Biomechanical evaluation of internal and external hexagon platform switched implant-abutment connections: an in vitro laboratory and three-dimensional finite element analysis

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element

analysis

Amilcar

C. Freitas-Júnior

a,b,∗

_,

_Eduardo

_P.

_Rocha

c

_,

_Estevam

_A.

_Bonfante

d

_,

Erika

O. Almeida

a,c

, Rodolfo

B. Anchieta

c

_,

_Ana

_P.

_Martini

c

_,

_Wirley

_G.

_Assunc¸ão

c

_,

Nelson

R.F.A.

Silva

a

_,

_Paulo

_G.

_Coelho

a

a_Department_of_Biomaterials_and_Biomimetics,_New_York_University_College_of_Dentistry,_NY,_USA b_Postgraduate_Program_in_Dentistry,_Potiguar_University_-_School_of_Health_Sciences,_Natal,_RN,_Brazil

c_Department_of_Dental_Materials_and_{Prosthodontics,}_São_Paulo_State_University,_Arac¸atuba_School_of_Dentistry,_Arac¸atuba,_SP,_Brazil d_Postgraduate_Program_in_Dentistry,_Unigranrio_University_-_School_of_Health_Sciences,_Duque_de_Caxias,_RJ,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received6September2011 Receivedinrevisedform 11May2012 Accepted17May2012 Keywords: Dentalimplants Platformswitching Biomechanics Reliability

Finiteelementanalysis

a

b

s

t

r

a

c

t

Objectives.Theaimofthisstudywastoassesstheeffectofabutment’sdiametershiftingon reliabilityandstressdistributionwithintheimplant-abutmentconnectionforinternaland externalhexagonimplants.Thepostulatedhypothesiswasthatplatform-switchedimplants wouldresultinincreasedstressconcentrationwithintheimplant-abutmentconnection, leadingtothesystems’lowerreliability.

Methods.Eighty-fourimplantsweredividedinfourgroups(n=21):REG-EHandSWT-EH (reg-ularandswitched-platformimplantswithexternalconnection,respectively);REG-IHand SWT-IH(regularandswitched-platformimplantswithinternalconnection,respectively). Thecorrespondingabutmentswerescrewedtotheimplantsandstandardizedmaxillary centralincisormetalcrownswerecementedandsubjectedtostep-stressacceleratedlife testing.Use-levelprobabilityWeibullcurvesandreliabilitywerecalculated.Fourﬁnite ele-mentmodelsreproducingthecharacteristicsofspecimensusedinlaboratorytestingwere created.Themodelswerefullconstrainedonthebottomandlateralsurfaceofthe cylin-derofacrylicresinandone30◦off-axisload(300N)wasappliedonthelingualsideofthe crown(closetotheincisaledge)inordertoevaluatethestressdistribution(svM)withinthe

implant-abutmentcomplex.

Results.TheBetavaluesforgroupsSWT-EH(1.31),REG-EH(1.55),SWT-IH(1.83)andREG-IH (1.82)indicatedthatfatigueacceleratedthefailureofallgroups.ThehigherlevelsofvM

withintheimplant-abutmentconnectionobservedforplatform-switchedimplants(groups SWT-EHandSWT-IH)wereinagreementwiththelowerreliabilityobservedforthe exter-nalheximplants, butnot fortheinternalheximplants. Thereliability90%conﬁdence intervals(50,000cyclesat300N)were0.53(0.33–0.70),0.93(0.80–0.97), 0.99(0.93–0.99)and 0.99(0.99–1.00),fortheSWT-EH,REG-EH,SWT-IH,andREH-IH,respectively.

∗ _{Corresponding}_author_at:_Department_of_Biomaterials_&_Biomimetics,_New_York_University_College_of_Dentistry,

345E24thStreet,Room812,NewYork,NY10010,USA.Tel.:+1558488402345. E-mailaddress:ac.freitas.jr@gmail.com(A.C.Freitas-Júnior).

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The postulatedhypothesiswaspartiallyaccepted.The higherlevelsofstressobserved withinimplant-abutmentconnectionwhenreducingabutmentdiameter(cross-sectional area)resultedinlowerreliabilityforexternalheximplants,butnotforinternalheximplants. ©2012AcademyofDentalMaterials.PublishedbyElsevierLtd.Allrightsreserved.

1. Introduction

Inthefirstyearafterimplantinsertionandloading,early peri-implant boneloss commonlyleads toa reductionin bone height,showntovaryasafunctionofqualityandquantityof bone,implantandabutmentdesigns,implant’ssurface struc-ture,insertiondepths,archregion,andotherfactors[1–3].An attempttohinderthisprocess hasresultedinthe develop-mentoftheplatformswitchingconcept,whichconsistsinuse ofanabutmentofsmallerdiameterconnectedtoaimplantof largerdiameter.Thisconnectionshiftstheperimeterofthe implant-abutmentjunctioninwardtowardsthecentralaxis (themiddleoftheimplant),potentiallyimprovingthe distri-butionofforcesandplacingtheimplant-abutmentgapaway fromtheperi-implantbone[4,5].Ithasbeensuggestedthat theinwardshiftoftheimplant-abutmentgapmayphysically minimizetheimpactoftheinflammatorycellinfiltrateinthe periimplanttissues,potentiallyreducingboneloss[2,6–11].

Fromabiomechanicalperspective,previousinvitrostudies [12–17]haveshownreducedlevelsofstressonperi-implant bone in platform-switched implants relative to matched implant-abutmentdiameters.Suchpotentialforcrestalbone levelpreservationhasbeenshowninanimal[18–20]and clin-icalstudies[11,21,22].

Ontheotherhand,complicationswithimplant-abutment connections is still a common clinical problem, especially in single-tooth replacements [18,23,24]. When considering platform-switched implants, previous studies [14,17] have shown an increased stress on the abutment and ﬁxation screw, which may compromise the system biomechani-cal performance.Controversially, several published studies [1,12,13,15,16,19,25–27]relatedtothemechanicsof platform-switched implants have been restricted to analyzing the stress distribution on peri-implant bone and not on the overall system biomechanical behavior. To date, studies evaluating the mechanical behavior of platform-switched implants considering the stress distribution in implant-abutment complex are scarce and restricted to computer simulations[14,17,28,29],whichdonotconsiderseveral clin-ical variables (inﬂuence of fatigue damage accumulation and wet environment) previously reported as important factors to reproduce clinically observed failure modes [30].

Sincethemainchallengesinthedevelopmentof implant-abutment connection designs comprises reducing the incidenceofmechanicalfailureswhileimprovingthe inter-face between soft tissue and implant-abutment junction [31,32], the evaluationofreliabilityand failure modes sup-portedbyevaluationofstressdistributionineachcomponent of platform-switched connections may provide insight into themechanical behaviorofdifferent conﬁgurations of implant-abutmentconnection.Therefore,the presentstudy soughttoassess theeffectofabutment’sdiametershifting

(regular and switched-platform) on reliability and failure modes of anatomically correct maxillary central incisor crownsvaryingthegeometryofimplantconnection(internal andexternalhexagon).Inordertoevaluatethestress distribu-tion withinimplant-abutmentcomplex (implant,abutment andﬁxationscrew),athree-dimensionalﬁniteelement anal-ysiswasperformedconsideringthevariables.Thepostulated hypothesis was that platform-switched implants would resultinincreasedstressconcentrationwithinthe implant-abutmentconnection,leadingtothesystems’lowerreliability when subjected to step-stress accelerated life testing (SSALT).

2. Materials

and

methods

2.1. Invitrolaboratorystudy:singleload-to-fracture

(SLF)andstep-stressaccelerated-lifetesting(SSALT)

Eighty-four commercially pure titanium grade 2 dental implants (SIN implants, São Paulo, SP, Brazil) were dis-tributed infour groups (n=21 each) varying the abutment diameter (switched or regular platform) and the type of implant connection (internal or external hexagon) (Fig. 1 and Table1): (1) SWT-EH(switching platform and external hexagonimplant);(2)REG-EH(regularplatformandexternal hexagonimplant);(3)SWT-IH(switchingplatformand inter-nalhexagonimplant);and (4)REG-IH(regularplatform and internalhexagonimplant).

All implants were vertically embedded in acrylic resin (Orthoresin, Degudent, Mainz, Germany), poured in a 25-mm-diameter plastictube, leaving the top platform in the same level of the potting surface (Fig. 2). All groups were restored with standardizedcentral incisor metallic crowns (CoCrmetalalloy,Wirobond® _280,_BEGO,_Bremen,_Germany)

cemented (RelyX Unicem,3M ESPE,St. Paul,MN, USA)on theabutments,whichpresentedidenticalheightbutdifferent diameters(Table1).

Formechanicaltesting,thespecimensweresubjectedto 30◦off-axisloading(Fig.2C).Threespecimensofeachgroup underwent single-load-to-fracture (SLF) testing at a cross-head speed of 1mm/min in a universal testing machine (INSTRON5666,Canton,MA,USA)withaflattungsten car-bide indenter applying the load on the lingual sideof the crown,closetotheincisaledge.Baseduponthemeanload tofailurefromSLF,threestep-stressacceleratedlife-testing profilesweredeterminedfortheremaining18specimensof each groupwhichwere assignedtoamild(n=9),moderate (n=6),andaggressive(n=3)fatigueprofiles(ratio3:2:1, respec-tively)[30,33].Mild,moderateandaggressiveprofilesreferto theincreasinglystep-wiserapidnessinwhichaspecimenis fatiguedtoreachacertainlevelofload,meaningthat speci-mensassignedtoamildprofilewillbecycledlongertoreach thesameloadofaspecimenassignedtoeithermoderateor

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Fig.1–Three-dimensionalmodelsofimplant-abutmentconnectionstobetestedinthepresentstudy.(A)and(B)SWT-EH

andREG-EH(switchingandregularplatformconnectedtoanexternalheximplant,respectively).(C)and(D)SWT-IHand

-REG-IH(switchingandregularplatformconnectedtoaninternalheximplant,respectively).

aggressiveprofiles.Inthepresentstudy,theprofilesstarted ataloadthatwasapproximately30%ofthemeanvalueof SLFandendedataloadthatwasapproximately60%ofthe samevalue.Therationaleforutilizingatleastthreeprofilesfor thistypeoftestingwasbasedontheneedtodistributefailure acrossdifferentsteploadsandallowsbetterprediction statis-tics, narrowing confidence bounds. The prescribed fatigue methodwasstep-stressacceleratedlife-testing(SSALT)under waterat9Hzwithaservo-all-electricsystem(TestResources 800L,Shakopee,MN,USA)wheretheindentercontactedthe crown surface,appliedthe prescribed load withinthe step profileandlifted-offthecrownsurface.Thus,duringSSALT each specimen was submitted to constant stress during a

predeterminedlengthoftime. Thestress onthisspecimen isthusincreasedstepbystepuntilfailure(bendingorfracture oftheﬁxationscrewand/orabutment)orsurvival(nofailure occurredattheendofstep-stressproﬁles,wheremaximum loadswereupto600N)[30,33].Baseduponthestep-stress dis-tributionofthefailures,thefatiguedatawereanalyzedusing apowerlawrelationshipfordamageaccumulationandthe uselevelprobabilityWeibullcurves(probabilityoffailurevs. cycles)atausestressloadweredeterminedforlifeexpectancy calculations by using the software Alta Pro 7 (Reliasoft, Tucson,AZ) [34]. Themaster Weibullcurves obtainedfrom theSSALTfatiguedatawereusedtodeterminethe reliabil-ity(theprobabilityofanitemfunctioningforagivenamount

Fig.2–(A)Componentassemblingfortheswitchingandregularplatform(fromlefttoright)restorationsintherespective

externalandinternalconnectiongroups:(1and2)SWT-EHandREG-EH(switchingandregularplatformconnectedtoan

externalheximplant,respectively);(3and4)SWT-IHandREG-IH(switchingandregularplatformconnectedtoaninternal

heximplant,respectively).(B)Implantconnectionconﬁgurationsembeddedinacrylicresin:(top,left)externaland(top,

right)internalhexagon;pouredina25-mm-diameterplastictube(bottom).(C)Mechanicaltestingset-up,wheretheload

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T able 1 – Char acter istics of the components used in the pr esent stud y . Components SWT -EH REG-EH SWT -IH REG-IH Implant External he x (SUR 5011) External he x (SUR 5011) Internal he x (SIHS 5511) Internal he x (SIHS 5511) 5.0 mm diameter by 11.5 mm length 5.0 mm diameter by 11.5 mm length 5.0 mm diameter by 11.5 mm length 5.0 mm diameter by 11.5 mm length Ø pr osthetic platform = 5.0 mm Ø pr osthetic platform = 5.0 mm Ø pr osthetic platform = 5.5 mm Ø pr osthetic platform = 5.5 mm Abutment Cemented (Al 4151) Ø platform = 4.1 mm Cemented (Al 5051) Ø platform = 5.0 mm Cemented (Al 4501) Ø platform = 4.5 mm Cemented (Al 5501) Ø platform = 5.5 mm Ø nec k’ s re g ion = 2.9 mm Ø nec k’ s re g ion = 2.9 mm Scr e w fixation scr e w (PTQ2008) fixation scr e w (PTQ2008) fixation scr e w (PTQH16) fixation scr e w (PTQH16)

oftimewithoutfailure,90%two-sidedconﬁdencebounds)of testedspecimensforcompletionofamissionof50,000cycles at210Nand300Nload[35]forgroupcomparisons.For the missionreliabilityandˇparameterscalculatedinthepresent study,the90%conﬁdenceintervalrangewerecalculatedas follows:

IC=E(G)±Z˛sqrt(Var(G)) (1)

whereCBistheconﬁdencebound,E(G)isthemeanestimated reliabilityforthemissioncalculatedfromWeibullstatistics,

Z_˛isthezvalueconcerningthegivenCBlevelofsigniﬁcance, andVar(G)isthevaluecalculatedbytheFisherInformation matrix[33,36].

Macro imagesoffailedsampleswere takenwitha digi-talcamera(NikonD-70s,Nikon,Tokyo,Japan)andutilizedfor failuremodeclassiﬁcationandcomparisonsbetweengroups. Inordertoidentifyfractographicmarkingsandcharacterize failure origin and direction ofcrackpropagation, the most representative failedsamplesofeach groupwereinspected ﬁrst underapolarized-lightmicroscope (MZ-APO stereomi-croscope,CarlZeissMicroImaging,Thornwood,NY,USA)and thenbyscanningelectronmicroscopy(SEM)(ModelS-3500N, Hitachi,Osaka,Japan)[37,38].

2.2. Three-dimensionalﬁniteelementanalysis

(3D-FEA)

Four virtual3Dmodelswere created usingcomputer-aided design(CAD)software(SolidWorks2010,DassaultSystèmes SolidWorks Corp., Concord, MA,USA) followingdesignand dimensionsobservedingroupsSWT-EH,REG-EH,SWT-IHand REG-IH.Each3DCADmodelrepresentedallcharacteristicsof theimplant-abutmentconnectioninordertoreproducethe experimentalconditionsprevailingasaresultofthe mechan-icaltests(Fig.1).Thecomponentsofthemodelsconsistedof amaxillarycentralincisorcrown(Co–Cralloy),a50␮m-thick [39]resincementlayer(RelyXUnicem),anabutment(titanium alloy),aﬁxationscrew(titaniumalloy),animplant(titanium alloy),and acylindercreatedintheCADsoftwarewiththe samedimensionsoftheplastictubesusedintheinvitro lab-oratorystudy (Fig.3A).Theanatomicallycorrectcrownwas generatedfrommicrocomputedtomographyimagesin.dicom format(␮CT40,ScancoMedicalAG,Bruttisellen,Switzerland) anditscementationsurfacewasdesignedtoﬁttheabutments inallgroups.Theimplantinsertionholeinthecylinder(acrylic resin)wasobtainedbyaBooleansubtraction(Fig.3B).

Thecomponentswereassembled,importedintoFEA soft-ware(AnsysWorkbench12.0,SwansonAnalysisInc.,Houston, PA,USA),meshed(Fig.3C)(numberofparabolictetrahedral elements [40]between254,513and 288,543;and numberof nodes between433,816and 492,803)and testedfor conver-gencepriortomechanicalsimulation.Itwasconsideredthat theconvergencecriterionbetweenmeshesreﬁnementwasa changeoflessthan6%inthemaximumsimulatedvonMises equivalentstress(vM)oftheimplant/abutment/screw

com-ponents[41].

TheFEAmodelassumptionswerethat:(1)allsolidswere homogeneous, isotropic and linearlyelastic; (2) therewere noslipconditions(perfectbonding)amongcomponents(set

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Fig.3–(A)3DCADmodelsoftheimplant-abutmentcomplexincludingﬁxationscrew,abutmentandimplant.(B)Complete

CADmodelwithacement-retainedcrownoveranimplantwhichwasembeddedintotheacrylicresincylinder.Thered

arrowrepresentsa30◦off-axisload(300N)appliedonthecrownsurface,andthebluearrowsaroundthecylinderrepresent

theﬁxation(fullconstraint)onthebottomandlateralsurfaceofthecylinderofacrylicresin.(C)Finiteelementmeshofthe

model.Ontherightthereisahighermagniﬁcation(2×)ofthemeshshowedintheboxedarea.

implant-abutment-screw, elastic modulus (E)=110GPa and Poisson’sratio(v)=0.35)[42];(3)therewasauniformcement layer(E=8GPa,v=0.33)[43];(4)therewasacrown(E=220GPa,

v=0.30) [44] with similar dimensions (13mm height with amesiodistal widthof8.8mm andbuccal-lingual width of 7.1mm) in all FEA models; (5) therewere no ﬂawsin any components;(6)theboundaryconditionsofthemodelwere deﬁned on the bottom and lateral surface of the cylinder ofacrylicresin(E=1.37GPa, v=0.30)1 _to_represent _the con-strainedofx,yand zdirections(displacement=0)(Fig.3B). Asinthemechanicaltests,one30◦off-axisload(300N)was appliedonthelingualsideofthecrown,closetotheincisal edge(Fig.3B).RegionsofhighervonMisesequivalentstress (vM)weredeterminedwithinimplant-abutmentconnection

forallmodels.

3. Results

3.1. Invitrolaboratorystudy(SLFandSSALT)

The SLF mean±standard deviation values for group SWT-EH was 1090.01N±140.49N, 1204.95N±49.78N for group REG-EH, 960.69N±113.85N for group SWT-IH and 818.8N±105.85NforgroupREG-IH.

Thestep-stress acceleratedfatigue allowsestimation of reliabilityatagivenloadlevel(Table2).Thecalculated reli-abilitywith90%conﬁdenceintervalsforamissionof50,000 cycles at300N showed that the cumulative damage from loadsreaching300Nwouldleadtorestorationsurvivalin53% ofspecimensingroupSWT-EH,whereas93%wouldsurvive ingroupREG-EH.Thesevaluesdepictastatistically signiﬁ-cantdifferencebetweengroupsSWT-EHandREG-EH.Onthe otherhand,theoverlapbetweentheupperandlowerlimits

1 _{Manufacturer’s}_information.

ofreliabilityvaluesingroupsSWT-IHandREG-IH indicates nostatisticallysigniﬁcantdifferenceinreliabilityof implant-supportedrestorationswithinternalconnections,regardless ofabutmentdiameter(switchingorregularplatform).Forthe givenmission,asurvivalof99%ofthespecimenswouldbe observedinbothgroups(SWT-IHandREG-IH).Asshownin Table 2, from99% to100%ofthespecimenswouldsurvive givenamissionof50,000cyclesat210N,indicatingno statis-ticallysigniﬁcantdifferenceinreliabilityamongallgroups.

Thestep-stressderivedprobabilityWeibullplotsata300N loadarepresentedinFig.4.TheBeta(ˇ)valuesandassociated upper and lower bounds derived from use level probabil-ity Weibullcalculation(probability offailurevs. numberof cycles) of 1.31 (0.75–2.28)and 1.83 (1.01–3.32)for platform-switchedimplants(groupsSWT-EHandSWT-IH,respectively), andˇvaluesof1.55(0.78–3.06)and1.82(1.02–3.25)forregular platformimplants(groupsREG-EHandREG-IH,respectively) indicatedthatfatiguewasanacceleratingfactorforallgroups. TheBetavaluedescribesfailureratechangesovertime(ˇ<1: Failurerateisdecreasingovertime,commonlyassociatedwith “earlyfailures”orfailuresthatoccurduetoegregiousﬂaws; ˇ∼ 1:failureratethatdoesnotvaryovertime,associatedwith failures ofarandom nature;ˇ>1:Failurerateisincreasing over time,associatedwithfailuresrelatedtodamage accu-mulation)[30,45,46].

3.2. Failuremodes

AllspecimensfailedafterSLFandSSALT.Failuremodesforall groupsarepresentedinTable3.Forrestorationsoverexternal heximplants(groupsSWT-EHandREG-EH)screwfractureat thethirdthreadregionwasthechieffailuremode(Fig.5C).In thesespecimens,abutments andimplants wereintactafter mechanicaltests.Forrestorationsoverinternalheximplants (groupsSWT-IHandREG-IH),screwandabutmentfractureat thenarrowestregion(cervicalcollaroftheabutment,diameter

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Table2–Calculatedreliability(upperandlowerlimits)fortestedgroupsgiventwodifferentmissions:50,000cyclesat 300Nloadand50,000cyclesat210Nload.

Output SWT-EH REG-EH SWT-IH REG-IH

50,000cycles@300N 0.53(0.33–0.70)a _0.93_{(0.80–0.97)}b _0.99_{(0.93–0.99)}c _0.99_{(0.99–1.00)}c 50,000cycles@210N 0.99(0.94–0.99)c _0.99_{(0.98–0.99)}c _1.00_{(0.99–1.00)}c _1.00_{(0.99–1.00)}c Thesuperscriptletters(a,bandc)depictsstatisticallyhomogeneousgroups.

Fig.4–Thisgraphshowstheprobabilityoffailureasafunctionofnumberofcycles(time)fortestedgroupssimulatinga missionof50,000cyclesat300N.NotetheleftpositionoftheSWT-EHgroup(green)relativetoREG-EHgroup(blue),and SWT-IHgroup(pink)relativetoREG-IHgroup(black),whichindicatestheneedformorecyclestofailureinregular-platform groupscomparedtotheswitched-platformgroups.

of2.9mm)(Fig.6BandC)wereobservedinallspecimensafter mechanicaltests.Noimplant fracturewasobservedinany group.

Observationofthepolarized-lightandSEMmicrographsof thescrew’sfracturedsurfaceallowedtheconsistent identifi-cationoffractographicmarkings,suchascompressioncurl, fatiguestriations and dimples, which allowedthe identifi-cationofflaworiginand thedirectionofcrackpropagation

(Fig.7).Asperourimaginganalysisofthespecimen’s frac-tured surface, all fractures were characterized bymaterial tearingand exhibitedgrossplasticdeformation,suggesting ductilefractures(Figs.6Cand7AandB).Theresulting duc-tilefracturesoccurredasstressesexceededthematerialyield strength leaving telltalefractographicmarksthat indicated crack propagation from lingual to buccal (Fig. 7C), where occlusalforcesnaturallyoccurintheanteriorregion.Although

Table3–Failuremodesaftermechanicaltesting(single-load-to-fracture(SLF)andstep-stressacceleratedlife-testing (SSALT))accordingtotheusedfailurecriteria.

Groups SWT-EH REG-EH SWT-IH REG-IH

SLF (n=3)

Screw:3fracture Screw:3fracture Screw:3fracture Screw:3fracture Abutment:3intact Abutment:3intact Abutment:3fracture Abutment:3fracture Implant:3intact Implant:3intact Implant:3intact Implant:3intact SSALT

(n=18)

Screw:18fracture Screw:18fracture Screw:18fracture Screw:18fracture Abutment:18intact Abutment:18intact Abutment:18fracture Abutment:18fracture Implant:18intact Implant:18intact Implant:18intact Implant:18intact

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Fig.5–Imagesillustratingthepeakofstressfortheﬁxationscrewinallgroups.(A)3DCADmodelwithabutmentin

transparencyshowingthecontactarea(blackarrow)atthethirdthreadregionofthescrew.(B)PeakofvonMisesequivalent

stress(vM)atthethirdthreadregionofthescrew.(C)Macropictureofthescrewfracturedatthethirdthreadregion.

Fig.6–Imagesillustratingthepeakofstressfortheabutments.(A)PeakofvonMisesequivalentstress(vM)locatedatthe

externalregion(lingualside)ofthecervicalcollaroftheabutment.(B)Macropictureofanabutmentfracturedatthe

narrowerregionoftheabutment(cervicalcollarregion).Inallspecimens,thefractureoccurredinthisregion.(C)SEM

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Fig.7–RepresentativefracturedscrewafterSSALTdepicting:(AandB)MacroimageandSEMmicrograph,respectively,

showingafractureoccurringatthethirdthreadregionviewedfromthescrew’slongaxis.(C)isaSEMmicrograph(60×)of

thefracturedsurfaceofsampleshownin(B).Thewhitedottedcircleshowsacompressioncurlwhichevidencesfracture

originattheopposingtensileside(whitebox),indicatingthedirectionofcrackpropagation(dcp)(whitearrow).(D)isa

highermagniﬁcation(250×)oftheboxedareain(C)showingthefractureorigin.(EandF)arehighermagniﬁcations(2000×

and1500×,respectively)ofthefracturedsurfaceshowingtypicalfractographicfeaturesofmetallicmaterials:(E)fatigue

striationsand(F)dimpledsurfaceappearance.

apartmayfailinabrittlemanner,ductilefracturemorphology isfrequentlyobservedawayfromtheorigin.Forexample, com-pressioncurlisafractographicfeaturerepresentativeofflexure failuresandresultsfromatravelingcrackchangingdirection asitentersacompressionfield[47].Usuallyitevidences frac-tureorigin atthe opposingtensileside (Fig.7D).Athigher magnifications(from500× to2500×),fatiguestriationswere observed(Fig.7E).Theyemanated outwardfromthe origin andmarkedsuccessivepositionsoftheadvancingcrackfront [37].Alsoinahighermagnification(1500×)adimpledsurface appearancecreated insomeareas onthefracturedsurface wasobserved,exemplifyingatypicalductilefractureinmetal alloys,commonlycreatedbymicrovoidcoalescence[37].

3.3. 3D-FEA

The values for vM within implant-abutment complex

(implant, abutment and ﬁxation screw) are presented in Table 4, and showed that the stress distribution on abut-ment and screw was strongly inﬂuenced by the abutment diameter(regularandswitched-platform)andtypeofimplant connection(externalandinternalhexagon).Whenreducing theabutmentdiameter,anincreaseinthevMof41.08%was

observedintheabutmentconnectedtoexternalheximplant (SWT-EH),whileanincreaseinthevMof53.27%wasobserved

intheabutmentconnectedtointernalheximplant(SWT-IH). Inthe ﬁxationscrew, increasesof19.67% and 11.57%were

observedinthevMforSWT-EHandSWT-IH,respectively.No

relevantdifferencesinthelevelsofvMwereobservedinthe

implantbodywhenconsideringthevariablesofthisstudy. Thehighest levelofstress was observedinthe ﬁxation screwforallmodels.Intheﬁxationscrew,thepeakofvMwas

concentratedatthethirdthreadregioninallgroups(Fig.5), whereasintheabutmentthepeakofvMwaslocatedonthe

lingualregionatthecervicalcollar(Fig.6A).

4. Discussion

The concept of platform switching is increasingly sought because it can be advantageous in several clinical con-ditions. Previous studies [8–11] have demonstrated that platform-switchedabutmentsmaynotonlyreducetheearly peri-implant bone loss and increase the biomechanical

Table4–vonMisesequivalentstress(vM)inMPa

withintheimplant-abutmentconnection.

Component Implant Abutment Screw

SWT-EH 228 182 365

REG-EH 225 129 305

SWT-IH 216 166 270

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ofmaxillarycentralincisorcrownsusingSSALT.Thismethod consistsonamechanicaltestforshorteningthelifeof materi-alsorhasteningthedegradationoftheirperformance.Unlike othermethods,theaimofsuchtestingistoquicklyobtain data which, properly modeled and analyzed, yielddesired informationon component lifeor performanceunder nor-maluse.Inaddition,theSSALTmethodallowstheprediction withconfidenceintervals (basedoncalculationofamaster Weibulldistribution)ofthelifeexpectancyofagivenmaterial underspecifiedloading.Wehaveusedalife-stress relation-shipmodelallowingtheextrapolationofauselevelprobability densityfunctionfromlifedataobtainedatincreasedstress levels. These models describe the path ofa particular life characteristic of the distribution from one stress level to another.FortheWeibulldistribution,thescaleparameter(eta) isconsideredtobestress-dependent.Therefore,thelife-stress modelfordatathatfitstheWeibulldistributionisassigned toeta.Ourresultsshowedthatfatiguedamageaccumulation acceleratedthefailures ofall testeddesignsinthepresent study, as evidenced by the resulting ˇ>1 (also called the Weibullshapefactor).Furthermore,astatisticallysignificant lowerreliability(givenamissionof50,000cyclesat300Nload) wasfoundforplatform-switchedimplantswithexternalhex (SWT-EH),butnotforplatform-switchedimplantswith inter-nalhex(SWT-IH).

These ﬁndings may be explained based in the associa-tionamongstressdistributionandsystem’sreliabilityaround theweakestcomponentoftheimplant-abutmentconnection: Theﬁxation screw.Thehigher levels ofstress (vM)inthe

abutmentscrewobservedfortheexternalhexagon connec-tionwasassociatedwithalowerreliabilityaftermechanical testingforbothregularandswitched-platformsystems(300N loadsimulation).However,itcanbeassumedthattheslight increase(11.57%)instresslevels(vM)observedinthe

ﬁxa-tionscrewwhenreducingabutmentdiameteroveraninternal heximplant (SWT-IH)wasnotsigniﬁcanttoresultinlower mechanicalreliability.Thelowervaluesforreliabilityobserved ingroupsSWT-EHandREG-EHwereduetolowerloads initi-atingprostheticcomponentfailurewhencomparedtogroups withinternalheximplants(SWT-IHandREG-IH).

Worthnotingisthatallpreviousconsiderationswere per-formed under mission of 50,000 cycles at 300N load. If a mission of50,000cycles at210Nloadisconsidered (mean valueforincisalbiteforce)[35],thecumulativedamagefrom loads reaching 210N would lead to restoration survival of

Thenarrowestpartofacomponentisusuallyitsweakestpart becauseitistheregionwherethemaximumstressesoccur, becauseofthesmallestcross-sectionalarea.Inthepresent studythepeakofvMwaslocatedattheexternalregionofthe

cervicalcollar(Fig.6)becauseaperfectbondingwas consid-eredbetweenabutmentandimplant.InourFEAsimulation therewasnoseparationofthesecomponentswhen submit-tedtotensileforcesandhighertensilestressesweregenerated atthisregion(externalareaofthecervicalcollar).Thosehigh tensilestressesarenotreal,giventhatinthephysicaltesting (SSALT) theabutments movesawayfrom the implant plat-form(atpalatalregion),butdoesnotpulltheimplant.Future simulations withmorecomplexmodels capabletoaddress suchlimitationare warranted.Moreover,ithasbeen previ-ouslyreportedthatthefailurelocationisrelatedtothedesign characteristicsoftheimplant-abutmentcombination,which iscommonlylocatedinthethreadedregionorareasthat rep-resentacriticalpointforprostheticcomponent’sendurance duetotheshiftingeometryalongitslengthandsubtle alter-ationincross-sectionalarea[23].

Despite the stress distribution observed in the 3D-FEA being obtained from single static loading, such as in SLF tests, whichdoesnotrepresentthecyclicloadingobserved inoralenvironmentandinfatiguetests(SSALT),ourresults suggest improved stress distribution within the implant-abutment connectionofregular-platformmodelsregardless of the methodology (invitro study or ﬁniteelement analy-sis).Thus,theimprovedstressdistributionmaypresumably be the reason forbetter mechanical behaviorof internally connected systems compared to the externally connected counterparts. Concerningthegeometryofimplant connec-tion(internalvs.external),higherreliabilitywasobservedin specimenswithinternalconnectionregardlessofthe abut-mentdiameter.Theseﬁndingsareinagreementwithother studiesthatpointedthatdeepjointsshowincreased stabil-ityfavoringstructuralstrengthofimplantsystems[24,32,48]. Itshouldbenoted,however,thatduetoengineeringdesign constraints such as minimum wall thickness for proper mechanicalperformanceofeachofthedifferentconnection systems,differencesinbothexternalandinternalfeaturesof the implant,abutment,and screwdesignswillexist.While fromaresearchstandpointitishighlydesirablethatonlythe connectionischangedwiththeconnectingscrewandimplant remainingthe same,suchinterplay isunfeasibleforwhen one is attempting to makeclinically relevantcomparisons

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(implantspresentingthesame diameter,length,andcrown size)betweenexternalandinternalconnectionsinmost com-mercially available systems, as alterations in the implant externalshapeisusuallyperformedbymanufacturersinorder tomaintaintolerancesforappropriateﬁtandwallthickness fortheinternalconnectionrobustness.

Accordingtotheliterature[7],therearepotential limita-tionsforusingplatform-switchedimplants,e.g.theneedfor componentsthathavesimilardesigns(thescrewaccesshole mustbeuniform)andtheneedforenoughspacetodevelop a proper emergence proﬁle. Considering that the replace-mentofsingle-unitedentulousspacesintheanteriorregion withimplant-supportedrestorationsisachallengingscenario interms oflong-termsuccessandesthetics,itiscrucialto acknowledgethefunctionalandmechanicallimitationsofthe implant-abutmentconnections.

5. Conclusions

Thepostulatedhypothesisthatplatform-switchedimplants would result in increased stress concentration within the implant-abutmentconnection,leadingtothesystems’lower reliability on laboratory mechanical testing was partially accepted.Thehigherlevelsofstressobservedwithin implant-abutmentconnectionwhenreducingabutmentdiameter,and thereforeitscross-sectionalarea,resultedinlowerreliability forexternalheximplants,butnotforinternalheximplants. Failuremodesweresimilarwhencomparingswitchingand regularplatforms.

Acknowledgements

ThisinvestigationwassupportedinpartbyResearchGrant 141870/2008–7fromCNPq–Brazil.Theauthorsarethankfulto

MarottaDentalStudio(Farmingdale,NY,USA)andSINimplants

(SãoPaulo,SP,Brazil)fortheirsupport.

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