Muscle contributions to medial tibiofemoral compartment contact loading following ACL

19 Pages·2017·2.61 MB·English

RESEARCHARTICLE
Muscle contributions to medial tibiofemoral
compartment contact loading following ACL
reconstruction using semitendinosus and
gracilis tendon grafts
JasonM.Konrath1☯*,DavidJ.Saxby1☯,BryceA.Killen1☯,ClaudioPizzolato1☯,
ChristopherJ.Vertullo1,2☯,RodS.Barrett1☯,DavidG.Lloyd1☯
a1111111111 1 SchoolofAlliedHealthSciencesandMenziesHealthInstituteQueensland,GriffithUniversity,GoldCoast,
Queensland,Australia,2 KneeResearchAustralia,GoldCoast,Queensland,Australia
a1111111111
a1111111111
☯Theseauthorscontributedequallytothiswork.
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*[email protected]
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Abstract
OPENACCESS
Background
Citation:KonrathJM,SaxbyDJ,KillenBA,
PizzolatoC,VertulloCJ,BarrettRS,etal.(2017) Themuscle-tendonpropertiesofthesemitendinosus(ST)andgracilis(GR)aresubstan-
Musclecontributionstomedialtibiofemoral
tiallyalteredfollowingtendonharvestforthepurposeofanteriorcruciateligamentrecon-
compartmentcontactloadingfollowingACL
reconstructionusingsemitendinosusandgracilis struction(ACLR).Thisstudyadoptedamusculoskeletalmodellingapproachtodetermine
tendongrafts.PLoSONE12(4):e0176016.https:// howthechangestotheSTandGRmuscle-tendonpropertiesaltertheircontributionto
doi.org/10.1371/journal.pone.0176016
medialcompartmentcontactloadingwithinthetibiofemoraljointinpostACLRpatients,and
Editor:GayleE.Woloschak,Northwestern theextenttowhichothermusclescompensateunderthesameexternalloadingconditions
UniversityFeinbergSchoolofMedicine,UNITED
duringwalking,runningandsidestepcutting.
STATES
Received:June10,2016
Materialsandmethods
Accepted:April4,2017
Motioncaptureandelectromyography(EMG)datafrom16lowerextremitymuscleswere
Published:April19,2017
acquiredduringwalking,runningandcuttingin25participantsthathadundergoneanACLR
Copyright:©2017Konrathetal.Thisisanopen usingaquadruple(ST+GR)hamstringauto-graft.AnEMG-drivenmusculoskeletalmodel
accessarticledistributedunderthetermsofthe
wasusedtoestimatethemedialcompartmentcontactloadsduringthestancephaseof
CreativeCommonsAttributionLicense,which
permitsunrestricteduse,distribution,and eachgaittask.Anadjustedmodelwasthencreatedbyalteringmuscle-tendonproperties
reproductioninanymedium,providedtheoriginal fortheSTandGRtoreflecttheirreportedchangesfollowingACLR.Parametersforthe
authorandsourcearecredited.
othermusclesinthemodelwerecalibratedtomatchtheexperimentaljointmoments.
DataAvailabilityStatement:Allanalyseddatais
presentedinthefigures.Allrawdatacanbefound
Results
inFigshareathttps://dx.doi.org/10.6084/m9.
figshare.3426887.v2andDOI:10.6084/m9.
Themedialcompartmentcontactloadsforthestandardandadjustedmodelsweresimilar.
figshare.3426887.
ThecombinedcontributionsofSTandGRtomedialcompartmentcontactloadinthe
Funding:Thefollowingstudywasfundedthrough
adjustedmodelwerereducedby26%,17%and17%duringwalking,runningandcutting,
agrantfromtheNationalHealthandMedical
respectively.Thesedeficitswerebalancedbyincreasesinthecontributionmadebythe
ResearchCouncil(NHMRC).Thegrantnumber
was628850,andtheURLishttps://www.nhmrc. semimembranosusmuscleof33%and22%duringrunningandcutting,respectively.
PLOSONE|https://doi.org/10.1371/journal.pone.0176016 April19,2017 1/19
MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
gov.au/.Thefundershadnoroleinstudydesign, Conclusion
datacollectionandanalysis,decisiontopublish,or
AlterationstotheSTandGRmuscle-tendonpropertiesinACLRpatientsresultedin
preparationofthemanuscript.
reducedcontributiontomedialcompartmentcontactloadsduringgaittasks,forwhichthe
Competinginterests:Theauthorshavedeclared
semimembranosusmusclecancompensate.
thatnocompetinginterestsexist.
Introduction
Thequadruplebundlehamstringgraftusingthesemitendinosus(ST)andgracilis(GR)ten-
donshasbecomeanincreasinglycommonorthopaedictechniqueforanteriorcruciateliga-
ment(ACL)reconstruction(ACLR).Thegraftpossessesexcellentmaterialstrength[1]and
hasminimalimpactonthekneeextensormechanism[2–4].However,followingharvest,the
sizeofthedonormusclesaresubstantiallyreduced[5]resultinginkneeflexorandinternal
rotationweakness[6–8].Althoughthereissomeevidenceofcompensatoryhypertrophyofthe
otherhamstringmuscles[6],thelossofmusclesizeinSTandGRlikelycompromisestheir
forceproducingcapability.Thiscouldinturn,haveimplicationsfortibiofemoraljointfunc-
tion,stabilityandloading.
Duringgait,themusclesthatspanthetibiofemoraljointplayacriticalroleinforwardpro-
pulsion,frontalplanetibiofemoralstabilityandcontactloading[9,10].Amuscle’scontribu-
tiontomedialcompartmentcontactloadingisstronglyassociatedwithitscapacitytostabilise
externalvalgusmoments,whilstamuscle’scontributiontolateralcompartmentloadingis
associatedwithitscapacitytostabiliseexternalvarusmoments[10].SincetheSTandGRare
commondonormusclesusedforreplacementoftheinjuredACLandhavelargemoment
armscapableofstabilisingexternalvalgusloads[11],thelossofSTandGRmusclesizemay
reducetheircontributiontobothmedialcompartmentcontactloadingandthestabilityofthe
tibiofemoraljoint.Previousstudieshavefoundthatthepeakkneeadductionmomentis
relatedtodiseaseseverity[12,13],however,giventhesubstantialcontributionsmadebymus-
clestothecontactloadingofthekneesarticularsurfaces[9,10],andtheirmechanicalrolein
stabilisingthejointagainstexternalloads,methodsthatestimatekneejointcontactloads
shouldincludethecontributionofthesurroundingmuscles.
Thereisemergingevidencethatcontactloadingofthetibiofemoraljointislowerthannor-
malfollowingACLrupture[14]andsubsequentreconstruction[15]andisassociatedwith
futureonsetofkneeosteoarthritis(OA)[15].KneeOAtypicallyaffectsthemedialcompart-
ment,withthelossofmedialcartilagebeinganimportantstructuralmarkerofdiseaseseverity
andprogression[16,17].Themagnitudeofthetibiofemoraljointcontactforcemaybeinflu-
encedbyexternalloadingconditions,kinematics,aswellasanindividualstask-specificmuscle
activationpatterns.Whencomparedtohealthycontrols,ACLreconstructedpatientshave
beenreportedtowalkwithsmallerkneeflexionanglesandkneeflexionexcursionduringgait
[18].Moreover,studiescomparingtibiofemoralmotionandloadingbetweentheACLRsand
controlshavereportedboththeinjuredandcontralateralsideshavesignificantdifferences
comparedtohealthyintactknees[19].Furthermore,Gardinieretal.[14]investigatedtibiofe-
moralcontactforcesinathleteswithacuteACLruptureandfoundthatpatientswalkedwith
decreasedjointcontactforceontheirinjuredkneecomparedtotheiruninjuredknee,which
persistsafterACLR[15].Howevernopreviousliterature,hasattemptedtoinvestigatethe
effectofdonormuscleatrophyontheircontributiontothejointcontactforce.Inordertoiso-
latetheeffectsofdifferentdonormuscle-tendonproperties,acomparisonisneededunderthe
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
sameexternalloadingconditions,kinematicsandunderlyingmuscleactivationpatternsthat
pertaintoeachindividual.
Directinvivomeasurementofjointcontactforcesisonlypossiblethroughtheuseof
instrumentedprostheticimplants.However,duetocostandinvasiveness,directmeasurement
isunfeasible.Analternativeapproachiscomputationalneuromusculoskeletal(NMS)models
thatprovideanon-invasivemethodtoestimatethetibiofemoraljointcontactforcesthatoccur
duringgait.Computationalmethodsmaybebroadlycategorisedaseitheroptimization-based
orelectromyography(EMG)drivenmodels.Alimitationofoptimization-basedmodelsisthat
theassumptionthatthenervoussystemrecruitsmusclesbasedonaknowncriterion(i.e.mini-
mizationofmusclestresses)maynotapplytoindividualswithjointpathologyorneurological
impairment[20].EMG-drivenmodelsaddressthisshortcomingbyusingmeasuredmuscle
activationpatternsasadditionalmodelinputs[21].Muscleactivationpatternstogetherwith
muscle-tendonkinematicsarethenusedasinputstoaHill-typemusclemodeltoderiveesti-
matesofmuscle-tendonforcesandmoments,aswellasjointcontactforces.Importantly,
EMG-drivenmodelestimatesoftibiofemoralcontactforceshavebeenvalidatedagainstdirect
measurementsfrominstrumentedkneeimplants[20,22,23].
Thepurposeofthisstudywastouseaneuromusculoskeletalmodellingapproachtodeter-
minetheeffectsofpreviouslyreportedalterationsinthemuscle-tendonpropertiesoftheST
andGRinACLR[5,6];ontheircontributionstomedialcompartmentcontactloadingwithin
thetibiofemoraljointexperiencedunderthesamemotionandexternalloadingconditions,
duringwalking,runningandsidestepcutting.WehypothesisedthattheSTandGRwould
contributelesstomedialcompartmentloadingofthetibiofemoraljointfollowingACLR,and
thatothernon-donormuscleswouldcompensateforthesereductions.Sincethedonormus-
clesareinvolvedinsupportingseveraldegreesoffreedom,itwasenvisagedthatestimating
thesetheoreticalcompensationstrategieswouldinformrehabilitationstrategiesinindividuals
thathaveundergoneaquadruplebundlehamstringauto-graftACLR.
Materialsandmethods
Participants
Twenty-fiveparticipants(20male,5female,meanage31±6years,meanbodymass84±
13kg)thathadundergoneaquadruplebundlehamstring(ST+GR)auto-graftACLRwere
recruited.Inclusioncriteriawere:(i)unilateralACLinjurysustainedwithoutanyconcomitant
kneeligamentinjury;(ii)between2–3yearspostaquadrupledST-GRgraft-ACLR;(iii)
between18–45yearsofage;(iv)theabilitytocomplywithtestingprotocol.Exclusioncriteria
were:(i)complexkneeinjurieswithadditionalligamenttears;(ii)previousorsubsequent
ACLinjuryorlowerextremitysurgery.EthicsapprovalwasobtainedthroughHumanRe-
searchEthicsCommitteeoftheUniversityofWesternAustralia(ReferenceCode:RA/4/1/
4150)withallparticipantsprovidingtheirwritteninformedconsentpriortoanytesting.
Surgicalprocedure
Patientswererecruitedfromtheclinicsoffourlocalorthopaedicsurgeons.Surgeonsfollowed
astandardisedprotocolforaquadruplebundlehamstringauto-graft.Followingtourniquet
applicationtothethigh,ananteromedialverticalincisionwasmadeoverthepesanserinus.
ThesuperiorborderofthepesanserinuswasthenincisedtovisualisetheSTandGRtendons.
Thetendonswereleftsecuredtotheirdistalattachmentpointsandanopen-endedtendonhar-
vester(Linvatec,LargoFL)wasusedtoreleasethetendonsproximallyfromtheirmuscular
attachmentpointsusingacutratherthanapushtechnique[24]toalengthof22cminfemales
and24cminmales.Thenaquadrupledgraftwasformedbyfoldingbothtendonsandwound
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
together.Thefemoraltunnelwascreatedviaatransportaldrillingtechnique,withfemoralfix-
ationofthegraftachievedbyaclosedloopEndobutton(Smith&Nephew,MemphisTN)and
tibialfixationachievedusingaroundcannulatedinterferencescrew(Smith&Nephew,Mem-
phisTN).Followingsurgery,allpatientsfollowedastandardisedearlymobilizationrehabilita-
tionprotocol[6].
Experimentalprotocol
Participantsinitiallyperformedaseriesofmaximalverticaljumps,isometriccontractions,as
wellasisokineticdynamometertrialsinordertoobtainmaximumEMGvaluesforeach
instrumentedmuscle.Participantswerethenfamiliarizedwitheachgaittask(walk,runand
sidestepcut)andsubsequentlyperformedaminimumofthreesuccessfultrialsofeachgait
task.Atrialwasconsideredsuccessfuliftherelevantfootlandedwhollyontheforceplatform
andwasperformedatthedesiredspeedsof2.0–2.5m/sforwalkingand4–4.5m/sforrunning
andsidestepcutting.Thesidestepcuttingwasperformed,usingthesurgicallegasthepivot
leg,toanangleof45˚fromtheapproachdirection.
Experimentaldatacollection
Motioncapture,forceplateandEMGdatawereconcurrentlyandsynchronouslyacquired
duringtheperformanceofeachtask.A10-cameraVICONMXmotionanalysissystem
(Vicon,Oxford,UK)wasusedtoacquirethemotionofretro-reflectiveskin-surfacemarkers
attachedtotheparticipants,andsampledat200Hz.Retro-reflectiveskin-surfacemarkers
wereplacedonprominentanatomicallandmarksinaccordancewiththeUWAmarkerset
[25],with3-markerclustersattachedtotheupper-limb,and10-markerclustersusedon
lower-limbsegmentstoimproveassessmentofkneemotion[26].Groundreactionforces
(GRF)weremeasuredfromtwoforceplates(AdvancedMechanicaltechnologyInc.,Water-
town,USA)samplingat1000Hz.EMGsfrom16musclesonthesurgicallimbweresampledat
1000Hzusingwirelesssensors(Zerowire,Aurion,Milan,IT)bipolarAg/AgClsurfaceelec-
trodes(Duo-Trode,Myotronics,USA).Themusclesinvestigatedwere:medialhamstring
group(semimembranosus(SM)/semitendinosus(ST));lateralhamstringgroup(bicepsfemo-
rislonghead(BFLH)andbicepsfemorisshorthead(BFSH));adductorgroup(AG);rectus
femoris(RF);vastuslateralis(VL);vastusmedialis(VM);gracilis(GR);tensorfascialatae
(TFL);sartorious(SR);gluteusmaximus(GMax);gluteusmedius(GMed);medialgastrocne-
mius(MG);lateralgastrocnemius(LG);soleus(SL);tibialisanterior(TA);andperoneals(PR).
Experimentaldataprocessing
DataprocessingwasperformedusingtheMOtoNMSsoftware[27]inMATLAB(TheMath-
works,Mass,USA).MarkertrajectoriesandGRFswerelow-passfilteredusingazero-lag,2nd
order,Butterworthfilterwithacut-offfrequencyof10Hzforwalkingand15Hzforrunning
andcutting.Static[28]andfunctional[25]taskswereperformedtoidentifyjointcentres.
EMGswereband-passfiltered(30–500Hz),fullwaverectifiedandthenlowpassfilteredwith
acutofffrequencyof6Hztoyieldlinearenvelopesforeachmuscle[21],andsubsequently
normalisedtotheirmaximumvalueidentifiedacrossalldynamictrials,functionaltasksand
dynamometertrialstorepresenttheactivationsof34musculotendinousunits(MTU)[29].
Kneejointcentresweredefinedusingmeanhelicalaxes[25],thehipjointcentresweredefined
usingHarringtonregression[30],andtheanklejointcentrewasdefinedasthemidpoint
betweenmedialandlateralmalleoli[31].
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
Thestandardmodel
Inordertoisolatetheeffectsofdifferentdonormuscle-tendonpropertiesunderthesame
externalloadingconditions,kinematicsandunderlyingmuscleactivationpatterns,wechose
tousethesurgicallegwithunadjustedmuscleparametersasthestandardmodel.Thestandard
modelwasusedtocomputeestimatesofthetibiofemoralcontactloadsduringthestance
phaseofeachtaskassumingnomorbiditytotheSTandGRusingtheEMG-drivenmodeof
thesoftwareCEINMS[32].CEINMShasbeendescribedindetailpreviously[32]andsowill
onlybedescribedinbriefhere.Themodelconsistedoffourcomponents:ananatomical
modelcreatedusingOpenSim[33]thatcontainedtheinsertionpointsandpathsoftheline
segmentrepresentationof34musculotendinousunits(MTU),anEMGtoactivationmodel
thatestimatedtheactivationoftheMTUsusingasecondorderdiscretenon-linearmodel
[21],amodifiedHill-typemusclemodelthatusedMTUactivationandkinematicstoestimate
MTUforcesandmoments,andacalibrationphase.EachMTUwasmodelledasacontractile
elementinserieswithacomplianttendon[34].Thetendonwasmodelledusinganon-linear
functionnormalisedtotendonslacklength(ls)[34].Thecontractileelementmodelconsistsof
t
genericforce-length,force-velocity,andparallelelasticfunctions,inwhichfinalMTUforce
(F ),isdependentoneachMTU’smaximumisometricforce(FMAX),optimalfibrelength
MTU m
(lo),andpennationangleatoptimalfibrelength(;o).
m m
CalibrationwasusedtooptimisetheMTUandactivationparametersforeachsubject.Cali-
brationconsistedoftwosteps:morphometricandfunctionalscaling[29,32,35,36].Themor-
phometricscalingadjustedtheparametersof(lo)and(ls)ofeachMTUtopreservethe
m t
dimensionlessmusclefibreandtendonoperatingcurveswhilerespectingtheoverallMTU
lengthacrossarangeoflower-limbjointangles[35,36].Thefunctionalscaling,partofthe
CEINMSframework,adjustedEMG-drivenmodelparameterssuchthattheleastsquareddif-
ferencesbetweenthemodelpredictedjointmomentsandtheexperimentallymeasuredjoint
momentswereminimised[21,29].Thecalibrationincludedjointmomentsfromhipadduc-
tion-abduction(HAA),hipflexion/extension(HFE),kneeflexion/extension(KFE),andankle
dorsi/plantarflexion(AFE)[29].Theexperimentaltrialsusedinthecalibrationprocedure
includedonewalk,onerunandonecut.Parametersincludedinthefunctionalcalibration
were:(i)activationparameters(C1andC2)whichadjusttheimpulseresponseofthesecond
orderfilter(ii)anon-linearshapefactor(A)whichaccountsforthenon-linearEMGtoforce
relationship[21],(iii)lo,(iv)ls,and(v)strengthcoefficientsfor12groupsofmusclesthatscale
m t
eachMTUsFMAXwithineachgrouptoaccountfordifferencesinmusclephysiologicalcross
m
sectionalareabetweenpeople[21,29].The12functionalmusclegroupsweretheuniarticular
hipflexors,uniarticularhipextensors,uniarticularhipadductors,biarticularhipadductors,
hipabductors,uniarticularkneeflexors,uniarticularkneeextensors,uniarticularankleplantar
flexors,uniarticularankledorsiflexors,biarticularquadriceps,biarticularhamstringsand
gastrocnemiusmuscles.Aftercalibration,theNMSmodeloperatedasanopen-looppredictive
systemforeachofthewalking,runningandcuttingtrialstocalculatemuscleforces,joint
momentsandkneejointcontactforcesasafunctionofmuscleactivationandmodelkinemat-
ics[32].
Theadjustedmodel
AversionofthestandardmodelwithmodificationstotheHill-typemuscle-modelparameters
fortheSTandGRwascreatedtorepresentdonormusclemorbidityfollowingahamstring
graftACLR[5,6].Musclevolumes(V )andpeakcrosssectionalareas(CSA )fromWilliams
m m
etal[5]andKonrathetal[6]werechosentorepresentSTandGRmorbidity,becausebothV
m
andCSA werereported.AlthoughWilliams[5]was6–9monthspost-surgeryandKonrath
m
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
Table1. MorphologicalchangestotheSTandGRfollowingACLR.
CSA(cm2) Volume(cm3)
Surgical Contralateral Surgical Contralateral
ST 8.8±3.6 11.4±3.3 114.8±67.6 214.9±70.4
GR 4.5±1.8 6.3±2.6 69.6±38.8 107.6±44
Crosssectionalarea(CSA)andvolume(mean±standarddeviation)(N=28)pooledfromWilliamsetal.(2004)(n=8)andKonrathetal.(2016)(n=20)for
theST/GRoftheSurgicalandContralaterallimb.
https://doi.org/10.1371/journal.pone.0176016.t001
[6]was2yearspost-surgery,theirvaluesweresimilar.Therefore,thesemorphologicalchanges
werepooledtogetherandusedtoadjusttheSTandGRparameters(Table1).TheCSAofthe
STandGRwerereducedinthesurgicallegofACLRpatientsrelativetothecontralateralleg
by33%and39%,respectively,andthecorrespondingmusclevolumeswerereducedby47%
and35%,respectively(n=28).
UsingtheV andCSA changestotheSTandGR,the(lo),(ls)andstrengthcoefficients
m m m t
wereadjusted.Physiologicalcrosssectionalarea(PCSA )isacommonlyusedmuscleparame-
m
ter,butwasnotreported,soweassumedPCSA =CSA .Therefore,theCSA ofanMTUis
m m m
directlyproportionaltothestrengthcoefficient(SC )multipliedby(FMAX),fromwhichwe
m m
developEq1.
CSASurg SCAdj
m ¼ m ðEq1Þ
CSACon SCNorm
m m
WhereCSASurgandCSAConrepresenttheaverageCSAofthemusclesofthesurgicallegand
contralateralleginACLRpatientsrespectively,whileSCAdjandSCNormrepresentthestrength
m m
coefficientsfortheadjustedmodelandstandardmodel.Fromthiswedevelopvaluesof(SCAdj)
m
fortheSTandGR.
Volume(V )ofamuscleisrelatedtoitscrosssectionalarea(CSA )multipliedbyoptimal
m m
musclefibrelength(lo).ThereforeloAdjcanbeapproximated,assuming;o isthesameinthe
m m m
surgicalandnormalcontralaterallegs,usingEq2
(cid:18) (cid:19)
(cid:0) (cid:1) VSurg 1
loAdj ¼ loCon m (cid:16) (cid:17) ðEq2Þ
m m VmCon CCSSAASmCmuorng
(cid:16) (cid:17)
WhereloConrepresentsthecontralateraloptimalfibrelengthrespectively,while VmSurg and
(cid:16) (cid:17)m VmCon
CSASmurg representtheratiosbetweensurgicalandcontralaterallegsV andCSA respectively.
CSACmon m m
UsingthenewloAdj,theadjustedtendonslacklengthwascalculatedusingthesameoptimiza-
m
tionmethoddescribedinthemorphometricscalinginwhichthedimensionlessmusclefibre
andtendonoperatingcurveswerepreservedwhilerespectingtheMTUlengthacrossarange
oflowerlimbjointangles[36].
Thefunctionalcalibrationwasthenrepeated,however,theadjustedlo (loAdj),adjustedls
m m t
andadjustedstrengthcoefficientsfortheSTandGRwerenotallowedtochange.Following
thiscalibration,newparameterswerecalibratedfortheother32MTUswithinthemodel,as
wellastheadjustedparametersfortheSTandGRrepresentingtheirmorbidity.Theopen-loop
predictionsystemwasthenruntoobtainMTUforcesandmomentsforeachofthe34MTUs
intheadjustedmodel.
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
Tibiofemoraljointcontactmodel
MTUforceestimatesfromthestandardandadjustedmodelwereincorporatedintoatibiofe-
moraljointcontactmodel[10,22]toestimatethecontactloadinthemedialcompartment
(FMC)(Fig1).Thecontactmodelwasbasedonthreeassumptions:(i)onlyforceswithacom-
ponentparalleltothelongaxisofthetibiaorthatgenerateavarus/valgusmomentaboutthe
kneejointcontributetoarticularloading,(ii)theseloadsactthroughonlyasinglecontact
pointoneachcondyle,separatedbydistance(d ),(iii)ligamentsdonotcontributetoloading
IC
Fig1.Tibiofemoraljointcontactmodel.Tibio-femoraljointcontactmodel(rightleg)usedtoestimate
medialcompartmentloads(FMC).Thepatellaisnotshown.Netmomentsaboutthelateraltibialcontactpoint
(MLC þMLC)weredividedbytheintercondylardistance(d ).
MTU ext IC
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
ofthearticularsurfaces.ThenetinternalMTUvarus/valgusmoments(MLC )aboutthelateral
MTU
contactpointsarefirstcalculatedbysummingtheproductofeachMTUsforce(F )multi-
MTU
pliedbyitsvarus/valgusmomentarm(rLC )aboutthelateralcondylefornMTUs,usingEq3.
MTU
P
MLC ¼ n F ðiÞrLC ðiÞ ðEq3Þ
MTU i¼1 MTU MTU
ThedifferencebetweenMLC andtheexternalmomentsaboutthelateraltibialcontact
MTU
points(MLC)canbeusedwiththeintercondylardistance(d )tocalculatethemedialcondyle
ext IC
contactforce(FMC),byassumingstaticequilibriumaboutthelateraltibialcontactpointinthe
frontalplane(Fig1),usingEq4.Thenetinternalandexternalmomentsaboutthelateralcon-
dylewerethenusedtoestablisheachmuscle’scontributiontothetotalmedialcompartment
loadexpressedasapercentage.
MLC þMLC
FMC ¼ MTU ext ðEq4Þ
d
IC
Statisticalanalysis
Arepeatedmeasuresgenerallinearmodel(GLM)wasusedtoassesstheeffectofmodel(stan-
dardversusadjusted)oneachindividualkneemuscle’saveragecontributionoverstancetothe
medialcompartmentloadforeachgaittask.Theeffectofmodelontheoptimalfibrelength,
tendonslacklength,strengthcoefficientandshapefactorforeachmusclewasalsotestedusing
thesamerepeatedmeasuredGLM.AllstatisticalanalysiswasperformedusingSPSSversion22
(SPSSInc,Chicago,Ill).Significancewasacceptedforp<0.05,buttoaccountformultiple
GLMcomparisons,BenjaminiandHochbergcorrectionswereapplied[37].
Results
STandGRhadsignificantlyshorteroptimalfibrelengths,longertendonslacklengthsand
reducedstrengthcoefficientsintheadjustedcomparedtostandardmodel(Fig2).Forthe
Fig2.Muscleparameterchanges.(A)Optimalfibrelength,(B)Tendonslacklength,(C)Shapefactorand
(D)Strengthcoefficientforeachkneemuscleinthestandard(white)andadjusted(black)model.Dataare
expressedasmean±onestandarddeviation.(*)denotesstatisticalsignificance.
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MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
medialnon-donormuscles,SM,MG,VMandSRsignificantlyincreasedtheiroptimalfibre
length,theSMandVMsignificantlydecreasedtheirtendonslacklength,whiletheSRsignifi-
cantlyincreasedtendonslacklengthintheadjustedmodel.Forthelateralnondonormuscles,
RF,VI,VL,BFSHandLGallsignificantlyincreasedoptimalfibrelength,VLandLGdecreased
theirtendonslacklength,whiletheBFSHincreaseditstendonslacklengthintheadjusted
model.
Thestandardandadjustedmodelproducednearidenticalestimatesofthemedialcompart-
menttibiofemoraljointcontactloadsaswellastherelativecontributionsoftheinternal
(MTUs)andexternalmomentstothemedialcompartmenttibiofemoraljointcontactloadfor
eachgaittask(Fig3).Therewerenosignificantdifferencesineitherthepeakoraveragemedial
compartmenttibiofemoraljointcontactloadsbetweenthestandardandadjustedmodel.Simi-
larly,therewerenosignificantdifferencesbetweentheexternalandinternalcontributionsto
themedialcompartmenttibiofemoraljointcontactloadsbetweenthestandardandadjusted
model.Medialcompartmentloadswerelowestinwalkingandhighestinrunning.External
momentswerethemajorcontributorstothemedialcompartmentjointcontactloadinwalk-
ingwhereastheinternalmomentswerethemajorcontributorsduringrunningandcutting
(Fig3).
ThecombinedcontributionsofSTandGRtomedialcompartmentloadintheadjusted
modelwerereducedby26%,17%and17%duringwalking,runningandcutting,andwerepri-
marilyoffsetbycorrespondingincreasesintheSMcontributionsof33%and22%during
Fig3.Medialcompartmentcontactloadsandrelativecontributions.Medialcompartment(MC)load
(Bodyweights),andrelativecontributionofnetinternal(solidlines)andexternalmoments(dashedlines)tothe
MCload(%)forstandard(red)andadjusted(blue)modelsfor(A)walking(B)runningand(C)cutting.Shaded
regionsindicate±onestandarderror.
https://doi.org/10.1371/journal.pone.0176016.g003
PLOSONE|https://doi.org/10.1371/journal.pone.0176016 April19,2017 9/19
MedialtibiofemoralcompartmentcontactloadingfollowingACLreconstruction
runningandcutting(Fig4)respectively,howevernoincreaseinSMcontributionswere
observedduringwalking.
Duringwalking,theaveragecontributionofST,GRandVItothemedialcompartment
loadthroughoutstanceweresignificantlyreducedintheadjustedversusstandardmodel(Fig
4).Duringrunning,thecontributionofGR,wassignificantlyreducedintheadjustedversus
standardmodel,whereasthecontributionofSMwassignificantlyincreased(Fig4).During
cutting,thecontributionsofGRandSRweresignificantlyreducedintheadjustedversusstan-
dardmodel,whereasthecontributionofSMwassignificantlyincreased(Fig4).
Fig4.Musclecontributionstomedialcompartmentcontactloadsaveragedoverstance.Musclecontributionstothetotalmedialcompartmentload
averagedoverstancephaseforthestandard(white)andadjusted(black)modelsduring(A)walking,(B)runningand(C)cutting.Errorbarsrepresent±one
standarddeviation.(*)denotesstatisticalsignificance.
https://doi.org/10.1371/journal.pone.0176016.g004
PLOSONE|https://doi.org/10.1371/journal.pone.0176016 April19,2017 10/19