Fractography of clinical failures of indirect resin composite endocrown and overlay restorations

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Article

Reference

Fractography of clinical failures of indirect resin composite endocrown and overlay restorations

SARATTI, Carlo Massimo, et al.

Abstract

tObjectives. Compare failure modes and fracture origins using fractography on recovered clin-ically fractured parts of indirect resin composite endocrowns and overlay restorations onendodontically treated teeth (ETT).Methods. Four endocrowns (3 molars, 1 premolar) and one overlay (molar) adhesively lutedon ETT were recovered after fracturing during function.

The time in service ranged between4 and 48 months. The composite materials were (i) CAD/CAM LAVA Ultimate (N = 1), (ii)Premise Indirect (N = 2), and (iii) Colombus (N = 2).

Fractography was performed by means ofdigital microscopy and SEM. Occlusal surfaces were checked for signs of fatigue degradationand contact wear. Cuspal plane angles were measured from profiles obtained from 3D digitalmicroscope images with respect to the horizontal plane of the occlusal central crown groove.Results. All five cases showed a wedge-opening mode I fracture, splitting the crown and toothin two parts through the crown's central groove. Classic brittle fracture features (arrest lines,twist and wake hackle) were easily identified on the fracture surfaces. Multiple origins werelocated [...]

SARATTI, Carlo Massimo, et al . Fractography of clinical failures of indirect resin composite endocrown and overlay restorations. Dental Materials , 2021

DOI : 10.1016/j.dental.2021.02.002 PMID : 33627233

Available at:

http://archive-ouverte.unige.ch/unige:150550

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

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dental materials 37 (2021)e341–e359

Availableonlineatwww.sciencedirect.com

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j ou rn a l h o m epa ge :w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a

Fractography of clinical failures of indirect resin composite endocrown and overlay restorations

Carlo M. Saratti

^a

, Giovanni T. Rocca

^a

, Stéphane Durual

^b

, Ulrich Lohbauer

^c

, Jack L. Ferracane

^d^,^e

, Susanne S. Scherrer

^b^,∗

aDivisionofCariologyandEndodontics,UniversityClinicsofDentalMedicine,UniversityofGeneva,Geneva, Switzerland

bDivisionofFixedProsthodonticsandBiomaterials,UniversityClinicsofDentalMedicine,UniversityofGeneva, Geneva,Switzerland

cResearchLaboratoryforDentalBiomaterials,DentalClinic1,UniversityofErlangen-Nuernberg,Erlangen, Germany

dDepartmentofRestorativeDentistry,SchoolofDentistry,OregonHealth&ScienceUniversity,Portland,OR,USA

eDivisionofBiomaterials&Biomechanics,OregonHealth&ScienceUniversity,Portland,OR,USA

a r t i c l e i n f o

Keywords:

Fractography Composite Fracture Restoration Crown Clinical Fatigue Wear ModeI

a bs t r a c t

Objectives.Comparefailuremodesandfractureoriginsusingfractographyonrecoveredclin- icallyfracturedpartsofindirectresincompositeendocrownsandoverlayrestorationson endodonticallytreatedteeth(ETT).

Methods.Fourendocrowns(3molars,1premolar)andoneoverlay(molar)adhesivelyluted onETTwererecoveredafterfracturingduringfunction.Thetimeinservicerangedbetween 4and48months.Thecompositematerialswere(i)CAD/CAMLAVAUltimate(N=1),(ii) PremiseIndirect(N=2),and(iii)Colombus(N=2).Fractographywasperformedbymeansof digitalmicroscopyandSEM.Occlusalsurfaceswerecheckedforsignsoffatiguedegradation andcontactwear.Cuspalplaneanglesweremeasuredfromproﬁlesobtainedfrom3Ddigital microscopeimageswithrespecttothehorizontalplaneoftheocclusalcentralcrowngroove.

Results.Allﬁvecasesshowedawedge-openingmodeIfracture,splittingthecrownandtooth intwopartsthroughthecrown’scentralgroove.Classicbrittlefracturefeatures(arrestlines, twistandwakehackle)wereeasilyidentiﬁedonthefracturesurfaces.Multipleoriginswere locatedalongthecentralgrooveinconjunctionwiththepresenceoffatiguecracks.Contact wearsurfacesshowedpittingandcracking.Cuspalplaneangleswerearound30–35^◦,except a50^◦palatalcuspslopefortheLavaUltimateoverlay.

Signiﬁcance.Fractographyonclinicalfracturesofresincompositeswasenlightening.Occlusal surfacefatiguedegradationfromcyclicloading,modeIfracturefromappliedmastication forcesoncuspalplanes,andstressconcentrationwithinthecrown’scentralgroove,indicate limitationsofuseofthesematerialsforendocrownsinposteriorteeth.

).

∗ Correspondingauthor.

E-mailaddresses:carlo.saratti@unige.ch(C.M.Saratti),giovanni.rocca@unige.ch(G.T.Rocca),stephane.durual@unige.ch(S.Durual), ulrich.lohbauer@fau.de(U.Lohbauer),ferracan@ohsu.edu(J.L.Ferracane),susanne.scherrer@unige.ch(S.S.Scherrer).

https://doi.org/10.1016/j.dental.2021.02.002

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1. Introduction

Endocrowns and overlays have been proposed to restore endodonticallytreatedteeth(ETT),relyingonadhesivetech- niquesasopposedtomoreinvasivepostandcorebuild-ups forretention[1–3].Bothendocrownsandoverlays(Fig.1)are fullcuspcoveragerestorationsofthemissingcrownsurface.

Anoverlayrestoresonlythecoronalpartofthetooth,while thepulpchamberisfullyﬁlledbyadirectlystratiﬁedresin- compositematerialbondedtothedentin.Anendocrownis acrown withacentralextensioninsidethepulpchamber.

Retentive undercuts ofthe pulpchambermay beremoved bythepreparationorpreviouslyﬁlledwithresin composite beforeadhesivelutingoftherestoration.

Asystematicreview[4],andtwo10to12-yearretrospec- tiveclinical evaluations[5,6] have been recently published supporting the use of endocrowns for rehabilitation of posterior ETT. Materials such as feldspar-based ceramic [6–8],polymer-inﬁltratedfeldspar-basedceramic[5],lithium- (di)silicate glass-ceramics [5,9] as well as resin-composite materials [10], have been clinically used for endocrowns.

Recently,developmentsinparticulateﬁlledresincomposites

Fig.1–Endocrownandoverlayrestorationschematicwith respecttopulpchamberbuild-up.

[11]havebroadenedtheiruseforindirectrestorationsowing toimprovedstrength,toughnessandfatiguebehaviorwhile maintainingamodulusofelasticitysimilartothatofdentin [12–15].Besideshavingacceptablemechanicalproperties,the mainadvantagescitedforresin-composite restorationsare easeofprocessing(indirectorCAD/CAMmade),highpolisha- bility,aswellasin-siturepairabilityincaseofaclinicalpartial fracture[16,17].

Theliteraturecontainsawealthofinvitrotestinginfor- mation regardingthese novelresin compositesfor indirect orCAD/CAMcrownrestorations,butclinicalfractureevents, althoughreportedinsurvivaldatafromlongitudinalstudies [5,18],arerarelyfractographicallydocumentedandanalyzed.

One exception is a recent paper by Lohbauer et al. [19]

reporting on fractured implant-supported CAD/CAM resin- compositecrownreconstructions.Fracturesare wellknown adverse outcomes for resin-composites in Class I and II restorationsand were reported tobethe mainreasons for failure, along with recurrentcaries,in asystematicreview andmeta-analysis[20].Therefore,whenin-servicefailureof largeadhesivelylutedresin-compositerestorations,suchas endocrowns, involve a fracture through the pulpchamber resultinginalossofthetooth,thevalueofacarefulfailure analysisoftherecoveredcrown-toothpartsinordertounder- stand the circumstances aroundsuch acatastrophic event becomesreadilyapparent[21,22].Asalreadywellexplained andoftenimplementedforceramicrestorations[22–25],fail- ureanalysisofrecoveredfracturedpartsusingfractographic means,suchasstereomicroscopyordigitalopticalmicroscopy with largedepth-of-ﬁeldcombined withSEM, provides key information astothe fracture mode, fracture origin(s) and direction ofcrack propagationbased on the recognitionof fracturesurfacemarkings.Therequestforrecoveredpolymer- based failed restorationsforfurther analysisand attempts tocorrelatethisinformationwithinvitrotestoutcomeshas

Table1–Recoveredindirectcompositeresinrestoration:brand,manufacturer,processing,composition,toothlocation (FDI/universalnumberingsystem,Uni)andmonths(mths)inservicebeforefracture.

Case Brand,manufacturer,processing

&restorationtype

PolymerizationandComposition FDI/Uni Mthsin service 1 LAVAUltimate

(3MESPE)

laboratory-processed CAD/CAMoverlay

Heatpolymerized,highlycross-linkedUDMAresin matrixreinforcedby79wt.%ofzirconia-silicaclusters (0.6–10␮m)+zirconia(4−11nm)+silica(<20nm).

27/15 4

2 PremiseIndirect(Kerr) laboratory-processed endocrown

Lightcured+heatpolymerizedbis-GMAandTEGDMA matrixreinforcedby84wt.%ofnano-hybrid

pre-polymerizedﬁllers(PPF)+ﬁneparticle barium-basedglass+nanosilica

47/31 47

3 PremiseIndirect(Kerr) laboratory-processed endocrown

Lightcured+heatpolymerizedbis-GMAandTEGDMA matrixreinforcedby84wt.%ofnano-hybrid

pre-polymerizedﬁllers(PPF)+ﬁneparticle barium-basedglass+nanosilica

26/14 39

4 Colombus

(CendresetMétaux) laboratory-processed endocrown

Lightcured+heatpolymerizedbis-GMA/UDMA monomercontaining77wt.%ofbariumglassﬁllersand pyrogenicSiO2

47/31 48

5 Colombus

(CendresetMétaux) laboratory-processed endocrown

Lightcured+heatpolymerizedbis-GMA/UDMA monomercontaining77wt.%ofbariumglassﬁllersand pyrogenicSiO2

14/5 15

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beenmadebyseveralauthors[21,26–28].Additionalinforma- tionregardingthecircumstancesofthefractureevent,timein service,toothlocation,crowndesignandprocessingmethod allaidinformulatingplausibleexplanationsforsuchunfa- vorableoutcomes.Forparticulatereinforcedresincomposite restorations,evidenceofdamageintheformofabrasivewear orfatigue(micro)crackingshouldbeinvestigatedonthecyclic loadedocclusalsurface,asthesephenomenawillgreatlycon- tributetotheoverallreductionofthemechanicalproperties ofthematerial[29,30].

Thispaperreports,fortheﬁrsttimeinthedentalliterature, extensivefractographicfailureanalysisofrecoveredclinical fracturedpartsofresincomposite endocrownsand overlay restorationsadhesivelylutedonendodonticallytreatedteeth.

Thefailureanalysiscomparesfracturemodes,crackorigins andlocations,surfacecontactwearandfatiguedegradation for three different resin-composite materials processed by eitherindirectlayeringtechniqueorCAD/CAM.Findingsare discussedand explainedwithrespecttobiomechanicsand material science considerations relevant to resin composites.

2. Materials and methods

2.1. Recoveredfracturedcrownparts

Four endocrowns (3 molars, 1 premolar) and one overlay (molar)adhesivelycementedonendodonticallytreatedteeth whichfracturedduringfunctionwererecoveredbythetreat- ingdentistsanddonatedtotheUniversityClinicsofDental Medicine of the University of Geneva for failure analysis.

The recovered endocrowns were indirect (layered) resin- compositeswhereas the overlay was anindirect CAD/CAM processedrestoration. Thetimes inservice before fracture rangedbetween4and48 months.Table1summarizes the recoveredrestorationsspecifyingbrandname,manufacturer, composition,processing,toothlocation(FDI/universalnum- bering systems) and months in service. When available, additionalinformationsuchasintra-oralphotographofthe insitufracturedrestoration,adhesivelutingprocedure,aswell asthecircumstancesofthefractureeventasdescribedbythe patientwerecollected.

Fig.2showsinsituimagesoftwoindirectresin-composite restorations split in half through the anatomical central groove: Case1(Fig. 2a) isaCAD/CAM made overlay (LAVA Ultimate)whichfracturedafter4months;Case4(Fig.2b,c) isanindirectcompositeendocrown(Colombus)whichfrac- turedafter48months.Inthisparticularcase,acarefulopening withanexplorer(Fig.2c)ofthecrackedendocrownrestora- tionwasneededforrecoveringofthelooserlingualfragment.

Suchdocumentationhelpsinidentifyingpossibledamageon thefracturedsurfacefromtheexplorer.

Allﬁve fracturecases involvednotonlythe restoration, butalsothetooth-rootstructureending,withatotallossof thetoothwhichhadtobeextracted.Whenbothhalvesofthe failedrestorationwereavailable,fractographywasperformed oneachofthem.

Fig.2–Insituclinicalimagesoftwoindirect

resin-compositerestorationssplitthroughthecentral groove.Fig.2a:CAD/CAMoverlay(LAVAUltimate)(Case1);

Fig.2b:Indirectcompositeendocrown(Colombus)(Case4).

Inthisparticularcase,acarefulopeningwithanexplorer (Fig.2c)ofthecrackedrestorationwasneededfor recoveringofthelooserlingualfragment.Such documentationisimportantasithelpsinidentifying possibledamagefromtheexploreronthefracturedsurface.

2.2. Fractographicfailureanalysis

Therecoveredfracturedparts (restorationwithtooth)were firstcleanedanddisinfectedinanultrasonicbathfor3min with5%sodiumhypochlorite,followedby3minindistilled waterandfinallyoneminuteinpureethanol.Astereomicro- scope(OlympusSZX9)andadigitalmicroscopewithalarge depthoffield(KeyenceVHX5000)wereusedfortheinitialdoc- umentation.Thefailureanalysisincludeddeterminingmode of failure, and identifying characteristic fracture markings (arrestlines,wakeandtwisthackle)onthefracturesurfaceto aidindeterminingtheoriginsoffracture.Inaddition,evidence ofocclusalsurfacecontactwearandsignsoffatiguecracking were recorded. Transilluminationwith an external quartz- tungsten-halogenlightsourcewasusedtorevealthepresence

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ofsuchfatiguecracksadjacenttoareas ofocclusalcontact wearandthefracturesurface.Apreliminarymappingofthe observedfeatureswasthensketchedout,indicatingareasof interestforfurtherhighmagniﬁcationanalysisutilizingthe SEM.Aftertheopticalmicroscopeevaluations,thespecimens weregluedonstubsandgoldcoated(20−100nm)fordetailed SEM(Sigma300VP,Zeiss)evaluationofthefractureorigins, characteristicsurfacecrackfeatures,andevidenceofocclusal surfacedegradationsuchaswear,fatiguecrackingandpitting.

3. Results

3.1. Fractographicfailureanalysis

Case1:CAD/CAM(LavaUltimate)resin-compositemolarNo.27/ 15overlay

The bonded overlay on tooth 27 had been made with afully digitalchair-side method, four monthsprior tothe fracture eventusing aCAD/CAMresin-composite bloc(LU).

Theinternalpulpchamberwasbuilt-upwithanano-hybrid resin-composite(T)(TetricEvoCeramA3,IvoclarVivadent)and the restorationcemented withaphotocuring hybridresin- composite(Tetric,IvoclarVivadent)usinganetch-and-rinse adhesive system(Optibond FL,Kerr Dental). Regarding the circumstanceofthefracture, thepatientreportedbitingon somethinghardduringmastication.

Fig.3illustratesthefracturemodeI(crackopening)through the occlusal central anatomical groove of the composite CAD/CAM restoration. Major occlusal contactsare marked withred arrows(and dotsinthe inset photo)asidentiﬁed bystereomicroscopy. Onthe mesialproximal view,thered arrowsindicatetheapproximateappliedloadingdirectionon thecuspalplanesduringchewinggeneratingmaximumstress levelswithinthecentralgroovefromwhichawedge-opening fracturestarted.

Fig.4showsa3Ddigitalmicroscopyocclusalviewofthe overlayincase1(Fig.4a)withthefracturerunningthrough thecentralgroovesplittingthecrownandtoothintwoparts.

The red arrows point to the multiple fracture origins on the occlusalcrown surface.Thefracture surface(Fig.4c)of therecoveredbuccalcrown-toothpartshowsmultiplecrack origins(redarrows)followedbynumeroussemicircularcon- centric arrest lines created atsuccessive times as aresult ofmultipledistinctloadingeventspropagatingthecrackon differentplanes. Thecrackpropagationdirections(dcp)are indicatedbyblackarrowsandrunapicallytowardstheroots.

TheLavaUltimate(LU)overlay’sthicknessis,initsthinnest part,2.3mm.Underneaththeoverlayrestorationisabuild- uplayerofresincompositeTetricCeram(Tetric)ﬁllinginpart ofthetoothpulpchamber.Transillumination(Fig.4b)reveals occlusal surfacefatiguecracks extendingalong the central mesio-distalgroove.Ontheocclusalcrownsurface,adom- inant oblong-shaped contact wearfacet of20 × 30 mmis visibleonthedisto-buccalcuspalplane(Fig.4a–c,)aswellas asmalleroneonthemesio-buccalcuspplane(Fig.4c).These wearsurfaceswillbefurtherdiscussedinFigs.5and6.

Fig.5showsSEMphotomicrographsofthebuccalfractured partoftheLavaUltimateoverlay.Redarrowsindicatemultiple fractureoriginslocatedwithinthecentralgroove.Directionof crackpropagation(dcp)movesapically.Highermagnifications ofthefatiguecracksinthevicinityoforiginNo1(whitebox)are illustratedinFig.5b,c.Someofthesecracksarepenetrating deep(>1mm)intothebulk(Fig.5a).Concentricarrestlines (al)spacedoutindicatingthatthecrackprogressedinstages, are visibleinFig. 5a. Inbetweenarrestlinesaremany fine twisthackle(th)whicharehacklethathaverotatedfromthe original crackplaneinresponsetoalateral rotationinthe axisofprincipaltension.Zone2onthemesio-buccalcuspal planeandzone3onthedisto-buccalcuspalplanearewear contactswithapittedappearanceasfillersfromthecomposite materialwereremovedbythewearprocess.Zone1(Fig.5a,b) correspondstotheunpolishedCAD/CAMmachiningmarks.

Further analysis ofthe fracture origin No1 isshown in Fig.6.Fatiguecracksarevisibleontheocclusalsurfacepene- trating∼200␮mdeepintothebulkofthecompositeoverlay withevidenceofsomecrackbranchingat90^◦.Multiplearrest lines(al)areshowingthestopandgoprocessoftheadvanc-

Fig.3–Case1showingafracturemodeI(crackopening)throughtheocclusalcentralgrooveofthecompositeCAD/CAM restoration.Majorocclusalcontactsaremarkedwithredarrows(anddotsintheinsetphoto)asidentiﬁedby

stereomicroscopy.Onthesideview(Mesial),theredarrowsindicatetheapproximateappliedloading(L)directiononthe cuspalplanesduringchewinggeneratingmaximumstresslevelswithinthecentralgroovefromwhichawedge-opening fracturestarted.

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Fig.4–Case1:LavaUltimate(LU)overlaywiththefracturerunningthroughthecentralgroove.Multiplefractureorigins(red arrows)(Fig.4a,b,c)arefollowedbynumerousconcentricarrestlines(al)alongthecrackpath(dcp)(Fig.c).Notethe occlusalfatiguecracks(Fig.b)andthelargewearfacetsonthecuspalplanes.Abuild-upofresincompositeTetricCeram (Tetric)constitutesarestorativebasebelowtheoverlay.

ingcrackprogressingapically.Multipletwisthackle(th)are presentbetweeneacharrestline.

Fig.7showsfurtherdetailsoftheLavaUltimatewearfacet onthedisto-buccalcuspalplaneinZone3(seealsoFig.4cand Fig.5a).Suchwearscarsareanongoingprocessofpolymer matrixcrackingandmatrix-ﬁllerinterfacedebonding(Fig.7c,

d)acceleratingthelocalizedlossofmaterialandincreasingthe pittingofthesurface.Theporousweb-likematerialinFig.7d maybeeitherpartofaresidualresinlayerandcohesivefail- ureinthematrixorsomesortoforganicdepositswhichwere noteliminatedbythecleaningprocessofthespecimenbefore addingathin20nmgold-coatingfortheSEManalysis.Poten-

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Fig.5–Case1(LavaUltimateoverlay).Multiplefractureorigins(redarrows)followedbynumerousconcentricarrestlines (al)alongthecrackpath(dcp).Fatiguecracksinthevicinityofthecentralgroovearepenetratingintothebulk.Zone2and3 ontheocclusalsurfaceshowwearcontactswithapittedappearance.Zone1correspondstotheunpolishedCAD/CAM machiningmarks.

tialdegradation/erosionoftheresinmatrixduetoexposure tooralandbacterialenzymescannotbeexcluded.

Case 2: indirect layered resin-composite molar No. 47 / 31 endocrown(PremiseIndirect)

Aswithcase1,thefracturesplitthecrownintwoparts through its central mesio-distal occlusalgroove in acrack openingmodeIfractureafter47monthsofintraoralfunction.

The recovered buccal part is viewed under a stereomicro-

scope(Fig.8a,b)anda3Ddigitalmicroscopeaftergold-coating (Fig.8c).Thelaboratoryhand-layeredendocrownisadhesively bondedtoapulpchamberbaselayerofhybridcomposite(Tet- ricEvoCeram).Twomainfractureoriginsmarkedbyyellow arrowswerelocatedontheocclusalsurface.CrackoriginNo 1isaprocessingholewithinthecentralgrooveofthecrown (Fig.8c).CrackoriginNo.2isconnectedtoanocclusalcontact damagedwearfacet(Fig.8b,c)onthe mesio-buccalcuspal

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Fig.6–Case1.DetailedviewoffractureoriginNo1inFig.5.Fatiguecracksarevisibleontheocclusalsurfacepenetrating

∼200␮mdeepintothebulkofthecompositeoverlaywithevidenceofsomecrackbranchingat90^◦.Multiplearrestlines(al) areshowingthestopandgoprocessoftheadvancingcrackprogressingapically.Twisthackle(th)arebetweeneacharrest line.

planeresultingfromocclusalcyclicloadingduringchewing (Fig.8b,c).Thefracturesurface(Fig.8a,c)showsmanyarrest linesnexttothefractureoriginsandtwisthackleattestingto thedirectionofcrackpropagation(dcp).

TheSEMimagesinFig.9a,bshowmoredetailsofcrack originNo1,aprocessingholeontheocclusalsurfaceatthe central groove from which wake hackle (wh, exiting from porosities)andtwisthackle(th)attesttothedirectionofcrack propagationdownwardstowardthetoothapex.Furtheraway onthemesio-buccalcusp,crackoriginNo2(Fig.9a,c)started fromamajorocclusalcontactasseeninFig.8b,ccontaining multiplefatiguecracksthatdevelopedundercyclicloading, resultinginaroughcrackedsurfacewithlossofcomposite material(Fig.9c,d).Fracturestartedfromoneofthesefatigue cracks and propagated in stepsas seen from the multiple arrestlinesandtwisthackle(Fig.9c).

Fig. 9a shows that the endocrown was not perfectly bonded to the tooth structure as well as to the composite build-up within the pulp chamber. Evidence of these delaminationswaspresentinthestereomicroscope images obtainedbeforegoldcoating(Fig.8a),verifyingthattheywere notartefactsoriginatingfromthevacuumcoatingprocedure.

Fracturepropagationmayhavebeenfacilitatedwhenpass- ing through these weakened interfaces (endocrown/tooth;

endocrown/build-up;build-up/tooth).

Case 3: indirect layered resin-composite molar No. 26 / 14 endocrown(PremiseIndirect)

Thefractureofthisrestorationoccurredafter39months duringmasticationandsplittheendocrownandtooththrough the central groove in a wedge opening type of fracture (Fig.10a).Forthisrecoveredpalatal(P)partwewillfocusonthe disto-palatalportionofthefracturesurface,astoomuchofthe restorationwaslostinthemesio-palatalapproximalregion duetotheextractionprocess.Thedisto-palatalpartshowsthe fractureorigin(yellowarrow)startingfromamajoroccluso- distalcontactweardamagedsurface(Fig.10a–c)characterized bymultiplefatiguecracks(Fig.10b,c).Onthefracturesurface, thecrackoriginisrelatedtofatiguecracksreachingupto400

␮mdeep intothe bulk(Fig. 10c).Oncethe fracturestarted, the crack path underwent several stops and starts as evi- dencedbythemultiplesemi-circulararrestlinescontaining finetwisthackleinbetween.Thedirectionsofcrackpropaga- tion(dcp)areindicatedbyblackarrows(Fig.10a).Onthedistal portion,thecrack movedfirst apicallyinadistaldirection, thenwasdeflectedmesially,beforemeetingtheothercrack frontarrivingfromtheoccluso-mesialsidebeforeresuming itspropagationtowardsthetoothapex.Majorchangesinthe crackingplanecanbeseenfromFig.10a,cwhichareoccurring duetovariousenergylevelsbeingimpartedtotherestoration duringthecyclicchewingimpacts,butalsofromthesupport- ingtooth structureredirectingthecrackpath. Thebuild-up

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Fig.7–Case1.HighermagniﬁcationsoftheLavaUltimatewearfacetonthedisto-buccalcuspalplaneinZone3(seealso Fig.5a).Suchwearscarsareanongoingprocessofpolymermatrixcrackingandmatrix-ﬁllerinterfacedebonding(Fig.7c,d) acceleratingthelocalizedlossofmaterialandincreasingthepitting.

compositebaseinthepulpchamberwasnotwellbondedto thetoothasseenfromthelargespacesatthemesialanddistal tooth-resininterface(Fig.10a).

Case 4: indirect layered resin-composite molar No. 47 / 31 endocrown(Colombus)

Theclinicalinsitufractureimagesofthiscaseareshown inFig.2.Thecrownsplitthroughthecentralgrooveinamode Ifractureduringchewingafter48months.Therecoveredlin- gualpartdocumentedinthefollowing3Ddigitalmicroscope andSEMimages(Fig.11)showstheendocrownbondedwitha self-adhesivelutingagent(RelyXUnicem)tothepulpcham- berandtoothstructurewithoutacompositebuild-uplayer.

Severalstartercracksareindicatedbyyellowandredarrows ontheocclusalsurfacewithinthecentralgroove.Themain crackorigin (redarrow)islocatedon thedisto-lingual cus- palplaneand connectedtoalarge oblong-shapedocclusal contact-loadsurface(Fig.11a).Twosmallerstartercracks(yel- low arrows) are visible on the mesio-lingual cuspal plane (Fig.11a)and propagatedtowardsthemesialmarginofthe endocrown.Close-upviewsofthefractureorigin(redarrow) (Fig.11b,c)showseveralspaced-outarrestlines(al)aswellas twisthackle(th)thatindicatethedcp(whitearrows)wasmov- ingdownwardstowardstheroots.Thecrackoriginisrelated toaprocessingholeontheocclusalsurface.

The pitted and worn occlusalcontact surface (Fig. 12a) fromwhichthemaincriticalcrackstarted(Fig.12b)contains veryfewmicrocracksincontrasttothepreviousPremiseIndi- rectcases2and3.Highermagniﬁcationofthefractureorigin (Fig. 12b) shows a smooth surface of the processing pore, fromwhichfracturestarted,differentfromthefatigued,pit- tedand worncontactsurface.Highermagniﬁcationsofthe pittedocclusalwearsurfaceshowslossofcompositematerial (Fig.12c)butwithrelativelyfewfatiguemicrocracks(Fig.12d).

Case 5: indirectlayered resin-composite premolarNo. 14 /5 endocrown(Colombus)

Thefractureoccurredofthisrestorationafter15months splittingtheendocrownandpartofthetoothintwothrough thecentralgrooveinmodefailureI(Fig.13a).Therecovered palatalpartshowsamainfractureorigin(redarrow)onthe occlusalsurfacerelatedtoaweakeningprocessingpore(0.5 mmindepth)(Fig.13b,c).Theinternalsurfaceoftheprocess- ingporeissmoothbecausetheresinispoorlycureddueto oxygenexposure.Onemajorarrestline(al)andmanytwist hackle(th)arevisibleonthefracturesurfacebelowthefrac- tureorigin (Fig.13b, c).Thedcp(blackarrows)moves from occlusaltoapical.Amajorocclusalcontactwearsurface(Fig.

a)indicatesarathercenteredloadingonthepalatalcuspal planeand wasnotincontactwiththe fractureorigin.The

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Fig.8–Case2.Mendocrown(PremiseIndirect)recoveredbuccalhalfafter47monthsofintraoralfunction.Twomain fractureorigins(yellowarrows)locatedontheocclusalsurfacearefollowedbymanyarrestlinesalongthecrackpath(dcp) onthefracturesurface.CrackoriginNo1isaprocessingholewithinthecentralgrooveofthecrown(Fig.8c).Crackorigin No.2isconnectedtoanocclusalcontactdamagedwearfacet(Fig.8b,c).

wear facet, as well as the extending fatigue crack on the occlusalsurface(Fig.13b),arefurtheranalyzedundertheSEM inFig.14.

Atlowmagniﬁcation(Fig.14a)theocclusalcontactwear surfaceshowspitting(littleholes).Thepittingisaresultof the lossof compositeand localized pull-out ofﬁller parti- clesduringcontactwearwiththeopposingtooth(Fig.14b) leavingarathersmoothvoidinthepolymermatrix.Further, verysmallporeswithinthepolymermatrixcontributetothe compositesurfacepitting(Fig.14b).Fromeachsideofthefrac-

tureorigin(Fig.13b),threechipfractures(Fig.14a)areseen.A fatiguecrackhasgrownhorizontallywithacurvature(Fig.13b, Fig.14a).Thetipofthefatiguecrack,magnifiedat10,000×in Fig.14cshowspolymercracking,crackbridging(br),transfiller fracture(tf),andfillerinterfacedebonding(fi))includingadis- lodgedfillerparticleinsidetheopencrack.Despitethelarge wearcontactsurface,fewlittlefatiguecracksarepresentcom- paredtothefindingsforthepreviouslydescribedcases2and 3.

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Fig.9–Case2(PremiseIndirect).DetailsofcrackoriginNo1(Fig.9a,b),aprocessingholeontheocclusalsurfaceatthe centralgroove.Wakehackle(wh),twisthackle(th)andarrestlines(al)attestthedcptowardthetoothapex.CrackoriginNo 2(Fig.9a,c)startedfromamajorocclusalcontact(seeFig.8b,c)containingmultiplefatiguecracksfromcyclicloading (Fig.9c,d).Fracturestartedfromoneofthesefatiguecracksandpropagatedinstepsasseenfromthemultiplearrestlines (al)andtwisthackle(th)(Fig.9c).

3.2. Cuspalplaneangles

CuspalplanesanglesweremeasuredforLavaUltimate(case 1)andColombus(case4and5).Proﬁlmeasurementsobtained from3DdigitalocclusalimagesareshowninFig.15.Fig.15a illustratestheLavaUltimatebrokenoverlay.Fig.15bindicates theproﬁlsectionsmadefromthecentralgroove(horizontal plane)tothecuspstips.RespectiveslopesforLavaUltimate were 35^◦ forthe mesio-buccal(MB) and disto-buccalcusps (DB)and50^◦ forthemesio-palatalcusp(MP).Measurements forColombuscase4areshowninFig.15c.Thecuspalplane anglesforthelingualrecoveredpartwererespectively35^◦for theMLand30^◦fortheDLcuspslopesontheirsteepestportion.

Fig.15drepresentstheColombuscase5palatalrecoveredpart withacuspalplaneangleof30^◦.ForthetwoPremiseIndirect compositeendocrowns,measurementcouldnotbeperformed withsufﬁcientconﬁdence.

4. Discussion

Brittlematerialssuchashighlyﬁlled resin compositeswill failwithlittletonoplasticdeformation.Fractureoftenoccurs catastrophically,meaningthattotalfailureoccursinasudden

andunexpectedway.Butfracture,beforebeingcatastrophic, mayalsooccurinstagesinwhichacrackwillgrowinsteps under cyclic loads ofvarying magnitudesand orientations suchasduringchewing,assistedbythepresenceoforalﬂuids.

Therefore,whenperformingafractographicanalysisonbro- kenpartsitisimportantdescribingtheoverallfracturepattern aswellasrecognizingthefractureorigin(s),thesource(s)from whichbrittlefracturebegan[22,31].

The fractographic failure analysis performed on four endocrowns and one overlay restored over endodontically treated teethrevealed several common key characteristics.

Firstly, allcases failedprimarilyin awedge-opening mode Ifracture throughthe centralocclusalgrooveofthecrown, splittingnot onlytherestoration, butthe supporting tooth structureaswell.Secondly,inallcasesmultipleoriginsoffrac- turewerelocatedattheocclusalsurfacewithinornearbythe centralgroove.Thirdly,largecontact-loadingareasoncuspal planesshowedsignsofwearandpittingtypicallywithsome fatiguecracks.

On the basis ofthese observations ofclinical fractures, the failureprocessesare discussedwithrespecttofracture mechanics theory and factors involved in the mechanical fatiguedegradationofdentalresincomposites.

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Fig.10–Case3(PremiseIndirect)molarendocrownwithtoothfractureafter39months.Ontherecoveredpalatalpartthe failureanalysisfocusesontheoccluso-distalfractureorigin(yellowarrow)startingfromamajorocclusalcontactdamage (Fig.10a–c)containingmultiplesurfacefatiguecracks(Fig.10b,c)someofwhichareextendingupto400␮mdeepintothe bulk(Fig.10c).Majorchangesinthecrackingplanecanbeseenfromtheblackarrows(dcp)Fig.10a,c.Thebuild-up compositebaseinthepulpchamberwasnotwellbondedtothetoothasseenfromthelargespacesatthemesialanddistal tooth-resininterface.

4.1. Fatiguecrackingdegradation

Fatigue degradation of resin-composites has been widely explainedin many key papers [29,32–34] and isthe major

cause of failure of a structure subjected to cyclic loading.

Theformationofmultiplefatiguecracks,inducingabreak- up ofthe composite structure with the resultant lowering ofthemechanicalproperties,hasbeenidentiﬁedwithinthe

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Fig.11–Case4:RecoveredlingualpartoftheColombusendocrown(molarNo.47/31)(seeFig.2c)whichfracturedafter34 months.Themaincrackorigin(redarrow)isrelatedtoaprocessinghole(Fig.11b,c)locatedontheDLcuspalplaneand connectedtoalargesaucer-shapedocclusalcontactsurface(Fig.11a).Twosmallerstartercracks(yellowarrows)arevisible ontheMLcuspalplane(Fig.11a).Multiplearrestlines(al)andtwisthackle(th)indicatethedcptowardsthetoothapex.

occlusalsurface.Duringtheearlystagesofocclusalfunction- ingoftherestoration,cracksofsubcriticalsizesareformed from repeated loading atlevels below the critical level for catastrophicfailureofthespeciﬁcmaterial.Thecrackscon- tinuetogrowandbranchduringthecyclicloadinginduced bychewing.Atsomepointinthefatigueprocess,theseini- tiallyseparatedindividualmicrocrackswillconnectwiththeir neighbors,forminglargermacrocracksextendingdeeper(i.e.

radialextension)intothebulkoftheresincompositerestora- tion. Thiswas clearly shown on the occlusal and fracture surfacesofCases1–3 (Figs. 5,6,10)whereocclusalfatigue crackspenetratedandevenbranchedinthesubsurfacebulk ofthecompositeresinmaterial.

Thisdeteriorationofthe materialduetosurfacefatigue crackingsignificantlylowerstheabilityoftheresincompos- ite towithstandfurtherchewing loadsand accelerates the processofcatastrophicfracture.Becausetherestorationsare constantlybathedintheaqueousoralenvironment,fluidscan penetrate throughthesurfacemicrocracksandbeabsorbed bytheresinmatrix.Theresultissomemicroscopicswelling ofthe resin matrix along with theformation ofinterfacial stressandhydrolysisofthesilane-mediatedbondbetweenthe inorganicfillerandthepolymermatrix[29,32,33,35].Evidence of water uptake of 43 ␮g/mm³ after 2 months of intraoral function wasmeasured on recoveredfractured partsof LavaUltimatecrownsbondedtozirconiaimplantabutments

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Fig.12–Case4(Colombus):SEMimagesofthemainfractureorigindescribedinFig.11.Theoriginisaprocessingpore(red arrow)(Fig.12a,b)havingasmoothsurface.Thepittedandwornocclusalcontactsurface(Fig.12a,c)containsveryfew microcracks(Fig.12d).

[19].Thus,crackformationmayresultfromacombinationof external(masticatory)damageaccumulation,internalresid- ualstressesinthemicrostructureofthecompositematerial fromprocessingaswellasfromhydrolyticdegradation[19,35].

Evidenceforthedeteriorationofthepropertiesofthecom- positehasbeenshowninseveralinvitrostudies.Forexample, theColombuscompositeusedinCase4and5,experienceda lossofstrengthof62%fromitsstaticcharacteristicﬂexural strengthvalueofS0=145MPawhensubjectedtoarotating- bendingcantileverfatiguetest(16.7Hzand10⁶cycles)under moistconditions.The50%survivalstress(S50)was54.6MPa [36].

Belli[37]showeda48%reductionfromtheinitialflexural strengthvalueof123MPaforLavaUltimate,thematerialused incase1,whenthecompositewasexposedto10⁴cyclesin aninvitrofour-point-bendingcyclicfatiguetest.Inanother study by Wendler[14], Lava Ultimate CAD/CAM disks lost closeto40% oftheirinitialstrengthwhensubjected to10⁵ cycles ofbiaxialloading(ball-on-three-balls). Other studies havedemonstratedalossofstrengthandtoughnessduring cyclicfatigueunderhydrolyticconditionswiththemagnitude beinginfluencedbythefillervolumefraction,fillerparticle sizes,qualityofthecross-linkingofthematrixandchemistry ofthemonomers[15,32,38–43].Theclinicalfracturesforcases

1–5occurred afterrespectively4,47,39,48and 15months.

Assuming3periodsof15minchewingperdayatarateof60 cyclesperminuteat1Hz[44],therespectiveloadingcycles would be324,000×(Case 1),3,807,000×(Case 2),3,159,000× (Case 3), 3,888,000× (Case 4) and 1,215,000× (Case 5). The extentofthelossofstrengthcreatedbytheclinicalfatigue degradationineachoftheclinicalcasespresentedcannotbe expressedwithcertainty,butconsideringthefatigue-induced micro/macrocracksvisibleontheocclusalsurface,aswellas thelargecontactwearfacetsformed,onecanhypothesizethat theserestorationsfailedmorerapidlythanexpecteddueto theiraccumulatedstructuraldegradation.

Fatiguefracturewilloccuroveravarietyofcracksizesasa functionofappliedstresses.Hence,failurefromasmallcrack willoccurunderhighappliedstress,whereas,fractureiniti- atedfromlargercrackswilloccurunderlowerappliedstress [29].

4.2. ModeIfracture(crackopeningmode)andfracture surface

Fracture mechanics are used to study fracture processes in materialsthat are eitherbrittleor have aductile-brittle behaviorwithsubstantialplasticity.Thelinearelasticfracture

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Fig.13–Case5:Indirectendocrownfractureonthe premolartoothNo.14/5after15months.Therecovered palatalpartshowsamainfractureorigin(redarrow)onthe occlusalsurfacewhichisaweakeningprocessingpore.On eithersideoftheoriginarechipdamages(Fig.13b)from whichafatiguecrackextends.Arrestlines(al)andtwist hackle(tw)areindicatingthedcp(blackarrows)(Fig.13b,c).

Amajorocclusalcontactwearsurface(Fig.13a)indicates therathercenteredloadingonthepalatalcuspalplane.

Again,thefailureisamodeIwedgeopeningtypewith propagationofthecrackfromocclusaltoapical.

Fig.14–Case5:Detailsofthecontactwearzonereveals pull-outofthereinforcingﬁllerparticles(Fig.14b)leavinga rathersmoothvoidinthepolymermatrix.Verysmallpores withinthepolymermatrixcontributetotheoverall

compositesurfacepitting(Fig.14a).Thetipofthefatigue crackinginFig.14a,magnifiedat10,000xinFig.14c,shows polymercracking,crackbridging(br),transfillerfracture(tf), fillerinterfacedebonding(fi)includingadislodgedfiller particleinsidetheopencrack.

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Fig.15–3DrepresentationoftherecoveredpartsforLavaUltimate(Fig.15a),Colombuscase4(Fig.15c)andColombuscase 5(Fig.15d).Cuspalplanesanglesmeasuredfromproﬁleswithrespecttothehorizontalplaneofthecentralgrooveas showninFig.15b,dprovidedanglesforLavaUlitmateof35^◦fortheMBandDBcusps,and50^◦fortheMPcusp.Colombus case4(Fig.15c)hadlingualcuspplanesof30^◦(DL)and35^◦(ML)ontheirsteepestportion.Colombuscase5(Fig.15d)hada palatalcuspplaneof30^◦.

mechanicsapproach(LEFM)dealswithbrittlefracture,deﬁned asoriginatingfromasharpcrackresultingfromprocessingor fromfatiguecrackingunderfunctionalcyclicloading.These small sharp cracks on the surface can rapidly grow when placedintensionorshearundervaryingamplitudeandalter- natingcyclicloads untilreachingacriticallengthatwhich thematerialwillfracturecatastrophically.Dentalparticulate- reinforcedresincompositesarehighlybrittleintheirbehavior [45],thusallowingapplicationofLEFMprinciplestostudytheir fracturephenomena.

ModeIfracturecorrespondingtoatensile stressnormal toaplaneofcrackingwasobservedforallﬁveclinicalfail- urecases.AcrackopeningfracturemodeIoccurredfromthe wedge-openingeffectoftheloadedcuspsfromtheantagonist toothoncethecriticalstressintensitylevelwasreachedatthe fractureorigin.Nevertheless,theloadingofthecrownscus- palplanesduringthechewingcyclesmayhavesomein-plane shearmodeIIloadingactingonthecrackatthesametimeit isunderopeningmodeIloading.Theinitialcrackorﬂawmay alsobeorientedatananglewithrespecttotheappliedcus- palload.Thus,itisverylikelythatacombinationofloading conditionsinmixedmodeI(normal)andmodeII(shear)have

actedintheinitialcrackpropagation.Thismaybeseenfrom thedifferentfractureplanesaftermajorarrestlinesaswellas thepresenceoftwisthackleformedundermodeI/IIloadings.

Attemptstodeducefracturestressusingquantitativefrac- tographywouldbeveryspeculativeasthecriticalcracklength is difﬁculttoidentifyforseveral reasons,including lossof informationduetobrokenoffmaterialattheorigin,possible R-curvebehaviorofthecomposite,andthepresenceofnumer- ousfractureorigins.However,evidenceofdeeppenetrating fatiguecracksintothebulk(400␮mupto1mm)indicates thattherestorationshadbeensubjectedtoquiteheavyloads duringchewing.

Arrest lines are observed from iterative loading creat- ingincrementalcrackgrowth[31].Onthefracturesurfaces, closely spaced-outconcentricarrestlinesnext tothecrack originsareindicatorsofamomentarystopofthecrackprop- agation. Afterresumingpropagation,the crackhas slightly changedplanesduetoachangeintheaxisofprincipleten- sion[31].Weassumethatduringcyclicfatigueloading,distinct loadingeventsreachathresholdenergythatcausesthecrack topropagateinstages,thoughtheenergyassociatedwithany oftheseeventsisnotsufﬁcientlyhightocausecatastrophic

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failure.Thismaybeanexplanationforthesenumerousreg- ularly spaced-out sharp lines visible inall molar fractures (cases1–4),regardlessofwhichresincompositereconstruc- tionmaterialwasused.

Manually layered endocrowns are prone to fabrication defects,mainlyairbubblesorpores.Hence,bothColombus cases4and5aswellasPremiseindirect(case3,originNo.1) hadprocessingporesof∼0.5mmdeep,whichactedascrack origins.Thesesmallcavities(i.e.holes)werealllocatedatthe occlusalsurfacenexttothecentralisthmusandsometimes indirectrelationwithcontactloadingsurfacedegradation.

These occlusal surface cavities, which remain unpolished aftercuring,mayalsobeincompletelypolymerizedduetothe presenceofanoxygeninhibitedlayerontheirsurface.Hence, theseprocessingdefectswillserveastheweakestlink,com- binedwithlocalized stress concentrationalong thecentral groovefromcuspalplaneswedgeopeningloadings.Evidence forthisexistsasthedegreeofconversionforColombuscom- posite(withabisGMA/UDMAmatrix)hasbeenmeasuredby FTIRtobe60%onthesurfaceand73%inthebulk[46].

AfracturetoughnessofKIc=0.56MPa√

mhasbeenreported forColombususingtheBraziliandisktest[46],whereasforthe heat-curedCAD/CAMblockofLavaUltimate,toughnessval- uesofK_Ic=1.14MPa√m(compacttensiontest)[14],and0.91 MPa√

m(notchlesstriangularprismNTP)[47]werereported.

Themanufacturer’sproductdescriptionindicatesaKIc=2.02 MPa√

m(SEVNB).Itisassumedthatthefracturetoughnessfor PremiseIndirectcompositewillliebetweenthatofColombus and Lava Ultimate, as it has a similar chemical composi- tionthoughaslightlyhigherﬁllercontentascomparedwith Colombus,butis,alsolightactivatedfollowedbyheatpoly- merization with a degree ofconversion of 58% after 24 h [48].Thesedifferencesintoughnesshoweverhavenotpre- ventedthecatastrophicclinicalfractureincludingthelossof thetooth.

Thepreferentialpathfollowedbythegrowingandpropa- gatingcrackfromtheocclusalfatiguedsurfacedemonstrated amixtureofpolymermatrixcracking,interfacialdebonding, cluster/ﬁllerfractureandevencrackbridging,allofwhichrep- resenttypicaltougheningmechanismsseeninmicrohybridas wellasnanoﬁlledcomposites[49].

Thefactthatdentalresincompositeswilldegradeintheir mechanicalpropertieswithmajorlossesofstrength(40–50%) afterbeingsubjectedtoonly10⁴⁻⁵cyclesunderfatiguetest- ing[14,37]requirescareinmaterialselectionaswellascrown design.Manyfactorswillincreasestressconcentrationand crackformation,suchascuspanglesandanatomicalgrooves inaV-notchshapeasdiscussedinthenextsection.

4.3. Cuspanglesandcentralgroovenotchdesign

ArecentpublicationbyShahmoradi[50]showedviaFEmod- elsofamonolithiccrowndesignthatcuspangles,thecrown’s centralgroovenotchdesign(sharpvsround)andthethick- nessoftherestorationgreatlyinﬂuencethecrack-initiation loadleadingtofracture.Their modelsclearlydemonstrated thatcuspangles≥40^◦(measuredfromthehorizontalplaneof thecentralgroovetothecuspalplane)createstressconcen- trationsinthecenterofthecentralgrooveforacontactload of150Nandacrownthicknessof1.5mm.Inourresearch,

thecuspanglesoftherecoveredfracturedcrownpartswere between30–35^◦ inmostofthe cases,exceptforthepalatal cuspofLavaUltimatewhichwasindeedquitesteep(i.e.50^◦).

Thesharpnessofthecentralgroovenotchcouldnotbever- iﬁed,asportionsofmaterialhadbeenlost,butforaCAD/CAM machinedcrownthecentralgrooveisneversharpduetothe limitations ofthe bur dimensions and machining process.

However,forthe artisanalmadecrowns,dependingon the layering techniqueused inthe laboratory, thecentral isth- musofthecrownscanbesharpandincorporatesomelarge processingporeswhichmayserveascrackorigins.Theﬁnal messagefromFEMliteratureisclear:formonolithicmaterials (allexceptzirconia)cuspslopesof20−30^◦(fromahorizontal grooveplane)aswellasaroundedcentralisthmusshouldbe incorporatedintothedesignofthecrown[50–52].

4.4. Compositecompositionandocclusalsurface degradation

Thecontactwearzonesontheocclusalsurfacewiththeantag- onisttoothwereeasilyidentiﬁedwiththestereomicroscope.

Whethertherewasexcessive wearcannotbeconcludedas noinitialevaluationwasavailable.However,themicroscopic analysis ofthe occludingsurfaceshowed localdegradation throughpitting,wearandsometimesmicro/macrocracking.

The differences between the ﬁve presented cases may be discussedinlightofthecompositionalandprocessingdiffer- encesofthematerials.

Increasingthefillercontentwithnano-particlesandclus- tersinthemicro-sizedrange,alongwithahighlycross-linked polymer matrix due tothe use of high pressure and heat forimprovingthedegreeofconversion,increasestoughness andstiffness[47,53].Hence,theresincompositewill,insuch formulations,behaveinamorebrittlemannerasmatrixdefor- mationislimited.Thismayexplainthepresenceofmultiple microandmacrocracksinthevicinityoftheisthmusnextto thefracturesiteforLavaUltimate.After4monthsofintrao- ral function, Lava Ultimate overlay showed pitted contact wearsurfaces resultingfrom breaking-offofmicroclusters offillerparticles,leavingmultiplelocalizeddeepcraterson theocclusalsurfaceatthecontactwearregion.Littletono microcrackingwasdetectedwithinthelargeslidingcontact surfaces,confirmingresearchfindingsfromaprevioussliding contacttest[54]forthismaterial.Unfortunately,noinforma- tionisavailableregardingtheantagonisttoothinvolvedinthe occlusalcontact.

Thelargeandsmoothlookingoblong-shapedcontactwear surfacesforbothColombuscases(4and5)maybeattributed tosomeinitialocclusaladjustmentswhichwerewellpolished, asnoobviousscratchesremainedonthesurface.Thepitted contactwearsurfacelookedsimilartothatoftheLavaUlti- materestoration.Theantagonistcontactingtoothhasenamel cuspsforCase4asveriﬁedfromavailableX-Rays.However, noinformationisavailableregardingtheantagonisttoothfor Case5.

PremiseIndirect(Case2and3),ahighlyﬁlledandcross- linked resin composite (84 wt.% [55]), showed very rough occlusalcontactsurfaceswithextensivecrackdamages.The existence of multiplewide open cracks most probably are attributedtohighercontactpressure andfrictionoccurring

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over3yearsofmechanicalfunction,acceleratingtheforma- tionofasurfaceandsub-surfacecracknetwork,drastically loweringthelifetimeoftherestoration.Theextensiverough- nessofthecontactwearsurfacesisduetoanongoingloss ofreinforcingﬁllerparticlesaccompaniedbybreaking-offof portionsofthepolymermatrix,leavingdeepopencracks.The availableX-Raysforbothcases2and3showedagainantago- nistteethwithenamelcusps.

Allthreeoftheresincompositematerialshavesimilarelas- ticmoduliconstants(around11−12GPa)andsimilarwt.%of ﬁllerparticles(77−84wt.%)[12,36,56].Somedifferencescer- tainlyexistintermsofdegreeofconversionwhichaddstothe pittingandmicro/macrocrackformation.Butthedominant degradation process willcome from the intraoral chewing conditions,contactpressure,occlusionimpactsandfrictional wear. Thefact thatthe strongest, toughestand best cured material,LavaUltimate,failedprematurelyafter4monthsis relatedtothewedge-openingforcesactingontherelatively steepcuspalplanes,andthefactthatitwasanoverlaywitha thicknessrangingbetween2.3–2.8mm,contrarytothemore voluminousandthickerendocrowns(∼3−4mm)intheother fourcases.

4.5. Clinicalconsiderations

Fractureofdirectlayeredcompositerestorationsonposterior teethhasbeenreportedtobeanimportantreasonforfail- urewithariskincreasefrom the secondyear onward[20].

For indirect CAD/CAM or laboratory layeredresin compositeendocrowns, thereare noclinical trialresults available.

Itis therefore more difﬁcult tobe more speciﬁc regarding thefailureanalysiswithalimitednumberoffracturecases.

Inaddition, allﬁvecasesinthis studywere locatedindif- ferentteethand presenteddifferentanatomies.Despite all beingclassiﬁableasadhesiverestorationstoETT,theywere allofdifferentthicknessesasaresultoftheirdifferentdesigns (overlayvsendocrown)orextensionsoftheendocoreinside the pulpchambers. Nevertheless, one should consider the effectivenessofETTwithresincompositesasthechoiceof reconstruction material in high stress regions inposterior teeth.

Asmentionedintheintroduction,theuseofnovelresin compositesforcrownreconstructionsonmolars,havebeen suggested[3,11].Themainargumentinfavorofresincom- positesvsceramicsarecosts,easeoffabrication,handlingand repairincaseofpartialfracture.

Overall,strength,fracturetoughness,andelasticconstants werenotverydifferentamongthethreeresincomposites.All resincomposites willdegradeassoon astheyare exposed to water and cyclic fatigue which is the case in the oral environment. This shows that for such large restorations improvementintoughnessduetoprocessinghasnotbeensuf- ficienttoavoidcatastrophicfracture.Althoughtoughnessisa highlyrelevantpropertytocharacterizedentalresincompos- itematerials[27],onlyaweakcorrelationwasfoundrelating toughnessandclinicalfracturesinamajorsystematicreview ofresincompositesonposteriorteeth[57].Linearelasticprop- erties (E, flexural ␴, KIc) were also shown to have limited clinical relevance. Initial properties were better associated withmicrostructuralfeaturessuchasfillerparticlesandpar-

ticleclusters,whereasfatigueresistancedependedmoreon thepolymericmatrix[15].

Thetoughestandstrongestmaterialoftheanalyzedfail- ure caseswasLavaUltimatewhichsince2015isno longer indicatedbythemanufacturerforcrownsbecauseofdebond- ingissues.Thefracturedoverlaywasperformedinearly2013, beforetherevisedrecommendationwasissuedby3M.

Colombuswhichwasonthemarketintheyear2000,was madespecificallyforindirectmolarrestorations.Endocrowns werenotspecificallymentioned.Cases4and5wereperformed in2002andthechoicetousethismaterialtoreconstructan ETTwasmadebytheclinicianusingavailableadhesiveluting techniques.PremiseIndirecthasbeenindicatedforcrowns andbridgeswith,however,thespecificrequestbytheman- ufacturertoincorporatetheirpolyethylenebraidedfibersto reinforcetherestoration.Thiswasnotdoneinthetwofrac- turedmolarcases(2and3).Fiberreinforcementismeantto strengthenthereconstructionandprovidetougheningmech- anismsofcrackarrestordeflection.Itcannotbesaidwhether thefracturedmolarreconstructionswouldhavesurvivedor atleastnotfracturedverticallywithultimatelythelossofthe toothhadfibersbeenusedinthesecases.Nevertheless,the positioningofthefiberbundlesornetiscrucialwithrespect tothecrown’sthickness.Atechnicalreportonhowtorecon- struct endodontically treatedteeth witha resin composite build-upofthepulpchamber,followedbyafibernetworkand thenanadhesivelybondedCAD/CAMresincompositeover- laywasdescribedbyRocca[58].Deeplocationofaglass-fiber net (StickNet)justabovethe compositepulpchambercon- tainingthecorebuild-up,wasshowntobeinefficienttostop theverticaldetrimentalfractureofLavaUltimateoverlaysof 3mmthickness,aswellasthetoothroot,althoughsomepar- tialcrackdeviationatthetoothhadoccurred[59].Considering thatall thefracturesanalyzedfractographicallyindicateda crackoriginationatthecentralisthmus,amoreocclusalposi- tioningofthe fiber netabove adome-shapepulpchamber compositebuild-upcould betterservethe purposeofcrack arrestordeflectionandavoidthelossofthetoothinthecase offractureduringmastication.

Itisbeyondthescopeofthispapertodiscusswhichmate- rialforendocrownreconstructionwouldbemostappropriate for endodontically treated teeth [3,4,11,60]. Unfortunately, clinicalevidenceonthesuitabilityofchairsideresincompos- itebasedCAD/CAMblocksiscompletelylacking.Despitethis lackofevidence,manufacturerstodaybroadlyindicatetheir materialsforthepreparationofsinglecrowns.Theendocrown indicationisnotalwaysspeciﬁedinrespectiveinstructionsfor use.

Attemptstocompare theperformanceofresincompos- itesrestorations(inlays,onlays,overlays)overceramicsusing a meta-analysis remained inconclusive due to the lack of standardizationorcriteriareported[61].Regardingceramics, lithiumdisilicateglassceramicswouldrepresentthematerial ofchoice,astheycanbewellbondedtotoothstructureand haveagoodcompromiseintermsofstrength(300−400MPa), fracturetoughness(2.2–3.0MPa√m)andelasticmodulus(100 MPa).

Itisworthreemphasizingthatthedominantforcesonthe cuspal planesofsuchreconstructions are critical,asthese are wedge-opening forces that form detrimental cracks at

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thecentralgroove.Hence,reconstructiondesignspossessing lesssteepcuspslopes,roundedcentralgroovesandocclusal contactmanagementtorelieve excessivechewing contacts shouldbethestandard.Certainly,basedonourﬁndings,devel- opmentofaresincompositematerialofferingimprovedwear andfatigueresistance,especiallyintheocclusallyloadedcon- tactregions,isdesirable.

5. Conclusion

Fractographic failure analysis of recovered clinically fractured molars and premolar endocrowns and overlay on endodonticallytreatedteethwassuccessfullyperformedwith stereomicroscopy, 3Ddigital microscopy and SEM analysis.

Fracture origins were located nearby the crown’s central isthmusontheocclusalsurfaceswithcatastrophiccrackprop- agation towardsthe root, splittingthe restoration and the tooth.Theresincompositereconstructionsfailedfromacom- binationofseveralfactors:(a)fatiguedegradationfromcyclic loading,(b)wedgeopeningmodeIfracturefromloadingson cuspalplanes, and(c)critical stressesconcentratinginthe vicinityofthecentralisthmuswhichactedassharpV-notches.

Clearevidenceofcontactwearwithpittingand sometimes extensivefatiguecrackingindicatedabrittlebehaviorunder functionalloading.Moreclinicaltrialsandfractographicfail- ureanalysesareneededtobetterunderstandpossibleclinical outcomelimitationsoftheuseofresincompositesforrecon- structionsofseverelydamagedendodonticallytreatedteeth.

Acknowledgement

Thisresearchdidnotreceiveanyspeciﬁcgrantfromfunding agenciesinthepublic,commercial,ornot-for-proﬁtsectors.

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