art hr ology
DESCRIPTION
Art Hr OlogyTRANSCRIPT
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Arthrology
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Introduction
Acompletestudyofhumanjointsincludessynchondroses,syndesmoses,symphyses,gomphoses,sutures,andsynovialjointsEachoftheseclassificationsincludesjointscapableofmovement.
Movementdefinitelytakesplaceatmostsyndesmoses,suchasthedistaltibiofibularjoint Thesymphysispubismoves,especiallyduringpregnancy Thereissomemovementoftheteethintheirsockets(gomphoses).
Infact,eventhesuturesoftheskullaremovable,atleastthroughthethirddecadeoflife,andsomeinvestigatorshaveclaimedthattheymovespontaneously,witharhythmindependentofheartrateorrespiratoryrate.
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Introduction
Basedonthisclaim,afewpractitionersactuallyapplytherapeuticmobilizationtothesuturesofthecranium However,forthesakeofsimplicity,andbecausetheemphasisofclinicalapplicationisonjointmobilizationtechniques,thediscussioninthischapterisrestrictedtothesynovialjoints,whicharethemostnumerousandmostfreelymovableofthevarioustypesofhumanjoints.
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Introduction
Themutualinfluencesofstructureandfunctionareemphasizedbecausethetwocannotbedealtwithadequatelyorunderstoodwhenconsideredindependently.
Thetraditionalanatomicconceptofsynovialjointsmustbeexpandedtoaphysiologicconcept;inadditiontothosestructuresthatanatomicallydefineajoint,thosestructuresresponsiblefornormalmovementatthejointmustalsobeincluded(Fig.31).
Withthisapproachthesynovialjointisconsideredthebasicunitofthemusculoskeletalsystemandusedasareferencefordiscussingnormalfunctionanddisordersofthissystem.
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Introduction
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KINEMATICS
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ClassificationofJointSurfacesandMovements
Thenatureofmovementatanyjointislargelydeterminedbythejointstructure,especiallytheshapesofthejointsurfaces Thetraditionalclassificationofsynovialjointsbystructureincludesthecategoriesofspheroid,trochoid,condyloid,ginglymoid,ellipsoid,andplanarjoints.
Itshouldbeapparenteventosomeonewithonlyabasicknowledgeofhumananatomyandkinesiologythatthisclassificationdoesnotaccuratelydefinetheshapesofjointsurfacesorthemovementsthatoccurateachtypeofjoint Theheadsofthefemurandhumerusdonotformtruespheresorevenpartsoftruespheres.
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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).
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ClassificationofJointSurfacesandMovements
Aginglymus,suchasthehumeroulnarjoint,doesnotallowatruehingemotiononflexionandextensionbutratherahelicalmovementinvolvingconsiderablerotation.
Thehumeroradialjoint,whichisatrochoidjoint,doesnotmoveaboutasingleaxis,orpivot,becausetheheadoftheradiusisoval,havingalongerdiameteranteroposteriorlythanmediolaterally.
Theinterphalangealjoints,thecarpometacarpaljointofthethumb,thehumeroulnarjoint,andthecalcaneocuboidjointscanbeconsideredassellar However,themovementsoccurringatthesejointsvarygreatly,asdotheshapesofthejointsurfaces.
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ClassificationofJointSurfacesandMovements
Therefore,althoughthisclassificationofjointstructuremayserveapurposefortheanatomist,initselfitisnotadequatefortheclinician,suchasthephysicaltherapist,whomustbeconcernedwiththefinerdetailsofjointmechanics.
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ClassificationofJointSurfacesandMovements
Asimilarproblemexistswithclassifyingjointmovement.Thetraditionalclassificationofjointmovementincludesthefollowing:
AngularIndicatinganincreaseordecreaseintheangleformedbetweentwobones(e.g.,flexionextensionattheelbow)
CircumductionMovementofabonecircumscribingacone(e.g.,circumductionatthehiporshoulder)
RotationMovementoccurringaboutthelongitudinalaxisofabone(e.g.,internalexternalrotationattheshoulder)
SlidingOneboneslidesoveranotherwithlittleornoappreciablerotationorangularmovement(e.g.,movementbetweencarpals)
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ClassificationofJointSurfacesandMovements
Therearetwoproblemswiththisclassificationsystemthatmakeitinadequateforthosecliniciansconcernedwithjointmechanics.
First,itdescribesmovementoccurringbetweenbonesbutignoresmovementoccurringbetweenjointsurfacesWhenmovementisdefined,whathappensatthejointisoftenignored Ananalogyistoconsiderthemovementofadoorbuttoignorethehinge.
Second,angularmovementsalmostneveroccurwithoutsomerotation;rotationnearlyalwaysoccurswithsomeangularmovement;glidingusuallyinvolvesangularandrotarymovement,andsoon.
Again,theclassificationneedstobeexpandedtotakeintoconsiderationthespecificsofjointmovement.
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ClassificationofJointSurfacesandMovements
Movementsoccurringbetweenbonesmustbedefinedinsuchawaythattheycaneasilyberelatedtomovementsoccurringbetweenrespectivejointsurfaces Therefore,itishelpfultodefinethemechanicalaxisofanyjointasalinethatpassesthroughthemovingbone,touchingthecenteroftherelativelystationaryjointsurfaceandlyingperpendiculartoit(Fig.32).
Osteokinematicmovement(movementoccurringbetweentwobones)cannowbedefinedaccordingtothemechanicalaxisratherthanaccordingtothelongaxisofthemovingbone,ashasbeendoneinthepast.
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ClassificationofJointSurfacesandMovements
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ClassificationofJointSurfacesandMovements
Byrelatingosteokinematicmovementtoarthrokinematicmovement(movementoccurringbetweenjointsurfaces),themovementofthemechanicalaxisofthemovingbonerelativetothestationaryjointsurfacecanbeconsidered.Inotherwords,onejointsurfacecanbeconsideredasstationaryanditsopposingjointsurfaceasmovingrelativetoit.
Thisrelativemovementisdefinedaccordingtothepathtracedbythelinerepresentingthemechanicalaxisofthejointonthestationarysurface Themechanicalaxisisdeterminedatthestartingpointofamovement;oncemovementhasbegun,itmaintainsthesamerelationshiptothemovingbonewhilemovingrelativetothestationarybone.
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JOINTSURFACES
Beforediscussingtypesofmovement,itisnecessarytodefinetheshapesofjointsurfacesbecausetheylargelydeterminethetypesofmovementsthatmayoccuratthejoint
Nojointsurfaceresemblesatruegeometricform;jointsurfacesareneitherspheres,ovals,orellipses,noraretheytruepartsofthese.
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JOINTSURFACES
However,anyjointsurfacecanbethoughtofasbeingpartofanovoidsurface,thatis,resemblingthesurfaceofanegg(Fig.33A).
AtypicalexampleisasagittalsectionofafemoralcondyleSomejointsurfaces,ratherthanrepresentingpartofasimpleovoid,mightbeconsideredacomplexovoid,orsellar,surface(Fig.33B).
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JOINTSURFACES Ifacrosssectionofanovoidsurfaceisexamined,itisclearthattheradiusofthejointsurfacechangesconstantly,formingacardioidcurve(Fig.34).
Asellarsurfaceisconvexinonecrosssectionalplaneandconcaveintheplaneperpendiculartoit,althoughthesurfacesofeachofthesecrosssectionsmayberepresentedbyacardioidcurve.
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JOINTSURFACES Referringagaintoasimpleovoid(Fig.35),theshortestdistancebetweenanytwopointsonthesurfaceistermedachord,andanyotherlineofcontinuousconcavitytowardthechordisanarc.
Athreesidedfiguremadeupofthreechordsisatriangle.
Athreesidedfigureinwhichatleastonesideisformedbyanarcisatrigone.
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JOINTMOVEMENTS
Anymovementinwhichthebonemovesbutthemechanicalaxisremainsstationaryistermedaspin (Fig.32A) Truespinofthehumerus,then,wouldbeamovementofflexioncombinedwithsomeabduction,becausetheglenoidcavityfacesslightlyforward Thebone,duringtruespin,rotatesaboutitsmechanicalaxis.
Whenthemechanicalaxisofthejointandthelongaxisofthemovingbonecoincide,suchasatthemetacarpophalangealandthefemorotibialjoints,thespiniswhatistraditionallytermedrotation atthejoint.
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JOINTMOVEMENTS
Whereaxesdonotnecessarilycoincide,suchasatthehipandshoulder,spindoesnotalwaysoccurwhenthejointrotates. Forexample,spinoccursduringinternalandexternalrotationattheshoulderwhenthehumerusisin90 ofabduction;themechanicalaxis(definedaccordingtothestartingpositionofmovement)coincideswiththelongaxisofthehumerus.
Amovementinwhichthemechanicalaxisfollowsthepathofachordistermedachordate,orpure,swing.
Iftheendofthemechanicalaxistracedthepathofanarcduringmovementofthebone,thebonehasundergoneanarcuate,orimpure,swing(Fig.32B).
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JOINTMOVEMENTS Animpureswingcanbethoughtofasapureswingwithanelementofspin,orrotation,aboutthemechanicalaxis.Thiselementofrotation,whichaccompanieseveryimpureswing,istermedconjunctrotation.
Habitualmovementsatanyjointareusuallyimpureswings Itfollowsthatmosthabitualmovements,orthosemovementsthatoccurmostfrequentlyatanyjoint,involvesomeconjunctrotation.
Itiswellknown,forexample,thatthetibiarotatesduringflexionextensionattheknee Ifonecarefullywatchestheulnaflexingandextendingonthehumerus,asimilarrotationisseen;theulnapronatesatthelimitsofextensionandsupinatesattheextremesofflexion.
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JOINTMOVEMENTS Althoughsuchconjunctrotationoccursateveryjoint,itoccurstoamuchgreaterdegreeatjointswithsellarsurfacesthanatthosewithsimpleovoidsurfaces.
Bynowitshouldbeevidentthatmostofwhathavebeentraditionallyconsideredangularmovementsareactuallyhelicalmovementsbecauseofthiselementofconjunctrotationthataccompaniesthem Thisrotarycomponentisanessentialfeatureofnormaljointmechanics.
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Arthrokinematics
ThestudyofwhathappensbetweenjointsurfacesonjointmovementisknownasarthrokinematicsWhenaboneswingsrelativetoanotherbone,oneoftwotypesofmovementmayoccurbetweenjointsurfaces.
Ifpointsatcertainintervalsonthemovingsurfacecontactpointsatthesameintervalsontheopposingsurface,onesurfaceissaidtorollontheopposingsurface(Fig.36A).
Thisisanalogoustoatireonacarcontactingtheroadsurfaceasthecarrollsdownthestreet;variouspointsonthetirecontactvariouspointsontheroad,thedistancebetweencontactpointsonthetireandroadbeingthesame.
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Arthrokinematics
ThestudyofwhathappensbetweenjointsurfacesonjointmovementisknownasarthrokinematicsWhenaboneswingsrelativetoanotherbone,oneoftwotypesofmovementmayoccurbetweenjointsurfaces.
Ifpointsatcertainintervalsonthemovingsurfacecontactpointsatthesameintervalsontheopposingsurface,onesurfaceissaidtorollontheopposingsurface(Fig.36A).
Thisisanalogoustoatireonacarcontactingtheroadsurfaceasthecarrollsdownthestreet;variouspointsonthetirecontactvariouspointsontheroad,thedistancebetweencontactpointsonthetireandroadbeingthesame.
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Arthrokinematics
If,however,onlyonepointonthemovingjointsurfacecontactsvariouspointsontheopposingsurface,slide istakingplace(Fig.36B) Thisisanalogoustoatireonacarthatisskiddingonice;thetireisnotturningbutismovingrelativetotheroadsurfaceitissliding.
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Arthrokinematics
Inmostmovementsathumansynovialjoints,bothslideandrolltakeplacesimultaneously Ifonlyrolltookplace,themovingbonewouldtendtodislocatebeforemuchmovementcouldoccur;ifonlyslideoccurred,impingementofjointsurfaceswouldpreventfullmovement(Fig.37).
Statedinanotherway,themovingbonemustrotateaboutaparticularcenterofmotion(orcentersofmotion)fornormalglidingtooccuratthejointsurfaces Iftheboneshouldmoveaboutanyothercentroidofmovementthanwhatisnormalforthatjoint,abnormalmovementwilloccurbetweenjointsurfaces.
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Arthrokinematics Conversely,ifnormalmovementdoesnotorcannotoccurbetweenjointsurfaces,themovingbonecannotmoveaboutitsnormalcentroidofmovement Thiswillbediscussedlater,inthesectiononanalysisofaccessoryjointmotions.
Clinically,ameniscustear,whichcausesabnormalmovementbetweenjointsurfaces,altersthenormalcentroidofmovementatajoint.
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Arthrokinematics
Thisoftenresultsinabnormalstressestothejointcapsule,whicharemanifestedaspainandmuscleguarding Atightjointcapsule,whichcausesalterationinthenormalcentroidofmovement,resultsinabnormalmovementbetweenjointsurfaces,usuallywithprematurecartilaginouscompressionbeforethemovementiscompleted.
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SummaryofJointFunction
Theterminologyofjointfunctionhasbeenexpandedtoaccommodatethemorespecificfeaturesofjointkinematics,includingtherelationshipsbetweenjointstructureandfunctionandthetypesofmovementsoccurringbetweenjointsurfaces.
Indoingsothefollowingosteokinematicterms,orthosetermsdefiningmovementbetweentwobones,havebeendefined:
MechanicalaxisLinedrawnthroughthemovingbone,atthestartingpositionofamovement,thatpassesthroughthecenteroftheopposingjointsurfaceandisperpendiculartoit
SpinMovementofaboneaboutthemechanicalaxis
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SummaryofJointFunction
PureswingMovementofaboneinwhichanendofthemechanicalaxistracesthepathofachordwithrespecttotheovoidformedbytheopposingjointsurface;alsocalledchordateswing
ImpureswingMovementinwhichthemechanicalaxisfollowsthepathofanarcwithrespecttotheopposingovoidsurface
ConjunctrotationElementofspinthataccompaniesimpureswing;alsotherotationthatmayoccurwithasuccessionofswings
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SummaryofJointFunction
Thefollowingarthrokinematicterms,orthosethatdefinethetypesofmovementoccurringbetweenjointsurfaces,havealsobeendefined:
RollMovementinwhichpointsatintervalsonthemovingjointsurfacecontactpointsatthesameintervalsontheopposingsurface
SlideMovementinwhichasinglecontactpointonthemovingsurfacecontactsvariouspointsontheopposingsurface
SpinTypeofslidethataccompaniesspinofabone;onehalfofthejointsurfaceslidesinonedirectionwhiletheotherhalfslidesintheoppositedirection(i.e.,themovingjointsurfacerotatesaboutsomepointontheopposingjointsurface)
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SummaryofJointFunction
DistractionSeparationofjointsurfaces CompressionApproximationofjointsurfaces;alwaysoccurswhenmovingtowardtheclosepackedposition
Theclosepackedpositionwasdefinedforajointinwhichthefollowingthreeconditionsexist:
1.Thejointsurfacesbecomemaximallycongruent.2.Thejointcapsuleandmajorligamentsbecometwisted,causingjointsurfacestoapproximate.3.Thejointbecomeslockedsothatnofurthermovementispossibleinthatdirection.
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ClinicalApplications:TERMINOLOGY
Accessoryjointmovements aresimplythosearthrokinematicmovementsthatmustoccurfornormalosteokinematicmovementtotakeplace Theseincludeslides,rolls,distractions,compressions,orconjunctrotations.
Considertheosteokinematicmovementofthehumerusmovingfromtherestingpositionwiththearmatthesidetotheclosepackedposition.
Thejointisconvexonconcave Theheadofthehumerusmustrollinthesamedirectioninwhichtheboneswings.
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ClinicalApplications:TERMINOLOGY
Itmustslideoppositethisdirectionorsomewhatinferiorlyandinward Becausetheclosepackedpositionisbeingapproached,thejointsurfacesarebecomingapproximated.
Itisamovementofimpureswing,soaconjunctrotation(inthiscasealateralrotation)mustoccur Ifanyoneoftheseaccessorymovementsdoesnotorcannotoccur,thenthisparticularswingofthehumeruscannotbeperformedpainlesslyorharmlesslythroughthefullrange.
Iffullosteokinematicmovementdoesoccur,itdoessoattheexpenseofthecapsuleorligaments,whichmustbeabnormallystretched,orofthearticularcartilage,whichmustbeabnormallycompressed.
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ClinicalApplications:TERMINOLOGY
Thetermcomponentmotions canbeusedsynonymouslywithaccessorymovements.
Forexample,lateralrotationofthetibiaisreferredtoasacomponentofkneeextension Likewise,spreadingofthedistaltibiaandfibulaisacomponentofdorsiflexionattheankle.
TheclinicianmustbeawareofthecomponentmotionsnecessaryforeachosteokinematicmovementatajointManyofthesearelistedinAppendix.
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ClinicalApplications:TERMINOLOGY
Jointplaymovements arethoseaccessorymovementsthatcanbeproducedpassivelyatajointbutcannotbeisolatedactively Theyincludedistractions,compressions,slides,rolls,orspinsatajointinaparticularposition.
Jointplaymovementsareusedwhenapplyingspecificmobilizationtechniquestorestoreaccessorymovementssothatfullandpainlessosteokinematicmovementmayberestored Forexample,inferiorglideoccursattheshoulderduringactiveelevation.
Itcanbeperformedpassively,butinitselfcannotbeperformedactivelybyvoluntarymusclecontraction;inferiorglideisajointplaymovementattheglenohumeraljoint.
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ClinicalApplications:TERMINOLOGY
Jointmobilization isaverygeneraltermthatmaybeappliedtoanyactiveorpassiveattempttoincreasemovementatajoint.
Inadditiontotraditionalmethodsofincreasingjointmovement,suchasactive,passive,andactiveassistedrangeofmotiontechniques,jointmobilizationincludesspecificpassivemobilizationtechniques.
Thesetechniquesareaimedatrestoringthosecomponentmovementsthatpermitpainfreeorharmlessosteokinematicmovement Theyareusedespeciallytorestorethosejointplaymovementsthatcannotbeisolatedactively.
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ClinicalApplications:TERMINOLOGY
Specificpassivemobilizationtechniquesaregraded(Fig.310).Grades1through4areoftenreferredtoasarticulation techniques,whicharepassiverhythmicoscillations.
Grade5isamanipulation techniquethatisahighvelocity,lowamplitude,passivethrust.
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ClinicalApplications:TERMINOLOGY
Thesegradesarerelativetothepathologicamplitudeofjointplaymovementthatexistsatthejointandnottothenormalamplitudethatshouldexist.
Therearetwomaincriteriafortheselectionoftheparticulargradetobeused:(1)thedegreeofpainorprotectivemusclespasmduringpassivejointplaymovement(irritability)and(2)thedegreeofrestrictionofjointplaymovement.
Thegreatertheirritability,thelowerthenumericgradeofmovementused Painandspasmmustbeavoided.
Manipulationisusedprimarilywhenaveryslight,minimallypainfulrestrictionexists.
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ClinicalApplications:TERMINOLOGY
Athirdcriterionofselectionmightapplyhere,namelytheskillandexperienceoftheoperator,becausemanipulativemaneuversshouldonlybeattemptedafterarticulationtechniqueshavebeenmasteredandaftermuchpractice.
ThetermsofjointmovementareshowninFigure311.
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ClinicalApplications:TERMINOLOGY
Specificaccessoryjointmotionsthatarelimitedmayberestoredbymanualoscillationsorthrusts Theprimarygoalofusingspecificjointmobilizationtechniquesisrestorationofnormal,painfreeuseofthejoint.
Theemphasisisnotonforcingaparticularanatomic(osteokinematic)movementatajoint,ashasbeendoneinthepastwithtraditionalmethodsofmobilization;rather,itisonrestoringnormaljointmechanicstoallowfull,painfreeosteokinematicmovementtooccur.
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ClinicalApplications:TERMINOLOGY
Inthisway,rangeofmotionisrestoredtothejointwithlessriskofdamagingthejointbycompressingisolatedportionsofarticularcartilage,andwithlesspainandmuscleguardingfromoverstretchingisolatedcapsuloligamentousstructures,asmaywelloccurifanosteokinematicmovementisforcedtheabsenceofnecessarycomponentmovements.
Thisistosaythatspecificpassivemobilization,correctlyapplied,isainsafer,moreefficient,andlesspainfulmethodofincreasingrangeofmotionatajoint.
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ClinicalApplications:TERMINOLOGY
Mennell,inhislectures,oftenusestheanalogyofadoorhavinglostmovementbecauseofafaultyhinge Effortstorestoremotionbypushinghardonthedoorarelikelytoresultinfurtherdamagetothehinge
Thelogicalmethodtoremedythesituationistodirectonesattentiontothehingetorestorenormalmechanicstothehinge,therebyrestoringnormalmovementofthedoor.
Thisdiscussionignoresthephysiologicconceptofthejoint Thisisdonesolelyforthesakeofsimplicity.
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ClinicalApplications:TERMINOLOGY
Obviously,whenrestoringnormaljointmechanicsisconsidered,attentionmustbegiventotheanatomicjointalongwiththosestructuresresponsibleforactivemovementofthejoint
Forexample,activeabductionattheshoulderisoftenlostbecauseoftheabsenceofinferiorglide Relativetotheanatomicjoint,thejointplaymovementofinferiorglidemaybelimited However,theproblemmayalsobephysiologic,inthatinferiorglidemaynotbeoccurringowingtoweaknessofthesupraspinatusmuscle.
Theseareverydifferentproblemsleadingtosimilarresults Thenatureoftheproblemmustbebroughtoutbyathoroughevaluation.
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ANALYSISOFACCESSORYMOTIONS
ClinicalAssessment.Manyofthemeansofdeterminingwhichaccessorymovementsarecomponentsofspecificosteokinematicmovementshavealreadybeendiscussed
Forinstance,thedirectionofrollisalwaysthesameasthatfortheswingofthebone Ifaconvexsurfacemovesonaconcavesurface,slidewilloccurinthedirectionoppositetotheroll;ifaconcavesurfacemovesonaconvexsurface,slideoccursinthesamedirectionastheroll Distractionoccurswhenmovingoutoftheclosepackedposition;compressionoccurswhenmovingintotheclosepackedposition Thesecomponentscanallbedeterminedforanyjointmovinginanydirection.
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ANALYSISOFACCESSORYMOTIONS
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ANALYSISOFACCESSORYMOTIONS
Someoftheothercomponentmotions,aslistedforeachjointintheAppendix,mustbememorizedordeducedanatomically.
Onewayofassessingthestateofaparticularaccessorymovement(itsamplitudeandirritability)isclinically,byevaluatingthejointplaymovements Theseexaminationmaneuversareessentiallythesameasthespecificmobilizationtechniques.
Ratherthanbeingperformedasagraded,therapeutictechnique,theyareusedtodeterminetheamplitudeofajointplaymovementandwhetherthemovementcausespainorspasm.
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ANALYSISOFACCESSORYMOTIONS
Theamplitudeofmovementandpossiblerestrictionmustbecomparedwiththeoperatorsconceptofnormalforthatmovement,atthatjoint,forthatbodytype.
ThisrequiresexperienceinevaluatingnormalaswellaspathologicjointsWheneverpossible,thepathologicjointmustbecomparedwithahealthycontralateraljoint.
Thedegreeofirritabilityisdeterminedbythepatientssubjectiveresponseandbythepresenceofprotectivemusclespasmwhenperformingtheexaminationmovement.
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ANALYSISOFACCESSORYMOTIONS
Proficiencyinclinicallyevaluatingjointplaymovementsandcorrelatingfindingstoknowledgeofaccessorymovementsatthejoint,aswellasothersymptomsandsignspresentingonexamination,isessentialtotheeffectiveapplicationofjointmobilizationtechniquesandmanagementofmusculoskeletaldisorders.
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NEUROLOGY JOINTNUTRITION LUBRICATION ResolvingProblemsofJointSurfaceWear
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NEUROLOGY
Theneuroanatomyand,especially,theneurophysiologyofjointsaresubjectsnotwellcoveredinthecurrentliterature FollowingtheoriginalworksofSherringtononneuromuscularphysiology,avastamountofinformationwascollectedconcerningtheroleofmuscleandtendonreceptorsininfluencingposture,movement,muscletone,andvariousreflexphenomena.
Littlehasbeendoneuntilrecentlytoidentifyspecificjointreceptors,andevenlesshasbeendonetodeterminetheirclinicalsignificance.
Becausethesubjectofjointneurologyisdirectlyrelevanttothemanagementofcommonmusculoskeletaldisorders,anoverviewispresentedhere.
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Innervation
Jointstendtoreceiveinnervationfromtwosources:(1)articularnervesthatarebranchesofadjacentperipheralnervesand(2)branchesfromnervesthatsupplymusclescontrollingthejointEachjointisusuallysuppliedbyseveralnerves,andtheirdistributionstendtooverlapconsiderably.
Ingeneral,aparticularaspectofajointcapsuleisinnervatedbybranchesofthenervesupplyingthemuscleormusclesthatwould,whencontracting,preventoverstretchingofthatpartofthecapsule.
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Innervation
Onenotableexceptionistheanteroinferioraspectoftheglenohumeralcapsule,whichisinnervatedbyabranchfromtheaxillarynerve.
Thenervefibersofanparticularnervearepurelyafferent,withtheexceptionofsmallvasomotorefferentstothebloodvessels.
Thefibersizesrangefromlargemyelinatedfiberstosmallmyelinatedandunmyelinatedfibers.
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Receptors
Jointreceptorstransmitinformationaboutthestatusofthejointtothecentralnervoussystem Thecentralnervoussysteminterpretstheinformationsentbythejointreceptorsandrespondsbycoordinatingmuscleactivityaroundthejointtomeetjointmobilityandstabilityrequirements.
Jointreceptorsfunctiontoprotectthejointfromdamageincurredbygoingintothepathologicrangeofmotion Theyarealsopartlyresponsiblefordeterminingtheappropriatebalancebetweensynergisticandantagonisticmuscularforcesandforgeneratinganimageofbodypositioningandmovementwithinthecentralnervoussystem.
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Receptors
Fourtypesofjointreceptorshavebeenidentified,eachservingarelativelyspecificroleinthesensorimotorintegrationofjointfunction.
TypeI:Postural DescriptionEncapsulatedendings,similartoRuffinicorpuscle LocationNumerousinthesuperficialjointcapsule;usuallyfoundinclustersofsix;locatedprimarilyintheneck,hip,andshoulder
RelatedfiberSmall(6to9m)myelinated(relativelyslowconduction) StimulusChangingmechanicalstressesinthejointcapsule;maybeactivatedbythepresenceofpositionalfaults;maybemoreactivewithtractiontechniquesthanwithoscillations
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Receptors
TypeII:Dynamic DescriptionThicklyencapsulated,similartopaciniancorpuscle LocationSparse(relativetotypeI);foundinjointcapsuleandligaments(deeperlayersandfatpads);primarilylocatedinthelumbarspine,hand,foot,andjaw
RelatedfiberMedium(9to12m)myelinatedStimulusSuddenchangesinjointmotion;maybemoreactivewithoscillationtechniquesthanwithtraction
ActionRapidlyadapng(actsfor12secondfollowingeachmoon),lowthreshold;dynamicmechanoreceptor
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Receptors
FunctionFiresonlyonquickchangesinmovement;providesinformationconcerningaccelerationanddecelerationofjointmovement;actsatinitiationofmovementasaboostertohelpovercomeinertiaofbodyparts;producesincreasedtoneinthemusclebeingstretchedandrelaxationinthemuscleantagonistictotheonebeingstretchedwhenthejointisatendrange,notactiveinmidrangeofmotion;inhibitspain
TypeIII:Inhibitive DescriptionThinlyencapsulated,similartoGolgiendorgan LocationPrimarilylocatedinintrinsicandextrinsicjointligaments,superficiallayersofthecapsule;inthelumbarspinenotdetectedinthelongitudinalposteriorligament,longitudinalanteriorligament,oriliolumbarligament
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Receptors
RelatedfiberLarge(13to17m)myelinated(fastconduction) StimulusStretchatendrange;moreactivewithfastmanipulationtechniques
ActionVeryslowlyadapting(actsforseveralminutesfollowingtheinitialstimulation),highthreshold;dynamicmechanoreceptor
FunctionMonitorsdirectionofmovement;hasreflexeffectonmuscletonetoprovideabrakingmechanismagainstmovementtendingtooverdisplacethejoint(movementtoofastortoofar);inhibitsmuscletone;respondstostretchatendofrange
TypeIV:Nociceptive DescriptionFreenerveendingsandplexus
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Receptors
LocationLocatedinmosttissues:fibrouscapsule,intrinsicandextrinsicligaments,fatpads,periosteum(absentinarticularcartilage,intraarticularfibrocartilage,andsynovium)RelatedfiberSmall(2to5m)myelinatedandunmyelinated(2m)(slowconduction)
StimulusSignificantmechanicaldeformationortension;directmechanicalorchemicalirritation
ActionNonadapting,highthreshold;painreceptors FunctionInactiveundernormalconditions;activewhenrelatedtissueissubjecttosignificantdeformationorothernoxiousmechanicalorchemicalstimulation;producestonicmusclecontraction
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ClinicalConsiderations
Itisapparentfromthepreviousdescriptionsthatstimulationofjointreceptorscontributestosenseofstaticposition(typeI),senseofspeedofmovement(typeI),senseofchangeinspeedofmovement(typeII),senseofdirectionofmovement(typesIandIII),regulationofposturalmuscletone(typeI),regulationofmuscletoneattheinitiationofmovement(typeII),regulationofmuscletoneduringmovement(coordination)(typeII),andregulationofmuscletoneduringpotentiallyharmfulmovements(typeIII).
Ofcourse,skinreceptors,connectivetissuereceptors,andmusclereceptorsalsocontributetomanyofthesesamefunctions.
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ClinicalConsiderations
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?
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ClinicalConsiderations
Oneparticularlyinterestingstudydemonstratesacaseinwhichmalocclusionofdentures,causingabnormalafferentdischargefromthetemporomandibularjointcapsules,resultedinanalmosttotalreflexinhibitionofthetemporalmusclesduringactiveocclusionbythepatient.
Restorationofnormaljointmechanics,byremodelingofthedentures,restorednormalmuscularactivity.AstudybyWykeshowedsignificantposturalchangesinaboywithapparentalterationofafferentimpulsesfromtheanklecapsuleafterinjurytothelateralaspectofthecapsule.
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ClinicalConsiderations
Theposturaldeficitpersisteddespiteanotherwisecompleterecovery,withrestorationofnormalstrengthandrangeofmotionandwithnoresidualpain Theboysonlycomplaintwasthatofoccasionalgivingwayoftheankle.
Freemanadvocatestheuseofcoordinationexercisesonabalanceboardforpatientswithchronicankleinstabilityintheabsenceofdemonstrablestructuralinstability Hereportsgoodresultswithsuchaprogram,attributingsuchgivingwayattheankletoalterationofnormaljointafferentflowfollowinginjurytothejoint,suchasfromananklesprain.
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ClinicalConsiderations
Mostphysicaltherapistshaveencounteredthecommonphenomenonofgrossquadricepsatrophyfollowingkneeinjurydespitepreventiveeffortstomaintainmusclefunction.
Althoughthereislittlecurrentliteratureonthesubject,itseemsreasonabletoattributethisproblemtoreflexmuscleinhibitionbyabnormaljointreceptorstimulation.
Asfarastechniquesoftreatmentareconcerned,itisinterestingtorelatewhatisknownaboutthefunctionofthesejointreceptors,andwhatisknownofarthrokinematics,totechniquesthathavealreadyevolved.
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ClinicalConsiderations
Considerthediagonalpatterncommonlyusedinproprioceptiveneuromuscularfacilitationtechniquesmovingthearmthroughflexion,abduction,externalrotationtoextension,adduction,andinternalrotation.
Partoftheexplanationofthispatternreferstomovingfromapositionofmaximumelongationandunspiralingoffunctionallyrelatedmusclestoapositionofspiralingandshorteningofthesesamemuscles
Inaddition,thispatterninvolvesmovingalljointssimultaneouslyfromaclosepackedtoaloosepackedposition
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ClinicalConsiderations
Indoingso,thejointcapsuleofeachjointmovesfromapositionofmaximumshorteningandspiralingtoapositionoflengtheningandunspiraling.
Studiesthusfaronanimalshaveindicatedthatmaximumafferentstimulationoccurswhenapproachingtheclosepackedpositionofajoint;thisistobeexpectedsinceitisthepositionofmaximumtighteningofthecapsuleandligamentsinwhichthereceptorslie.
Thetechniquesofproprioceptiveneuromuscularfacilitationevolvedwithprimaryconsiderationoftheneurophysiologyofmuscles,usingmovementpatternsthatcombineactionsoffunctionallyrelatedmusclestobringaboutamutualfacilitationofeachmuscleinthechain.
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ClinicalConsiderations
Itisnowsuggestedthatthesepatternsalsocombinefunctionallyrelatedjointmovementsthataddtothefacilitativeeffectofthepatternsonthemusclesinvolvedthroughjointreceptorstimulation.
Italsoseemsprobablethatthejointreceptorsplayasignificantroleinothertechniquesoffacilitation,suchasquickstretch,thattendtostimulatethetypeIIreceptor.
Itisimportanttoconsiderthefunctionofjointreceptorswhenusingjointmobilizationorothertreatmenttechniquesinvolvingjointmovement Theeffectivenessofeffortstoincreasemovementatajointwillnaturallybecompromisedbyanymusclecontractiontendingtorestrictjointmovement.
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ClinicalConsiderations
Itisnowsuggestedthatthesepatternsalsocombinefunctionallyrelatedjointmovementsthataddtothefacilitativeeffectofthepatternsonthemusclesinvolvedthroughjointreceptorstimulation.
Italsoseemsprobablethatthejointreceptorsplayasignificantroleinothertechniquesoffacilitation,suchasquickstretch,thattendtostimulatethetypeIIreceptor.
Itisimportanttoconsiderthefunctionofjointreceptorswhenusingjointmobilizationorothertreatmenttechniquesinvolvingjointmovement Theeffectivenessofeffortstoincreasemovementatajointwillnaturallybecompromisedbyanymusclecontractiontendingtorestrictjointmovement.
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ClinicalConsiderations
Emphasis,then,mustbemadeonavoidingreflexmusclecontractionsthatwouldtendtopreventorrestrictadesiredjointmovementForthisreasonandforotherobviousreasonspainmustbeavoidedduringjointmobilization,becauseitiswellknownthatpainatajointtendstoelicitareflexmuscleresponsetorestrictmovementatthejoint.
SuddenjointmovementtendstostimulatefiringofthetypeIIIreceptors,whichsetsupareflexmusclecontractiontorestrictfurthermovement.
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ClinicalConsiderations
GradualinitiationofmovementtendstostimulatethetypeIIreceptor,whicheffectsasmallfacilitativemuscularresponse.
Passiveandactivemobilizationtechniquesarebestperformedrhythmically,withoutsuddenchangesinspeedordirectionofmovement.
AmanipulationmustbeperformedsoquicklythatitiscompletedbeforethereflexmuscularresponseproducedbystimulationofthetypeIIIreceptorcanacttointerferewiththemovement Similarly,itmustbeperformedthroughaverysmallamplitudetominimizethenumberoftypeIIIreceptorsstimulated.
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ClinicalConsiderations
WithrespecttothetypeIVpainreceptors,itisworthemphasizingthatarticularcartilage,fibrocartilage(e.g.,menisci),synovium,andcompactboneareessentiallyaneural.Thisiswelldocumentedinanatomicstudiesandclinically.
Intheanatomicjointsthemajorpainsensitivestructuresarethefibrouscapsule,ligaments,andperiosteum.Thiscarriessomeimportantclinicalimplications Itsuggeststhatpathologicconditionsthatmightalterjointmechanics,suchthatthearticularcartilageundergoesunduecompressionstress,maygounnoticedbythepatientintheinitialstages.
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ClinicalConsiderations
Infact,thepatientmaynotenothinguntileitherjointmechanicsarealteredsufficientlytoplaceanabnormalstressonthejointcapsuleoruntilthejointcartilageundergoessufficientdegeneration,causingalowgradesynovitiswithresultantpressureonthecapsulefromeffusion
Thismayexplainwhypersonswithfrozenshouldersorosteoarthrosisofotherjointsoftendonotpresenttoaphysicianuntilthediseasehasprogressedconsiderably.
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ClinicalConsiderations
Italsosuggeststhatcliniciansmustlearntoexamineroutinelyforsubtlechangesinjointmechanicsratherthanconsideringonlygrossrangeofmotion,strength,andcomplaintsofpainbythepatient.
Patientspresentingwithveryearlysymptomsorsignsofosteoarthrosiscouldenjoycompletearrestorreversalofthejointproblemifproperlymanaged,ratherthanresigningthemselvestofuturejointreplacement.
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JOINTNUTRITION
Inadditiontobeinganeural,articularcartilageisforthemostpartavascular.Thisisalsotrueofintraarticularfibrocartilage Because,ingeneral,bodytissuesdependonbloodsupplyfornutrition,thesestructureswouldseemtobeatadisadvantage.
Itisgenerallybelievedthatthearticularmarginsdoreceivesomenutrientsfromthehighlyvascularizedsynoviumandperiosteumadjacenttothem.
Themenisciatthekneealsoreceivenutrientsattheirperipheralcapsularattachments,anditissuggestedthatthedeeplayersofarticularcartilagearefedbythebloodsupplytothesubchondralbone.
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JOINTNUTRITION
However,theproblemofnutritiontothemoresuperficial,centrallylocatedportionsofthearticularcartilageandtothemorecentrallylocatedpartsoftheintraarticularfibrocartilageremains.
Thesecartilaginousareasaretheprimaryarticulatingsurfaces,notthemoreperipheralareasordeeperlayers Itisgenerallyagreedthatnutritiontotheseregionsoccursbydiffusionandimbibitionofsynovialfluid.
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JOINTNUTRITION
Thisisauniquesituationbecausenutrientsmustcrossatleasttwobarrierstoreachthechondrocytesembeddedwithinthecartilage
First,theymustpassfromthecapillarybedofthehighlyvascularizedsynovium.
Second,theymustthendiffusethroughthesuperficialmatrixlayersofthecartilaginoussurface,beforereachingthecellwallofthechondrocyte.
Thus,synovialfluidservesamajorfunctionasasourceofnutritionforarticularcartilageandintraarticularfibrocartilage.
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JOINTNUTRITION
Intermittentcompressionanddistractionofjointsurfacesmustoccurforanadequateexchangeofnutrientsandwasteproductstotakeplace Ajointthatisimmobilizedundergoesatrophyofarticularcartilage,justasajointinwhichthereisprolongedcompressionofjointsurfaceundergoessimilaratrophicchanges.
Thethreeprimarymechanismsbywhichsynovialjointsundergonormalcompressionanddistractionarethefollowing:(1)weightbearinginlowerextremityandspinaljoints,(2)intermittentcontractionofmusclescrossingajoint,and(3)twistinganduntwistingofthejointcapsuleasthejointmovestowardandawayfromtheclosepackedpositionduringhabitualmovements.
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JOINTNUTRITION
Withrespecttothelastmechanism,itisnecessarytorecallthatasthejointapproachestheclosepackedposition,itssurfacesnotonlybecomecompressedbutalsoapproachapositionofmaximalcongruency.
Thus,compressionnormallyoccursinapositioninwhichgreaterareasoftheopposingjointsurfacesareincontact Thisensuresthatrelativelylargeportionsofthejointsurfacesundergoadequateexchangeofnutrients.
Fromapathologicstandpoint,ajointthathaslostmovement,suchasfromatightjointcapsule,doesnotreceiveanormalexchangeofnutrientsoverthepartsofthejointsurfacesthatnolongercomeintocontact.
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JOINTNUTRITION
Thisisespeciallytrueinthecaseofatightjointcapsule,sincemovementstowardtheclosepackedpositioninwhichthereismaximaljointsurfacecontactareusuallythemovementsthataremostrestricted.
Attritionalchangesinarticularcartilagerelatedtoagingareobservedintherelativelynoncontactingportionsofthejointsurfaces.
Severalreasonsforthismaybepostulated First,thesearetheareasofarticularcartilagethatundergolessdeformationwithuseofthejointovertime;asaresult,therateanddegreeofexchangeofnutrientfluidsarelessintheseareas.
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JOINTNUTRITION
Also,withagethereisareductionofthechondroitinsulfatecomponentofcartilaginoustissue Becausethefluidbindingcapacityofarticularcartilageislargelydependentonitschondroitinsulfatecontent,adecreaseinthisconstituentmightinterferewithnormalnutritiontothetissue.
Furthermore,becauselossofjointrangeofmotionoccurswithadvancingage,theexchangeofnutrientstoportionsofthearticularcartilageisreduced.
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Lubrication
Inadditiontoservingasanutritionalsourceforarticularcartilage,synovialfluidalsoactsasalubricanttopreventunduewearofjointsurfacesfromfriction.
Instudyinglubricationofhumanjoints,however,notjustthepropertiesofsynovialfluidandhowtheyaffectmovementandfrictionbetweentwosurfacesareconsidered.
Inadditiontherearetheshapeandconsistencyofthejointsurfacesaswellasthetypesofmovementthatoccurbetweenjointsurfaces.
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Lubrication
Manymodelshavebeenproposedforhumanjointlubrication.Someoftheearliermodelstendtoignoremanyoftheuniquepropertiesofhumanjoints.
Themorerecentmodelsevolvedwiththesophisticationofengineeringprinciples,whicharebetterabletodealwithsomeofthecomplexfactorsinvolvedinhumanjointlubrication However,itisgenerallyagreedthatnoonemodelofjointlubricationappliestoalljointsunderallcircumstances.
Themajormodeoflubricationinaparticularjointmaychange,dependingonfactorssuchasloadingandspeedofmovement.
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Lubrication
Synovialfluidhasessentiallythesamecompositionasbloodplasma,exceptfortheadditionofmucin.
Mucinismucopolysaccharidehyaluronicacid,whichisalongchainpolymer.
Theviscouspropertiesofsynovialfluidareattributedtohyaluronicacid.
Themostimportantpropertytobeconsideredinthisrespectisthethixotropic ornonnewtonianqualityofsynovialfluid;theviscositydecreaseswithincreasedshearrate(increasedspeedofjointmovement).
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ModelsofJointLubrication
Ananalogycannotaccuratelybedrawnbetweenamachinemodeloflubricationandthelubricationofsynovialjoints Oneofthemajorreasonsforthisisthatthephysicalpropertiesofarticularcartilagedifferconsiderablyfromthephysicalpropertiesofmostmachinecomponents.
Articularcartilageisporousandrelativelyspongelikeinthatithasthecapacitytoabsorbandbindsynovialfluid Articularcartilageisalsoviscoelastic;thedeformationrateishighoninitialapplicationoftheloadandlevelsoffwithtime.
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ModelsofJointLubrication
Whentheloadisremoved,theinitialreformationrateishighanddecreasesovertime(Fig.314).
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ModelsofJointLubrication
Althougharticularcartilageappearsquitesmoothandshinymicroscopically,itis,infact,relativelyroughmicroscopicallyArticularcartilagealsohasthetendencytoadsorblargemolecules,suchashyaluronicacidinsynovialfluid,toitssurface.
Theearlymodelofjointlubricationdescribedahydrodynamic,orfluidfilm,situation(Fig.315).
Inthiscase,synovialfluidfillsinthewedgesofspaceleftbythejointsurfaceincongruencies Onmovementbetweensurfacesthesynovialfluidisattractedtotheareaofcontactbetweenthesurfaces.
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ModelsofJointLubrication
Thisoccursbecauseof(1)thepressuregradientproducedbythemovementand(2)thefactthatrelativemovementtendstopulltheviscousfluidinthedirectionofthemovingsurface Theresultofthisisthemaintenanceofalayeroffluidsbetweenjointsurfacesduringmovement.
Anyfrictionoccurringasaresultofmovementoccurswithinthefluidratherthanbetweenjointsurfaces Thismeetstherequirementsofagoodlubricationsystembecauseitallowsfreemovementandpreventsweartothejointsurfaces.
Thissystemworkswellduringmovement;however,itwouldtendtofailunderveryslowvelocityorunderheavyloading
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ModelsofJointLubrication
Thehydrodynamicmodel,however,cannotbecompletelyrepudiatedbecausethepreviousdescriptiondoesnotconsidertheviscoelasticityofjointsurfaces.Thismodelcanbemodifiedtoanelastohydrodynamicsystem(Fig.315).
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ModelsofJointLubrication
Becauseofthenatureofarticularcartilagetodeform,notalltheenergyofheavyloadinggoestodecreasingthethicknessofthelayeroffilmbetweenthesurfaces,thusincreasingfrictionbetweenthesurfaces.
Instead,deformationofthejointsurfacesoccurs,increasingtheeffectivecontactareabetweensurfacesandthusreducingtheeffectivecompressionstress(forceperunitarea)tothelubricationfluid Thisallowstheprotectivelayeroffluidtoremainataboutthesamethickness.
Thus,theelastohydrodynamicmodeldescribesasystemthatwithstandsloadinginthepresenceofmovement.
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ModelsofJointLubrication
ThismodelofjointlubricationcanbeexpandedbyincludingtheconceptsofboundarylubricationandweepinglubricationWithanymaterialsundergoingrelativeshearbetweentwosurfaces,frictionistheresultoftheirregularitiesofthesurfaces;thegreatertheirregularities,thegreaterthefriction.
Effectivelubricationmustreducethisfrictiontoaminimum,thusreducingwearofthesurfacestoaminimum.Inthecaseofboundarylubrication,thelubricantisadsorbedtothesurfaceofthematerial,ineffect,reducingtheroughnessofthesurfacesbyfillingintheirregularities.
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ModelsofJointLubrication
Becausearticularcartilageisabletoadsorblongchainmoleculesofhyaluronicacid,thesemoleculesareabletofillintheirregularitiesandtocoatthesurface Anyfrictionoccurringastheresultofshearmovementoccursbetweenmoleculesofthelubricantratherthanbetweenthejointsurfacesthemselves.
Thisprobablyservesasanadjuncttotheelastohydrodynamicsystem,especiallyincasesofextremeloadingsufficienttodecreasesignificantlythethicknessofthelayeroffluidmaintainedbytheelastohydrodynamicmodelunderlighterloads.
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ModelsofJointLubrication
Bytheuseofthismixedlubricationmodel,whichcombineselastohydrodynamicconceptswithboundarylubricationconcepts,thedemandsofhumansynovialjointsaremet.
Thesystemallowsmovement,changeindirectionofmovement,loading,andvariationsincongruenciesofjointsurfaces Ittakesintoconsideration,atleastingeneralterms,thepropertiesofthelubricant(synovialfluid)andthesurfacematerials.
Thereisstillconsiderablecontroversyovertherelativeimportanceofeachofthelubricationmodelsundervariousconditions,butmostauthorsagreewiththegeneralconceptspresentedpreviously.
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ModelsofJointLubrication
Becauseitisstillunknownhoweachmodelcontributestonormaljointlubrication,verylittleinvestigationintothemutualeffectsofpathologicjointconditionsandjointlubricationhastakenplace.
Abreakdowninsomeaspectofthelubricationsystemislikelytocauseoraddtotheprogressionofjointdisease,suchasdegenerativejointdisease
Ontheotherhand,certainjointdiseasesresultinchangesinstructureandfunctionofjointconstituents Forinstance,thereisalossofjointcartilageindegenerativejointdiseaseandchangesinsynovialfluidviscosityinrheumatoidarthritis.
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ResolvingProblemsofJointSurfaceWear
IthasbeenemphasizedthatsynovialjointsurfacesareincongruentBecauseoftheincongruencythatexistsinmostpositionsofmovement,arelativelysmallcontactareaexistsbetweenjointsurfaces.
Thewedgesofspacethatsurroundthiscontactareaarenecessaryforahydrodynamiclubricationsystemtooperateeffectively;withoutthesespacesthelubricantcouldnotbedrawn,orforced,betweenthecontactingsurfaces.
Onemaywonderifsuchasmallareaofcontactmightincreasethelikelihoodofwearbetweenjointsurfaces,sinceloadingforcesfromweightbearingandmusclecontractionwouldbedistributedoverasmallsurfacearea,thusincreasingthecompressivestresstothejoint.
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ResolvingProblemsofJointSurfaceWear
Ineachofthesejoints,movementtendstoberestrictedtoonearcofmovementthatisdeterminedalmostentirelybytheshapesofthejointsurfaces Itwouldseemthatduringmovementatthesejoints,thecontactingareaononejointsurfacewouldconsistentlyfollowarutontheopposingsurface,increasingthelikelihoodofexcessivewearintherutorattheareaofthesurfacecontactingtherut.
Inthesejointstheproblemofexcessivewearisresolvedinanumberofways First,thesejointsurfacesare,relativelyspeaking,themostcongruentinthebody,sothatforcesaredistributedoverasomewhatlargerarea Consider,forexample,theclosefitbetweentheulnaandthetrochlearsurfaceofthehumerus.
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ResolvingProblemsofJointSurfaceWear
Second,thecontactareaoneachsurfaceisconstantlychangingthroughoutanarcofmovement Achangeincontactareasoccursinonesensebecauseacombinationofrollandslidetakesplacebetweenjointsurfaces.
Inanothersense,thecontactareachangesbecausecontactalternatesfromthebottomofthevalleytothesidesoftheslopesononesurfaceandcorrespondinglyontheopposingsurface Forinstance,withthekneeinfullextension,thearticularsurfaceofthepatellamakescontactwiththefemuratastripextendingmediolaterallyacrossthemiddleofthepatellarsurface Inflexion,however,onlythemedialandlateralmarginsofthepatellamakecontactwiththefemoralcondyles
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ResolvingProblemsofJointSurfaceWear
However,atjointssuchastheknee,amorecomplexsituationexistsThekneeissignificantlyincongruentcomparedwiththejointsdiscussedpreviously;itmustbetoallowsomedegreeofrotationtooccurindependentlyofflexionorextensionorinconjunctionwiththem.
Thekneemustalsowithstandheavierloadingfromweightbearinginawidevarietyofpositions Thus,thekneeisoftenrequiredtoundergoheavyloadinginpositionsofflexion,inwhichthesurfacesareveryincongruentasmallareaofcontactwithstandsrelativelylargecompressiveforces.
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ResolvingProblemsofJointSurfaceWear
Thus,theremightbeconcernthatinasituationofheavyloading,relativelylowvelocity,andsmallcontactareabetweensurfaces,thelubricationsystemwouldnotbesufficienttopreventexcessivefriction(shear)andwearbetweenjointsurfaces.
Thismightverywellbethecaseinajointsuchasthekneethatmustundergosuchconditionsduringnormaldailyactivities,suchasclimbingstairs,squatting,andlifting Theremightalsobeconcernaboutthetendencyforthefemurtoslipforwardonthetibiaundersuchconditions,againbecauseoftheincongruencyofjointsurfacesandthelackofintrinsicstability.
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ResolvingProblemsofJointSurfaceWear
Theknee,then,isajointthatmustallowmovementofspinbetweenthetibiaandthefemurandswingbetweenthetibiaandfemurbecauseofthefunctionaldemandsplacedonit Forthistobepossible,thejointsurfacesmustbesufficientlyincongruent.
However,becauseofthisincongruenceandbecauseofnormalheavyloadinginavarietyofpositions,thekneeappearssusceptibletoexcessiveshearforcesbetweencontactingjointsurfacesduringmovement,excessivecompressiveforcesbetweencontactingsurfacesonstaticloading,andintrinsicinstabilitywhenloadedinflexion.
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ResolvingProblemsofJointSurfaceWear
Itisprobablethattheintraarticularmenisciservetocompensateforwhatwouldotherwisebeunsatisfactoryengineeringatthekneejointunsatisfactoryinthatthejointwouldnotwithstandthenormalforcesappliedtoitwithoutgivingwayorundergoingprematurewearingofjointsurfaces.
Underheavystaticloading,themenisciacttoincreasetheeffectiveloadbearingsurfaceareaatthejoint,thusreducingtheforceperunitarea.
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ResolvingProblemsofJointSurfaceWear
Duringmovementwithheavyloading,theyagainacttoincreasetheloadbearingsurfacearea,buttheyalsomaintainawedgeshapedintervalsurroundingtheareaofcontactintowhichthelubricantfluidcanbedrawn.
Becausethemenisciaresemicartilaginous,theycanalsoabsorbsynovialfluidWithincreasedloading,fluidcanbesqueezedoutfromthemenisciandthearticularloadbearingsurface,contributingtoaweepinglubricationphenomenon Also,asthemeniscirecedebeforetheadvancingcondylesduringmovement,theycanacttospreadalayeroflubricantoverthejointsurfacesjustbeforecontact.
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ResolvingProblemsofJointSurfaceWear
Thereisalsoconsiderableslideofthefemoralcondylesalongthetibialsurfaceduringthecompleterangeofflexionextension Thisfeaturealsoreducesthelikelihoodofexcessivewearonthetibialsurfacesbydistributingtheloadbearingsurfaceoveralargerarea.
Thedegreeofslidecouldnotoccurnormallywithouttheextrinsiccontrolprovidedbythecruciates,norwithouttheintrinsicstabilityprovidedbythemenisci Thistypeofmotion,inwhichtheareaofcontactofaparticularjointsurfaceconstantlychangeswithmovementatthejoint,isnecessarytoallowfortheintermittentcompressionofarticularcartilageessentialtonormalnutritionandlubrication.
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ResolvingProblemsofJointSurfaceWear
Fibrillationofarticularcartilageinnormalhipandshoulderjointsoccursfirstinnonweightbearingsurfaces.
Also,asignificantaccelerationofdegenerativechangesoccursinweightbearinganimaljointsinwhichajointisimmobilizedbutfulluseofthelimbisallowed.
Thesearebothexamplesoftheeffectsonarticularcartilageoftheloadbearingcontactareanotbeingdistributedoveralargeareaoftheopposingsurfaces Thisisperhapsapartialexplanationforthefrequencyofdegenerativearthritisoccurringinhumanhipjoints;arelativelysmallsurfaceareaisusedforweightbearing,whilemuchofthearticularcartilagereceiveslittleornocompression.