University of Birmingham

Investigating the limits of resin-based lutingcomposite photopolymerization through variousthicknesses of indirect restorative materialsHardy, C. M. F.; Bebelman, S.; Leloup, G.; Hadis, M. A.; Palin, W. M.; Leprince, J. G.

DOI:10.1016/j.dental.2018.05.009

License:Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND)

Document VersionPeer reviewed version

Citation for published version (Harvard):Hardy, CMF, Bebelman, S, Leloup, G, Hadis, MA, Palin, WM & Leprince, JG 2018, 'Investigating the limits ofresin-based luting composite photopolymerization through various thicknesses of indirect restorative materials',Dental Materials, vol. 34, no. 9, pp. 1278-1288. https://doi.org/10.1016/j.dental.2018.05.009

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Investigatingthelimitsofresin-basedlutingcompositephotopolymerizationthroughvariousthicknessesofindirectrestorativematerials

C.M.F.Hardya,b,c,⁎

chloe.hardy@uclouvain.be

S.Bebelmanc

G.Leloupa,b,c,d

M.A.Hadise

W.M.Paline

J.G.Leprincea,b,c,d

aSchoolofDentalMedicineandStomatology,Universitecatholique (Pleasechangeallthe"a"lineto"SchoolofDentalMedicineandStomatology,atCliniquesUniversitairesSaint-Luc,UniversitecatholiquedeLouvain,Belgium")deLouvain,Belgium

bAdvancedDrugDeliveryandBiomaterials(ADDB),LouvainDrugResearchInstitute(LDRI),UniversitécatholiquedeLouvain,Brussels,Belgium

cBio-andSoft-Matter(BSMA),InstituteofCondensedMatterandNanoscience(IMCN),UniversitécatholiquedeLouvain,Louvain-la-Neuve,Belgium

dCRIBIO(CenterforResearchandEngineeringonBiomaterials),Brussels,Belgium

eBiomaterialsUnit,UniversityofBirmingham,CollegeofMedicalandDentalSciences,InstituteofClinicalSciences,SchoolofDentistry,5MillPoolWay,BirminghamB57EG,UK

⁎Correspondingauthorat:SchoolofDentalMedicineandStomatology,UniversitecatholiquedeLouvain,Belgium.

Abstract

Objective

sTodeterminethelimitationsofusinglight-curableresin-basedlutingcomposites(RBLCs)tobondindirectceramic/resin-compositerestorationsbymeasuringlighttransmittancethroughindirectrestorativematerials

andtheresultingdegreeofconversion(DC)oftheluting-compositesplacedunderneath.

Methods

Various thicknesses (0–4mm) and shades of LAVAZirconia and LAVAUltimatewere prepared and used as light curing filters. A commercial, light curable RBLC, RelyX Veneer (control)was comparedwith four

experimentalRBLCsofthefollowingcomposition:TEGDMA/BisGMA(50/50or30/70wt%,respectively);camphorquinone/amine(0.2/0.8wt%)orLucirin-TPO(0.42wt%);microfillers(55wt%)andnanofillers(10wt%).RBLCs

coveredwiththeLAVAfilterwerelight-curedfor40s,eitherwiththedual-peakBluephaseG2oranexperimentaldeviceemittingeitherintheblueorvioletvisibleband.ThesampleswereanalyzedbyRamanspectroscopyto

determineDC.Lighttransmittancethroughthefilterswasmeasuredusingacommonspectroscopytechnique.

Results

All the factorsstudiedsignificantly influencedDC (p<<0.05).RBLCswith increasedTEGDMAcontentexhibitedhigherDC.Only smalldifferenceswereobservedcomparingDCwithout filtersand filters≤1mm

(p>>0.05).Forthicknesses≥2mm,significantreductions inDCwereobserved(p<0.05).Transmittancevaluesrevealedhigher filterabsorptionat400nmthan470nm.Aminimal threshold of irradiancemeasured

throughthefiltersthatmaintainedoptimalDCfollowing40sirradiationwasidentifiedforeachRBLCformulation,andrangedbetween250–500mW/cm2.

Significance

ThisworkconfirmedthatoptimalphotopolymerizationofRBLCsthroughindirectrestorativematerials(≤4mm)andirradiationtimeof40sispossible,butonlyinsomespecificconditions.Thedeterminationofsuch

conditionsislikelytobekeytoclinicalsuccess,andallthefactorsneedtobeoptimizedaccordingly.

1IntroductionClinicalstudiesdescribehighperformanceofbondedceramicrestorations(esthetics,goodsurvivalrate),notonlytorestoreanteriorteeth[1–4],butalsoforextensiveposteriorrestorations[5–7].Forbothindications,the

bondingqualityisessentialtoprovideclinicaleffectiveness,especiallyforpartialrestorations.Weaknessesinthebondinginterfacemayleadtoearlyclinicalfailures;mainlylossorfractureoftherestoration,butalsopossiblyfavorthe

occurrenceofotherissuessuchassecondarycaries,post-operativesensitivityormarginaldiscolourationduetomarginalleakage[8].

Traditionally,adual-cureresin-basedlutingcomposite(RBLC)ispreferredfortheplacementofindirectrestorations,toensureeffectivepolymerizationeventhroughthickand/oropaquerestorations.Thedual-curechemistry

supposedlycombinestheassuranceof‘dark’chemicalcuringwithsomeofthenumerousadvantagesprovidedbypurelylight-curablesystems.Thelatternotablyincludeimprovedhandlingaspects,suchasasinglepastewithoutthe

needformixing,bettercontrolofworkingtime,fastersetting,easierexcessremovalorimprovementoftheinterfacecolourstability[9].Despitetheseadvantages,veryfewworksinvestigatedtheuseofpurelylight-curablecomposites

toluteindirectrestorations[10,11].Onereportedthepossibilitytoreachan“adequate”polymerizationofaconventionalresincomposite(describedas80%ofthemaximummaterialmicrohardness)whenlightcuredthrough7.5mm

thick‘endocrowns’[11].Anotherrevealedhigherbondstrengthvalueswhenlightcurableresincompositeswereusedtolute4mmthickinlayscomparedwiththeuseofadual-cureresincomposite[10].Suchobservationsmaybe

explainedby twomajor elements: firstly, light curable resin compositesusually containmore fillers thandual cure resin cements [12], hencehigher intrinsicmechanical properties [10]. Secondly, photopolymerization processes

probablygenerateahigherconcentrationoffreeradicals,whichcanbeprofitableduringtheautoaccelerationstepindimethacrylateresins.Duringsuchstep,anynewgrowthcentrecreatedindeedleadstoefficientchainpropagation

sincethelowmobilityofthebuildingpolymerchainsreducesthelikelihoodofbimoleculartermination[13].Thisreinforcesthepotentialinterestofutilizingsolelylight-curablechemistriesnotonlytoluteveneers[14]orthininlays,

butalso thickerposterior restorations, suchasendocrowns [10,11].The importanceof effectivephotopolymerization in light-curableRBLCs, andeven those systems that includeautopolymerization chemistries, is highlighted in

numerousworks[15–18].Forexample,apreviousstudyhasreportedathree-folddecreaseinmicrohardnessofdual-cureRBLCswhenlightcuredthroughthick(4mm),comparedwiththinner(2mmorless),ornouseofindirect

ceramicfilters[15].Asimilarobservationwasmadewhenmeasuringthedegreeofconversionofadual-cureRBLC,withatwo-tofour-folddecreaseofconversionthroughopaque2mmceramicfilters[17].Theautopolymerization

step in a dual-cured system seems therefore insufficient to ensure optimal polymerization of luting composites.Hence, undercuring of dual-curematerials beneath thick indirect restorations remains a risk,which is potentially

worsenedwithsystemsthatuselight-curablechemistriesalone.Effectivepolymerizationofthelatter,isindeednecessarytoensureoptimalphysico-mechanicalproperties[13,19]andcolourstability[20,21],therebyreducingtherisk

ofinterfacialfailure.

Light transmittance through a tooth-coloured indirect restoration is significantly affected bymaterial type. Veneers are commonly fabricatedwith feldspathic glass (porcelain), which exhibit relatively high translucency,

however,moreopaquematerialsexist,especiallythosefabricatedusingmoremodernCAD/CAMprocesses,includingresin-basedcomposites,particlereinforcedceramiccomposites(e.g.lithiumdisilicatesandleucite-basedceramics)

andpolycrystallineceramics(e.g.aluminaandzirconia),thelatterofwhichareexpectedtobetheleasttranslucent(notwithstandingmodernattemptstoincreasetranslucencyofmonolithicpolycrystallinecrownsbyadjustingthe

phasestabilisationdopant,grainsize,andsoforth).Therefore,iflighttransportislimitedbytheopacityoftheindirectmaterial,otherinherentmaterialchemistriesthatcircumventtheneedforeffectivepolymerizationusinghigher

irradianceiscertainlyworthyofconsideration.

Theinterestofusingalternativephotoinitiatorsystemstotheclassicalcombinationofcamphorquinone/amine(CQ),suchasType1acylphosphineoxides,hasbeenextensivelydescribedfordirectrestorativeresincomposites.

Notably,higherfinalDCandhighermechanicalpropertieshavebeenreportedusingcuringtimesshorterthan3s[22–26].Thiswasexplainedbyhighermolarabsorptivityandquantumyieldefficiency[27,28],whicharepotentially

keyaspectsasregardslight-curingthroughindirectmaterials.Indeed,lowtransmittanceisiexpectedthroughthickindirectmateriallayers,whichexplainstherelativelylongirradiationtimesthatwereusedwhenlutingwithlight-

activated(non-dualcure)resin-composites(from40s[10]toseveralcyclesof90s[11]).

Consequently,theaimofthisworkwastodeterminethelimitsofRBLCphotopolymerizationbymeasuringlighttransmittancethroughvariousthicknessesofindirectrestorativematerialsandtheresultingdegreeofconversion

(DC)oftheresincementsplacedunderneath.ExperimentalRBLCsofvariousmonomerratiosandphotoinitiatorcontent,aswellasfiltersofdifferentmaterialsandshadeswereconsidered.

2MaterialsandMmethodsThethicknessoflargeindirectrestorationssuchasoverlaysorendocrownsisinhomogeneous(Fig.1A,B).Inordertoexperimentallymodelsuchvariabilityofthicknessinareproduciblemanner,variousCAD/CAMblockswere

usedtoprepare10mmdiameterdisc-shapedfiltersof4differentthicknesses:0.5,1,2and4mm(±0.01mm).

TheCAD/CAMblocksweremadeofeitherapolycrystalline,yttria-stabilisedzirconiaceramic,LAVA-Zr(shadesA3anduncoloured−–Zr-A3andZr-U)oraresincompositeblockwithdispersedfillers,LAVAUltimate(shadesA3

andMC2–Ult-A3andUlt-MC2––Ult-A3andUlt-MC2–thelatterreportedasmoreopaque)(3M-ESPE,StPaul,MN,USA).FiveRBLCswerelight-curedthroughthesefilters:fourexperimentalformulationsandRelyXVeneer(3M-ESPE,St

Paul,MN,USA), a commercially-available light-curable RBLC used as control. Experimental formulationswere prepared tomimic the commercialmaterial (Table 1). The experimental formulations contained two proportions of

conventionalmonomersTEGDMAandBisGMAinratiosof50/50and30/70wt%,respectively.Eachresinblendcontainedeithercamphorquinone/amine(0.2/0.8wt%−–CQ)orLucirin-TPO(0.42wt%−–Lu-TPO)asthephotoinitiator.

Thedifferentcomponentswereweighedusinganelectronicanalyticalbalance(ANDFR-300-MKII,A&DINSTRUMENTSLIMITE,Abingdon,U.K.−–accuracy±100μg)andplacedsequentiallyinopaqueplasticpottopreventlight

exposure.Bariumglassmicrofillersandfumedsilicananofillerswereaddedinamountsof55/10wt%,respectively.Silanatedfillerswereused,bothforthenano-andmicro-scaleparticles.

Table1Compositionofresin-basedcompositecements.

alt-text:Table1 (Thistableisincomplete.Pleasecheckthedocumentattachedtoyourquerietocompleteit.)

Resin Fillers Monomers Photoinitiator

RelyXVeneer(3RelyXVeneer(3MESPE,StPaul,MN,USA)

Silanetreatedceramic(55–65wt%), Silanetreatedsilica(1–10wt%)andBisGMA10–20%oftotalweight

Titaniumdioxide<1wt%)

Silanetreatedsilica(1–10wt%)and TegDMA10–20%oftotalweight

Ethyl4-dimethylaminobenzoate(EDMAB)(<1wt%)

Reactedpolycaprolactonepolymer(1–10wt%) * BisGMA10-20%oftotalweightTegDMA10-20%oftotalweight*TitaniumDioxide<1wt%)Ethyl4-DimethylAminobenzoate(EDMAB)(<1wt%)Benzotriazol(<1%wt)DiphenyliodoniumHexafluorophosphate(<1wt%)*enzotriazol(<1wt%)

* Diphenyliodoniumhexafluorophosphate(<1wt%)

*

CQ50/50 Bariumglassfillerssilanated(G018-186/K6,d50=3±1μm,SchottAG,LandshutGermany)andmethacrylsilanetreatedfumedsilica(12nm,AEROSIL®R7200Aerosil7200,EvonikIndustries,Germany)inamountsof55/10wt%respectively.

50/50wt%ofBis-GMAandTegDMAresin(Sigma–Aldrich)

Camphorquinone(SigmaAldrich,CASNumber10334-26-6)asthephotoinitiatoranddimethylaminoethylmethacrylate(SigmaAldrich)asco-initiator,intheproportionsof0.2/0.8wt%

CQ30/70 70/30wt%ofBis-GMAandTegDMAresin(Sigma–Aldrich)

Fig.1ExampleofLavaUltimateE(3M-ESPE)overlayofvariablethickness(A),andthesameoverlayirradiatedwithBluePhaseG2(Ivoclar-Vivadent) (Pleaseadd(B));(C)experimentalsetuptopolymerizeRBLCthrougharestorativematerialfilter(thicknesses

rangingfrom0.5to4mm).

alt-text:Fig.1

TPO50/50 50/50wt%ofBis-GMAandTegDMAresin(Sigma–Aldrich)

Lucirin-TPO(TPO,fromBASF)0.42wt%asthephotoinitiator

TPO70/30 70/30wt%ofBis-GMAandTegDMAresin(Sigma–Aldrich)

*aAccordingtomanufacturersinformations.

Bis-GMA:BisphenolAglycerolatedimethacrylate,CASNumber:1565-94-2.

TEGMA:TriethyleneGlycolDimethacrylate,CASNumber109-16-0.

Fillerswereincorporatedsequentiallyusingadualasymmetriccentrifuge(Speedmixer,FlackTek,USA)for30secondsat3500rpmforthenanofillers,andat2500rpmforthemicrofillers.Themixingprocedure(rpm,time,

etc.)waspreviouslyoptimizedinotherwork[25].

Lightsourceswereeitherthedual-peakBluephaseG2(BPG2,Ivoclar-Vivadent,Schaan,Liechtenstein;curingtipdiameter=10mm;“Highpower”)oralight-curingdevice(AURA,Lumencor,USA;curingtipdiameter=6mm)

emittingeitherintheblue(AURAblue;455–485nm)orintheviolet(AURAviolet;395–415nm);theirradianceforbothspectraloutputswascalibratedandsetataround1000mW/cm2.Theirradiancevaluesweremeasuredwiththe

ThorlabsOpticalPowerandEnergyMeterPM100USBat1020mW/cm2forAURAviolet,1030mW/cm2forAURAblueand1119mW/cm2fortheBPG2.Therelationshipbetweentheabsorptionspectraofthephotoinitiators(CQandLu-

TPO)andthecuringlightsemissionspectrawerecompared(Fig.2;[24]).

Light transmittancewasmeasuredthroughthevarious filtersusingaUV–visspectrometer (USB4000,OceanOptics,UK;n=3).The spectrometerwascoupled toa200μmoptical fibreandanopalineglassCC3cosine

corrector(3.9mmdiameterofcollectionarea,OceanOptics,UK)andcalibratedwithaNationalInstituteofStandardsandTechnology(NIST)traceablelightsource(MikropackDH2000/OceanOptics,UK).Followingcalibration,the

integrationtimewassetautomaticallywithaboxcarwidthof0andspectraaverageequals1.TheLAVAfilterswereinterposedcentrallybetweenthetipofthelightcuringunitandthecosinecorrector.Thelightdevicewasfixedina

standardizedposition,withthesurfaceofthetipparallelandincontactwiththefiltersurfaceandthefilterparallelandascloseaspossibletothecosinecorrector.Theabsoluteirradiancewascalculatedastheintegralbeneaththe

curve.

Thelighttransmittancewasmeasuredwithandwithoutapolyesterfilm,andthetransmittanceprofileshowedthattherewasnosignificantdifference(p<0,05).

TheRBLCwereplacedin1mmthick,5mmdiameterTeflonmolds,coveredoneachsidewithapolyesterfilm(≅0.1mmthick),compressedbetweentwoglassslidestoextrudeexcess,andcoveredbyaceramicfilterthroughwhich40slightirradiationwasperformedwiththelight-tipparallelandindirectcontact(Fig.1C).

Afterphotopolymerization,thesamples(n=3)werestored‘dry’,inthedarkatroomtemperatureforoneweek,beforebeinganalyzedbyRamanspectroscopy(DXRRamanMicroscope,ThermoScientific,Madison,WIUSA)to

Fig.2Emissionspectrumofinvestigatedcuringlights(presentedinrelativeirradiance,100%beingthemaximumspectralirradiance),comparedtothemolarabsorptivityofthetwophotoinitiatorsincludedintheinvestigatedmaterials,i.e.Lu-TPOandCQ.

Thedottedlinesrefertotheleft-Y-axis(Molarabsorptivity)andtheplainlinestotheright-Y-axis(Relativeirradiance).

alt-text:Fig.2

determinethedegreeofconversion(DC,in%)[29]ontheupperRBLCsurface(n=3).Briefly,afrequencystabilizedsinglemodediodelaserexcitedthesamplesthrougha50x×microscopeobjective.Thespectrawereacquiredinthe

areaof1600cm−1,usinga50slit,a60sirradiation,5accumulations,andagratingof400lines/mm.ThecalculationofDCwasbasedonthedecreaseinintensityofthepeakcorrespondingtothemethacrylateC

Cgroupat1640cm−1 comparedwith anunpolymerized sample. The aromatic peak at 1610 cm−1was used as the internal standard [30].Given the small thickness of theRBLC layer (around 25 μm [31]), itwas assumed that

theDCmeasurementattheupperRBLCsurfaceofthe1mmthicksampleswasrepresentativeoftheconversionofthewholeRBLClayer.

StatisticalanalyseswereperformedwiththeJMPPro12software(SAS).One-wayANOVAwereperformedfollowedbymultiplecomparisonswithalevelofsignificanceofp=0.05;whennormaldistributionofthedatacould

notbeverified,thenon-parametricWilcoxontestwasused;whennormalitywasverified,HSDTukey’stestwasused.

3ResultsAninverselogarithmicrelationshipwasobservedbetweentransmittanceandfilterthickness.Afterlogarithmictransformationoftransmittance,linearcorrelationswithcorrelationscoefficientsbetween0.89and0.97were

observed(Fig.3).Fig.3alsoconfirmsthepreviousaffirmationthatthetransmittanceissignificantlylowerforAURAvioletthanforBPG2andAURAblue,especiallyforZr-A3.Lighttransmittanceforallthematerialsarerelatively

similarbetweenAURAblueandBPG2,althoughtransmittanceisgenerallyhigherforBPG2.

Lighttransmittancethroughtheceramicfilterswassignificantlyaffectedbythickness(p<0.0001),type,shade(p=0.0205forLAVA-Zr)aswellasbylighttype(p<0.0001),butnotforeithershadeofUltimate(p=0.4218)

(Wilcoxontest)(representativeexamplesinFig.3,fullresultsinTable2)

Table2Lighttransmittancedependingonfilterthickness,in%ofmaximumtransmittance(withoutinterpositionofanyfilter)*.

alt-text:Table2

Fig.3Transmittance(Lntransformation)asafunctionoffilterthickness,forthedifferentlights(AURAblueintwodifferentwavelengthsandBPG2).Forallfourtypesoffilters,linearcorrelationsweredrawn,andareassociatedwiththefollowing

correlationcoefficients:AURAviolet−–Ult-A3(R2=0.97),Ult-MC2(R2=0.96),Zr-A3(R2=0.91),Zr-U(R2=0.96);AURAblue−–Ult-A3(R2=0.98),Ult-MC2(R2=0.97),Zr-A3(R2=0.89),Zr-U(R2=0.92);BPG2––Ult-A3(R2=0.96),Ult-MC2(R2=0.93),

Zr-A3(R2=0.80),Zr-U(R2=0.90).Foralltransmittancedata,standarddeviationscanbefoundinTable2.

alt-text:Fig.3

*Similarupper-caselettersandvariousshadesofgreyconnectinthesamerowresultswhicharenotsignificantlydifferent(basedonTuckey’stest,p=0.05).Lower-caselettersconnectinthesamecolumn(andforagivencuringlight)resultswhicharenotsignificantlydifferentatagivenfiltrethickness(basedonTukey’stestp=0.05).**Notransmittancevaluecouldbemeasured.

Overall,lighttransmittancedecreasedsignificantly(p<0.05)witheachadditionalceramicfilterthickness(Table2).Thetransmittancewasgenerallylowthroughall4mmthicknessfilters,i.e.between10.9and17.5%for

BPG2,between2.4and5.0%forAURAblue,andbetween0.42and1.93forAURAviolet(Fig.3andTable2).

Thecomparisonoflighttransmittancebetweentheceramicandresincompositeofsimilarshade(A3)revealedalowertransmittanceateachthicknessforeachmaterial,respectively(purpleandredcurves,respectively,inFig.

3;Table2)foreachofthecuringlights.

TheeffectoffiltershadewasmoreobviousforLAVA-Zr,withasignificantlylowertransmittancethroughZr-A3thanZr-Uateachthickness(purpleandorangecurves,respectively,inFig.3)andforallthreelights(Table2).For

LAVA-Ult,atendencyofhighertransmittancewasobservedforUlt-A3ascomparedtoUlt-MC2,butthedifferenceswerenotstatisticallysignificantinallconditions(redandgreencurves,respectively,inFig.3;Table2).

BPG2seemedtobeassociatedwithhigherpercentageoftransmittancethanAURAforeachthicknessandmaterialtype.ThesameobservationcouldbedoneforAURAblueascomparedtoAURAviolet,thelatteryieldingthe

lowesttransmittancevalues(Table2).

RegardingDC,thetypeofRBLC(p=0,0001,Wilcoxontest),photoinitiatorandmonomerscontents(p<0,0001), filtertype(p=0,0005), filtershade(p=0,0061 forLAVA-Zr)and light (p<0,0001) aswell as thickness

(p=0,0001)allsignificantlyinfluencedthevalues,exceptfortheshadeofUltimate(p=0.5349)(Wilcoxontest).

InordertoidentifytherelationshipbetweenlighttransmittanceandRBLCconversion,DCwasplottedagainsttransmittance(Fig.4).Thereby,itispossibletoidentifythetransmittancethresholdnecessarytomaintainoptimal

DCafter40sirradiationforeachRBLCformulationandcuringlight.Forallconditions,DCcurvesinflectionwaslocatedbetween250and500mW/cm2.

ForexperimentalCQcompositions,BPG2andAURAbluearecomparableforCQ50/50,andBPG2wasslightlymoreefficientforCQ70/30(blueandredcurves,respectively,inFig.4).ForexperimentalLu-TPO-basedmaterials,

AURAvioletismoreefficientateachleveloftransmittancethanBPG2(greenandpurplecurves,respectively;Fig.4).Forthecommercialproduct,Rely-XVeneer,BPG2yieldsmuchhigherDCateachleveloftransmittance(orange

curveinFig.4).

WhencomparingcuringlightsefficiencyforeachRBLC(Fig.4),itappearsthatBPG2andAURAbluehaverelativelycomparableefficienciesinexperimentalCQ-basedmaterials,whileBPG2isclearlymoreefficientforthe

supposedlyCQ-basedRely-XVeneer.ForLu-TPO-basedmaterials,AURAvioletappearsasmoreefficientthanBPG2atlowirradiance.

Withoutanyfilter,theDCvaluesrangedbetween57.2and74.7%.DCofLu-TPO-RBLCwassignificantlyhigherthatCQ-basedoneatsimilarmonomerratio(p<0.05).DCof50/50TEGDMA/Bis-GMAwassignificantlyhigher

thanDCofthe70/30ratioforasimilarphotoinitiatorsystem.ThelowestDCwasobservedforthecommercialcontrolmaterialRely-XVeneer(Fig.4andTables3–5).

Table3DegreeofconversiondependingonfilterthicknessforBluephaseG2*.

alt-text:Table3

Fig.4Degreeofconversion(%)inrelationwiththemeasuredtransmittance(mW/cm22);smoothedsplinedcurveswithlambda=0.1).ComparisonofRBLCcompositionsforeachcuringlight.ForallDCdata,thestandarddeviationsareinTable3,4and5s3–5.

alt-text:Fig.4

*Similarupper-caselettersandvariousshadesofgreyconnectinthesamerowresultswhicharenotsignificantlydifferent(basedonTuckey’stest,p=0.05).Lower-caselettersconnectinthesamecolumnresultswhicharenotsignificantlydifferentatagivenfiltrethickness(basedonTukey’stestp=0.05).

Annotations:A1. Pleasesuppressthislineinthemiddleofthebox

A1

A2. Thisthicklineshouldnotbethere.Pleasechangewithtablesattachedtoyourquerie

Table4DegreeofconversiondependingonfilterthicknessforAURAblue(468nm+-10nm)*.

alt-text:Table4

*Similarupper-caselettersandvariousshadesofgreyconnectinthesamerowresultswhicharenotsignificantlydifferent(basedonTuckey’stest,p=0.05).Lower-caselettersconnectinthesamecolumnresultswhicharenotsignificantlydifferentatagivenfiltrethickness(basedonStudentttest,p=0.05).

Table5DegreeofconversiondependingonfilterthicknessforAURAviolet(400nm+-10nm).

alt-text:Table5

Meandegreeofconversionforthelight-curingunitAura.400nm.expressedinpercents.Theresultsinasameraw,unconnectedwiththesameletteraresignificantlydifferent(p-value0.05),fromthetestKruskal-

Wallis.Minusculelettersshowsignificantlydifferencebetweendifferentresincomposition.withinasamematerial(p-value0.05.fromtheStudent’st-test).*0isnotameasuredvalue,becausetheDCwassolowthanwecouldnotmeasureit.

WhiletheeffectofmonomerratioisimportantontheabsoluteDCvalueatagivenfilterthickness,DCvalueswithincreasingfilterthicknessandagivenphotoinitiatorsystemdecreasesimilarlyfor50/50and70/30ratios

(Tables3–5).Notably,thedropscorrespondingtosignificantdifferencescanbeobservedatsimilarfilterthicknesses.

AlthoughDCwithoutfiltersandwiththinfilterswerehigherforLu-TPO-basedRBLC,thetrendreversedwhenusingthick(4mm)filterswithBPG2(Table3).WhenusingAURAhowever,whereirradiancewascomparable

betweenvioletandbluepeaks,suchdifferenceswerenotobserved,DCofLu-TPO-basedmaterialsremaininghigherat4mm,exceptforZr-A3(Tables4and5).

Regardingfiltertype,asobservedfortransmittance,thecomparisonofDCbetweenceramicandresincompositefilterswasachievedforsimilarshade(A3),withasignificantlylowerDCforZr-A3thanforUlt-A3(Tables3–5)

foreachofthethreecuringlights.

Inrelationtowhatwasobservedfortransmittance,theeffectoffiltershadeonDCwasmoreobviousforLAVA-Zr,withasignificantlylowerDCthroughZr-A3thanZr-Uandforallthreelights(Tables3–5).ForLava-UltDCwas

higherthroughUlt-A3thanthroughUlt-MC2(Tables3–5),althoughdifferenceswerenotstatisticallysignificant(p>0.05).

Regardingthesameshadeforthetwodifferentmaterials,asignificantdifferencewasobservedbetweenZr-A3andUlt-A3fortheDCobtainedthroughthicknesses>2mm.

With regard to thickness, the use of filters≤ 1mm resulted in few significant differences inDCwhen comparedwithRBLCswithout filters. For thicknesses≥ 2mm,more significant reductions in DCwere observed,

particularlyat4mm,dependingontheceramicfilter/light/photoinitiatorcombination.Ingeneral,thecriticaldecreaseinDCwasobservedbetween2and4mm.However,forsomecombinations,nosignificantdecreaseinDCwas

observed,evenfor4mm-thickfilters(p>0.05),i.e.Rely-XVeneer−AURAblue−Ult-A3,CQ70/30–AURAblue−Zr-U,CQ50/50–BPG2––AURAblue–Ult-A3,CQ70/30–AURAblue–Zr-U,CQ50/50–BPG2– (Pleasereplaceby:i.e.Rely-XVeneercured

byAURAbluethroughUlt-A3;CQ70/30curedbyAURAbluethroughZr-U;CQ50/50curedbyBPG2throughZr-U.)Zr-U(Tables3–5).

4DiscussionThecurrentworkconfirmed thatoptimalphotopolymerizationofRBLCs through indirect restorativematerials (≤4mm)and irradiation timeof40s is possible, but only for specific conditions.Thedeterminationof such

conditionsislikelytobekeytoclinicalsuccess,andallthefactorsstudiedinthepresentwork(filtermaterialtype,thicknessandshade,monomercomposition,photoinitiatorcontent,etc.)significantlyimpactedbothtransmissionand

conversion.

ThefirstobviouslimitationofthisprocedurewastoachievesufficientlighttransmittanceforoptimalpolymerisationoftheRBLCthroughtheindirectrestoration.Thesignificantimpactofmaterialshadeontransmittance

observedhereconfirmedthefindingsofpreviousstudies,i.e.thatdarkershadesledtolowertransmittancebothinceramicsandresincomposites[17,32,33].Thisinturnresultedinlowerconversionorlowermechanicalpropertiesof

RBLC[16–18].

The inverse logarithmic relationshipbetweenmaterial thicknessand transmittancedescribed inotherworks [32]wasconfirmedhere (Fig.3), the slopebeing specific to each curing light/material combination.Themost

importantdecreaseintransmittanceobservedatshorterwavelengths(AURAviolet)withtheinvestigatedmaterialsisalsoinaccordancewithpreviouswork[34],andmayrepresentalimitationforthesesystems.However,thismaybe

materialspecificanddependoneachparticularfiller,resincompositionandratio.Nevertheless,theeffectsofincreasedvioletlighttransmittancethroughindirectmaterialsareworthyoffurtherinvestigation,especiallyforRBLCs

containingphotoinitiatorchemistriesthatabsorbasshorterwavelengthbands.

ThesecondobviouslimitationtolightcuringRBLCsthroughthicklayerswasunderstandingtheexactdefinitionof“sufficient”transmittance.Perhapsasensibleapproachwouldbealighttransmittancehighenough(fora

givenirradiationtime,here40s)inordertoreachaDCcomparabletothatobtainedwithoutanyfilter.Suchthresholdcouldbeidentifiedinthecurrentworkasrangingbetween250and500mW/cm2(Fig.4).Suchpresentationofthe

dataavoidsarduousline-by-lineanalysisofthedatatables(Tables3–5),whichoftenresultsinconclusionsthatareonlyrelevanttoeachcombinationoffiltertypes,shades,curinglights,etc.TheabilitytoachieveanoptimalDCpurely

bylightcuringdependsonthecombinationofirradianceandirradiationtime.Ithasbeendescribedbeforethatthereis“noapparentlowerlimittotheirradiancethatmaygiveeffectivepolymerization,atleastdownto25mW/cm2”

[35].Thiswasreportedfordirectrestorativecomposites,inthicklayers(2mm).InthecontextofRBLCs,wherelayersaround25μmareused[31],thisstatementbecomesevenmorerelevant.Asfortheupperlimitofirradiationtime,

itwouldbedeterminedasthetimeaclinicianiswillingtodevotetothelightcuringprocedure,orriskofover-heatingthepulp.Therefore,thequestionisnotwhetheralight-curableRBLCcanbecuredoptimallythroughthickindirect

restorations,butrather,inwhatirradiationtime(providedthataminimumirradiancereachesthematerial)?Forthepresentwork,thisparameterwassetat40s;otherworks(e.g.Ref.[36]indicatedthatmaximumthicknessforan

efficientlightcureduring20sthroughceramicfiltersis2mm.Therefore,itwouldbemoreappropriatetoconsidertransmittanceratherthanfiltermaterialtype,shadeorthickness,andtoadapttheirradiationtimetoprovidespecific

indicationsforeachlutingmaterial.Curingtimeexposureisthemostcriticalparameterforoptimizingdegreeofconversion.

Withinthecurrentcuringparameters(40sirradiationtime),DCvariedsignificantlywithmonomerratio,photoinitiatortypeandcuringlight.Consideringthemonomerratio,anincreasedlowmolecularweightmonomers

(TEGDMA) content led, as expected, to higher DC for RBLCs light cured under similar conditions [37,38]. This can be explained by their high molecular mobility, which enables additional propagation in the later stages of

polymerizationreaction,i.e.whenitbecomesdiffusion-controlled[37,38].DespitetheattempttoformulatetheexperimentalRBLCsinacomparablefashiontoourcontrolcommercialmaterial(Rely-XVeneer),itappearedthatthe

optimalDCofthelatterwasinallcasesinferiortotheexperimentalformulations.Reasonsforthismayincludetheeffectofproprietarycompounds,pigmentsandothercompoundsthatactascompetitiveabsorbers,and/oraless

favorablephotoinitiatorandco-monomercombination.Furthermore,althoughco-monomerratiohadanimpactontheabsoluteDCvalues,ithadnoinfluenceontheevolutionofDCvalueswithincreasingfilterthickness,whichpurely

dependsonlighttransmission.

Regarding the photoinitiator type, DC results confirmed previously described trends that for similar irradiance, resin composites or adhesives using Lu-TPO showed higher DC than their CQ counterparts (Fig. 4)

[20,22,24,26,39,40]. Similarly, Lu-TPO-based light cured systemswere associated firstwith a lower release of un-reactedmonomers, hencewith a lower cytotoxic potential [24], and secondwith superiormechanical properties

comparedtoCQ-counterparts[23].Highermechanicalpropertiesmayleadtoamoreeffective,durableandstablebondinginterface(tooth-RBLCandRBLC-indirectrestorativematerial)overtimewithareducedsolubility[24],which

remainstobeverified.

ThehigherDCcombinedwithhighermechanicalpropertiesofLu-TPO-basedcompositesweresaidtoresultinahighercross-linkingdensity,acharacteristicwhichwasalsosuggestedtoaccountforahighercolorstability

[20,41]andan improvedresistance tohydrolyticdegradation [20] comparedwithCQ-basedmaterials.This further supports thepotential ofusingLu-TPO-basedRBLCmaterials, to reduce the riskof interfacialdegradationand

discoloration.Suchdetrimentaleffectshavebeendescribedinagreaterproportionwhenusingdual-curedmaterialscomparedwithpurelylightcuredtypes[9].Otherfactorsexplainingthehighercolorstabilitymayalsorelatetothe

oxidationprocessoftheaminepresentintheCQinitiationsystem,whichcausesdiscolouration[21].Duringphotopolymerization,aminesmayalsoformby-productsthatcanalsocauseyelloworbrowndiscolouration[42].Theabsence

ofamineinthecatalystsystembasedonLu-TPOcouldbeassociatedwithreducedshadealterationafteraging.Insummary,theuseofLu-TPOasphotoinitiatorinRBLCshaspotentialtoimproveclinicaloutcomes,bothintermsof

bondingefficiency,bondingstabilityaswellascolorstability.Thelatterisparticularlyimportantincaseofthinveneers,sincethefinalcolouroftheserestorationsafterbondingtoteethwereclearlyshowntobeinfluencedbythe

shadeoftheunderlyingstructures,includingthelutingsystem[43].

Finally,thelightspectrumandspectralirradiancehasanimportantimpactintermsofcuringefficiency.DespitetheincreasedtransmittedirradianceofBPG2(Fig.3),whichisprobablyduetothehighercombinedirradiance

ofthetwodifferentspectrumpresentintheBPG2light,DCvaluesandprofilesweresimilarforCQ-basedmaterials,andhigherforAURAvioletatlowerirradiances(Fig.4).Thiscouldbeexplainedbythefactthatwhilebluepeaks

betweenBPG2andAURAbluewererelativelycomparable, thevioletpeak inBPG2correspondstoonly20%of thetotal irradiance(Fig.2).Thisresults ina lowertransmittanceof the (∼410nm)violetspectrumatagivenBPG2

transmittancevalue.Moreover,theemissionpeakofAURAvioletwaslocatedatshorterwavelengths(∼405nm),providingamoreeffectiveoverlapwithLu-TPOabsorptionspectrumthanthevioletpeakoftheBPG2(Fig.2).Forthe

commercialproductRely-XVeneer,thehigherDCresultsassociatedwithBPG2comparedwithAURAblueateachleveloftransmittancewaslikelyaresultofthebroaderspectrumofthebluepeakoftheBPG2comparedtoAURAblue,

andconsequently,alargeroverlapwiththeabsorptionspectrumofCQ.AnotherlesslikelyexplanationwouldbethepresenceofanadditionalphotoinitiatingsysteminRely-XVeneerabsorbingatlowerwavelengths,whichwouldthen

benefitfromthevioletemissionoftheBPG2.

5SignificanceThecurrentinvestigationconfirmedthat,underspecificconditions,optimalphotopolymerizationofRBLCscouldbeachievedthroughindirectrestorativematerials(≤4mm)andanirradiationtimeof40s.Suchanapproach,

whichisassociatedwithbothclinicaladvantagesandfundamental improvements inmaterialproperties,maybeviable,however,multiplefactorssuchasmonomercomposition,photoinitiatorcontent, filtermaterialandthickness

(studiedhere),andprolongedcuringtime(> 40s)shouldbeoptimizedaccordingly.

TheLu-TPO-basedRBLCprovidedhigherconversioncomparedwiththetraditionalCQsystem,providedthatsufficientlyhighirradianceinthevioletwavelengthrangewasused.

Finally,theperformanceofsuchanapproachintermsofbondstrength,bondstability,andultimatelyclinicalefficiencyshouldbeverified.

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QueriesandAnswersQuery:“Yourarticleisregisteredasaregularitemandisbeingprocessedforinclusioninaregularissueofthejournal.IfthisisNOTcorrectandyourarticlebelongstoaSpecialIssue/Collectionpleasecontactm.renaud@elsevier.comimmediatelypriortoreturningyourcorrections.”Answer:ok

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Query:Asperjournalspecificationmaximum10keywordsareallowed.Kindlyprovidekeywords.Answer:Degreeofcure;Degreeofconversion;Lighttransmittance;Lighttransmission;Polymerisationkinetics;Resinbasedlutingcomposite;Indirectrestorativematerials;Lucirin-TPO;Irradiationtime;Camphorquinone

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