Rev. bras. ortop. vol.51 número1

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

Original

Article

Effect

of

removal

and

reinsertion

of

force-closed

stems

on

deformation

of

total

hip

arthroplasty

夽

Sandro

Griza

a,∗

,

Luiz

Sérgio

Marcelino

Gomes

b

,

André

Cervieri

c

,

Telmo

Roberto

Strohaecker

d

a_Universidade_Federal_de_Sergipe,_São_Cristóvão,_SE,_Brazil

b_Servic¸o_de_Cirurgia_e_{Reabilitac¸ão}_Ortopédica_e_{Traumatológica,}_Batatais,_SP,_Brazil c_Universidade_Luterana_do_Brasil,_Canoas,_RS,_Brazil

d_Universidade_Federal_do_Rio_Grande_do_Sul,_Porto_Alegre,_RS,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received2December2014 Accepted30March2015 Availableonline21January2016

Keywords:

Arthroplasty,hip Prosthesisdesign Mechanicalphenomena

a

b

s

t

r

a

c

t

Objectives: Thisstudyinvestigatedremovalofaforce-closedstem,doneinordertoimprove acetabularexposureduringrevision,withreinsertionafterwards.Itisunknownhowmuch thisproceduremodifiesthestem/cementinterface.

Methods:Threetaperedstemmodelswereimplantedintocompositefemurs.Straingauges wereembeddedinthemedialaspectofthecementmantleandinseveralpositionsonthe outersurfaceofthefemurs.Thedeformationwasmeasuredduringstaticloading,which wasappliedattwodifferenttimes:afterimplantationandafteronemillionloadingcycles, followedbystemremovalandreinsertion.Thettestwasperformed.Thedifferencesin deformationwerecompared(atp≤0.05)betweenthetwostaticloadingtimesandamong

thethreestemdesigns.

Results:Nosignificantdifferencesindeformationwerefoundafterthetwoloadingtimesfor thethreemodels.Nosignificantdifferencesintheinitialdeformationsofthethreemodels werefoundformostofthegaugesattachedtothefemurs.

Conclusions: Reinsertionoftheforce-closedstemdoesnotaltertheloadtransmissionfrom thestemtothecementandtothesurfaceofthefemur,evenafteronemillionloadingcycles. ©2016SociedadeBrasileiradeOrtopediaeTraumatologia.PublishedbyElsevierEditora Ltda.Allrightsreserved.

Efeito

da

remoc¸ão

e

reinserc¸ão

de

hastes

tipo

force-closed

nas

deformac¸ões

da

artroplastia

total

de

quadril

Palavras-chave:

Artroplastiadequadril Desenhodeprótese

r

e

s

u

m

o

Objetivos:Estudodaremoçãodehastedotipoforce-closedeasuareinserçãoposteriorpara aumentaraexposiçãodoacetábulodurantearevisão.Nãoéconhecidooquantoesse pro-cedimentomodificaainterfacehaste/cimento.

夽

WorkperformedwithinthePostgraduateProgramonMetallurgical,MiningandMaterialsEngineering,PhysicalMetallurgyLaboratory, DepartmentofMetallurgy,UniversidadeFederaldoRioGrandedoSul,PortoAlegre,RS,Brazil.

∗ _{Corresponding}_author_.

E-mails:[email protected],[email protected](S.Griza).

http://dx.doi.org/10.1016/j.rboe.2015.03.016

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Fenômenosmecânicos

sômetrosdedeformaçãoforamembebidosnoaspectomedialdomantodecimentoeem diversasposiçõessobreasuperfícieexternadosfêmures.Asdeformaçõesforammedidas durantecargasestáticas,asquaisforamaplicadasemdoisdiferentesmomentos:apósa implantaçãoeapósummilhãodeciclosdecarga,seguidopelaremoçãoereinserção.O testetfoifeito.Asdiferençasentreasdeformaçõesforamconfrontadascomp≤0,05entre

osdoismomentosdecargaestáticaeentreostrêsprojetosdehastes.

Resultados: Nãoforamencontradasdiferençassignificativasnasdeformaçõesapósosdois momentosdecargaparaostrêsmodelos.Nãoforamencontradasdiferençassignificativas nasdeformaçõesiniciaisdostrêsmodelosparaamaioriadosextensômetrosaderidosaos fêmures.

Conclusões: Areinserc¸ãodehastedotipoforce-closednãoalteraatransmissãodecargada hasteparaocimentoeparaasuperfíciedofêmur,mesmoapósummilhãodeciclos.

Introduction

Polished,tapered,cementedandglue-freestemsarewidely appliedintotalhiparthroplasty. Thesestemsfunctionasa conicalinterferenceassembly,inamannerknownas “force-closed”.1_{Theoretically,}_given_that_there_is_no_strong_chemical

bondbetween themetal stem and the polymercement, it shouldbepossibletoremovethestemfromthecementand obtainthesameinterfacialinteractionafteritsreinsertion.

In the caseof acetabularrevision, removal ofthe stem isofinterestbecausethisincreasesthe degreeofexposure ofthe acetabulum and reduces the duration of the opera-tion.Naborsetal.2_conducted_a_10-year_clinical_follow-up_on

24casesofacetabularrevision,inwhichforce-closedstems wereremovedandreinserted.Nabors etal.2 _and_Bell_et_al.3

alsoevaluatedtherotationalstabilityofthestemsthrough mechanicaltestsanddidnotfindanyevidentlossofstem sta-bilitycausedbythereinsertion.Nonetheless,changesatthe interfacebetweenthestemandcementmayoccurafterstem reinsertion.4_The_small_interfacial_spaces_that_may_arise_after

theresidualtensionsinthecementhaverelaxedpossiblydo notproduceanysignificantchangestotherotationalstability ofthestemovertheshortterm.However,thesespacesmay producechangestoloadtransmissionfromthestemtothe cement.Normanetal.5_discovered_that_changes_to_the

interfa-cialinteractionbetweenthestemandcementhadaprofound influenceonalterationstothedeformationsthatwere trans-ferred to the cement and to the femur. Measurements of electricalresistancethroughtheextensometrytechnique(i.e.

usingstraingauges)canbeusedtodetectsuchalterationsto thesedeformations.

The present study had the objective of ascertaining whethertherewereanychangestodeformationseitherinthe cementorinthefemurafterremovalandreinsertionofthe stem.Ifthedeformationsoftheprimaryarthroplastydidnot differsignificantlyfromthedeformationsafterthe reimplan-tation, thiswould beastrong indicationthat theinterface betweenthestemandcementhadbeenpreservedandthat removalandreinsertionwasasafeprocedurefroma mechan-icalpointofview.

Materials

and

methods

Commerciallyavailableforce-closedstemsmadeofstainless steel(ASTMF138)weresuppliedbythemanufacturer(MDT Implantes,RioClaro,SP,Brazil).Thestemsdifferedfromeach otherregardingtheirtransversegeometryandtheirtapering angles and planes(Fig. 1). Theimportantgeometric differ-encesbetweenthestemswerethefollowing:groupA(Spoac®_): proximalthicknessof12.25mm,taperingof1◦₁₅′_and_circular cross-sectionalgeometry;groupB(Maxima®_):_proximal thick-nessof12mm,doubletapering(4◦₃₀′ _and₁◦ _on_the _lateral andmedialfaces,respectively,and3◦₁₂′_in_the_lateral_plane) andrectangularcross-sectionwithroundedcorners;groupC

(SpoacNC®_):_proximal_thickness_of₁₃_mm,_triple_tapering₍₃◦_, 3◦₃₀′_and₃◦₅₃′_,_{respectively,}_in_the_frontal,_lateral_and trans-verseplanes)andarectangularcross-section withrounded corners.Twostemsfromeachgroupwereimplantedinlarge syntheticfemurs(3306PacificResearchLabs).

Implantation

The appropriatestem sizewas selected bymeans of tem-plates and the medullary cavity was obstructed by means ofapolyethylenerestrictor.Bonecement(SimplexP, Styker-Howmedica-Osteonics) was introduced into the medullary cavityinaretrogrademanner,usingasyringe.The implan-tationwasperformedbyanexperiencedsurgeon(LSMG).

Measurementsondeformations

Extensometers measuring electrical resistance (i.e. strain gauges)wereattachedbothtothefemursandtothecement, inspecificpositionsofthetestbody,inordertoenable mea-surementoflargedifferencesindeformationwhenalterations totheinterfacebetweenthestemandcementoccurred.5

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Fig.1–Thethreestemmodels.Fromlefttoright,conicalstem(groupA),doublytaperedstem(groupB)andtriplytapered stem(groupC).Thecentralfigureshowscross-sectionsthroughthestems.Thefigureontherightshowsanextensometer (straingauge)attachedtothecementlayerclosetothetipofastem.

extensometers(Kyowa KFG02120C111-N15-C02). The exten-someterswereattachedtothecementlayersandtothefemurs usingsimilarprotocols.6 _Cement_layers_were_applied_at_the

proximalanddistallevelsofeachstembeforeits implanta-tion,soastoenableattachmentoftheextensometers.The cementlayersweresandeddownuntilathicknessof1mm wasreached.Theextensometerswereappliedtothelayers onthemedialfaceofthestems.Oneoftheseextensometers attachedtoastematdistallevelbeforeimplantationis pre-sentedinFig.1.Theextensometerswerethenallembeddedin thecementlayerduringtheimplantationandtheyremained insidethislayerafterremovalofthestems.Thepositionsof theextensometersarepresentedinFig.2.

Thedeformationsweremeasuredbymeansofan acqui-sition board (HBM MGCplus). All the extensometers were calibratedbymeansofprecisionelectricalresistors(Vishay Micro-measurements).

Load

The distalcondyles ofthe femurswere fixedin asupport devicesoastoensureposteriorinclinationof9◦ _and_lateral inclination of10◦_. _The _condyles_were _embedded_in _PMMA resin aftertheyhad been properly fixedusing screws.The testbodieswere loadedinto aservohydraulictestmachine (MTS810,MTSCorporation,USA).Staticloadswereapplied totheheadsofthestemsafterimplantationandafter reim-plantation. Tenstatic loadblockswere appliedatarateof 2300N/minuntilreaching2300N,andthis wasfollowedby one minute of load-bearing and a further minute of load relief. Eachdeformation valuewas represented bythe dif-ference betweenmaximumandminimumvaluesproduced because of the static load blocks. The mean deformation causedbythetenloadblockswasusedforanalysis.Sine-wave cyclicalloadslimitedbetweenminimumpeaksof230Nand

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maximumpeaksof2300Nwereappliedaftertheinitialstatic loads.Loadingfrequenciesgoingfrom7Hztoamaximumof onemillioncycleswereused.

Removalandreinsertion

Theprocedureofremovalandreinsertionofthestemswas conductedafterapplicationofthecyclicalloads.Thestems were removed by means of an appropriate extractor tool. Threehoursaftertheirremoval,thestemswerecarefully rein-sertedintothecavityofthecementlayerandweresubjectedto afinaltimeofstaticloading.Theremovalandinsertion proce-durewasconductedbythesamesurgeonwhohadperformed theprimaryimplantation.

The deformations were measured during static load application, both afterthe primary implantationand after reinsertion of the stems. The differences in deformation between the two loading times and between the initial deformationsmeasuredinthethreestemmodelswere com-paredby means ofthe t test, using asignificance level of

p≤0.05.

Results

Four embedded extensometers were damaged during the primaryimplantation.Thedamagewasdonetotwo exten-sometersinatestbodyingroupB,oneextensometeringroup A(position2)andoneextensometeringroupC(position1). Nonetheless,itwaspossibletomakecomparisonsbetween positions1and2,respectively,ingroupsAandC.No signifi-cantdifferencesinthedeformationsmeasuredinthecement andinthefemurwerefoundincomparingthetwoloading times(Table1).Fig.3showsthedeformationsmeasuredby the embeddedextensometersand also thosemeasured on themedialandlateralsurfacesofthefemurs.Thecomparison betweenthedeformationsbeforeandafterreimplantationcan beseeninthefigure.

Acomparisonbetweenthethreestemmodelsregarding the initialdeformations ofeach positionmeasured on the femurswasalsomade.Table 1showsthatingroup A,the deformations tended to decrease after reinsertion of the conical stems, while in group C, the deformations in the medialandlateraldirectionsinthetriplytaperedstemsalso decreased.On theother hand,it couldbeseen thatinthe doublytaperedstems(groupB),thedeformationstendedto increaseafterreinsertion,atmanyofthepositionsmeasured, except in the lateral direction. However, significant differ-ences between stems B and C were observed only in the medial2positionandintheposteriorposition(Table1and

Fig. 4). None ofthe other comparisons between the three stemspresentedanysignificantdifferences.Fig.3alsoshows thattherewasgreaterdeformationclosetothe topsofthe stems (medial 2 and lateral 3 positions, in all three stem models.

Giventhatonlytwomeasurementsensorsshowed signifi-cantdifferencesbetweenstemsBandC,weperformedlinear regressionanalysisinwhichthethreestemmodelsweretaken tobelongtoasinglegroup.Thisregressionalsopresenteda

strongcorrelation,withacoefficientoflinearityof0.91and T

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–3000 –2500 –2000–1500–1000 –500 500 1000 1500 2000 Lateral 1

Lateral 2

Lateral 3 Medial 2

Strain (µm/m) Strain (µm/m)

2 1

–1627 ₁₇₆

339

331 313 171 435

829

1012

1113 1144 765

941

1267

1490

1607 1626 1368

1424

–1758

–2012 –1997 –1951 –2030

–564

–667

–698 –682

–1894

–2117

–2476 –2348

–1793 –1913

Medial 1

0 0

Stem A, deformations after reinsertion

Stem A, initial deformations

Stem B, deformations after reinsertion

Stem C, deformations after reinsertion

Stem B, initial deformations

Stem C, initial deformations

Fig.3–Deformationsmeasuredusingtheembeddedextensometersandthoselaidoutonthemedialandlateralfacesof thefemurs.

–1200 –1000 –800 –600 –400 –200 0 –1500 –1000 –500 0

–727

–561 –683 –740

–664 –754

–789 –1087 –1228

–814 –803 Stem A, initial deformations

Stem C, initial deformations Posterior

Fig.4–Deformationsmeasuredontheanteriorandposteriorfacesofthefemurs.

inclinationfromthestraightlineof1.03forthedeformations measuredusingtheembeddedextensometers,anda coeffi-cientof0.98andinclinationof0.96forthedeformationson thesurfaceofthefemurs(Fig.5).

–3000 –3000 –2500 –2000 –1500 –1000 –500 500 1000 1500 2000

0

–2000 –1000

Embedded extensometers Surface of femur

0 –1000

yo=0.9568x –92.713 R2_=0.9787 ye=1.0262x +30.257

R2_=0.9149

Initial deformations (µm/m)

Deformations after reinsertion (

µ

m/m)

–2000

Fig.5–Linearregressioncomparingdeformationscaused bytheinitialstaticloadsandbythestaticloadsafter reimplantation,forthethreestemgroups.Thesymbol“ye” referstotheembeddedextensometers,whilethesymbol “yo”referstotheextensometerslaidoutontheexternal surfaceofthefemurs.

Discussion

Inthepresentstudy,thepotentialforalterationstothe defor-mationstransferredfromthestemtothecementlayerandto thefemur,causedbyremovalandreinsertionofthestem,was investigated.Althoughthispracticehasbeenusedinseveral situations inordertofacilitateexposureofthe acetabulum inrevisionsurgicalprocedures,withoutanyadverseclinical consequences,itstillneedstobeconfirmedthatthispractice doesnotinterferewitharthroplastyfromamechanicalpoint ofview,inordertoavoidproblemsthatmightariseoverthe longterm.Threeestablishedmodelsofforce-closedstemwere testedwiththeaimofascertainingwhetherallthreemodels had the same mechanical behavior. These stem only pre-sentedsubtledifferencesindesign.Force-closedstemssuch astheExeterstemhaveshownexcellentlong-termresults.7_In

thesemodels,thestemmigratesbecauseoftheabilityofthe cementtoflowandthisenablesloadtransmissionthrough thecementtothefemurinamorehomogenousmanner.8,9

Subtledesignmodificationstoforce-closedstemshavebeen conceivedoverrecentdecades.Examplesofsuchalterations includethosethatculminatedintheconceptionofthe dou-blytaperedExeterUniversalstemandtriplytaperedC-stem.10

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taperinganglesandplanesmayinterferewiththerigidityand stabilityofthestem.Theymayalsoaffecttheloadtransmitted fromthestemtothecementandfinallytothebonetissue.

Thetechniquesusedinourstudyhavebeenwidelyused in tests that seek to predict differences in the mechanics of arthroplasty.6,11,12 _Moreover, _alterations _to _the _interface

betweenthestemandcementcaneasilybedetectedthrough usingextensometers.5_The_quantity_of_one_million_load_cycles

waschoseninordertoencouragecementflowcharacteristics andstemmigration,inanattempttosimulatewhatoccurs

invivo.

Someextensometersembeddedinthe cementwerelost duringthe study.Nonetheless,it wasstillpossibletomake acomparisonbetweenpositions1and2inrelationtostems AandC,respectively.Thecomparisonsbetweenthe deforma-tionsduetotheinitialloadsandthoseafterthereinsertion,for theembeddedextensorsandforthoselaidoutonthesurface ofthefemurs,didnotshowanysignificantdifferences.

Thecomparisonsofthedifferentpositionsonthefemurs betweenthethreestemmodelsregardingthedeformations duetotheinitialloadsindicatedthatthereweresignificant differencesbetweenstemsBandCinthemedial2position andontheposteriorface.Thesedifferencesmayhavebeen related to the subtle differences in stem design. Although thesethreestemmodelspresentgeometricaldifferences,this hasnotbefoundtoproduceanysignificantdifferencesintheir clinicalperformances.1,9 _The_design_differences_did_not_give

risetodifferencesinthemajorityofthedeformationsthatwe measured.Moreover,consideringthatwemade21 compar-isonsofdeformationbetweenthethreemodels,ourfindingof significantdifferencesatonlytwopositionsisaverysmall pro-portion.Forthisreason,thelinearregressionanalysiscould beperformedwiththethreemodelsinasinglegroupof force-closedstems.Theregressionanalysisshowedthattherewasa strongcorrelationbetweenthedeformationsmeasuredbefore andafterthereinsertion.

Somelimitations ofthe present study need tobe high-lighted.Syntheticfemursaredifferentfromnaturalfemurs. However,various studies haveshown that the mechanical propertiesofsyntheticfemursaresimilartothoseofnatural femurs.Furthermore,syntheticfemursreducethevariationof theresults,sincetheyaremanufacturedfromastandardized model.

Thegreatestlimitationofthepresentstudywasthatonly twotestbodiesfromeachstemgroupwereassessed.However, sincenosignificantdifferenceswerefoundincomparingthe deformationsbeforeandafterreimplantation,inanyofthe extensometerpositionsforthesetwotestbodies,andsince the linearregression analysis presentedstrong correlation, wedecidedtoconcludetheexperimentswithonlytwotest bodiespergroup.Inthelinearregressionanalysis,thethree modelswereputtogetherinasinglegroupandthenumber oftestbodiesforthisanalysisthusincreasedtosix.Ifthere wereanyalterationsattheinterfacebetweenthestemandthe cement,thesewouldbeseenthroughdispersionofthelinear regressionand/ordistancingoftheunitinclinationfromthe straightline,andfromtheinterceptofthelineonthe coordi-nateaxis.InclinationandR2_that_are_close_to_one_(variations lowerthan 10%)and smallinterceptvalues(valuessmaller thantwoordersofmagnitudeofthedeformationsmeasured)

indicated good concordancebetween the two timesof the statictests.6

Another importantlimitationisthat inclinicalpractice, particlesandfluidsmayenterthecementcavityand/orthe stembeforestemreinsertion,andthiswasnotinvestigatedin thepresentstudy.Therefore,theresultsareapplicableonlyin casesinwhichboththecavityandthestemcanbeprotected fromtheseparticlesandfluidsbeforereinsertion.

Theresultsfromthisstudyprovideagoodindicationofthe permanenceoftheinterfacebetweenthestemandcement, andalsotheinterfacebetweenthecementandfemurafter thereinsertion.Somestudieshaveindicatedthatalterations totheseinterfacespromotelargedifferencesindeformations, both ofthe femurand of thecement.5 _{Crowninshield} _and

Tolbert13 _conducted _a _study _on _both _glued _and _non-glued

stem/cementinterfaces.Theyusedathinlayerofepoxyresin topromotereliablegluingofthestemtothecement. Alter-ationstotheinterfacialrelationshipwerereflectedinchanges tothedeformationmeasuredinthecementthatwereashigh as100%.Studieshaveshownthatforce-closedstemsmigrate because ofthe abilityofthe cementto flow,although this does notrepresent anindicationofloosening ofthestem. On the contrary,cement flow may ensure good interfacial contactbetweenthestemandthecement.Thisbehavior pro-motes load transferthroughthe cement tothe femurina morehomogenousmanner,andthistheoreticallypreserves the quality ofthe femurforlong periods.1,9,14–16 _There _are

nostrongchemicalbondsbetweenthemetalstem andthe polymercement.Therefore,asshowninourstudy,itis possi-bletoremovethestemandthenobtainthesameinterfacial interactionafterthereinsertion.

This study showed thatthe greatest deformations were closetothedistallevel(medial2andlateral3positions),for allthreemodels.Thedeformationsinthecementshowed val-uesgreaterthan1000␮m/m.Theseresultsarecomparableto thoseofseveralotherstudiescitedinthisarticle.

Conclusions

Taking into account the experimentalconditions that had beenproposedforthepresentstudy,reinsertionofthesame force-closed stem did not alter the transmission of defor-mation atthe interface betweenthe stem and cement or, consequently,tothesurfaceofthefemur.Removaland rein-sertionofthesteminthecementlayerdidnotsignificantly alterthemechanicsofthearthroplasty,evenafteronemillion cycles.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgements

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