The effect of preparation taper on the resistance to fracture of monolithic zirconia crowns

dental materials 37 (2021)e427–e434

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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

The effect of preparation taper on the resistance to fracture of monolithic zirconia crowns

Christian Schriwer

, Nils Roar Gjerdet

, Dwayne Arola

, Marit Øilo

aDepartmentofClinicalDentistry,FacultyofMedicine,UniversityofBergen,Norway

bMaterialsScienceandEngineering,UniversityofWashington,USA

cDepartmentofRestorativeDentistry,DentalSchool,UniversityofWashington,USA

a r t i c l e i n f o

Articlehistory:

Received9November2020 Receivedinrevisedform 15March2021

Accepted28March2021

Keywords:

Zirconia Dentalcrowns Taper

Convergenceangle Fractureresistance Fractographicanalyses Cementthickness

a bs t r a c t

Objective.Monolithiczirconiacrownshavebecomeaviablealternativetoconventionallay- eredrestorations.Theaimofthisstudywastoevaluatewhetherthetaper,andthuswall thickness,oftheabutmentorpre-definedcementspaceaffectthefractureresistanceor fracturemodeofmonolithiczirconiacrowns.

Methods.Amodeltoothwaspreparedwithataperof15^◦andashallowcircumferential chamferpreparation(0.5mm).Twoadditionalmodelsweremadebasedonthemastermodel withataperof10^◦and30^◦usingcomputer-aideddesignsoftware.Twentymonolithic3rd generation translucentzirconiacrownswereproducedforeachmodelwithpre-defined cementspacesettoeither30␮mor60␮m(n=60).Theestimatedcementthicknesswas assessedbythereplicamethod.Thecementedcrownswereloadedcentrallyintheocclusal fossaat0.5mm/minuntilfracture.Fractographicanalyseswereperformedonallfractured crowns.

Results.Theloadatfracturewasstatisticallysignificantdifferentbetweenthegroups(p<

0.05).Thecrownswith30^◦taperfracturedatlowerloadsthanthosewith10^◦and15^◦taper, regardlessofthecementspace(p<0.05).Thefractureoriginfor47/60crowns(78%)was inthecervicalarea,closetothetopofthecurvatureinthemesialordistalcrownmargin.

Theremainingfracturesstartedattheinternalsurfaceoftheocclusalareaandpropagated cervically.

Significance.Thefractureresistanceofthemonolithiczirconiacrownswaslowerforcrowns withverylargetapercomparedto10and15^◦tapereventhoughthecrownwallswerethicker.

©2021TheAuthor(s).PublishedbyElsevierInc.onbehalfofTheAcademyofDental Materials.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.

org/licenses/by/4.0/).

1. Introduction

Material choice, preparation and cementation technique affecttheclinicalperformanceofall-ceramicdentalcrowns

Abbreviations:Y-TZP,yttria-stabilizedtetragonalzirconiapolycrytalline;3D,three-dimensional;FEA,finiteelementanalyses;CAD/CAM, computer-aided-design/computer-aided-manufacturing.

∗ Correspondingauthorat:Aarstadveien19,NO-5009Bergen,Norway.

E-mailaddress:[email protected](C.Schriwer).

in terms of fracture rates, retention, function and/or aes- thetic [1–3]. The use of yttria-stabilized zirconia (Y-TZP) is increasingduetoitshighflexuralstrength,fracturetoughness andgoodbiocompatibility[4–6].Highersinteringtemperature andlongerdwell-timeresultinlargergrainsize[7–9].Higher

https://doi.org/10.1016/j.dental.2021.03.012

0109-5641/©2021TheAuthor(s).PublishedbyElsevierInc.onbehalfofTheAcademyofDentalMaterials.Thisisanopenaccessarticle undertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

Fig.1–Overviewofthestudygroups.A:Thethreedifferentmodelswithtaperof10^◦,15^◦and30^◦.B:Twentycrownswere producedofeachmodel;tenwithacementspaceof30␮m(left)andtenwith60␮m(right).Theouterlimitofthecrowns remainedunchangedforallgroups.

amountofyttriaresultsinahigherproportionofcubicphase crystals[6,10].Thesealterationofthematerialcomposition andproductionmethodhasresultedinmoretranslucentzir- coniamaterials(oftenreferredtoas3rdgenerationzirconia), whichareaestheticallyacceptableevenwithoutaveneering layer,“monolithiczirconiacrowns”[6].Butthealterationsthat increasethe translucencycause areductioninthe flexural strengthandfracturetoughness[9].

Fracturesandlossofretentionare themainreasons for complicationswithzirconiacrowns[11].Theintroductionof monolithiczirconiacrowns hasreducedthe problemswith chippinganddelamination[12,13].Fromabiologicalperspec- tivetheidealsituationistoremoveaslittletoothsubstanceas possible.Thisusuallyconflictswithrequiredmaterialthick- nesstoachieve acceptableaesthetics and flexuralstrength [14]. Elimination ofaveneering layerreduces theneed for preparationdepth.Materialthicknessocclusallyof3Y-TZPof 0.5and1.0mmstillgiveshigherloadsatfracturethana1.5 mmthicknessoflithiumdisilicatereinforcedglassceramic [9,15];a thicknessof1 mmhassimilar strength tothat of traditionalmetal-ceramiccrowns[13].Thefewstudiesregard- ing thickness of monolithic zirconia crowns have focused solelyonocclusalthickness[13,16].Severalstudiesindicate allceramiccrownsmainlyfailfromfractureinitiatedinthe crownmargins[17].Itisthusprobablethatmargin-orwall thicknessisofgreaterimportanceforfractureresistancethan occlusalthickness.

Asthecrowncontourisprimarilydrivenbyanatomicstruc- tures,aestheticsandhygienicdesign,thecementspaceand thetoothabutmentpreparationarethevariablesthatcanbe adjustedwhilemaintainingthecrown’smaterialthickness.

Thecementspaceispre-definedbytheoperatoranddental technicianwhenmanufacturingthecrown.Theinternaland marginalfitistheactualfitwhenplacingthecrown.There is no consensusabout the clinically acceptableinternal or marginalfitofthecrown,butmostpublicationsrecommend amarginalfitbelow120␮m[18,19].Poormarginalfithasclin- icalimplicationsforadhesionoforalbacteriathatcanleadto secondarycariesandgingivalinflammationandsubsequent deteriorationofperiodontalhealth[20–24].Toonarrowaxial cementspacepreventscementflowatthecrownmargin,and thus result intoothick of occlusalcementlayer and axial discrepancyofthecrownatthecervicalmargin.

Preparation techniques for dental crowns have varied greatly over time [25,26], ranging from preparations with smalltaper(convergenceangle)andsharpedges,depending on mechanicalretention,torounded edgesandlargetaper depending on adhesive cement. Design of the restoration, sizeanddistributionofmaterialflaws,residualstress,degra- dation, ceramic-cement interfacialfeatures, wallthickness, elasticmoduliofthematerialandforcesappliedareallcon- tributing factorstocrackinitiation andpropagation[27,28].

There is stilllimited understanding ofhow specificdesign variablescontributetofractureresistanceofmonolithiczirco-

dental materials 37 (2021)e427–e434

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Table1–Thematerialusedwithbrandname,productionmethod,materialcompositionandgrainsize.Thedataare fromthemanufacturer.

Code Brandname Productionmethod Materialcomposition Grainsize

DDX2 DentalDirektDDCubeX² Soft-machined ZrO2+HfO2≥90%,Y2O3<10%,Al2O3<0,1%,otheroxides≤0.005%. 0,36␮m

niacrowns.Bothcementthicknessandtaperoftheabutment affectthemechanicalretention,seatingandmarginalfit,but itisuncertainwhetheritalsoaffectsthefractureresistance [25]. Theconsequences ofa larger taper willinevitably be increasedwallthicknessgiventhesameanatomicalformof theoutercontour.Therefore,theaimofthisstudywastoeval- uatewhetherpre-definedcementspaceortaper,andthuswall thicknessoftherestoration,affectthefractureresistanceof monolithiczirconiacrowns.

2. Material and methods

Asyntheticsecondpremolartooth(KaVoDentalGmbH,Bib- erach,Germany)ofthelower jaw,#45,waspreparedwitha circumferentialshallowchamferof0.5mm,ataperof15^◦and roundededges.Thedepthandtaperweremeasuredrepeat- edly during preparation by light microscopy (Leica Model TM-505/510,MitutoyoAmericanCorporation,Illinois,USA).At 10× magnificationwithadesignatedsoftware(LeicaAppli- cationsuiteV4.4)untilcorrectdepthand15^◦wereobtained.

AnA-siliconeimpression(Affinis,3MESPE,Minneapolis,USA) wasusedtoproduceastone modelofthepreparationand adjacentteeth(TannlabA/S, Oslo,Norway).Themodelwas digitally scanned to produce a 3D model (3Shape Dental Designer,Copenhagen,Denmark).Thetaperwascontrolled tobe15(±2)degreesandtwoadditionalmodelswith10^◦and 30^◦taperweredigitallydesignedbasedonthemastermodel inthesoftware.Thefinishlineandthechamferdepthwere identicalinallgroups.

The pre-defined cement space, hereby referred to as cementspace,wasdigitallysettoeither30␮mor60␮monall threemodels(Fig.1).Tenidenticalmonolithiczirconiacrowns 5Y-TZP(DDCubeX²,DentalDirektGmbH,Spenge,Germany) werepreparedforeachofthesixtestgroupsaspresentedin Table1,resultinginatotalof60crowns.Theproceduresfol- lowedmanufacturer’srecommendations.Physicalmodelsof thethreedigitaldesignswereproducedbyadditivemanufac- turing(ProJet3510MP,3DSystems,RockHill,USA).Twenty identicalepoxymodelswereproducedforeachdesign(EpoFix, StruersA/S,Ballerup,Denmark).Oneepoxymodelwasmade tofiteachoftheindividualcrowns.

Allthecrownswereinspectedforcracks,marginchipping andotherdefectsusingalightstereomicroscope(LeicaM205 C,Heerbrugg,Switzerland)equippedwithaLEDringlightat 10×magnification.Picturesweretakenfordocumentationat 10×and20×magnificationifdefectsweredetected.Thequal- ityofthecrownmarginswasgradedaccordingtoa5-graded scale[17].Forthe20crownsmatchingthemastermodel,the estimatedcement thickness wasmeasured using a replica methodmeasuringsiliconefilmthicknessforcementthick- nessasdescribedindetailelsewhere[29].

Thecrownswerecementedtotheirrespectiveepoxymodel with standardized pressure (110N), using glass-ionomer cement (Fuji One, GC Corporation Tokyo, Japan). Excess

cement was removed after setting,and the crowns placed in distilled waterat 37 ^◦C for24 (±1) h. Thecrowns were subsequentlyloadedcentrallyattheocclusalsurfacewitha horizontalsteelcylinderwithadiameterof13mmusinga servo-hydraulicmaterialtestingsystem(MTS852MiniBionix II, Minnesota,USA)at0.5mm/minuntilfracture. Thesteel cylinderwascushionedwitha3mmthickrubberdiscofhard- ness90 ShoreAtodistribute theload evenlyand toavoid contactdamageduringloading.Thespecimenswere main- tainedinwateratroomtemperatureduringloading.Theload atfracturewasrecorded.Eachcrownwasinspectedusingthe aforementionedstereomicroscopetoassessthefracturemode andorigin.

Thestatisticalanalysiswasperformedusingacommercial softwarepackage(Stata13.1.StataCorpLLC,CollegeStation, USA). Resultsforthe loadatfracture wereevaluated using a Kruskall–Wallis test, supplemented with Kruskall–Wallis equality-of-populationsranktesttoanalysethe differences amongthegroups.Aone-wayANOVAtestwasusedtoassess theinternalfit.Spearman’srankcorrelationtestwasusedto evaluatecorrelationsbetweenvariables.Thesignificancelevel wassetto0.05.

3. Results

3.1. Crownmarginquality

There wasno significant differenceinthe gradeofmargin defectsbetweenthegroups(p>0.05,Fig.2).Furthermore,there wasnosignificant correlationbetweenthegradeofdefects andloadatfracturewithinthegroups(p>0.05).Thesizeand numberofdefectsinthesamplewere,ingeneral,small.

3.2. Cementthickness

Theestimatedcementthicknessvaluesofcrowns with15^◦ taperanddifferentsettingsforcementspacemeasuredbythe replicamethod,areshowninFig.3.Therewasnosignificant differenceintheocclusalcementthicknessorthemarginal cementthicknessbetweenthegroupswithdifferentcement space(p>0.05).Thegroupwith60␮mcementspacehadsig- nificantlyreducedcementthicknesscomparedtothegroup withcementspaceof30␮m(p<0.05).

3.3. Loadatfracture

There were significant differences in the load at fracture amongthetestedgroups(p<0.05)asshowninFig.4.Rangefor thedifferentgroupswere10^◦(1087N–2583N),15^◦(642N–2495N) and30^◦(771N–1769N).Thespecimenswitha30^◦taperhada significantlylowerloadatfracturethantheothergroups(p<

0.05).Therewerenosignificantdifferencesinloadatfracture betweenthegroupswithdifferentcementspaceandidentical taper(p>0.05).

Fig.2–Tukey’sboxplotillustratingthe“gradeofdefect”ofthegroups.Thebottomandtopoftheboxrepresentthefirstand thirdquartiles,andtheinternalhorizontallinesrepresentthemedian.Thewhiskersrepresentthemaximumand

minimumdatawithinthe1.5interquartilerange(IQR).Thedotsrepresentoutliersoutside3IQR.Therewasnosignificant differencebetweenthegroupsregardingthegradeofdefects(p>0.05).

3.4. Fracturemode

Resultsfromthefractographicanalysesshowedthattheorigin offracturefor47ofthe60crownswasinthecervicalarea.

Thefracturestartedclosetothetopofthecurvatureinthe mesialordistalcrownmargin(Fig.5).Thereafter,thecrack propagatedtowardsandthroughtheocclusalareafollowing thepathofleastresistancetotheoppositecervicalarea.In theremainingthirteenofthecrowns,theorigin offracture wasintheocclusalarea.Specifically,thesefracturesstarted attheinternalsurfaceandpropagatedtotheexternalsurface andtotheapproximalcervicalareaonbothsidesasshownin Fig.6.

4. Discussion

Fracture resistanceis akey requirement to the survivalof all-ceramic crowns.The aim of this study was to identify whethertaperoftheabutmentorpre-definedcementspace affecttheloadatfracture,aclearmetricoffractureresistance.

Theresultsindicatedthatthetaperaffectedloadatfracture, whereaspre-definedcementspacedidnot.Furthermore,an increaseofthetaperto30^◦decreasedtheloadatfracturecom- paredwithataperof10^◦and15^◦eventhoughthecrownswalls weremuchthicker.

Previous studies indicate that increased material thick- nessincrownwalls,ingeneral,increasestheloadatfracture [30–32].Tosomeextent,thepresentresultscontradictthese studiesasthecrownwallsweremuchthickerinthe30^◦taper groupcomparedtothe10and15^◦groups.Themarginthick- nesswas,however,identicalinallgroups.Thereisapositive associationbetweentheelasticmodulusofthematerial,wall thicknessandthecompressivestrength.Inaddition,thereis substantial evidenceindicatingthatathickercorematerial resultsinreducedriskoffractures[33–36].Thesestudieshave, however,mostlyfocusedonbi-layeredstructuresandadirect comparisonisdifficult.

Theimportanceofthecrowngeometryonthemagnitude ofmarginalstresshasbeen emphasizedthrough resultsof previousfiniteelementanalyses(FEA)[33,37].Whenaforce is applied on top ofthe crown, most ofthe stress is dis- tributed tothe occlusalarea andsomestressisdistributed throughtheaxialwallsofthecrown[1].Theocclusalareaof thepreparationandwallthicknesswillvaryasafunctionof the taper.Higherstressdevelopsattheocclusalareawhen thetaperofthepreparationissmalland,subsequently,when the occlusalareaisincreased[38].Areduced occlusalarea and,subsequently,alargertaperthereforeresultsinlarger stresseswithintheaxialwallsofthecrownwhencompared toasmallertaperandlargerocclusalarea[39].Inaddition, theheightofthepreparationinfluencesthestressatthecer- vicalareas[37].Onestudyindicatesthatsmallertaperresults

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Fig.3–Tukey’sboxplotofthecementthicknesseswiththedifferentpre-definedcementspaces.Therewasnosignificant differenceoftheocclusalandmarginalcementthicknessamongthegroups(p>0.05).Thegroupwithcementspaceof60

␮mhadsignificantlyreducedaxialcementthicknessthanthegroupwithcementspaceof30␮m(p<0.05).Seelegendto Figure2forexplanationofbox-plot.

inlower stress[40],but anotherindicatestheopposite[38].

Neitherofthesepreviousstudiesexplainorexploretheconse- quencesofareductioninstressattheocclusalareaoncrown fractures(locationandmode).Thefractographicanalysesin the present study showed that most fracture origins were locatedcervicallyasevidentinFig.5. Thus,understanding thestressinthisareacouldbedecisive.Theobservedfracture modesweresimilartothosereportedfromstudiesconsisting offractographicevaluationsofclinicallyfailedzirconiacrowns [41].

Throughocclusalloadingofthecrown,theabutmentmate- rialwillundergocompressionaswellastransverseexpansion asaresultofPoisson’seffect.ThePoisson’sratioofepoxyand dentinissimilar(∼0.3).Therefore,thedegreeofbulgingofthe epoxymodelisexpectedtobethesameasthatofteethdur- ingmasticationwhensubjectedtothesameocclusalload.The bulgingcausesthedevelopmentofhoopstressthatislargest atthecrownmargin.Thatstressstatefacilitatestheinitiation ofcracksperpendiculartothishoopstressandperpendicular tothecrownmarginasseeninclinicalfailures[42,43].

Thetapersofthisstudywere10^◦,15^◦and30^◦.Thetheoret- icallyidealtaperhasvariedfrom2to22[44,45],butclinically valuesrarelymeetstheseparameters[46].Asystematicreview from2015showsthatdailyreportedpracticesliebetween18.2 and23.9[47].Ourchoiceoftapersischosentolieintheouter limittogiveanindicationoftheimpactofthedifferenttapers.

Previousstudiesofcrownsproducedusingthecomputer- aided-design/computer-aided-manufacturing (CAD/CAM) techniquehaveshownacceptableinternalfit,comparableto otherlaboratorytechniques[48,49].Inthepresentstudythe marginalcementspacesetinthetwogroupswereidentical, sothefindingthatbothgroupshadequalmarginalcement thicknesswasexpected.Thegroupwithcementspaceof60

␮mhadthelowestvaluesofthecementthicknessindicating good seating. Good seating combined with a small taper increasesthe mechanicalretentionand minimizestheloss oftoothsubstance,therebyimprovingclinicalsuccess[24].

Anincreasedclearanceinthegroupwith60␮mcement space mayexplainthis finding [50].Theimportanceofthe cementandcementthicknessonthefactureresistanceare notconclusive[1,35,51,52].Theelasticmodulusoftheepoxy (10.5GPa)andtheglass-ionomercement(7−8GPa)arequite similar,whiletheyttria-stabilizedzirconiahasahighermod- ulus(205GPa).Theabsenceofacorrelationbetweentheload atfractureandinternalfitinFig.4canbeexplainedbythe similarcharacteristicofthetwomaterialsusedasabutment andcement.

Ingeneral,thecrownmarginqualitywasgood.Thisfind- ingcouldexplainthelackofcorrelationbetweenmarginflaws andtheloadatfracture.However,thisfindingisnotfullyin agreementwiththatofThompsonetal.[27]wheremachining defectsaffecttheloadatfracture.Thepreparationusedinthe

Fig.4–Tukey’sboxplotoftheloadatfractureofthe differenttapersandcementthickness.Thegroupswith samesuperscriptlettersarenotsignificantlydifferentfrom eachother(p>0.05).SeelegendtoFigure2forexplanation ofbox-plot.

Fig.5–Afractographicmapforarepresentativecrown(30^◦, 60␮m).Thefractureoriginwaslocatedatthecervical margin(whitearrow).Theoriginissurroundedbyaflat smootharea(fracturemist).Thethinblackarrowsindicate thedirectionofthehacklelinesradiatingfromthefracture mist,andtheCCshowscompressioncurlsmarkingthefar endofthefracturepathwheretensionalforceconvertsto compressiveforce.

presentstudywasmadeonasinglemodelwithanevenand smoothfinishline.Assuch,universalconclusionscannotbe drawn.Consideringthevariablespresentintheclinic,every preparationwillhaveanindividualappearanceandthereare manyvariablesthatcontributetothestressdistributioninthe crown[33].Thepresentresults,however,indicatetheimpact ofthepreparationtaperforceramiccrowns.Thespecimens thatweretestedinthisstudyweremadeofzirconiawitha highpercentageofcubiccrystals(49%).Thisreducesitsflex- uralstrengthandfracturetoughnesswhencomparedtothe moretraditionalY-TZP[9].Nevertheless,theloadatfracture inthepresentfindingsexceedsthatexpectedfrommastica- tionforces.Thisstudywasaninvitrostudy,wheretherewasa

Fig.6–Afractographicmapillustratingthefractureofa representativecrown(15^◦,30␮m).Thefractureoriginwas locatedattheinternalsurfaceoftheocclusalarea(white arrow).Thethinblackarrowsindicatedirectionhackle lines,andtheCCshowscompressioncurl.

singleloadtofracturewithaconstanttemperature.Oralfunc- tionwithmasticationforcesatdifferentanglesandchanging temperatures makesdirectcomparisontoclinicalvaluesis difficult[53].Thetestedspecimenswereneitherexposedto artificialaging.Thesimilarappearanceofthefracturemodes inthe presentstudy andthoseobservedinclinicallyfailed crownsindicates,however,thatthestresssituationsaremore closely relatedthanmany otherinvitrotrials withocclusal contactdamages.Furtherstudiesarenecessarytoassessthe effectofagingcomparedtoimmediateloading[54].

5. Conclusion

Largepreparationtaperreducedtheloadatfractureofmono- lithic zirconia crowns. A larger pre-defined cement space improvedseatingbutdidnotaffecttheloadatfracture.

Acknowledgments

The authors gratefully acknowledge Odd Johan Lundberg, HeleneHofstad,SteinAtleLieandTannlabforcontributing tothisarticle.

ThisresearchwasfundedbyUniversityofBergen,Norway.

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