suchthatthearticularcartilageundergoesunduecompressionstress,maygounnoticedbythepatientintheinitialstages.


Infact,thepatientmaynotenothinguntileitherjointmechanicsarealteredsufficientlytoplaceanabnormalstressonthejointcapsuleoruntilthejointcartilageundergoessufficientdegeneration,causingalowgradesynovitiswithresultantpressureonthecapsulefromeffusion

Thismayexplainwhypersonswithfrozenshouldersorosteoarthrosisofotherjointsoftendonotpresenttoaphysicianuntilthediseasehasprogressedconsiderably.


Italsosuggeststhatcliniciansmustlearntoexamineroutinelyforsubtlechangesinjointmechanicsratherthanconsideringonlygrossrangeofmotion,strength,andcomplaintsofpainbythepatient.

Patientspresentingwithveryearlysymptomsorsignsofosteoarthrosiscouldenjoycompletearrestorreversalofthejointproblemifproperlymanaged,ratherthanresigningthemselvestofuturejointreplacement.

JOINTNUTRITION

Inadditiontobeinganeural,articularcartilageisforthemostpartavascular.Thisisalsotrueofintraarticularfibrocartilage Because,ingeneral,bodytissuesdependonbloodsupplyfornutrition,thesestructureswouldseemtobeatadisadvantage.

Itisgenerallybelievedthatthearticularmarginsdoreceivesomenutrientsfromthehighlyvascularizedsynoviumandperiosteumadjacenttothem.

Themenisciatthekneealsoreceivenutrientsattheirperipheralcapsularattachments,anditissuggestedthatthedeeplayersofarticularcartilagearefedbythebloodsupplytothesubchondralbone.

JOINTNUTRITION

However,theproblemofnutritiontothemoresuperficial,centrallylocatedportionsofthearticularcartilageandtothemorecentrallylocatedpartsoftheintraarticularfibrocartilageremains.

Thesecartilaginousareasaretheprimaryarticulatingsurfaces,notthemoreperipheralareasordeeperlayers Itisgenerallyagreedthatnutritiontotheseregionsoccursbydiffusionandimbibitionofsynovialfluid.

JOINTNUTRITION

Thisisauniquesituationbecausenutrientsmustcrossatleasttwobarrierstoreachthechondrocytesembeddedwithinthecartilage

First,theymustpassfromthecapillarybedofthehighlyvascularizedsynovium.

Second,theymustthendiffusethroughthesuperficialmatrixlayersofthecartilaginoussurface,beforereachingthecellwallofthechondrocyte.

Thus,synovialfluidservesamajorfunctionasasourceofnutritionforarticularcartilageandintraarticularfibrocartilage.

JOINTNUTRITION

Intermittentcompressionanddistractionofjointsurfacesmustoccurforanadequateexchangeofnutrientsandwasteproductstotakeplace Ajointthatisimmobilizedundergoesatrophyofarticularcartilage,justasajointinwhichthereisprolongedcompressionofjointsurfaceundergoessimilaratrophicchanges.

Thethreeprimarymechanismsbywhichsynovialjointsundergonormalcompressionanddistractionarethefollowing:(1)weightbearinginlowerextremityandspinaljoints,(2)intermittentcontractionofmusclescrossingajoint,and(3)twistinganduntwistingofthejointcapsuleasthejointmovestowardandawayfromtheclosepackedpositionduringhabitualmovements.

JOINTNUTRITION

Withrespecttothelastmechanism,itisnecessarytorecallthatasthejointapproachestheclosepackedposition,itssurfacesnotonlybecomecompressedbutalsoapproachapositionofmaximalcongruency.

Thus,compressionnormallyoccursinapositioninwhichgreaterareasoftheopposingjointsurfacesareincontact Thisensuresthatrelativelylargeportionsofthejointsurfacesundergoadequateexchangeofnutrients.

Fromapathologicstandpoint,ajointthathaslostmovement,suchasfromatightjointcapsule,doesnotreceiveanormalexchangeofnutrientsoverthepartsofthejointsurfacesthatnolongercomeintocontact.

JOINTNUTRITION

Thisisespeciallytrueinthecaseofatightjointcapsule,sincemovementstowardtheclosepackedpositioninwhichthereismaximaljointsurfacecontactareusuallythemovementsthataremostrestricted.

Attritionalchangesinarticularcartilagerelatedtoagingareobservedintherelativelynoncontactingportionsofthejointsurfaces.

Severalreasonsforthismaybepostulated First,thesearetheareasofarticularcartilagethatundergolessdeformationwithuseofthejointovertime;asaresult,therateanddegreeofexchangeofnutrientfluidsarelessintheseareas.

JOINTNUTRITION

Also,withagethereisareductionofthechondroitinsulfatecomponentofcartilaginoustissue Becausethefluidbindingcapacityofarticularcartilageislargelydependentonitschondroitinsulfatecontent,adecreaseinthisconstituentmightinterferewithnormalnutritiontothetissue.

Furthermore,becauselossofjointrangeofmotionoccurswithadvancingage,theexchangeofnutrientstoportionsofthearticularcartilageisreduced.

Lubrication

Inadditiontoservingasanutritionalsourceforarticularcartilage,synovialfluidalsoactsasalubricanttopreventunduewearofjointsurfacesfromfriction.

Instudyinglubricationofhumanjoints,however,notjustthepropertiesofsynovialfluidandhowtheyaffectmovementandfrictionbetweentwosurfacesareconsidered.

Inadditiontherearetheshapeandconsistencyofthejointsurfacesaswellasthetypesofmovementthatoccurbetweenjointsurfaces.

Lubrication

Manymodelshavebeenproposedforhumanjointlubrication.Someoftheearliermodelstendtoignoremanyoftheuniquepropertiesofhumanjoints.

Themorerecentmodelsevolvedwiththesophisticationofengineeringprinciples,whicharebetterabletodealwithsomeofthecomplexfactorsinvolvedinhumanjointlubrication However,itisgenerallyagreedthatnoonemodelofjointlubricationappliestoalljointsunderallcircumstances.

Themajormodeoflubricationinaparticularjointmaychange,dependingonfactorssuchasloadingandspeedofmovement.

Lubrication

Synovialfluidhasessentiallythesamecompositionasbloodplasma,exceptfortheadditionofmucin.

Mucinismucopolysaccharidehyaluronicacid,whichisalongchainpolymer.

Theviscouspropertiesofsynovialfluidareattributedtohyaluronicacid.

Themostimportantpropertytobeconsideredinthisrespectisthethixotropic ornonnewtonianqualityofsynovialfluid;theviscositydecreaseswithincreasedshearrate(increasedspeedofjointmovement).

ModelsofJointLubrication

Ananalogycannotaccuratelybedrawnbetweenamachinemodeloflubricationandthelubricationofsynovialjoints Oneofthemajorreasonsforthisisthatthephysicalpropertiesofarticularcartilagedifferconsiderablyfromthephysicalpropertiesofmostmachinecomponents.

Articularcartilageisporousandrelativelyspongelikeinthatithasthecapacitytoabsorbandbindsynovialfluid Articularcartilageisalsoviscoelastic;thedeformationrateishighoninitialapplicationoftheloadandlevelsoffwithtime.


Whentheloadisremoved,theinitialreformationrateishighanddecreasesovertime(Fig.314).


Althougharticularcartilageappearsquitesmoothandshinymicroscopically,itis,infact,relativelyroughmicroscopicallyArticularcartilagealsohasthetendencytoadsorblargemolecules,suchashyaluronicacidinsynovialfluid,toitssurface.

Theearlymodelofjointlubricationdescribedahydrodynamic,orfluidfilm,situation(Fig.315).

Inthiscase,synovialfluidfillsinthewedgesofspaceleftbythejointsurfaceincongruencies Onmovementbetweensurfacesthesynovialfluidisattractedtotheareaofcontactbetweenthesurfaces.


Thisoccursbecauseof(1)thepressuregradientproducedbythemovementand(2)thefactthatrelativemovementtendstopulltheviscousfluidinthedirectionofthemovingsurface Theresultofthisisthemaintenanceofalayeroffluidsbetweenjointsurfacesduringmovement.

Anyfrictionoccurringasaresultofmovementoccurswithinthefluidratherthanbetweenjointsurfaces Thismeetstherequirementsofagoodlubricationsystembecauseitallowsfreemovementandpreventsweartothejointsurfaces.

Thissystemworkswellduringmovement;however,itwouldtendtofailunderveryslowvelocityorunderheavyloading


Thehydrodynamicmodel,however,cannotbecompletelyrepudiatedbecausethepreviousdescriptiondoesnotconsidertheviscoelasticityofjointsurfaces.Thismodelcanbemodifiedtoanelastohydrodynamicsystem(Fig.315).


Becauseofthenatureofarticularcartilagetodeform,notalltheenergyofheavyloadinggoestodecreasingthethicknessofthelayeroffilmbetweenthesurfaces,thusincreasingfrictionbetweenthesurfaces.

Instead,deformationofthejointsurfacesoccurs,increasingtheeffectivecontactareabetweensurfacesandthusreducingtheeffectivecompressionstress(forceperunitarea)tothelubricationfluid Thisallowstheprotectivelayeroffluidtoremainataboutthesamethickness.

Thus,theelastohydrodynamicmodeldescribesasystemthatwithstandsloadinginthepresenceofmovement.


ThismodelofjointlubricationcanbeexpandedbyincludingtheconceptsofboundarylubricationandweepinglubricationWithanymaterialsundergoingrelativeshearbetweentwosurfaces,frictionistheresultoftheirregularitiesofthesurfaces;thegreatertheirregularities,thegreaterthefriction.

Effectivelubricationmustreducethisfrictiontoaminimum,thusreducingwearofthesurfacestoaminimum.Inthecaseofboundarylubrication,thelubricantisadsorbedtothesurfaceofthematerial,ineffect,reducingtheroughnessofthesurfacesbyfillingintheirregularities.


Becausearticularcartilageisabletoadsorblongchainmoleculesofhyaluronicacid,thesemoleculesareabletofillintheirregularitiesandtocoatthesurface Anyfrictionoccurringastheresultofshearmovementoccursbetweenmoleculesofthelubricantratherthanbetweenthejointsurfacesthemselves.

Thisprobablyservesasanadjuncttotheelastohydrodynamicsystem,especiallyincasesofextremeloadingsufficienttodecreasesignificantlythethicknessofthelayeroffluidmaintainedbytheelastohydrodynamicmodelunderlighterloads.


Bytheuseofthismixedlubricationmodel,whichcombineselastohydrodynamicconceptswithboundarylubricationconcepts,thedemandsofhumansynovialjointsaremet.

Thesystemallowsmovement,changeindirectionofmovement,loading,andvariationsincongruenciesofjointsurfaces Ittakesintoconsideration,atleastingeneralterms,thepropertiesofthelubricant(synovialfluid)andthesurfacematerials.

Thereisstillconsiderablecontroversyovertherelativeimportanceofeachofthelubricationmodelsundervariousconditions,butmostauthorsagreewiththegeneralconceptspresentedpreviously.


Becauseitisstillunknownhoweachmodelcontributestonormaljointlubrication,verylittleinvestigationintothemutualeffectsofpathologicjointconditionsandjointlubricationhastakenplace.

Abreakdowninsomeaspectofthelubricationsystemislikelytocauseoraddtotheprogressionofjointdisease,suchasdegenerativejointdisease

Ontheotherhand,certainjointdiseasesresultinchangesinstructureandfunctionofjointconstituents Forinstance,thereisalossofjointcartilageindegenerativejointdiseaseandchangesinsynovialfluidviscosityinrheumatoidarthritis.

ResolvingProblemsofJointSurfaceWear

IthasbeenemphasizedthatsynovialjointsurfacesareincongruentBecauseoftheincongruencythatexistsinmostpositionsofmovement,arelativelysmallcontactareaexistsbetweenjointsurfaces.

Thewedgesofspacethatsurroundthiscontactareaarenecessaryforahydrodynamiclubricationsystemtooperateeffectively;withoutthesespacesthelubricantcouldnotbedrawn,orforced,betweenthecontactingsurfaces.

Onemaywonderifsuchasmallareaofcontactmightincreasethelikelihoodofwearbetweenjointsurfaces,sinceloadingforcesfromweightbearingandmusclecontractionwouldbedistributedoverasmallsurfacearea,thusincreasingthecompressivestresstothejoint.


Ineachofthesejoints,movementtendstoberestrictedtoonearcofmovementthatisdeterminedalmostentirelybytheshapesofthejointsurfaces Itwouldseemthatduringmovementatthesejoints,thecontactingareaononejointsurfacewouldconsistentlyfollowarutontheopposingsurface,increasingthelikelihoodofexcessivewearintherutorattheareaofthesurfacecontactingtherut.

Inthesejointstheproblemofexcessivewearisresolvedinanumberofways First,thesejointsurfacesare,relativelyspeaking,themostcongruentinthebody,sothatforcesaredistributedoverasomewhatlargerarea Consider,forexample,theclosefitbetweentheulnaandthetrochlearsurfaceofthehumerus.


Second,thecontactareaoneachsurfaceisconstantlychangingthroughoutanarcofmovement Achangeincontactareasoccursinonesensebecauseacombinationofrollandslidetakesplacebetweenjointsurfaces.

Inanothersense,thecontactareachangesbecausecontactalternatesfromthebottomofthevalleytothesidesoftheslopesononesurfaceandcorrespondinglyontheopposingsurface Forinstance,withthekneeinfullextension,thearticularsurfaceofthepatellamakescontactwiththefemuratastripextendingmediolaterallyacrossthemiddleofthepatellarsurface Inflexion,however,onlythemedialandlateralmarginsofthepatellamakecontactwiththefemoralcondyles


However,atjointssuchastheknee,amorecomplexsituationexistsThekneeissignificantlyincongruentcomparedwiththejointsdiscussedpreviously;itmustbetoallowsomedegreeofrotationtooccurindependentlyofflexionorextensionorinconjunctionwiththem.

Thekneemustalsowithstandheavierloadingfromweightbearinginawidevarietyofpositions Thus,thekneeisoftenrequiredtoundergoheavyloadinginpositionsofflexion,inwhichthesurfacesareveryincongruentasmallareaofcontactwithstandsrelativelylargecompressiveforces.


Thus,theremightbeconcernthatinasituationofheavyloading,relativelylowvelocity,andsmallcontactareabetweensurfaces,thelubricationsystemwouldnotbesufficienttopreventexcessivefriction(shear)andwearbetweenjointsurfaces.

Thismightverywellbethecaseinajointsuchasthekneethatmustundergosuchconditionsduringnormaldailyactivities,suchasclimbingstairs,squatting,andlifting Theremightalsobeconcernaboutthetendencyforthefemurtoslipforwardonthetibiaundersuchconditions,againbecauseoftheincongruencyofjointsurfacesandthelackofintrinsicstability.


Theknee,then,isajointthatmustallowmovementofspinbetweenthetibiaandthefemurandswingbetweenthetibiaandfemurbecauseofthefunctionaldemandsplacedonit Forthistobepossible,thejointsurfacesmustbesufficientlyincongruent.

However,becauseofthisincongruenceandbecauseofnormalheavyloadinginavarietyofpositions,thekneeappearssusceptibletoexcessiveshearforcesbetweencontactingjointsurfacesduringmovement,excessivecompressiveforcesbetweencontactingsurfacesonstaticloading,andintrinsicinstabilitywhenloadedinflexion.


Itisprobablethattheintraarticularmenisciservetocompensateforwhatwouldotherwisebeunsatisfactoryengineeringatthekneejointunsatisfactoryinthatthejointwouldnotwithstandthenormalforcesappliedtoitwithoutgivingwayorundergoingprematurewearingofjointsurfaces.

Underheavystaticloading,themenisciacttoincreasetheeffectiveloadbearingsurfaceareaatthejoint,thusreducingtheforceperunitarea.


Duringmovementwithheavyloading,theyagainacttoincreasetheloadbearingsurfacearea,buttheyalsomaintainawedgeshapedintervalsurroundingtheareaofcontactintowhichthelubricantfluidcanbedrawn.

Becausethemenisciaresemicartilaginous,theycanalsoabsorbsynovialfluidWithincreasedloading,fluidcanbesqueezedoutfromthemenisciandthearticularloadbearingsurface,contributingtoaweepinglubricationphenomenon Also,asthemeniscirecedebeforetheadvancingcondylesduringmovement,theycanacttospreadalayeroflubricantoverthejointsurfacesjustbeforecontact.


Thereisalsoconsiderableslideofthefemoralcondylesalongthetibialsurfaceduringthecompleterangeofflexionextension Thisfeaturealsoreducesthelikelihoodofexcessivewearonthetibialsurfacesbydistributingtheloadbearingsurfaceoveralargerarea.

Thedegreeofslidecouldnotoccurnormallywithouttheextrinsiccontrolprovidedbythecruciates,norwithouttheintrinsicstabilityprovidedbythemenisci Thistypeofmotion,inwhichtheareaofcontactofaparticularjointsurfaceconstantlychangeswithmovementatthejoint,isnecessarytoallowfortheintermittentcompressionofarticularcartilageessentialtonormalnutritionandlubrication.


Fibrillationofarticularcartilageinnormalhipandshoulderjointsoccursfirstinnonweightbearingsurfaces.

Also,asignificantaccelerationofdegenerativechangesoccursinweightbearinganimaljointsinwhichajointisimmobilizedbutfulluseofthelimbisallowed.

Thesearebothexamplesoftheeffectsonarticularcartilageoftheloadbearingcontactareanotbeingdistributedoveralargeareaoftheopposingsurfaces Thisisperhapsapartialexplanationforthefrequencyofdegenerativearthritisoccurringinhumanhipjoints;arelativelysmallsurfaceareaisusedforweightbearing,whilemuchofthearticularcartilagereceiveslittleornocompression.

Art Hr Ology

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