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Abstract: This study investigated the radiopacity values of glass ionomer- and resin-based bulk-fill restoratives of different thicknesses using digital radiography. Two glass ionomer-based and three resin-based bulk-fill restoratives, and a conventional composite were studied. Five disc-shaped specimens were prepared from each of these materials at three different thicknesses; specimens of enamel and dentin with the same thicknesses were also prepared. Materials were placed over a complementary metal oxide-semiconductor sensor together with the tooth specimen and an aluminum step-wedge, and then exposed using a dental X-ray unit. The images were analyzed using a software program to measure the mean gray values (MGVs), which were converted to equivalent aluminum thicknesses. Two-way ANOVA was used to investigate the significance of differences among the groups. The GCP Glass Fill specimens showed the lowest radiopacity values, and the Quixfil specimens had the highest values. All materials had higher radiopacity values than enamel and dentin, except for GCP Glass Fill, which had a radiopacity similar to that of enamel. The resin-based bulk-fill restoratives had significantly higher radiopacity values than glass ionomer-based restoratives. All of the tested materials showed radiopacity values higher

than that of dentin, as recommended by the ISO. (J Oral Sci 57, 79-85, 2015)

Keywords:dentalmaterials;bulk-fillrestoratives;radiopacity; digital radiography.

IntroductionThe radiopacity of dental materials used for restorations isveryimportantforradiographicdiagnosis,particularlybecause radiopaque materials allow for better radio-graphic detection of secondary caries in posterior teeth (1,2). A material with adequate radiopacity makes itpossible to detect secondary caries and distinguish them fromtherestorativematerial,assistingintherecognitionoffaultyproximalcontours,voids,inadequatemarginaladaptation,andinterfacialgaps(3,4).Forthisreason,theradiopacity of dental materials has been studied exten-sively.Several authors have discussed the question of how

a radiopaque restorative material should function foroptimal diagnosis. Studies of radiopacity normally compare a material with enamel, dentin, or aluminum(2,5). The International Standard Organization (ISO)4049(6)andISO9917(7)havepublishedaradiopacityprotocol and guidelines for resin composite and glass ionomer cements. According to the ISO (6,7), if amanufacturer claims their product to be radiopaque,the radiopacity of a 1.0-mm-thick specimen should beequal to or greater than the same thickness of aluminum. Aluminumisthereferenceofchoicebecauseitsreportedradiopacity is similar to that of dentin and it can be

JournalofOralScience,Vol.57,No.2,79-85,2015

Original

Comparative study of radiopacity of resin-based and glass ionomer-based bulk-fill restoratives using digital radiography

Bilal Yasa1),EbruKucukyilmaz2),ElifYasa3),andElifT.Ertas4)

1)DepartmentofRestorativeDentistry,FacultyofDentistry,IzmirKatipCelebiUniversity,Izmir,Turkey2)DepartmentofPediatricDentistry,FacultyofDentistry,IzmirKatipCelebiUniversity,Izmir,Turkey

3)DepartmentofRestorativeDentistry,FacultyofDentistry,SifaUniversity,Izmir,Turkey4)DepartmentofDentomaxillofacialRadiology,FacultyofDentistry,IzmirKatipCelebiUniversity,

Izmir,Turkey

(ReceivedNovember19,2014;AcceptedMarch12,2015)

Correspondence to Dr. Bilal Yasa, Department of RestorativeDentistry, Faculty of Dentistry, Izmir Katip Celebi University,Izmir35640,TurkeyFax:+90-232-325-2535E-mail:bilalyasa@hotmail.comdoi.org/10.2334/josnusd.57.79DN/JST.JSTAGE/josnusd/57.79

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machinedeasilyandaccurately(1,8).Forthesamethick-ness,theradiopacitiesofAlanddentinareapproximatelyequivalent, and enamel has approximately twice theradiopacityofAl(9).Moststudiessuggestthatamaterialwith a radiopacity equal to or slightly greater than that of enamel would be ideal for detection of secondary caries inradiographs(10-15).Currently,bulk-fillrestorativesareclinicallypreferred

because they are easy to handle. Instead of using theconventionalincrementalplacementtechnique,bulk-fillrestorativesallowforhomogeneousincrementthicknessesupto4mm,eliminatingtheneedforthetime-consuminglayering processes (16). Resin-based composites andglass ionomer-basedmaterials can be used as bulk-fillrestoratives (17). Resin-based composites that differin terms of particle size, distribution, volume fraction,viscosity,andapplicationmethods,arealreadyavailablecommercially. Similarly, glass ionomer-based bulk-fillrestoratives have been introduced for clinical applica-tionsfollowingrecentrefinementstotheircomposition.Infact,glassionomer-basedmaterialshavebeenwidelyusedinrestorativedentistrysince1972(18).Oneofthedisadvantagesoftheearlyglassionomerswasthattheylacked suitable radiopacity, which made it difficult toradiographically differentiate any recurrent caries from therestoration(19).Becauseofthisinsufficiency,manu-facturers improved glass ionomer-based materials byincorporatingfillersorbyusingradiopaquecompoundsto enhance the degree of radiopacity. Pedrosaetal.(20)havesuggestedthattheradiopacity

of materials is an important aspect of studies aimed at the evaluation of new commercial materials, and forpreventionof imagingmisdiagnosis. It is assumed thatthecompositionofbulk-fill restorativesdoesnotdiffermarkedlyfromthatofcurrentlyused,incrementallyfilledconventionalresincompositesandglassionomer-basedmaterials.However,thedifferingchemistryofthemono-mericresinformulationsandfillercharacteristics(type,volumefraction,density,andparticlesizeanddistribu-tion) of bulk-fill restoratives may affect radiologicalcharacteristics such as the depth of cure and mechanical

properties, which Finan et al. (21) have shown to besignificantlyimpacted.Toourknowledge,nopublishedpapershaveyetcomparedglass-ionomerandresin-basedbulk-fill restoratives.Therefore, the aim of the presentstudy was to evaluate the radiopacities of resin- andglass ionomer-based bulk-fill restoratives using digitalradiography,andtocomparethemwithenamel,dentin,and a conventional composite at different thicknesses.Our working hypothesis was that there would be asignificant difference in the radiopacity of resin-basedandglassionomer-basedbulk-fillrestorativesatdifferentthicknesses.

Materials and MethodsSpecimen preparationThepresentstudywasapprovedbytheResearchEthicsCommitteeof the IzmirKatipCelebiUniversity,underreport number 2014-70. Three resin-based bulk-fillrestoratives, two new commercial glass ionomer-basedbulk-fillrestoratives,andaconventionalcompositemate-rialwereselectedforthisstudy.Theselectedmaterials,manufacturers,andchemicalcompositionsare listed inTable 1. The sample size was calculated considering 80%

powerandasignificancelevelof0.05usingdata(effectsize=4.08)obtainedfromthestudybyLachowskietal.(2).Althoughthedatainthisstudysuggestedthatatotalofninespecimenswouldbesufficientfortheanalysis,aworst-casescenariowasproposedwitha0.93effectsize.According to thisworst-case scenario, the total samplesizewas calculated to be 30 (n = 5) considering 95% poweratasignificancelevelof0.05.Plasticringmoldswithaninternaldiameterof5mm

and depths of 1 mm, 2 mm, and 4 mmwere used topreparestandardizedspecimens.Fivespecimensofeachmaterial at each depth were prepared in accordance with the manufacturers’ instructions. The mold was placed on amicroscopeglassslab,andthematerialswereinsertedintothemolduntilitwasoverfilled.Thenamylarmatrixstrip was placed on the top and a second glass slab was positioned over the strip to flatten the surface.Chemi-

Table 1MaterialsusedinthisstudyMaterial Radiopaquefillercontent-Filler%(wt/vol) Manufacturer BatchNoQuixfil composite resin-based Zirconiumoxide,Silicondioxide-86/66 DentsplyDeTrey,Konstanz,Germany 121000SDRBulkFill composite resin-based Bariumaluminofluoroborosilicateglass,

Strontiumaluminofluorosilicateglass-68/44DentsplyDeTrey,Konstanz,Germany 1001086

FiltekBulkFill composite resin-based Zirconia/silica,ytterbiumtrifluoride-64/42 3MESPE,StPaul,MN,USA N435626EquiaFil glass-ionomer-based Fluoro-aluminosilicateglass-NA GCCorp.,Tokyo,Japan 1203121GCPGlassFill glass-ionomer-based Fluoro-aluminosilicateglass-NA GCPDental,Vianen,Netherlands 7211953FiltekZ550 conventionalcompositeresin Silanetreatedceramic,Silanetreatedsilica85/70 3MESPE,StPaul,MN,USA N334740

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cally activated materials were allowed to set for theperiod of time recommended by each manufacturer. The light-activatedmaterialswerecuredwitha lightsourceas recommended by themanufacturer (Valo, UltradentProducts, South Jordan, UT, USA).After the sampleshadbeenremovedfromthemold,thethicknesseswerechecked with a digital caliper in order to ensure stan-dardization.Afterpreparation,thespecimenswerestoredundermoistconditionsat37±1ºCuntiltheywereusedfor radiographic experiments. One freshlyextractedhumanmolarwasused in this

study to obtain enamel and dentin specimens. It wasprepared by longitudinal sectioning using a low-speed

diamondsaw(Isomet1001,BuehlerLtd.,LakeBluff,IL,USA),andslicesmeasuring1mm,2mm,and4mminthickness were obtained. The slices were kept in distilled water until used for radiographic experiments.

Digital radiographyOnespecimenofeachmaterialtogetherwiththealuminumstepwedge and a tooth specimenwere positioned overtheCMOS(complementarymetal-oxide-semiconductor)sensor (DIGORA Toto, SOREDEX, Milwaukee, WI,USA).The aluminum step-wedgewasmade of 99.5% pure aluminum alloy, with the thickness varying from0.5to10mminuniformstepsof0.5mm.Radiographicimageswereobtainedat65kVpand7mA.Theexposuretimewassetat0.32swithafocustotargetdistanceof30cm(Myray,CeflaDentalGroup,Imola,Italy).Figure 1 shows radiographic images of the enamel,

dentin,aluminumstep-wedge,andbulk-fill restorativesatdifferentthicknessesontheCMOSsensor.Themeangrayvalues (MGVs)ofeachof themate-

rials and tooth slices were measured on the digital radiographsusingasoftwareprogram(AdobePhotoshopCS3Extended,ver.10.0,AdobeSystems,SanJose,CA,USA)infivedifferentregions,eachwitha10×10pixelarea,toreducethemeasurementbias.Carewastakentoavoid areas containing air bubbles or other anomalies,andmeasurementsweretakenbyanevaluatorwhowasblindedtotheidentitiesofthematerials.AftertheMGVsof the observed steps of the aluminum step-wedge oneachimagehadbeencalculated,aregressioncurveequa-tionforeachimagewasdefinedfortheMGVsoffurtherstepsthatcouldnotbeobservedontheimagebecauseofthe limiteddimensionsof theCMOS.Thus,MGVsforeachof thestepsof thealuminumstep-wedge,varyingfrom 0.5 to 15mm in uniform steps of 0.5mm,wereobtained.(Fig.2).TheMGVsforeachofthematerialsand tooth slices were then converted into millimetersofaluminum(mmAl)using the followingequation,asreportedbyLachkowskietal.(2):

A×0.5 +mmAlbelowmaterial’sMGV

B

where:A:MGV of thematerial - theMGV of the aluminumstep-wedge increment immediately below the material’s MGV.B:MGVofthealuminumstep-wedgeincrementimmedi-atelyabovethematerial’sMGV-MGVofthealuminumstep-wedge increment immediately below the material’s MGV.0.5:0.5-mmincrementsofthealuminumstep-wedge.

Fig. 1 Radiographic images of tested restorative materials,enamel,dentin,andaluminumforthicknessesof1mm,2mm,and4mm.Noticethatincreasedthicknessesofthematerialswascorrelatedwithsignificantincreasesintheirradiopacity.

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The following example illustrates the application of this equation to calculate the radiopacity one 4-mm-thickQuixfilsample:MGVofthesample=193.68MGVofthealuminumstep-wedgeincrement(14.5mm)immediatelybelowthatofthesample=189.61MGV of the aluminum stepwedge increment (15mm)immediatelyabovethatofthesample=195.54Usingtheequation:A(193.68–189.61)=4.07B(195.54–189.61)=5.93(4.07×0.5⁄5.93)+14.5mmAl=14.84mmAl

Statistical analysisDatawereanalyzedstatisticallyusingtwo-wayanalysisof variance (ANOVA, IBM SPSS statistics ver. 20.0,IBMCorp.,Armonk,NY,USA) andTukey’s testwiththelevelofsignificancesetat0.05,todeterminethepres-ence of statistically significant differences between themeanvaluesofthetestedmaterials.

ResultsTwo-way ANOVA of the radiopacity testing datarevealed that the radiopacitywas significantly affectedbytherestorativematerialtypeandthethicknessofthematerial (P < 0.05), and the interaction effect betweentheevaluatedfactorswassignificant(P<0.05)(Table2).Themeanradiopacityvaluesandthestandarddeviationsofthematerials,enamel,anddentinareshowninFig.3.There was a statistically significant difference in

radiopacity values between the resin-based and glassionomer-based groups (P<0.05).Theradiopacityvaluesincreasedsignificantlywiththespecimenthicknesses(P <0.05).

Therewas a large variation among the radiopacitiesof resin-basedandglass ionomer-basedbulk-fill restor-atives, ranging from 1.95 to 5.04 at 1mm, to 3.44 to8.51at2mm,and6.53to14.73at4mm.Quixfilhadthehighest radiopacityvalueatall thicknesses,whileGCPGlassFillshowedthelowestvalueatallthicknesses.All of the radiopacities of the resin-based bulk-fill

restoratives showed a significant difference in compar-ison with enamel and dentin (P<0.001).For theglassionomer-based bulk-fill restoratives, the radiopacityvaluesofEquiaFilweresignificantlyhigherthanthoseof enamel and dentin, while the radiopacity values ofGCPGlassFillweresignificantlyhigherthanonlydentin(P < 0.05). The radiopacity values of GCP Glass Fillwerenotsignificantlydifferentfromthoseofenamelatthicknesses of 1 mm (P=1.000)and2mm(P=0.211).Atathicknessof4mm,theradiopacityofenamelwassignificantly higher than that of GCP Glass Fill (P < 0.001). For the resin-based bulk-fill restoratives, theradiopacityvaluesweresignificantlyhigherthanthoseofenamel and dentin (P<0.05).When the radiopacities of bulk-fill restorativeswere

comparable to that of the conventional composite, theglass ionomer-based bulk-fill restoratives showed asignificant difference (P < 0.05), but the resin-basedrestorativesdidnot(P>0.05).

DiscussionSecondary caries are generally located on the proximal gingival margin in 80-90% of cases, and radiopaquedental restorative materials allow better radiographicdetectionof secondarycaries (1). Inorder toallow fora correct diagnosis, restorativematerialsmust have anoptimalradiopacitytocontrastwithsecondarycaries(2).

Fig. 2ExampleofthecalculationoftheMGVsforeachstepofthealuminumstep-wedgeusingaregressioncurveequationononeoftheimagesofa4-mmsample.

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Therefore,materialswith a radiopacity lower than thatof enamel are not suitable for use as an initial increment. Thefirst incrementhas tobe adequately radiopaque tomake the tooth restorationmargin clearly visible (22).Otherwise,highlyradiopaquematerialsmaymaskcaries

lesionsbecauseofsuperimposition(23).Moreover,highradiopacity near a less radiopaque area can cause the MachBandeffect,whichproducesavisualillusionthatenhancesthecontrastbetweenalightandadarkerarea,making the dark borderline area darker. This effect might

Fig. 3MeanmmAl radiopacity values of bulk-fill restoratives incomparison to enamel anddentin. (Vertical black lines indicate thatthemeanvalueshavenostatisticallysignificantdifferencesfromeachotherwhenanalyzedusingTukey’stest,P>0.05.)

Table 2Two-wayANOVAforthematerialtype,thicknessandinteractiontermaccordingtotheradiopacitydata(P<0.05)

Sum of Squares df MeanSquare F PCorrectedmodel 922.797 23 40.122 6285.897 <0.0001Intercept 2346.381 1 2346.381 367610.224 <0.0001Thickness of material 540.717 2 270.358 42357.353 <0.0001Materialtype 322.284 7 46.041 7213.240 <0.0001Materialtype×thicknessofmaterial 59.796 14 4.271 669.160 <0.0001Error 0.306 48 0.006Total 3269.484 72Correctedtotal 923.103 71

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be misinterpreted as caries (1,20). Some authors havesuggested that a material with a moderate radiopacity is moreappropriate,andthatanoptimalradiopacitywouldbeslightlygreaterthanenamel(i.e.,2mmeq.Al)(10-15).In thepresent study, the radiopacityof theQuixfil andSDRBulkFill farexceeded the radiopacityofenamel,making them more likely to obscure the presence of a caries lesion, and therefore less suitable. On the otherhand,theradiopacitiesofEquiaFilandFiltekBulkFillseemedtobemoreappropriate.Additionally,becauseithas radiopacityvalues thataresimilar toor lower thanthat of enamel,GCPGlassFill,when used as thefirstincrement in the gingival part of posterior restoration,may result in areas that are misinterpreted as secondary cariesonaradiograph.Therefore,GCPGlassFillshouldnotbeusedasthefirstincrementinthegingivalpartofrestorations.

The radiopacity of a resin material is related to its content (weight and volume percent) and chemicalcomposition,andcanbe influencedbyspecimenthick-ness(20).Inthepresentstudy,anincreaseinthicknesswas generally correlated with an increased radiopacity of allmaterials,beingconsistentwiththefindingsofotherstudies (2,18,24).When added to the radiopaquefiller,elements with a higher atomic number (e.g., barium,zinc, aluminum, strontium, zirconium, silicon, yttrium,ytterbiumandlanthanum)increasetheradiopacityofthematerials because they enhance the capacity to absorb X-rays(2,5,20).Quixfilcontainsradiopaquefillers,suchas zirconia and silica, with a high percentage by bothweightandvolume(86%,66%),andthereforeitshowedthehighestradiopacityvaluesamongthematerialsinthisstudy. Oneofthedisadvantagesofconventionalglassionomer

is its lack of radiopacity.To overcome this deficiency,radiopaquesecondaryfillershavebeenadded(25-27).Inthepresentstudy,EquiaFilshowedsignificantlyhigherradiopacityvaluesthanGCPGlassFill.Althoughbothofthesematerialshavethesameradiopaquefillercontent,differences may be caused by filler volume fractions,which are not clearly reported by the manufacturers. ISOstandards(6,7)requiretheminimumradiopacity

ofrestorativematerialstobeequaltoorgreaterthanthatofanequivalentthicknessofAl.Aluminumwaschosenas a reference because it has the same radiopacity as dentin(9).Thedentinandenamelradiopacityvaluesinour studywere 1.10 and 1.96 at 1mm, 2.01 and 3.65at 2mm, 4.33 and 7.16 at 4mm eq.Al, respectively.Thevaluesforenamelanddentinareinagreementwithpreviousstudies,wherethedentinradiopacitywascloseto1mmeq.Alandtheenamelradiopacitywascloseto2

mmeq.Al(3,5,22,28).In the present study, a digital radiographic system

was used. The main advantage of this radiographicdigital system is that development procedures are notrequired.Traditionalfilmdevelopment,unlessperformedcarefully,canproducesignificantvariations in thefinalradiograph.Incontrast,adigitalmethodprovidesmoreconsistentresults(8).Inaddition,itprovidestheMGV,which is calculated directly by the computer software with the same standard for all specimens. The obtained MGV is used to convert the radiopacity values tomillimetersofaluminum(2).In thisstudy,asimplifiedequationreportedbyLachowskietal. (2)wasusedforconversion toMGVs. However, when this equation isused, theMGVforeachstepof thestep-wedgeshouldbe calculated.Therefore, using the originally observeddata(MGVsforthefirstelevenstepsofthestep-wedge)(Fig. 2), the regression equation was defined for eachimageinordertocalculatetheMGVsforthestepsofthealuminum step-wedge.Varying the radiographic exposure time and target

distance are factors that affect the radiopacity of restor-ative materials. Nevertheless, Gu et al. (29) using adigitalX-raysysteminwhichtheexposuretimevariedfoundthatthisdidnotsignificantlyaffecttheradiopacitymeasuredatatargetdistanceof30cm,andthatvaryingthe target distance did not significantly affect the radi-opacity as long as the samples were properly exposed (8).Inthepresentstudy,thetargetdistancewassetto30cm andwas not changed during the experiment. Froma theoretical standpoint, while an underexposed imagehas a background fog, overexposed images ‘black out’objects of low radiopacity (29). In the present study,theexposuretimewaslongenoughtovisualizea1-mmthicknessofaluminumwhilenotproducingverymuchbackground fog. Further studies on the radiopacity ofbulk-fillrestorativeswillbenecessaryforevaluatingtheeffect of different combinations of exposure times and targetdistancesusingdifferentX-raysystems.Within the limitations of this study, the radiopacity

values of resin-based and glass ionomer-based bulk-fill restoratives appear to differ considerably, and it isimportant that any restorative material has sufficientradiopacitytoallowdetectionofsecondarycaries.Allofthe testedbulk-fill restoratives in this studypassed theISOrequirementsforradiopacity.

Declaration of interestTheauthorsreportnoconflictsofinterest.Theauthorsaloneareresponsible for the content and writing of the paper.

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