Rev. bras. ortop. vol.52 número3

SOCIEDADE BRASILEIRA DE ORTOPEDIA E TRAUMATOLOGIA

w w w . r b o . o r g . b r

Review

article

Importance

of

preclinical

evaluation

of

wear

in

hip

implant

designs

using

simulator

machines

夽

Rafael

Mello

Trommer

_,

_Márcia

_Marie

_Maru

InstitutoNacionaldeMetrologia,QualidadeeTecnologia(Inmetro),DivisãodeMetrologiadeMateriais,DuquedeCaxias,RJ,Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received20May2016 Accepted5July2016

Availableonline30December2016

Keywords:

Arthroplasty,replacement,hip Hipprosthesis

Prosthesisdesign

a

b

s

t

r

a

c

t

Totalhiparthroplasty(THA)isasurgicalprocedurethatinvolvesthereplacementofthe damagedjointofthehipbyanartificial device.Despitetherecognizedclinicalsuccess ofhip implants,wearofthe articulatingsurfacesremainsasone ofthecriticalissues influencingperformance.Commonmaterialcombinationsusedinhipdesignscomprise metal-on-polymer(MoP),ceramic-on-polymer(CoP),metal-on-metal(MoM),and ceramic-on-ceramic(CoC).However,whenthedesignofthehipimplantisconcernedbesidesthe materialsused,severalparameterscaninfluenceitswearperformance.Inthisscenario, wherethesafetyandefficacyforthepatientarethemainissues,itisfundamentalto eval-uateandpredictthewearrateofthehipimplantdesignbeforeitsuseinTHA.Thisisoneof theissuesthatshouldbetakenintoaccountinthepreclinicalevaluationstepofthe prod-uct,inwhichsimulatedlaboratorytestsarenecessary.However,itisfundamentalthatthe appliedmotionsandloadscanreproducethewearmechanismsphysiologicallyobservedin thepatient.Toreplicatetheinvivoangulardisplacementsandloadings,specialmachines knownasjointsimulatorsareemployed.Thisarticlefocusesonthemaincharacteristics relatedtothewearsimulationofhipimplantsusingmechanicalsimulators,giving infor-mationtosurgeons,researchers,regulatorybodies,etc.,abouttheimportanceofpreclinical wearevaluation.Acriticalanalysisisperformedonthedifferencesintheprinciplesof oper-ationofsimulatorsandtheireffectsonthefinalresults,andaboutfuturetrendsinwear simulation.

©2016SociedadeBrasileiradeOrtopediaeTraumatologia.PublishedbyElsevierEditora Ltda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

夽

StudyconductedattheInstitutoNacionaldeMetrologia,QualidadeeTecnologia(Inmetro),DivisãodeMetrologiadeMateriais,Duque deCaxias,RJ,Brazil.

∗ _{Correspondingauthor.}

E-mail:rmtrommer@inmetro.gov.br(R.M.Trommer).

http://dx.doi.org/10.1016/j.rboe.2016.07.004

Importância

da

avaliac¸ão

pré-clínica

do

desgaste

em

projetos

de

implantes

de

quadril

usando

máquinas

simuladoras

Palavras-chave:

Artroplastia,substituic¸ão, quadril

Prótesedoquadril Desenhodeprótese

r

e

s

u

m

o

Aartroplastiatotaldoquadril(ATQ)éumprocedimentocirúrgicoqueenvolveasubstituic¸ão daarticulac¸ãodanificadaporumdispositivoartificial.Apesardoreconhecidosucessoclínico dosimplantesdequadril,odesgastedassuperfíciesarticularesaindaéumadasquestões críticasqueinfluenciamodesempenho.Ascombinac¸õesdemateriaiscomunsusadasnas prótesesincluemmetalsobrepolímero(MsP),cerâmicasobrepolímero(CsP),metalsobre metal (MsM)e cerâmica sobrecerâmica (CsC). Noentanto,em relac¸ãoao desenhodo implantedequadril,alémdosmateriaisutilizados,váriosparâmetrospodeminfluenciar oseudesgaste.Nestecenário,ondeaseguranc¸aeeficáciaparaopacientesãoas princi-paisquestões,éfundamentalavaliarepreverataxadedesgastedomodelodeimplantede quadrilantesdesuautilizac¸ãoemATQ.Estaéumadasquestõesquedevemserlevadasem contanaetapadeavaliac¸ãopré-clínicadoproduto,naqualtestesdesimulac¸ãoem labo-ratóriossãonecessários.Noentanto,éfundamentalqueosmovimentosecargasaplicados possamreproduzirosmecanismosdedesgastefisiologicamenteobservadosnopaciente. Máquinasespeciais,conhecidascomosimuladoresdearticulac¸ão,sãoutilizadasprareplicar osdeslocamentosangularesecargasinvivo.Esteartigoenfocaasprincipaiscaracterísticas relacionadasàsimulac¸ãodedesgastedeimplantesdequadrilpormeiodesimuladores mecânicos,fornecendoinformac¸õesacirurgiões,pesquisadoreseórgãosreguladores, den-treoutros,sobreaimportânciadaavaliac¸ãopré-clínicadodesgaste.Foifeitaanálisecrítica sobreasdiferenc¸asnosprincípiosdefuncionamentodossimuladoreseseusefeitosnos resultadosfinais,bemcomosobreastendênciasfuturasnasimulac¸ãodedesgaste.

©2016SociedadeBrasileiradeOrtopediaeTraumatologia.PublicadoporElsevier EditoraLtda.Este ´eumartigoOpenAccesssobumalicenc¸aCCBY-NC-ND(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Thereplacement of the damaged hip joint byan artificial device, known as Total HipReplacement (THR), is a surgi-calprocedure widely performed inOrthopedics inthe last decades.1_{THRhasanexcellentcost/effectivenessratio,onceit}

improvesthefunctionalstateandqualityoflifeofthepatients inareliableway.2_{Thehipimplantdesignsoftenusea}

com-binationofmetal-on-polymer(MoP),metal-on-metal(MoM), ceramic-on-ceramic(CoC)orceramic-on-polymer(CoP)asthe materialsofthefemoralheadandliner.Althoughthe clini-calsuccessandgreattechnologicaladvancesinhipimplants arewellrecognized,thematerialsusedinTHRsystemshave beencontinuouslysubjectofresearchanddevelopment.3_The

wear ofthe articulating components, causing the primary failureofthe implantduetoosteolysisand aseptic loosen-ing,remainsasanimportantdrawback.4–6_{Themainproblem}

relatedtowearisthegenerationofdebris,whichcanincitea highlyinflammatorybiologicalresponsethatcanleadto sub-sequentlocalizedperiprostheticboneloss,andconsequently, re-surgeryisrequired.7 _{Particularlyforultra-highmolecular}

weightpolyethylene(UHMWPE)cupliners,highwearoccurs mostlyinthesuperior-lateralportionoftheliner,8 _andthe

consequentdebrisgenerationintothebodybecomesamain factortolimitingthelifeoftheimplant.1

Tribology(frictionandwear)ofthebearingsurfaces, associ-atedwithbiocompatibility,aretwocriticalaspectsresponsible fortheclinicalsuccessofahipimplant.Researchand devel-opmentofnewmaterialsofhipimplantsisafundamentalkey

inreducingwear.Itisimportanttokeepinmindthatwhena newdesign(includingmaterials,geometry,etc.)isbeing con-sideredtobeusedinaTHRsystem,thecomponentswillbe exposedtoseveralloadingsandmovements,givingoriginto awiderangeofmechanicalcontactstressesduringthedaily activities.6_{Itmakesthepre-clinicalvalidationacriticalstage}

inthe developmentofanewdesign.Pre-clinicalvalidation isconsideredbysomeresearchersandmanufacturersasan extensionintheriskanalysistask.9

Themostacceptedpreclinicalmethodtoevaluatethewear performanceofahipimplantdesigninlaboratorycomprises theuseofsingularmachinesthatsimulatesthephysiological loadingsandmovementsclinicallyobserved.Thesemachines areknownashipjointsimulators,andprovideimportant out-comesabouttheexpectedbehaviorofahipimplantinclinical use.

This work reviewsthe main characteristics ofthe wear behaviorofhip implantsobtainedthroughtheresultsfrom simulator tests performed under standard protocols, and explainstheimportanceinthepreclinicalevaluationofnew hipimplantdesignsbeforeitsclinicaluse.

Why

simulate

wear

in

a

hip

implant?

safetyand efficacyofthepatient, adeeperinvestigationof theperformanceoftheproductisevennecessary.

Itisnotonlythetribologyperformanceofthehipdesign coupling,butalsothefixationoftheprostheticcomponents that influencethe long-termclinical performanceof artifi-cialhipjoints.10 _{Onthe otherhand,the arisingofmodern}

implants,inassociationwithsurgicaltechniquerefinements, alsowithcementless fixationmethods, havemovedtothe wear properties of the materials of the bearings surfaces the main responsibility for the durability of the total hip arthroplasty.11_{Forthesereasons,wearofthehipimplantis}

oneofthedesignattributeswidelyinvestigatedin laborato-riesandcompaniesaroundtheworld,alsoclinicallyduring thelifeofthepatient,and inretrievedimplants.Thestudy ofthe performanceof the implant in wear tests is a tool forthedevelopmentofhipdesigns,onceitallowsassessing wearrates,debrissizesandshapespriortoimplantation.12

The simulation of wear in laboratory is mainly motivated bythe possibilitytoassessingtheperformanceofdifferent hipimplantdesignsbycomparingtheirwearratesandwear mechanisms.Differenttypesofmaterialcombinationsmay resultindifferentwearmechanisms.Thereareseveral mecha-nismsofwear,suchasabrasive,adhesive,fatigueandabrasive wearmechanismbythirdbodyparticle.Thewearmechanisms canbeassessedafterthe weartestsbylaboratoryanalyses withadvancedtechniquesofmaterialscharacterization.

Consideringthepracticaloperationofthehipjoint,itiswell knownthatthewearisgovernednotonlybythematerial prop-ertiesanddesign,butalsobythelubricantfilm.13_Moreover,

fromthepointofviewofengineering,wearofahipimplant isalsoregardedasafunctionofthekinematicsconditions,14

whichcanalsoaffect thelubricationfilm.Thecorrect sim-ulationofthekinematicsenvironmentiscrucialinorderto correctlyreproducethelubricationconditionofthejoint,and thus,thewearbehaviorofthehipimplant.

Regardingthechoiceofthepairofmaterialsofthejoint, oneshallconsidertheadvantagesanddisadvantages(risks) relatedtothe intendedtribologicalperformanceofthehip implant.Forexample,combinationsthatusesoftarticulating material(such asUHMWPE)againstahardbearingsurface (metallicalloy orceramic) willproducespecificwearrates, whicharecorrelatedtothespecificpropertiesofthematerials, associatedwiththe specificjointdesign(geometry). There-fore,itisessentialtodeterminetheacceptablelevelofwear ofthematerialcombination.Itisatthislevelthatthewear simulation inlaboratory, using appropriateequipment and protocols,associatedtoacontrolledenvironmentthat repli-catesthehumanmusculoskeletalsystemandkinematics,is consideredasapowerfultoolforthecorrectevaluationofwear performance.

Preclinical wear tests also ensure that the hip implant designhasa satisfactory performanceand safety interms ofdurability,whichincreasestheconfidenceoftheproduct tothepatientsandsurgeons.3_{Afurtheradvantagethathip}

implant wearsimulationcan bringout isthepossibility of assessingtheengineeringfactorsthatwouldaffectthewear oftheimplantinvivo.Thesurfacefinishing(roughness)ofthe bearingsurfacesisregardedasakeyparameterthatinfluences thewearbehavioroftheimplant.15,16_{Forexample,weartests}

canbeperformedinsimulatorstocomparethewearrateof

thelinerasafunctionofdifferentsurfaceroughnessvaluesof thefemoralhead.17

Simulated tests can also be performed to evaluate the effect ofscratchedfemoralheads onthe wearofthe liner. Theexistenceofscratchesintheheadisregardedasoneof themechanismsresponsibleforabrasivewearinUHMWPE.14

Additionally, roughnessand scratcheson thefemoralhead surfacearecloselyrelatedtothetypeofmaterialused.Metals suchasstainlesssteelandCoCralloy(CoCr)aremore suscep-tibletoscratchingthanceramics,oncethesematerialshave lowerhardnessincomparisontoceramics.

Thewearperformanceoftheimplantregardingthe influ-enceofusingdifferentmaterialsinthefemoralhead,suchas ceramics andmetals,canalsobeevaluatedfromsimulated weartests.Simulatorshavebeenusedsincelongdateto eval-uatethewearmechanismsofthesematerials,givingvaluable informationabouttheadvantagesanddisadvantagesofeach material.15,18,19

The diameter of the hip joint is another design aspect criticaltowearsinceitisrelatedtothecontactstressfield pro-ducedduringthehipoperation.Weartestscanbeperformed insimulatorstoevaluatetheinfluenceoffemoralhead diam-eteronthewearrateofliners,mainlythosefabricatedwith UHMWPEmaterial.Itisseenthat,forUHMWPE,the volumet-ricwearcancriticallydependonthediameterofthefemoral head.19_{Forexample,thewearofaUHMWPEliner}

articulat-ing againsta32mm-femoral headishigher thanthe wear observed forlinersarticulating againstfemoralheadswith smallerdiameters.20_{Theadvantageofusingahigher}

diame-terisrelatedtothehigherrangeofmovement(ROM)obtained withoutimpingementoccurrence,leadingtobetterstability oftheimplantanddecreaseofdislocationrisk.20–24_In

oppo-site,increasingthefemoralheadsizeleadstoanincreasein theslidingdistanceandvelocity,whichareparameters rec-ognized to have influenceon wear.22 _{However, tests using}

wearsimulatorshavedemonstratedthatfemoralheadswith largediametersonlycausewearwhenthelinerisfabricated from non-crosslinkedUHMWPE. In the case ofcrosslinked UHMWPE,wearsimulatedtestshaveshownthatwearisnot necessarilydependentonthesizeofthefemoralhead.25

Researchanddevelopmentonorthopedicartificialjoints, making use of experiments in joint simulators, are vast. Regardless of the increasing use of diverse materials, UHMWPE isoneofthemost appliedmaterialsforimplant joints,and hasbeen used asbearing materialforartificial joints since the 1960s.17,26,27 _{Since the first generation of}

UHMWPE,thesearchofsolutionsforwearrelatedproblems hasbeenfocusedinchangingitsmechanicalproperties. Mod-ifications inthe structure ofUHMWPE havebeen made to improve the wear resistance of the hip implant material. OnewaytoimprovethemechanicalpropertiesofUHMWPE, whereasindirectly,isthesterilizationbygammairradiationin substitutionofethyleneoxide(EtO).Suchirradiationprocess leadstothe crosslinkingofthepolymerchainsand conse-quentlyincreasesthewearresistanceofthematerial.28_Highly

cross-linkedUHMWPElinerspresentlowwear,bymorethan 50%, comparedwith noncrosslinkedones.29 _The

It is now obvious that hip wear simulation tests are a powerfultooltoconfirmtheimprovementinwearresistance beforeclinical use. In fact, it was already verifiedthat hip wearsimulationassociatedwithdeepmaterialanalysis,from macrotomicro/nanoscalephenomena,arehelpfulto iden-tifythe amountofwearandthemateriallossmechanisms ofUHMWPEappliedinTHR.Usingadvancedtechniquesfor atomiclevelanalysesoftheUHMWPEcanrevealaspectsof materialstructuremodificationsrelatedtowearperformance. Thewear performance can become worst if the UHMWPE material undergoes strain softening while running under multi-directionalmechanicalstressfield.30,31

Hip

simulators

machines

Theuseofadvancedmaterials,aswellasimprovementsin thedesignofthe hips,hasbeen consideredasalternatives toreducethewearofimplants.However,toconfirmthata newmaterialordesigniseffectivetoreducewear,areliable comparativeprocedureisrequired.Usually,afirstevaluation involvessimplifiedteststoassessthewearandcoefficientof friction(COF)ofthematerialsintendedtobeusedasbearing materials.Briefly,theequipmentusedtoperformsimplified testsisatribometer, andthemainadvantageofusingthis kindofmachineisthelowcost,simplicityandready availabil-ityofresults.Itallowsrankingdifferentmaterialswithrespect totheirwearrateandcoefficientoffriction.Thisrepresents tribologicalperformanceevaluationbutonlyatapreliminary level;thatis,itisnotpossibletoevaluatetheeffectofsome otherimportantfeatures,suchasthedesignofthehipjoint, onwear.Thislimitationisrelatedtotheelementarygeometry employedtothecontactingsurfaces,theappliedloadranges andtheenvironmentalconditions,whichdonotexactly repro-duceinvivoconditions.17

Tests performed insuitable equipment,where boththe design and the material wear properties can be simulta-neouslyassessedbythe useofrealhipjoint implants,can bringimportantpre-clinical outcomes about the wear per-formance ofthe implant.The worldwideaccepted method to assess the design and material wear properties of hip implantsinvolvestheuseofasimulatormachine.Asalready is in its own name, a simulator replicates the conditions of loadings, angular displacements (abduction/adduction, internal/external rotation and flexion/extension) and envi-ronment,as observed in the human gait. In this way, the predictionoftheinvivowearratesoftotaljointreplacements isobtainedinamoreclinicallyrelevantway.32,33_{Nevertheless,}

althoughtheattributesofthehipjointdesignareevaluated inthehipsimulatortest,itshouldbeclearthattheexpertise ofthesurgeonaswellastheclinicalaspectsofthepatient cannotbetakenintoaccount.

Joint simulatorshave been widelyused for along time worldwide,notonlytoevaluatedesignsforwearperformance, but alsoto conduct R&D inhip implants,and not onlyby laboratoriesbutalsobythemanufacturingcompanies.Inthe last case,it means thatit is possibletopromptly evaluate thewearperformanceofanewhipdesignattheownsiteof thecompany.Theinterestinhavingahipsimulatoronsite is a good indicative that worldwide companies, especially

the American and European ones, are making particular investments to improving the tribological performance in theirownmanufacturedproducts.

In Brazil, the scenario is quite different. The hip joint simulatorsaremostlylocatedinresearchinstitutesand uni-versities,meaningthattheevaluationofthewearratesofthe hipimplantdesignscannotbepromptlyobtainedatthe com-panysite.ThisfactcaninhibitactionsdirectedtoR&Dunless that thecompanyisinconnectiontothe testerentityina well-establishedjoint-workrelationship.Thiskindofaction shouldallowmakinginnovativeadvancementsinthe ortho-pedicproductsectorinBrazil.

Severaltypesofjointsimulatorsareavailableinthemarket toperformweartests.Themaindifferenceamongthe simu-latorsisthemechanicalprincipleofoperation,leadingtothe existenceofdifferentmodelsandmanufacturersaroundthe world.28 _{Todate,mostofthehip simulatormachineshave}

hydraulic or electromechanicaloperation.Anexample ofa hydraulicsimulatoristheoneplacedattheMaterials Metrol-ogyDivision (Dimat)intheNational InstituteofMetrology, QualityandTechnology(Inmetro).ItispresentedinFig.1A, illustratingthegeneralviewofthemachineandthedetailof aworkstation(Fig.1B)wherethetestspecimens(femoralhead andliner)aremounted.

Thisparticularsimulatorhasprinciplesofoperationbased inhydraulicforcesthatareresponsibletosupplytherequired conditionstooperatetheloadingsandangulardisplacements. Oneinterestingfeatureofthisequipmentisthesix-axisload cellinstalledineachworkstation,whichallowsmeasurement offorcesandmomentsinx, yandzCartesiancoordinates. Thesetofforcesandmomentsareproducedasaresultofthe designofthehip implantwhentheloadand displacement curves are appliedduringthe test.Itsassessmentcangive informationontheresistiveforcesactingduringthehip opera-tion,suchasthoserelatedtofriction.Itshouldberemembered that, assmall isthefriction forceacting duringthe opera-tion ofthe joint,better willbe felt the implant inside the body.

Despite the existence ofinternational standardizedtest methodsdescribingtheloadingsanddisplacements,the prin-ciple of mechanical operation of the simulator plays an importantroletothedifferencesinthewearratesobserved among differentlaboratories. Insteadof the threetypesof angularmotionperformedbythejoint,hipsimulatorscanuse simplifiedconditions.Forexample,onlytheflexion/extension and internal/external rotation can be applied to the test specimen in somesimulators, being not able toapply the abduction/adduction. Inthis case,this typeofsimulatoris usually knownas biaxial rockingmotion(BRM), wherethe motion is applied in the femoral head through the rota-tionofablockmountedunderthefemoralheadwith23◦_of inclination.32_{BRMsimulators,insomecircumstances,apply}

onlystaticload,butevenwithsuchdiverseloadingcondition, theyhaveshowntoproducevalidresults.18

Fig.1–(A)Generalviewofahydraulicmulti-stationhipsimulator;(B)detailofaworkstationwithahipjointtestspecimen (withoutlubricant).

Hip

wear

simulation

methodology

The in vivo tribological aspects of hip replacements are complex,highlyvariable anddependon severalconditions inherentlyfoundinthepatients.Thismakesasinglestandard pre-clinical test unableto simulate all the involved condi-tions in laboratory.34 _{However, standardization is an issue}

tobeconsideredsoastocreateabasisforquality determi-nation.In terms ofhip wearperformance evaluation,it is necessarytoestablishastandardoperationalcycling concern-ingtheangularmovementsandtheloadings,havinginmind the existenceof different machines in several laboratories aroundtheworld.Theoretically,thiswouldallow reproducibil-ity among the international laboratories, and thus, make comparisonofthe wearratesofhip implantdesigns possi-ble,independentlyofthecharacteristicsofthetester.TheISO 14242standard,initsparts1and3,35,36_specifiesthe

condi-tionstobeusedfortheweartestingoftotalhip-jointimplants. Itgivestherangesfortheangularmovements betweenthe articulatingcomponents,thepatternoftheappliedload,the speedcycle,the testingduration,thesampleconfiguration, andthetestenvironment.Themethodsofwearassessment, basedongravimetrictechniquesand/orchanges in dimen-sionalformofthecomponents,arealsospecifiedintheISO 14242-2standard.37

Theranges ofthe angular movements and the applied forcespecifiedintheISO14242-1standard35_{wereestablished}

with basis in the human gait and forces also considering therelativemovement betweenthepelvicandthe femoral components.12,28_{Theappliedforcereportedinthis}

standard-izedprotocol wasdefinedbasedon simpleactivitiesofthe dailylife,forexample,walking.Ithasbeenshownthat dur-ingwalking activity the loadpresents valuesbetweenzero

tofour timesthe patientbody,inthat onecycle oftesting comprisesacompressiveloadingwithamaximumvalueof 3000Nandminimum loadof300N,angulardisplacements offlexion/extension(F/E),abduction/adduction(AB/AD) and internal/externalrotation(IR/OR).Theanglesgoto2◦_IR,10◦ ER,25◦_F,18◦_E,4◦_ABand7◦_{AD,respectively.}

AnotherimportantparameterspecifiedintheISO14242-1 standard35_{ishowthelubricationbetweenthefemoralhead}

and the acetabularliner is promoted duringthe test. Itis importanttohaveinmindthatthephysiologicalconditionof lubricationofthehumanjointisrelatedtothesynovialfluid. In thesimulatorweartest thein vivolubricationcondition isreplicatedbytheuseofcalfserumdilutedwithdeionized water.Particularly forthis typeoflubricant,it isimportant topayattentiontotheproteinmassconcentration.Aprotein massconcentrationof(30±2)g/Lhad beenspecifiedinthe ISO14242-1standard.35_{However,ithasrecentlychangedin}

the waythat, ifthe wearmechanisms observedinthe hip joint components testedinthe simulatorare notclinically reproducibleas thoseobserved inretrieved joints,a differ-entproteinmassconcentrationinthelubricantmaybeused. Thishighlightsoneofthemainconcernsofhipwear simula-tion,whichisthereproductionoftheclinicalmechanismsas observedinvivo.

Topredictthewearofthehipimplantasitwouldbeinvivo, itisspecifiedaminimumoffivemillioncycles,usingtheforce andangularmovementrangesaccordingtotheISO14242-1 standard.35_{Itisassumedthatonemillionofcyclesofmotion}

of the hip joint replicate approximately the average num-berofnormalwalkingfootstepsofanaveragepatientinone year.18,19,38

test represents one year of clinical use of the hip pros-thesishas beenconsidered bymany researchersas anold conjecture.Thisistrue,especiallyforthesocalled hard-on-hard bearing combinations, such as CoC and MoM. These areTHRalternativesthathavebeenincreasinginyoungand activepatients.39 _{Tothis case,theexpertsbelongingtothe}

standardizationcommitteessuchasISOandASTMare nowa-daysmakingeffortstodevelopingspecificstandardizedtest methods.Thetestprocedures forhard-on-hardjointsmust anticipatetheperformanceundermoreadverseworking con-ditions,inordertobecapabletodiscernthehipperformance correctly.However,forthesoft-on-hardcombinationof mate-rials,whichisoneofthemostemployedworldwide,thetest procedureestablishedintheISO14242-1standard35_is

consid-eredadequateforperformanceevaluationofthehipjoint. Thepracticalresultofaweartestistherateinwhichthe hipcomponentwearsasafunctionofthenumberofrunning cyclesunder theangularmovements andforces appliedin thesimulator. Usually, lossofmaterial(in mass)is plotted asafunctionofspecificintervalsoftesting,suchas500,000 cycles.Anexampleofgraphicalrepresentationofthe cumu-lativemasslossasafunctionofthenumberofcyclesisshown in Fig. 2. In this particular example, UHMWPE acetabular liners (componentsthat lose material dueto wear) articu-lated againsta28mm femoralheads.Thehead wasmade ofCoCrMoalloy(comprisingthreespecimenpairs,resultsin

Fig.2A)andofstainlesssteel(threespecimenpairs,results inFig.2B)uptofivemillioncycles.31 _{Fromtheslopeofthe}

graphicalrepresentation,andassumingalinearcurveusing theleastsquaremethod,itispossibletoestimatethewear rateofeach individualsample.37 _{Itcanbeseenfrom Fig.2}

thatthe averagewear rateofthe setofUHMWPE samples isapproximately48mg/106_{cycles(estimatedas51mm}3_/106

cycles),independentofthematerialusedinthefemoralhead. Tohighlightthe importanceofweartestsusing simula-torsinordertocomparethewearratesamongdifferenthip designs,anexampleisthehipdesigncomprisingthefemoral headandliner(28mmdiameter)madefromnanocrystalline diamond(NCD)onsiliconnitridesubstrate,inwhichthewear ratewas0.02mm3_/106_cycles.40_{Comparingthewearratesof}

thetwohipdesignsabovementioned,itispossibletoobserve enormousdifferenceinthewearrateamongthem.

The graphical plot is also useful to obtain information aboutthewearbehaviorofthehipimplantdesignalongthe timeofoperation.Dependingonthematerialcombination, itispossibletoobservetwodifferentbehaviors: therun-in wearandthesteady-statewear.Thefirstoneischaracterized bythewearthatoccursduringtheinitialphaseofthetest, whichisclinicallyequivalenttotheinitialrunningperiodof theinvivoimplantedhip.Inthisphase,wearratesare usu-allyhighbecausetheheadandcupgointoconformitywith eachotherwhilewearoccurs.Thetopographyofreal contac-tingsurfacesisnotidealandsomeofthesurfaceasperities canbeinitiallyhigh.Thesearewornawayintherunning-in period,thusmakingthesurfacesconformal.Afterthe tran-sientperiod,thesteady-statewearisattained,characterized byaconstantwearrate,typicallylessthantherun-inwear rate.

Thedeterminationof the wearrate playsan important roleintheevaluationofhipdesigns,sinceitallowsthedirect

350

Avg wear rate = (48 ± 9) mg/106 _cycles

Avg wear rate = (48 ± 9) mg/106 cycles 300

250

200

150

100

50

0

0 1 2

Number of cycles (×106₎

CoCr femoral head 28 mm

Stainless steel femoral head 28 mm

3 4 5 6

350

300

250

200

150

100

50

0

0 1 2

Number of cycles (×106₎

Mass loss (mg)

Mass loss (mg)

3 4 5 6

Sample 1 Sample 2 Sample 3

A

B

Fig.2–Exampleofwearplotsasafunctionofnumberof cyclesforUHMWPEacetabularlinersthatarticulated against(A)CoCrMoalloyand(B)stainlesssteel.

rankingofmaterialcombinationsintendedtobeusedinhip implants.AsspecifiedintheISO14242-237_{standard,thewear}

behaviorisobtainedafterfittingalinebyleastsquaremethod throughoutthecumulativemasslossdata,asafunctionof thenumber ofloadingcycles.Thewearrateisthensimply calculatedastheangularcoefficient(slope)ofthefittedline. Ithasbeenseenthatthisprocedureisquiteconvenientforthe wearratedeterminationofhipimplantsmadeofconventional UHMWPE.However,someotheraspectsshouldbeconsidered forthewearratedetermination,asithasbeenseenworldwide, bywell-establishedhipwearsimulationplayers.Itisespecially criticalinthecaseofwearwithtwodistinctrun-inandsteady statebehaviors.Inthiscase,itiseitherpossibletocalculatea run-inwearrateseparatedfromthesteady-statewearrate, or aunique wearrate usingthe entiresetofdata. Clearly, thewearratescalculatedinthisparticularsituation canbe quitedifferent.Thus,careshouldbetakenwhenreportingthe averagewearrateofaparticularhipimplantdesign.

butalsoastheeffectofdifferencesintestprotocols, hard-ware setupand lubricant composition.41 _{All these aspects}

mustbeconsideredwhencomparingresultsproducedfrom differenttestinglaboratories.Theestablishmentofan indi-vidualdatabaseforeachlaboratoryplaysanimportantrole instudyingwearofhipimplants.However,itisahardtask oncethewearsimulatedtestisexpensiveandtime consum-ing,requiringseveralyearstoobtainareliableandpowerful database.

Moreover,itshouldbeemphasizedthat,forprecise eval-uation ofany implant design, oneshould consider several aspectsofthematerialsusedinthejointcomponents,going fromphysical,chemicalandmechanicalcharacterizations,to thebiocompatibilityassessment,besidesthepreclinicaltests forperformanceassessmentoftheproduct.However,inspite ofthe huge set of dataobtained from the tests and anal-yses concerningtheevaluation oftheimplant design, it is alwaysimportanttocallattentiontothefactthattheseresults areconcernedtotheevaluationofthe product;thatis,not consideringthesurgicalperformanceandthepatienthealth condition.Thus,theyshouldbeinterpretedcautiouslywith respecttothedirectapplicabilitytoclinicalconditions,and cannotbeassumedasafinaldecisiononthesuccessofahip designafterimplantation.

Concerning the preclinical wear tests for performance assessment for the particular case of Brazil, the national literatureonlyreportscasestudiesoffailedimplanted pros-thesis,thecausesgoingfrommaterialsoutofspecification, throughstructuralnon-conformityofthecomponents.Wear ofimplantsisnotreported, probablybecausethe implants could not even reach to the stage to develop wear. How-ever, problematic issues concerning material control and straightmechanicalstructuredesignverificationscanbe over-comewithtechnologicalmodernizationofBrazilianimplant manufacturers,alsowiththe helpofincreasingnumber of specializedtestinglaboratories.Inthisway,failurescoming fromnon-conformityinmaterialsandstructuredesignscan besurpassedbyothers,nowcomingfromwear.Inthisway, thewearbehaviorcanbecomeakeyfactorfordictatingthe performanceoftheproduct.Wecallattentionthatperforming teststopredictwearofhipimplantdesignsshouldovercome theexistingobstaclessuchasmaterialcontrolandstraight mechanicalstructuredesign,inordertomakeadvanceson thesafety andhealthofpatients,and gainingbenefitfrom thedataobtainedintheweartests,toimprovetheimplant’s market.

Future

of

hip

simulation

wear

tests

Testingofhipimplantsisadynamicarea.Recently,retrieved implantshaveprovidedinformationabouttheinvivo mech-anismsofwearinhipimplants.Severaltestprotocols have alsobeen developedin laboratoriesaround theworld, and theinfluenceofdesign,aswellasparametersofloadingand movementsonthewearperformance,havebeenaddressed. Furthermore,in the last decades, the tribological research ofthematerialcombinationsusedintotalhipreplacements hasbeenconsideringamoresimplifiedmodelofloadsand

movementsthanthatestablishedintheISO14242standard, andbasedonanormalhumangait.12

However, it iswell known that, inmany situations, the femoralhead and acetabularcup are notsubmittedto the stressesobservedinanormalgaitonly,sincehipimplants have been considered to be used in younger and more activepatients.Inaddition,consideringthedevelopmentsof new materials (ceramics, UHMWPE with crosslinking, etc.) to improve wear resistance, the current loads and move-ments prescribed inISO 14242-1 standard35 _{forwear tests}

maybecomelimitedtoevaluatewearofmodernhipimplants designs.Therecent clinicalfailuresbylocalizedwearfrom edgeloadingandabnormalsounds(squeaking)identifiedin several hip implant designs have also contributed to the need of moreaggressive hip wear test.42 _{Also,increase in}

thelifespanofthehipimplantisexpectedbybothsurgeons andpatients;thisleadstotheneedofimprovementsinthe wearteststowardestablishinghigherlevelsofconfidenceof thedevicesagainstparticularconditionsofelevatedworking time.Thetimedurationand therangesofmovements and loadingsestablishedintheISOstandard14242-135_donot

rep-resentaspectsofcurrenteverydaylifeactivitiesofmoreactive patients, thustheweartestprotocolshouldbemodifiedto meettherequirementsofmoderndailylifeactivity.12

Inthisscenario,anewtermintheareaofhipwear simula-tionhasbeenarisingrecently,namedas“adverseconditions test”.This includessteep inclinationangleofthe acetabu-larcup,higherloadinglevel,highermotionspeed,thirdbody abrasion and microseparation.43 _{Adverse conditions tests}

are powerfulonce itallowsidentification ofmaterialswith improved performance and the elimination ofhip designs withinadequatewearresistanceundermoreaggressive oper-ation,priortotheirclinicaluse.

Adverseconditionstestshavebeenconsideredparticularly fortheevaluationCoCandMoMbearings,oncethecurrentISO 14242-135 _{standardtest protocolusuallycannotdistinguish}

differenthard-on-harddesignsconcerningwear.Besidesthat, insomeTHR,itispossiblethataperfectpositioningofthe implant cannot beachieved.This implicatesinan adverse conditionthatcanleadtoincreasedwearofthearticulating surface.Hipsimulatormachinescanalsobeemployedto eval-uatethestabilityperformanceoftheimplantbyreplicating suchadverseconditionsrelatedtoincorrectpositioningofthe implantthatmayoccurduringthesurgery.

Regardingthewearmeasurement,asdiscussedbefore,the wearrateisanimportantresultobtainedwithhip simula-tion tests. However,inaccurate measurement ofmass loss ofthetestspecimensaffectstheconfidenceofthe informa-tion disseminatedaboutthewearqualityoftheprostheses beingtested.Toovercomethispossibledrawback,geometrical approaches using three-dimensional Coordinate-Measuring Machine (CMM) havebeen coupledtohip wear simulation teststoobtainmoreaccuratemeasurementofwear.31,44

Geo-metricalapproacheshavebeenconsidered asanadditional analysistohipwearsimulationbecauseithastheadvantage ofidentificationofthelocationofwearanditscorresponding depthprecisely.

have been used, mainly dueto the high hardness, chemi-calinertness,lowcoefficientoffrictionandtolerancebythe organism.45_{Itshouldbepointedoutthat,despitetheexpected}

superiorwearperformance,retrievedceramiccouplesinsome caseshaveshownalong,narrowareaofdamage,whichhas beencalledstripewear.46_{Stripewearisassociatedwithedge}

loading,whichisthecontactbetweentheheadandtheedge oftheliner.Thiscontactresultsfrommisalignmentbetween thecenteroffemoralheadandthecenteroftheacetabular cup.Itisrelatedtothemigrationofthefemoralcomponent, producingdistaltranslationandinternalrotation,insuchway thatthefinalpositionofthecomponentisretroverted.46

Vari-ationintranslationalpositioningofthecentersofthehead andcup,whichisnotdetectedonradiographs,isafrequent clinicallyoccurrenceandcanresultinsubstantialincreasein wearrate.35_{Clinically,aloosesofttissuetensioncanallow}

separationofthebearingduringtheswingphaseofgait,and whentheheelstrikeoccursitcanproduceedgeloadingbefore relocationoccurs.

However,CoC bearingshave shownan excellent perfor-mance under standard hip simulator test conditions and do not present usual stripe pattern of wear as found in retrievals.39 _{The wear identified under standard hip}

sim-ulation tests is demonstrated to be very low in simulator studies.45 _{Modificationoftheweartestprotocolestablished}

intheISO14242-1standard35 _{hasbeenconsideredbymany}

researchers, to including micro-separation as an artificial featureofthe wear test, inorder topromote edgeloading occurrence.Inthis way,it isexpectedthatceramiccouples afterthewearsimulationinlaboratorypresent stripewear, whichclinicallyreplicatesthewearfeatureasfoundinvivo.35

Final

remarks

Hipsimulator machines are apowerful tool forpreclinical evaluationofhipdesigns,anticipatingtheintendedwear per-formance of the bearing materials. There is need of deep studies concerning the wear performance ofhip implants availableintheBrazilianmarket.

Knowledgeoftribologicalprinciplesinassociationwithhip simulatorwear tests have contributed to the development anddurabilityincreaseofartificialhipjoints.Resultsofwear testscancontributetoincreasetheconfidenceofthesurgeon onaspecifichipdesign.However,evenwiththe technolog-icaladvances,thesuccessofatotalhipreplacementdevice stilldependsontwoimportantfactors:thesurgeonandthe patient’sphysiologicalcondition.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

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