Report on fractures of trilayered all-ceramic fixed dental prostheses

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Reference

Report on fractures of trilayered all-ceramic fixed dental prostheses

BELLI, Renan, SCHERRER, Susanne, LOHBAUER, Ulrich

Abstract

Three cases of trilayered fixed dental prostheses fractured in vivo are fractographically analyzed to identify causes of failure. In all cases fractures initiated from a preexistent crack/defect in the zirconia framework produced previous to the fusion firing of a lithium disilicate veneering ceramic. In two cases zirconia framework pre-cracks were found to have been infiltrated with the intermediate fusion glass that diffused therein during sintering. This report highlights the susceptibility of zirconia to grinding damage and its role in limiting the lifetime of zirconia-based fixed dental prostheses.

BELLI, Renan, SCHERRER, Susanne, LOHBAUER, Ulrich. Report on fractures of trilayered all-ceramic fixed dental prostheses. Case Studies in Engineering Failure Analysis , 2016, vol. 7, p. 71-79

DOI : 10.1016/j.csefa.2016.10.001

Available at:

http://archive-ouverte.unige.ch/unige:99759

Disclaimer: layout of this document may differ from the published version.

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Report on fractures of trilayered all-ceramic ﬁ xed dental prostheses

Renan Belli

^a,

*, Susanne S. Scherrer

^b

, Ulrich Lohbauer

^a

aFriedrich-AlexanderUniversitätErlangen-Nürnberg(FAU),DentalClinic1–OperativeDentistryandPeriodontology,ResearchLaboratory forDentalBiomaterials,Glueckstrasse11,91054Erlangen,Germany

bDivisionofProsthodontics-Biomaterials,UniversityClinicofDentalMedicine,UniversityofGeneva,Barthélemy-Menn19,1205Geneva, Switzerland

ARTICLE INFO

Articlehistory:

Received2May2016

Receivedinrevisedform30August2016 Accepted27October2016

Availableonline9November2016

Keywords:

Dentalceramic Zirconia Fracture Fractography Dentalbridges Crowns Trilayers

ABSTRACT

Threecasesoftrilayeredfixeddentalprosthesesfracturedinvivoarefractographically analyzedtoidentifycausesoffailure.Inallcasesfracturesinitiatedfromapreexistent crack/defectinthezirconiaframeworkproducedprevioustothefusionfiringofalithium disilicateveneeringceramic.Intwocaseszirconiaframeworkpre-crackswerefoundto have been infiltrated with theintermediatefusion glass thatdiffused thereinduring sintering.Thisreporthighlightsthesusceptibilityofzirconiatogrindingdamageandits roleinlimitingthelifetimeofzirconia-basedfixeddentalprostheses.

ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Thepaceofinnovationintheﬁeldofprostheticdentistrycomesintowwithmanufacturersofdentalmaterials,usually guidedbytheirabilitytoadaptengineeringmaterials(e.g.,alloys,ceramics,composites)forprocessingroutinessuitablefor dentalapplications.ThemostrecentadvanceinthisrespectisbestepitomizedbytheintroductionofCAD/CAMtechnology inthe90s,settorevolutionizeprosthetictherapies.Inthe2000sthistechnologyreachedmaturity,featuringtodaythe requireddigitalresourcesnecessaryforhighprecisionscanning,modelingandmachining.Anaturalconsequencewasthe developmentofpre-sintered(e.g.,alumina,zirconia)andfully-sintered(e.g.,partiallycrystallizedglasses)ceramicmaterials, deliveredinsmallblocksforsingle-unitreconstructionsoraswideblanksformulti-unitpurposes.Thesearemachinedtothe desiredshape/sizeandusedsoloasmonolithicpiecesorcombinedtoothermaterialstoproducelayeredstructures.

In a commercialstrategy todisseminatetheuseofin-ofﬁceCAD/CAM machinery,processingtechniques havealso evolvedtoallowpractitionerstoproducemorecomplexlayeredchairsidestructures.Justrecentlyatechniquehasbeen introducedadvocatingthefusionofthemachinedversionoflithiumdisilicate(LS2)glass-ceramicoverlaysontozirconia (ZrO2)frameworksthroughtheuseofafusionglassasanalternativetotheotherwiselaboratory-produced“hot-pressed” technique.Thepartially-crystallizedLi₂O

^2SiO2glass-ceramicoverlayconstructisﬁredontoasinteredZrO₂frameworkviaa

*Correspondingauthorat:ResearchLaboratoryforDentalBiomaterials,DentalClinic1—OperativeDentistryandPeriodontology,Glueckstrasse11, D-91054Erlangen,Germany.

E-mailaddress:rbelli@dent.uni-erlangen.de(R.Belli).

http://dx.doi.org/10.1016/j.csefa.2016.10.001

2213-2902/ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

ContentslistsavailableatScienceDirect

Case Studies in Engineering Failure Analysis

j o u r n a lh o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c s e fa

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thinlayerofglassslurryactingasafusionglass.Fusionoccursduringaseparateﬁringthatsimultaneouslycrystallizesthe lithiummetasilicate(Li2O

^SiO²⁾^precursory^phase.^The^result^is^athree-layerstructureconsistingofmaterialswithdifferent thermo-mechanicalproperties(LS₂-glass-ZrO₂).Intheshort-term,dentalbridgesproducedfollowingthistechniquehave showneithercomparableclinicalperformance [1]orhigher fracturerates[2] comparedtoconventionalhand-layered veneered-ZrO2.Theclinicalperformanceofsuchconstructsisyettobefullyassessedinthelong-term,butkeyinformation regardingfailurecanalreadybeprovidedbyfractographicanalysisofbrokenconstructs.Thisreportanalyzesthreeofsuch trilayerstructuresfracturedinvivoinordertobringinsightastotheirmechanicalbehaviorandfailuremechanisms.

2.Background

Inarecentcontribution[2]wereportedontheclinicalfractureratesofvariousall-ceramicrestorativesystemswithina 3.5yearsperiodbyanalyzingalargedatasetofnearly35thousandrestorationsproducedinadentalCAD/CAMmachining centerinGermany.AsignificantlylowerfractureratewasfoundfortheLS₂-glass-ZrO₂system(19from3.095restorations) whenusedassingle-unitprosthesesincomparisontomonolithicLS2structures(111from9.053restorations).Conversely, forextendedindications(i.e.,3-to5-unitfixeddentalprostheses)theLS2-glass-ZrO2systemshowedasignificantlyhigher numberofcatastrophicfractures(21from535restorations)thanaconventionallyhand-layeredveneered-ZrO₂system(3 from 364 restorations). From the total of 491 fracture events, the authors had access to the original fragments of approximately30cases,3ofthembeingoftheLS2-glass-ZrO2trilayersystem.Thefractographicanalysesofthese3casesare reportedinthepresentpaper.ThesewillbepresentedseparatelyasCase1,2and3illustratedbyFigs.1–2,3–4and5–7, respectively.

Accordingtoinformationobtainedfromthemachiningcenter,allthreeﬁxeddentalprothesesweremanufacturedfrom thesamematerialsandfollowedthesameprocessingprotocol.TheCAD-CAMcreatedframeworkwasﬁrstmachinedoutofa greenblank(e.maxZirCAD,Ivoclar-Vivadent,Schaan,Liechtenstein),driedandsinteredat1500Cfor2h.Apre-crystallized lithiumdisilicateglassCAD-CAMblock(e.maxCAD,Ivoclar-Vivadent)wasusedforthemachiningoftheoverlaystructure.

ForthefusionoftheZrO2frameworkwiththeLS2overlay,themanufactureroftheLS2(Ivoclar-Vivadent)recommendsthe useofaproprietaryfusionglasswithadjustedfiringprogram,particlesizeandcoefficientofthermalexpansion(CTE)of 9.510⁶K¹.Divergingfromthisrecommendation,analternativefusionglass(DCMhotbondfusio6,DCMGmbH,Rostock, Germany)withaslightlyhigherCTE(9.810⁶K¹)wasused,reachingamaximumsinteringtemperatureof770C.Since thecrystallizationfiringofthepre-crystallizedLS₂blockmustreachamaximumtemperatureof840C,weassumethatthe crystallizationandfusionfiringswereundertakenseparately.

Case1representsasingle-unitﬁxedprosthesis(or“crown”)replacingtheseconduppermolarontheleftside(FDI numberingsystem:#27).Thefracturewasreportedbythedentist24monthsafterinstallation;thebrokenpartfragment waslostbythepatientbuttheremainingcrownstillonthetoothstructurewasremovedbythedentistanddeliveredtothe laboratory’sclaimdepartment.TheappearanceofthefracturedcrownisillustratedinFigs.1–2.Thefractureincludedthe zirconiaframeworkmarginonthedisto-palatalside,exposedsomeoftheZrO2frameworkshowingdelaminationof a substantialportionoftheLS2overlayceramic.

Case2correspondstoasingle-unitcomponent(or“crown”),aimedtorestorethesecondlowermolarintheleftside(FDI

#37).ThefractureillustratedinFig.3occurredafter19monthsfrominstallationandinvolvedmostofitsbuccalside.Nosign ofoverlayspallingwasdetectedtothenakedeye.

InCase3adoublefractureisanalyzedina3-unitﬁxeddentalprosthesisintheupperleftside,withthesecondpremolar(FDI

#25)astheunsupportedponticandtheﬁrstpremolar(FDI#24)andﬁrstmolar(FDI#26)asabutmentssupportedbynatural teeth(Fig.4).Themesialfractureinvolvingthemesio-palatalmarginandthemargininthemiddleofthebuccalpremolarside.

Thetrajectoryofthedistalfracturelinkedthebucco-distalconnectortothepalatalmarginofthemolarcrown.Thisresultedin threefragments,aslabledinFig.4.Atthispointinsufﬁcientinformationwasavailabletomakeassumptionsregardingthe orderofevents,thatis,whichfracturetookplaceﬁrstorinwhichdirection.Neitherinformationaboutthecementation procedure(adhesiveornot)normaterialsusedforthispurposehavebeenrecordedontheclaimsheets.

3.Fractographicanalysis

Therecoveredfragmentswerecleanedin3vol%sodiumhypochloriteand90vol%ethanolsolutionsinanultrasonicbath toremoveorganicdebrisandcontaminants.Photographsandstereomicroscopicimagesweretakentoregisterthevisual appearanceofthefragmentsbeforeprocessingforscanningelectronmicroscopy(SEM).Fragmentswerethensputter-coated withgoldandanalyzedinanSEM(LeitzISISR50,Akashi,Japan)forfracturemarkingsthatwouldallowthedeterminationof cracktrajectoriesandfractureorigins.

3.1.Case1

TwoimportantaspectsinCase1are:(i)thesemi-lunarshapedfractureinvolvingtheZrO₂frameworkatthecrown margin;and(ii)thedelamination/fractureof theoverlayLS2 glass-ceramicinvolvingtheentiredisto-palatalcusp.The locationofthefractureoriginwilldeterminethechronologyofeventsandclarifywhichcomponentfracturedﬁrstandhow oneledtoanother.

72 R.Bellietal./CaseStudiesinEngineeringFailureAnalysis7(2016)71–79

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Ageneralinspectionontheocclusalsurfaceofthecrownrevealedawornsurfaceregionindicatingocclusalcontact activitycompatibletothereportedservicetime.Surfacesmoothnesswaslostonthoseareasexposingthemicrostructureof theglass-ceramic(Fig.1a).Alsonotedweregrindingmarkingsfrommanualadjustmentsastheyresemblethosestemming fromdiamond-embeddedrotaryinstrumentsvisibleonthepalatalandocclusalsurfaces.Thethreecomponents(thatis, zirconiaframework,fusionglassandlithiumdisilicateoverlay)seemtohavebeenleveledoffatthemarginspostfusion firing.Rotatingfinishing/polishinginstrumentshavemostprobablybeenusedtoasufficientdegreetomaintainmaterial integrity,sincenoextensivedamageisseenhere.However,thelayercorrespondingtothefusionglasspresentedcracksthat didnotseemtoberelatedtothemainfracture(Fig.1b).

Observableunderelectronbackscatter-modeintheSEM,thefracturesurfacewherethedelaminationoccurredshowed remnantsofthefusionglassattachedtothezirconia’soutersurfacebutalsoregionsofexposedframework(Fig.1d).This suggestsascenarioofweakinterfacialbondbetweenthefusionglassandboththeZrO2andtheLS2.Defectivewettingofthe ZrO2bytheglassmeltshouldalsobeconsideredasaprocessingproblem.Zonesofporosityhavebeenobservedintheglass layer,pointingtoexcessiveairentrapmentandinsufﬁcientairdiffusionoutoftheglassthroughthenarrowpathwaytoward theedges.ThefracturesurfaceoftheLS2overlaydidnotrevealanyfeatureindicativeofocclusalcontact-inducedcone crackinglinkedtoapossiblefractureorigin.MarkingsonthefracturesurfaceoftheLS2overlayweregenerallydifﬁcultto readduetothecoarsemicrostructure.Wakehacklemarkings(exampleseeninFig.1c)onthefusionglasslayerindicatethe directionofcrackpropagationasshownbytheblackarrows.Thesemarkingspointbacktotheedgesofthesemi-lunar fractureintheZrO2framework(redarrow).InFig.2thesemi-lunarfracturesurfaceoftheZrO2isshownfromacervicalview aswellasfromanangleshowingthegroundintagliosideofthezirconiaframework.Wakehackleandtwisthacklelinesare

Fig.1.Case1.Atrilayered(LS2-glass-ZrO2)crownifshownwithafractureonthemargininvolvingtheZrO2frameworkandpartoftheLS2overlay.Thered arrowpointsthefractureorigin.IntheSEMoverviewofthecrownthedirectionofcrackpropagation(dcp)isindicatedinblackarrows.Differentrelevant regionsaremagniﬁed.In(a)aregionontheocclusalsurfaceoftheoverlayisshownpresentingtracesofocclusalwearexposingthemicrostructureoftheLS2

material;Fig.1bshowsacrackrunningthroughthefusionglasslayerthatconnectstheoverlaytotheframeworkIn(c)wakehacklelineshelptodeﬁnethe dcpinthefusionglass.Fig. 1disabackscatteredelectronimageofthecenterofthedelaminatedarea,showingthatthefractureruneitherintheglasssolder layeroratitsinterfacewiththeZrO2framework.

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identifiedindicatingthedirectionofcrackpropagation(blackarrows).Inthecenterofthesemi-lunarfracturesurface(black rectangle)asemi-ellipticalcrack(whitearrows)isdirectlyrelatedwiththebottomintagliosurfaceoftheframework.Higher magnificationsinFig.2showinsidethisfracturedsurfaceasemi-ellipticalcriticalcrackatthebaseoftheframework.An evenhighermagnificationallowsmeasurementsofinitialandcriticalcracksizes,markedwithwhitearrows.Theinitial crackmeasuresapprox.20

m

^mⁱⁿ^length^and80

m

^mⁱⁿ^width^(aînitial).Ûnderânâpplied^tensile^stress^the^crack^grew^toâ

criticalsizeofapprox.130

m

^m^long^(a^critical)^and220

m

^m^wide.^Concentricârrest^linesâreîndicativeôfâhalt-and-go propagationofthecrackfront.Hacklelinesvisibleontheentirefracturesurfacerunnormaltothethicknessplane,fromthe internal(intaglio)surfacetowardtheglass-meltinterface.Theoverallcrackpropagationwithinthezirconiasemi-lunar fractureismarkedbytheblackarrowsbasedontherecognitionofhackleandtwisthacklefeatures.Thus,thefracturestarted withintheareaofthecriticalcrackinthecenterofthesemi-lunarzirconiafractureandpropagatedoutwardtowardthe zirconiamargins.Thecrackprogressedthroughtheweakerinterfaceforsomedistanceuntilbreakingthroughthethickness ofthelithiumdisilicateoverlay.ThecompressioncurlinFig.1indicatestheendofthefractureevent.

3.2.Case2

ThefractographicanalysisofCase2alsodealswithasemi-lunar-likefractureofthebuccalmargin.Thefracturespans fromthedistaltothemesialbuccalmargin(Fig.3).Acompressioncurllocatedonthebuccalmesialsidewaseasilyvisibleon thestereoandSEMgeneraloverviewimageandindicatesbending(ﬂexure)ofthestructure.Acompressioncurlrepresents theendofafractureeventunderbendingstressesandisnecessarilyboundwithanopposingsideofthefracturesurface subjecttotensilestressesinwhichtheoriginshouldbelocated(i.e.disto-buccalmargin).Theglobaldirectionofcrack propagationfromdistaltomesial(blackarrows)isconﬁrmedbythepresenceofhackle,wakehackle,andtwisthacklelines alongthefracturepath(Fig.3aandb).Particularlywithinthefusionglasslayer,wakehackleemanatingfromporesareeasily spottedandprovideaprecisemappingofthedirectionofcrackpropagation,tracingbacktotheoriginonthebucco-distal margin.InFig.4theexactfractureinitiationoriginisnotpinpointedbutanapproximately2mmwideand350

m

^m^deep

mirrorisseen(whitedashedlines)followedbymanyvelocityhackleextendingintothelithiumdisilicateoverlay.The localizeddirectionofcrackpropagationisindicatedwithblackarrows(zirconiaandLS₂)andredarrows(fusionglass).The distalmargin edgeincluded inthefracture initiationprocessis somehowrounded, whichis unusual.Noevidence of reshapingoftheintagliosidecouldbenoted.Justabovetheroundedzirconiaintagliocoremargintracesofthefusionglass (Fig. 3c and d) infiltrating thezirconia coreare an indicationof existing small cracks in thegreen statemilled and subsequentlysinteredzirconiaframeworkwiththefusionglassmeltinfiltratedpreexistingcracksduringthefusionfiring.

Thismayinfactclosesmallexistingcracksinthezirconiaframeworkbutitalsoindicatesthatmachiningofzirconiaisnot Fig.2.Case1.Cervicalviewofthesemi-lunarfractureontheframeworkmargin.Redarrowspointthefractureorigin.Blackarrowsindicatethedirectionof crackpropagation(dcp).Atthezenithofthesemi-lunarfracturesurfaceathesemi-ellipticalcrackcanbeobservedhavingconcentricarrestlinesindicative ofsubcriticalcrackpropagationduetocyclicloading.Inthispoint,seenfromtheinnerofthecrown,tracesofgrindingdamagefromdiamondburadjusting procedurescanbedetected.Thedirectionofgrindingcoincideswiththeorientationofthecrack.Fromthesemi-ellipticalcrackaninitialandcriticalcrack sizescanbeestimated.

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exemptofsmallsurfacedefectsunlesspolishedoffwhichisneverthecasewithframeworkscomingoutofCAD-CAMmilling units.Ontheocclusalsidenearthecompressioncurl,achipfracturepropagatedintotheLS2overlay(seeareaaboveFig.3a) asaresultofhighcontactloading.Thelocalizedfracturehoweverremainedconﬁnedwithinthelithiumdisilicateoverlay thicknessasseenbyseveralarrestlines.Thiscontactdamagefractureisconsideredasasecondaryeventoccurringafterthe mainfracture.

3.3.Case3

Sincetwofracturestookplaceatoppositesidesofadentalbridgeconstruct,itisassumedthatonecameﬁrstleadingto instabilityofthestructure andeventuallyleadtothesecondfracture(Fig.4).Sinceonefractureproduces usuallytwo matchingfracturesurfaces,themostreadablefragmentwasselectedforthefractographicanalysesalthoughallpieceswere cautiouslylookedat.

Fig.3.Case2.BuccalviewofthefracturedcrownonthegypsummodelandaSEMoverviewofthefracturesurface.Thefractureoriginatedontheright- handsideandpropagatedtowardstheleft-handsideoftheimage(photographandoverviewSEM).Redarrowspointthepossiblefractureoriginsites,on theouterorontheintagliosurfaceoftheZrO2framework.InFig.3aandbarrestlinesontheLS2overlayandZrO2framework,alongwithwakehacklelines ontheglasslayerconﬁrmthistrajectory,indicatedbyblacksarrows(dcp=directionofcrackpropagation).Figs.3canddshowalongcrackontheouter surfaceoftheZrO2frameworkthatwasinﬁltratedwiththeglasssolderduringthefusionsintering.Wakeandtwisthacklelinesindicatedthatthiscrackwas involvedinthefractureinitiation,whetherasoriginorbyweakeningtheframework.

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Thedistalfracture(ontooth#26),illustratedinFig.6bythefracturesurfaceinFragment1,showsﬁnehackleandarrest linesonthezirconiacoretracingthecracktrajectorybacktothecervicaldistalmarginatthebuccalsidenearthedistal bridgeconnector.Inthezirconiaframeworknoevidentsignofa preexistentdamageinducedorprocessingdefectwas detected.Afractureorigin(redarrow,Fig.6a)waslocatedatasharpangleofthezirconiaframeworkmarginonthedisto- buccalside.Onthezirconiaframework,thecrackranfromthedistalmarginmovingupwardtotheocclusalsurface,crossed themid-sectionof themolarsegment and ended atthepalatal margin.Justabovethe zirconiaorigin,anothercrack originatedintheouterfusionglasslayer(Fig.6b).Thecrackpropagatedalongthefusionglassparalleltothezirconiafracture pathwithinthethicknessoftheLS2butmovedupwardtoexitocclusallyasseenbythepresenceofacompressioncurl, indicatingthatsomebendingwasinvolvedin thefractureprocess.Hence,thefractureof thelithiumdisilicateoverlay occurredsimultaneouslytothezirconiafractureandoriginatedwithinthefusionglass.Itisworthnotingthatthefusion glassspilledoverthemargins,inﬁltratingaroundtheframeworkedgesandispresentinexcessattheoriginsite.Theouter surfaceofthelithiumdisilicateatthefracturesiteoftheglassisveryroughandmayhavecontributedtofractureinitiation.

Themesialfracture,illustratedinFig.7wasanalyzedonFragment2.Onthebucco-mesialfracturesurface,manyfine hackleandacleararrestlinesinthezirconiaframework(Fig.7b)indicatethatthecrackinitiatedandpropagatedfromthe oppositedisto-palatalside.Aclose-upviewofthisside(Fig.7a)showsacomplexfracturesurfaceexposingacrackedzirconia frameworkintwodirections,aninterfacefractureofthefusion-glassaswellasofthelithiumdisilicateoverlay.Onthatside, theotherfragment halfwas not matching,indicating some lostmaterial andtherefore somevaluablefracture origin information.Nevertheless, wakehackle linesonthe fracturesurface ofthe glasslayer indicatethecrack propagation direction(blackarrows)movingfromthemarginupwardstowardstheLS2overlay.Theoriginismarkedbyaredarrowvery closetotheedgeandontheintagliosideofthezirconiaframeworkwhichshowsgrindingmarksfromreshapingaswellas glassspillingover.Thecrackmovedwithinthethicknessofthezirconiaandlithiumdisilicateupwardtowardtheocclusal sideandcontinuedtopropagateasindicatedbythebackarrowsuntilexitingonthemesialmargin.Thepresenceofglassnot onlyspillingoverbutalsoinfiltratingthezirconiaintagliomayindicatethepresenceofexistingframeworkcracksnearthe edgebutalsoshowssomeprocessingissuestobemasteredincludingirregularwettingleavingvoids(pores)attheLS2-ZrO2 Fig.4. Case2.SEMmagnificationoftheright-handsidefractureedgeinFig.3.ThefracturemirrorisoutlinedbythedottedlineintheZrO2framework.Black arrowsindicatethedirectionofcrackpropagation(dcp)intheframeworkandintheLS2overlay.Redarrowsindicatethedcpinthefusionglasslayer.

Observetheroundededgeattheintagliosurfaceoftheframework,wherethefractureoriginated.

Fig.5.Case3.Occlusalviewofthefractured3-unitbridgeonthegypsummodel(Fig.5a).Twofracturesarevisible;themedialfractureonelement#24and thedistalfractureonelement#26,resultinginthreefragments,asindicatedontheimage.Fig.5bpresentstheviewofthemedialfractureformedby Fragments1and2whileFig.5ctheviewofthedistalfractureformedbyFragments1and3.

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interface.Grindingmarksin theinner surface of thezirconiaframework aretypicalfor reshapingprocedures bythe laboratorybecauseofseatingfrictionontheworkingmodel.Suchreshapingwithdiamondbursmayaddtodegradethe zirconiaandcreatesomevulnerabilityespeciallynearthemargins,wherehooptensilestressesusuallybuildup.

Thefractographymaynotprovideusananswerastowhichfractureoccurredﬁrst.However,someconsiderationsmaybe expressed.Hence,thejaggedfracturesurfaceofthemesialfracture(Fig.7)maybefrommultiplecrackingduetoalocalized instabilityofthepartiallybrokenstructurewhiletheposteriorpartofthebridgewasstillnotfractured.Somerocking movementuntiltotalfractureoccurredmayexplainthedifferenceinthefracturesurfacebetweenthemesialfractureof Fragment2(Fig.7)andthedistalfractureonFragment1.Unfortunately,noinformationastotheremovalprocessbythe dentistisavailablewhichmightalsobeinvolvedinadditionaldamage.

4.Causesoffailureanddiscussion

InallthreefailedtrilayeredLS2-glass-ZrO2ﬁxeddentalprosthesesrecoveredfromapoolof40fracturecasesreportedin [2],thefractureoriginwasfoundtobepreexistentsurface/marginalcracksintheZrO₂framework.Surfaceandmarginal defectsmayhaveresultedfromthemachiningofthegreenbody,butlargecrackinginthisstagewouldnothavebeen sustainedwithoutfurthercatastrophicfailureuponsintering.Mostprobably,thepre-cracksintheframeworkwereinﬂicted post-sinteringoftheZrO2bydiamond-embeddedrotatinginstruments(dentalburs)utilizedforreshapingpurposes.Classic grindingdamagewasfoundaroundpreexistingcracksinCase1and3.Theproblematicsurfacegrindingdamagehasbeen recentlyexploredfordentalZrO2[3],showingthatdefectsupto28

m

^mⁱⁿ^depth^may^be^generated^merely^from^grinding

with75

m

^m^diamond^particles,potentiallyreducingthestrengthofZrO2bynearly40%.

InCase1thefinalshapeoftheframeworkfracture,originatingfromagrindinginducedflawontheintagliosurface,along withthedirectionalmarkingsobserved,stronglysuggestaloadingscenarioofalocalizedstressconcentrationattheintaglio surfacenearthemargin.Conceptually,adentalcrownissupportedbyanabutmenttoothusuallyhavingaslightconical geometrywiththebasetowardthemargin.Consequently,thecircumferentialmarginaledgeexperienceshoopstresses Fig.6.Case3.ViewofFragment1distalfracture(bottomleft)andSEMoverview.Theredarrowpointstheoriginofthisfracture,attheframeworkmarginof element#26closetothedistalconnectoronthevestibular(orbuccal)side.Blackarrowsindicatethedirectionofcrackpropagation(dcp).Fig.6aisa magnificationoftheoriginsiteintheframework,showingthatthefractureinitiatedfromasharpcornerintheframeworkmargin.Noclearstructuraldefect istobeseenthere.In(b)theoriginoftheoverlayfractureisshownrightabovetheoriginsiteintheframework.Fracturemarkingsin(c),(d)and(e)servedto identifythedirectionofcrackpropagation.

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uponocclusalloading,andanon-uniformmarginaladaptationmayconcentratestressesinareaswherethegapbetween toothandframeworkisnarrower.Thetrajectorytowardthemargindespitethecrackhavingitslong-axisparalleltoit, supportsthistheory.Addedtothat,hacklelinesshowthatthecrackpropagatedconcurrentlytowardtheouterframework surface,anindicationthattheintagliosurfacewassubjectedtotensilestressesalsointhecervico-occlusaldirection.

Withthedimensionsoftheinitialand criticalcracksizesinhandaiandac,respectively,thestressesnecessaryfor inducingunstablefracturecanbeestimatedas:

s

ⁱ¼ KIc

Y_ip ¼ffiffiffiffia_i 4:5 1:55 ffiffiffiffiffiffiffiffiffiffiffiffiffi

20

m

p ¼649MPa ð1Þ

and

s

^c¼ KIc

Ycp ¼ffiffiffiffiffiac 4:5 1:2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

130

m

p ¼328MPa: ð2Þ

whereMPa)K_Icistheframeworkfracturetoughness(approx.4.5MPapmfor3Y-TZP[4–6])andYthegeometryfactorof semi-ellipticalsurfacecracks.Thearrestlinesinthesemi-ellipticalcrackconcentrictotheinitialgrindingdamagearetypical signsofsubcriticalcrackgrowth,duringwhichtheinitialcracksizeaigrewtoac,thecriticalcracksizebeforeunstable fracture. Consequently, the maximum applied stress during the loading cycles,

s

appl,max, assuming constant stress amplitude,remainedbelow

s

ⁱ^,^probablyⁱⁿ^the^range³²⁸^MPa

s

^appl,max<648MPa.Becauseachangesasitgrows,Ymust alsochange(calculatedusingtheNewmanandRajuequation[7]).Acleardepreciationofthestructuralstrengthofthe framework resulted fromthe grinding damage (typical strength of 3Y-TZP is 900–1100MPa). This has also a severe implicationtothefatiguelifeoftheconstruct.Duetoitssusceptibilitytostresscorrosioninahumidenvironment,3Y-TZP canundergosubcriticalcrackgrowthatstressintensitylevelsbelowthecriticalstressintensityfactorforunstablefracture, withthethresholdKtharound0.45KIc[8].Thismeansthatsubcriticalgrowthoftheinitialgrindingdamageaicouldhave Fig.7.Case3.ViewofFragment2distalfracture(bottomleft)andSEMoverview.Thiswasthemainfractureevent.Theredarrowpointsthelocationofthe fractureorigin.Blackarrowsindicatethedirectionofcrackpropagation(dcp).AtthebottomleftcornerofFragment2,anareaofextensefragmentationisto beseen.Fig.7aisamagniﬁcationofthisarea,andshowsthepresenceofthefusionglasscoveringthebulkcross-sectionoftheframeworkextendingallthe waytotheintagliosurface,wheretracesofgrindingarevisible.Othercracksandsurfacedefectsarepresentaroundthisarea.In(b)anarrestlinehelpsto determinethetrajectoryofthecrackfromtheleft-handsidetotheright-handsideoftheimage.

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startedatstressvalueslowerthanhalfthatof

s

^c^,âround²⁹²^MPa,â^stress^magnitude^that^falls^close^to^thatôf

s

^c^.^A^realistic

loadcyclewouldattainitsmaximum

s

âppl,max^slightly^higher^than³²⁸^MPa^down^to⁰^MPaâtûnloading.

InCases2and3pre-crackswerefound,respectively,ontheouterZrO₂surfaceandonthemarginextendingtothebulk.

Thesewereonlydetectedduringfractographybecausetheywerefilledwiththefusionglass,whichinfiltratedthecracks duringthefusionsintering.Incase2theglassmeltinfiltratinglocallythezirconiaattheinterfacewasnotinvolvedwiththe originofthecrack.Howeverincase3,thefusionglassattheimmediatevicinityofthecrackoriginatthemesialmarginwas connectingwithaveryroughoutersurfaceofthelithiumdisilicateoverlay(Fig.6b).Thisleadsonetowonderhowthe polishingprocessisdoneinthelaboratoryafterthefusionglassfiring.Fromafractographyperspective,thefusionglasslayer, duetoitshighporecontentandlackofmicrostructure,showedvaluablesurfacemarkings,especiallywakehacklelines, whichhelpeddefinecrackpropagationdirections.

Asallfractureoriginsdevelopedatthezirconiaframework,theprocessinganddesignplaysamajorrole.Hence,several processingweaknessescanbenoted.First,thezirconiaframeworkshowedsystematicallygrindingmarksfromreshaping withdiamondburs,inclosevicinitywiththecrackorigin.Second,poorwettingandpresenceofporositiesinthefusionglass combinedwithirregularthicknessesandspillingoverinsidethezirconiaframework(intaglio)areindicatorsofdifﬁcultiesin controllingthequalityofthefusionprocess.Thefractureoftheframeworkledinevitablytothefractureoftheoverlay constructduetohighstressesatfailure,aspredictedbyinvitroexperiments[9],withrarecasesofinterfacialdelamination.

Inthatparticularstudy,LS2-glass-ZrO2bendingbeamsyieldedstrengthvaluescomparabletomonolithicZrO2.Here,the trilayerednatureoftheconstructscannotbeaccountedforanystructuralcompromise.Yet,comprehensivethermalstress assessmentsarelackingand shouldbepursuedfora betterunderstandingofinternalstressesin suchconstructs. The differencesinclinicalfractureratesrecordedbetweensingle-unitand3-to5-unitprosthesesin[2]mightberelatedtoa higherprobabilityofdefectstakingplaceinmulti-unitconstructsduetothehigheramountofsitestobesubjectedto processingdefects,irregularfusion-glasslayer,thermalresidualstressandgrinding.Reshapingisusuallyundertakenonthe intagliosurfaceduetoapoorﬁttosupportingabutments,aknownprobleminmulti-unitprostheses.

5.Conclusionsandrecommendations

Manydentalprosthesesfailduetoprocessingdefectsintroducedduringfabricationandpriortoinstallation[10–12].The threecasespresentedhereshoweddefectsintheZrO2whichledtothefracturesunderserviceatsigniﬁcantlyearliertimes thanforecastedforundamageddentalzirconia[8].Thishighlightstheapparentlackofqualitycontrolduringtheprocessing ofZrO2exercisedbytechnicians/dentists,especiallyregardingsurfacestate,grindingzirconiaceramicsasifequivalentin handlingasmetals.Frameworkdimensionshavingverythincross-sectionsarecontributingtothis scenario.Ifsurface damageisnottobeprevented,thicker–or“anatomical”–frameworkgeometriesshouldbeemployedtoassureextended lifetimes.Theglassmelttofuseazirconiaframeworkwithanestheticandstrong lithiumdisilicateoverlaymayneed improvementsifthistechnologyistobecontinuedtobeuseindentallaboratories.

Themaindifﬁcultyofexecutingfailureanalysisofcasesthatareprovidedbyamillingcenterinreturnofclaimsfrom dentistsisthatnoinformationisavailableregardingthedescriptionofthefractureeventbythepatient,northetechnique usedtorecoverthebrokenpartsinsitu.Thesearevaluabledescriptionsthathelpinterpretthefracturesurfaceandfracture process.Inaddition,dentaltechniciansarerarelyincludedinthefailureanalysis,whoshouldinthefuturebeaninteractive discussionpartner.

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