d e n t a l m a t e r i a l s 2 8 ( ) e218 e228 Available online at

element

analysis

Amilcar

C.

Freitas-Júnior

,

Eduardo

P.

Rocha

,

Estevam

A.

Bonfante

,

Erika

O.

Almeida

, Rodolfo

B.

Anchieta

,

Ana

P.

Martini

,

Wirley

G.

Assunc¸ão

,

Nelson

R.F.A.

Silva

,

Paulo

G.

Coelho

a_{DepartmentofBiomaterialsandBiomimetics,NewYorkUniversityCollegeofDentistry,NY,USA} b_{PostgraduatePrograminDentistry,PotiguarUniversity-SchoolofHealthSciences,Natal,RN,Brazil}

c_{DepartmentofDentalMaterialsandProsthodontics,SãoPauloStateUniversity,Arac¸atubaSchoolofDentistry,Arac¸atuba,SP,Brazil} d_{PostgraduatePrograminDentistry,UnigranrioUniversity-SchoolofHealthSciences,DuquedeCaxias,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.

∗ _{Correspondingauthorat:DepartmentofBiomaterials&Biomimetics,NewYorkUniversityCollegeofDentistry,}

345E24thStreet,Room812,NewYork,NY10010,USA.Tel.:+1558488402345.

E-mailaddress:[email protected](A.C.Freitas-Júnior).

0109-5641/$–seefrontmatter©2012AcademyofDentalMaterials.PublishedbyElsevierLtd.Allrightsreserved.

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

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 wasappliedat30◦tothelongaxisoftheimplant.

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

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 _{torepresent 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’sinformation.}

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

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

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 micrographoftheregionoffracture.

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

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 (in vitro 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

(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.

r

e

f

e

r

e

n

c

e

s

[1] BozkayaD,MuftuS,MuftuA.Evaluationofloadtransfer

characteristicsoffivedifferentimplantsincompactboneat

differentloadlevelsbyfiniteelementsanalysis.Journalof

ProstheticDentistry2004;92:523–30.

[2] HermannF,LernerH,PaltiA.Factorsinfluencingthe

preservationoftheperiimplantmarginalbone.Implant

Dentistry2007;16:165–75.

[3] ManzMC.Factorsassociatedwithradiographicverticalbone

lossaroundimplantsplacedinaclinicalstudy.Annalsof

Periodontology2000;5:137–51.

[4] Lopez-MariL,Calvo-GuiradoJL,Martin-CastelloteB,

Gomez-MorenoG,Lopez-MariM.Implantplatform

switchingconcept:anupdatedreview.MedicinaOral,

PatologiaOralYCirugiaBucal2009;14:e450–4.

[5] LazzaraRJ,PorterSS.Platformswitching:anewconceptin

implantdentistryforcontrollingpostrestorativecrestal

bonelevels.InternationalJournalofPeriodonticsand

RestorativeDentistry2006;26:9–17.

[6] BaumgartenH,CocchettoR,TestoriT,MeltzerA,PorterS.A

newimplantdesignforcrestalbonepreservation:initial

observationsandcasereport.PracticalProcedures&

AestheticDentistry:PPAD2005;17:735–40.

[7] GardnerDM.Platformswitchingasameanstoachieving

implantesthetics.NewYorkStateDentalJournal

2005;71:34–7.

[8] CappielloM,LuongoR,DiIorioD,BugeaC,CocchettoR,

CellettiR.Evaluationofperi-implantbonelossaround

platform-switchedimplants.InternationalJournalof

PeriodonticsandRestorativeDentistry2008;28:

347–55.

[9] LuongoR,TrainiT,GuidonePC,BiancoG,CocchettoR,

CellettiR.Hardandsofttissueresponsestothe

platform-switchingtechnique.InternationalJournalof

PeriodonticsandRestorativeDentistry2008;28:551–7.

[10] DuarteAR,RossettiPH,RossettiLM,TorresSA,Bonachela

WC.Invitrosealingabilityoftwomaterialsatfivedifferent

implant-abutmentsurfaces.JournalofPeriodontology

2006;77:1828–32.

[11] CanulloL,FedeleGR,IannelloG,JepsenS.Platform

switchingandmarginalbone-levelalterations:theresultsof

arandomized-controlledtrial.ClinicalOralImplants

Research2010;21:115–21.

[12] SchrotenboerJ,TsaoYP,KinariwalaV,WangHL.Effectof

microthreadsandplatformswitchingoncrestalbonestress

levels:afiniteelementanalysis.JournalofPeriodontology

2008;79:2166–72.

[13] BaggiL,CappelloniI,DiGirolamoM,MaceriF,VairoG.The

influenceofimplantdiameterandlengthonstress

distributionofosseointegratedimplantsrelatedtocrestal

bonegeometry:athree-dimensionalfiniteelement

analysis.JournalofProstheticDentistry2008;100:

422–31.

[14] TabataLF,AssuncaoWG,AdelinoRicardoBaraoV,deSousa

EA,GomesEA,DelbenJA.Implantplatformswitching:

biomechanicalapproachusingtwo-dimensionalfinite

elementanalysis.JournalofCraniofacialSurgery

2010;21:182–7.

[15] VargasLC,AlmeidaEO,RochaE,FreitasJrAC,AnchietaRB,

KinaS,etal.Regularandplatformswitching.Bonestress

analysiswithvaryingimplantdiameters.JournalofOral

Implantology2011.

[16] Rodriguez-CiuranaX,Vela-NebotX,Segala-TorresM,

Rodado-AlonsoC,Mendez-BlancoV,Mata-BuguerolesM.

Biomechanicalrepercussionsofboneresorptionrelatedto

biologicwidth:afiniteelementanalysisofthree

implant-abutmentconfigurations.InternationalJournalof

PeriodonticsandRestorativeDentistry2009;29:479–87.

[17] MaedaY,MiuraJ,TakiI,SogoM.Biomechanicalanalysison

platformswitching:isthereanybiomechanicalrationale?

ClinicalOralImplantsResearch2007;18:581–4.

[18] JungRE,PjeturssonBE,GlauserR,ZembicA,ZwahlenM,

LangNP.Asystematicreviewofthe5-yearsurvivaland

complicationratesofimplant-supportedsinglecrowns.

ClinicalOralImplantsResearch2008;19:119–30.

[19] BeckerJ,FerrariD,HertenM,KirschA,SchaerA,SchwarzF.

Influenceofplatformswitchingoncrestalbonechangesat

non-submergedtitaniumimplants:ahistomorphometrical

studyindogs.JournalofClinicalPeriodontology

2007;34:1089–96.

[20] CochranDL,BosshardtDD,GrizeL,HigginbottomFL,Jones

AA,JungRE,etal.Boneresponsetoloadedimplantswith

non-matchingimplant-abutmentdiametersinthe

caninemandible.JournalofPeriodontology2009;80:

609–17.

[21] HurzelerM,FicklS,ZuhrO,WachtelHC.Peri-implantbone

levelaroundimplantswithplatform-switchedabutments:

preliminarydatafromaprospectivestudy.JournalofOral

analyses.JournalofPeriodontology2009;80:1125–32.

[26] ChangCL,ChenCS,HsuML.Biomechanicaleffectof

platformswitchinginimplantdentistry:a

three-dimensionalfiniteelementanalysis.International

JournalofOralandMaxillofacialImplants2010;25:295–304.

[27] CrespiR,CappareP,GherloneE.Radiographicevaluationof

marginalbonelevelsaroundplatform-switchedand

non-platform-switchedimplantsusedinanimmediate

loadingprotocol.InternationalJournalofOraland

MaxillofacialImplants2009;24:920–6.

[28] PessoaRS,VazLG,MarcantonioJrE,VanderSlotenJ,DuyckJ,

JaecquesSV.Biomechanicalevaluationofplatformswitching

indifferentimplantprotocols:computedtomography-based

three-dimensionalfiniteelementanalysis.International

JournalofOralandMaxillofacialImplants2010;25:911–9.

[29] CanayS,AkcaK.Biomechanicalaspectsofbone-level

diametershiftingatimplant-abutmentinterface.Implant

Dentistry2009;18:239–48.

[30] CoelhoPG,SilvaNR,BonfanteEA,GuessPC,RekowED,

ThompsonVP.Fatiguetestingoftwoporcelain-zirconia

all-ceramiccrownsystems.DentalMaterials2009;25:

1122–7.

[31] PjeturssonBE,TanK,LangNP,BraggerU,EggerM,Zwahlen

M.Asystematicreviewofthesurvivalandcomplication

ratesoffixedpartialdentures(FPDs)afteranobservation

periodofatleast5years.ClinicalOralImplantsResearch

2004;15:625–42.

[32] KhraisatA,StegaroiuR,NomuraS,MiyakawaO.Fatigue

resistanceoftwoimplant/abutmentjointdesigns.Journalof

ProstheticDentistry2002;88:604–10.

[33] NelsonW.Acceleratedtesting:statisticalmodels,testplans

anddataanalysis.NewYork:JohnWiley&Sons;2004.

[34] ZhaoWEE.Ageneralacceleratedlifemodelforstep-stress

testing.IEEETransactionsOnReliability2005;37:1059–69.

[40] deAlmeidaEO,RochaEP,FreitasJrAC,FreitasJrMM.Finite

elementstressanalysisofedentulousmandibleswith

differentbonetypessupportingmultiple-implant

superstructures.InternationalJournalofOraland

MaxillofacialImplants2010;25:1108–14.

[41] HuangHL,HsuJT,FuhLJ,TuMG,KoCC,ShenYW.Bone

stressandinterfacialslidinganalysisofimplantdesignson

animmediatelyloadedmaxillaryimplant:anon-linear

finiteelementstudy.JournalofDentistry2008;36:

409–17.

[42] HuangHL,FuhLJ,KoCC,HsuJT,ChenCC.Biomechanical

effectsofamaxillaryimplantintheaugmentedsinus:a

three-dimensionalfiniteelementanalysis.International

JournalofOralandMaxillofacialImplants2009;24:455–62.

[43] CoelhoPG,BonfanteEA,SilvaNR,RekowED,ThompsonVP.

LaboratorysimulationofY-TZPall-ceramiccrownclinical

failures.JournalofDentalResearch2009;88:382–6.

[44] ErkmenE,MericG,KurtA,TuncY,EserA.Biomechanical

comparisonofimplantretainedfixedpartialdentureswith

fiberreinforcedcompositeversusconventionalmetal

frameworks:a3DFEAstudy.JournaloftheMechanical

BehaviorofBiomedicalMaterials2011;4:107–16.

[45] ReliaSoft.TheWeibullDistributionandBeta.2010.

[46] SilvaNR,deSouzaGM,CoelhoPG,StappertCF,ClarkEA,

RekowED,etal.Effectofwaterstoragetimeandcomposite

cementthicknessonfatigueofaglass-ceramictrilayer

system.JournalofBiomedicalMaterialsResearchPartB,

AppliedBiomaterials2008;84:117–23.

[47] QuinnGD.Fractographyofceramicsandglasses.

Washington:U.S.GovernmentPrintingOffice;2007.

[48] MaedaY,SatohT,SogoM.Invitrodifferencesofstress

concentrationsforinternalandexternalhex

implant-abutmentconnections:ashortcommunication.