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AbstractIntroduction:Thepurposeofthisstudywastoexaminetheeffectofdifferenttemperatures(3˚C,22˚C,37˚C,and60˚C)oncyclicfatigueofthreedifferentnickel-titaniumrotaryfiles.Methods:Threegroupsofnickel-titaniumrotaryfiles(EdgeFile[EdgeEndo,Albuquerque,NM],VortexBlue[DentsplyTulsaDentalSpecialties,Tulsa,OK],andESX[BrasselerUSA,Savannah,GA])ofsize#25with.04taperand25mmlengthweretestedinametalblockthatsimulatedacanalcurvatureof60˚and5mmradiusofcurvature.Theblockwassubmergedinacontrolled-temperaturewaterbathfilledwithwaterat3˚C±0.5˚C,22˚C±0.5˚C,37˚C±0.5˚C,and60˚C±0.5˚C.Thirtyfilesfromeachexperimentalgroupweretestedintheblockateachofthefourtemperaturecycles,androtatedat500rpm.Timetofilefracturewasrecorded,andconvertedtonumberofcyclestofracture(NCF).Statisticalanalysiswascompletedusingaone-wayANOVAwithpost-hocTukeytest.Results:VortexBluefilesshowedasignificantdecreaseinNCFastemperatureincreasedfrom3˚Cto60˚C.ESXfilesshowedasignificantdecreaseinNCFastemperatureincreasedfrom3˚Cto37˚C.EdgeFilefilesshowedasignificantincreaseinNCFfrom3˚Cto22˚C,andasignificantdecreaseinNCFfrom22˚Cto37˚C.Foreachtemperaturetested,EdgeFilefilesshowedhigherNCFthanVortexBluefiles,andVortexBluefilesshowedhigherNCFthanESXfiles.Conclusions:Inthisinvitrostudy,temperaturewasfoundtohaveasignificanteffectonthecyclicfatigueoftheNiTirotaryfilestested.Ateachtestedtemperature,NCFofEdgeFilefileswashigherthanNCFofVortexBluefiles,whichwashigherthanNCFofESXfiles.Futurecyclicfatiguestudiesshouldconsiderbeingconductedatbodytemperature.Keywords:temperature,cyclicfatigue,nickel-titanium,files

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Introduction Nickel-titanium(NiTi)endodonticrotaryfilespossesssuperelasticity,whichisofgreatbenefitduringinstrumentationofcurvedendodonticcanals(1).Theseinstrumentshaveanaustenitephaseathighertemperatures,anduponcoolingorapplyingstresses,cantransformintothemartensitephase.Thisphasetransitionallowsforgreaterflexibility.Thetransitionphasetemperaturesoffilescandifferdependingonthealloysandheattreatmentsusedforprocessing(1).

ApilotstudyconductedattheUniversityofDetroitMercyin2015showedEdgeFilefilestohavesignificantlygreatercyclestofracturecomparedtoVortexBlueandEndoSequencefilesinasimulated60˚curveinametalblockatroomtemperature.HeatedNaOCl(sodiumhypochlorite)irriganthasbeenshowntobeadvantageousfordisinfection(2,3).Theuseofheated1%NaOClsolutionat60˚Cwasfoundtohavesignificantlygreaterdissolutioncapacityofhumanpulptissuecomparedwith20˚Csolution(2).Additionally,heatedNaOClat45˚CshowedgreaterkillingofE.faecalisthanat20˚C(2).ClinicallythisheatingmaycausetheNiTifiletotransitiontowardsthestifferaustenitephase.ThiscanmakethefilemoresusceptibletofatiguecrackpropagationcomparedtoaNiTifilepredominantlyinthemartensitephase(4).Coolingcanallowforgreaterflexibilityofnickeltitanium.Nitinolstentswereshowntobemoreeasilymountedandremovedoncecooledwithice-waterat4˚C,duetoincreasedpliabilityofthestentoncecooled(5).OnestudyusedNiTiwiretocreaterotaryinstruments,andtestedthematthreedifferenttemperatures;ahighernumberofcyclestofracturewasseenat10˚Ccomparedto50˚C(6).AnotherstudytestedavarietyofrotaryNiTifilesavailableinthemarketat20˚Cand37˚C,andfoundadecreaseinthenumberofcyclestofractureat37˚Ccomparedto20˚C(7).

Theincreasedductilityofafileinitsmartensitephaseallowsittobehighlyelastic.Thisalsoimprovesdampingproperties,resistancetocrackgrowthduetofatigue,andahigherresistancetocyclicfatigue(4).TheuseofheatedorcooledirrigantsmayfavortheNiTiphasetransitionoftheinstrumenttowardsmoreausteniticormartensiticphase,respectively,whichcanaffectitscyclestofracture.

ThepurposeofthisstudywastoexaminetheeffectoftemperaturechangesonthecyclicfatigueofEdgeFile,VortexBlue,andESXrotaryNiTiinstruments,testedinametalblocksubmergedinawaterbath.MaterialsandMethods Onehundredtwentynickel-titaniumrotaryendodonticfileswereusedforeachofthethreeexperimentalfilegroups:EF(EdgeFilefiles;EdgeEndo,Albuquerque,NM),VB(VortexBluefiles;DentsplyTulsaDentalSpecialties,Tulsa,OK),andESX(ESXfiles;BrasselerUSA,Savannah,GA).Allfilesweresize#25with.04taperand25mmlength,andweresterilizedbeforeuse.Theentirelengthofeachfilewasplacedintoasimulatedcanalinametalblock,whichwassubmergedinawaterbath.Thefileswererotatedat500rpm(whichiswithintherecommendedrangeofrotationalspeedsinthedirectionsforuseofeachfilesystem)withtheuseofanendodonticmotor(TulsaE3motor).Themetalblockwasmadeoftemperedsteelwithamilledcanalthatsimulateda60˚canalcurvatureand5mmradiusofcurvature,andawidthof1.5mm.Adigitalthermometerwasusedtomeasurethe

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temperatureofthewaterbeforeinitiatingtestingofeachfile.Withineachgroupof120files,fileswererandomlysubdividedfortestingatthefourdifferenttemperaturecycles,with30filespertemperaturecycle.Thefourdifferenttemperaturecycleswereicewater(3˚±0.5˚C),roomtemperature(22˚±0.5˚C),bodytemperature(37˚±0.5˚C),andhotwater(60˚C±0.5˚C).ApictureofthesetupisshowninFigure1.Avideorecordwasmadeofeachfilerotation,andtimetofracturewasrecordedinseconds.Thiswasidentifiedbyvisualand/oraudiblesoundoffracture.TimetofracturewasconvertedintoNCF.

Figure1.PictureofwaterbathandelectricmotorsetupResults Aone-wayANOVAwithpost-hocTukeytestwasusedtocomparethecyclicfatigueoftheinstrumentsatdifferenttemperatures.TheresultsaregraphicallyshowninFigure2.Whenthetemperaturecycleincreasedfrom3˚Cto22˚C,theaverageNCFsignificantly(p<0.01)decreasedforbothVBandESX,from4842(SD=1136)and932(SD=163)to2062(SD=358)and466(SD=66),respectively.However,theNCFsignificantly(p<0.05)increasedforEFfrom6185(SD=2143)to7243(SD=2088).Whenthetemperaturecycleincreasedfrom22˚Cto37˚C,allfilesshowedasignificant(p<0.01)decreaseintheNCF,toaveragesof1233(SD=218),271(SD=55),and1675(SD=384)forVB,ESX,andEF,respectively.Furtherincreasingthetemperaturefrom37˚Cto60˚Ccausedasignificant(p<0.01)reductionintheNCFforVBtoanaverageof651(SD=111).ESXandEFalsohadreductionsintheirNCFtorespectiveaveragesof218(SD=45)and901(SD=201),

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butthesewerenotstatisticallysignificant.Ateachtemperature,EFhadasignificantly(p<0.01)higherNCFthanVB.VBhadasignificantly(p<0.01)higherNCFthanESX.

Discussion ThefindingsofthisstudyshowedthattemperaturehasasignificanteffectonthenumberofcyclestofractureforallrotaryNiTiendodonticfilestested.Thisisinagreementwithpreviousstudies(6,7)whereincreasingthetemperaturewasfoundtodecreasetheNCF.However,thisstudyexaminedagreatervarietyoftemperaturesthantheotherstudies.Thisisofimportance,becausecyclicfatiguestudiesareusuallyconductedinanexvivosettingwheretemperatureisnotconsideredasavariable.Futurestudiescanbemademoreclinicallyrelevantbybeingconductedatbodytemperatureinsteadofroomtemperature.TheNCFfortheESXandVortexBluefilesnearlyhalvedwhenthetemperaturewasincreasedfromroomtemperaturetobodytemperature,whilethatforEdgeFilefilesdecreasedbyoverfourtimes.ThislargedifferenceinNCFisapparentwhencomparedtotheotherfiles.Forexample,whencomparedtoVortexBluefiles,atroomtemperaturetheNCFofEdgeFilefileswasover3.5timeshigher.Whenheatedtobodytemperature,theNCFofEdgeFilefileswasonly1.35timeshigherthanVortexBluefiles;however,thiswasstillsignificant.ThemetallurgicpropertiesofendodonticfilesappeartoaffecttheirNCFinadditiontotemperature. Filetypeswereselectedtogetavariationinmetallurgy.TheEdgeFilefilesarepredominantlyinamartensiticphaseatroomtemperature,whichisresponsible

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Figure2.EffectofTemperatureonCyclicFatigue

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fortheirlackof‘shapememory.’FurthercoolingoftheEdgeFilefilesto3˚CresultedinadecreaseintheNCF.ThisdecreaseinNCFmaybeduetoamechanismsimilartohowtraditionalmetalsbecomemore‘brittle’astheycool,causingthemtofracture.Heatingmostlikelycausedatransitiontoanausteniticphase,reducingtheNCF.AlthoughtherewasareductionintheNCFfrom37˚Cto60˚C,thiswasnotfoundtobestatisticallysignificant.BoththeESXandVortexBluefilesatleastdoubledtheirNCFwhentheywereplacedinicewater,sincetheymaypossessmoreausteniticphaseatroomtemperaturecomparedtoEdgeFilefiles.Bycoolingthem,theytransitionedtoamoremartensiticphase.ThedifferenceinNCFforESXfilesfrom37˚Cto60˚Cwasnotsignificant.Thismaybeduetothefilesalreadybeinginmostlyanausteniticphaseatbodytemperature;consequently,furtherheatingdidnothaveaneffect. OurfindingssuggestthatanirrigantchilledbelowroomtemperatureseemsfavorabletoincreaseNCF,exceptforfilesthatarealreadyinamartensiticphasesuchasEdgeFilefiles.Veraetal.(8)examinedusing2.5˚Cirrigantexvivoandfoundthatirrigationfor5minutesresultedina10˚Cdecreaseinexternalroottemperatureafter30seconds.Theyconcludedthatusingthechilledirriganttodecreaseexternalroottemperaturemightbeawaytoreduceinflammationintheperiradicularregion.InadditiontoincreasingNCF,anotherbenefitofacoldirrigantmaybetoprovideananti-inflammatoryeffect.

UsingaheatedNaOClirrigant,however,hasbeenshowntobeeffectivefordisinfectionandtissuedissolution(2,3).Accordingtoourfindings,theuseofaheatedirrigantduringfileinstrumentationmayincreasetheincidenceoffileseparationduetothereductioninNCFthatmayresult.Furthermore,exposureofNiTifilestoheatedNaOClmaycausemorecorrosionleadingtofasterseparation(9).Therefore,aheatedNaOClirrigantmaybebestsuitedasafinalstepintheirrigationprotocol.InaninvivostudybydeHemptinneetal.(10),roomtemperatureNaOClsolutioninjectedintoacanalincreasedtemperatureto30.9˚Cafteronly10seconds.Thetemperatureofaheatedsolutiondecreasedfrom56.4˚Cto45.4˚Cafter5secondsandtookaminutetodecreasedownto37˚C.Thisrapidequilibrationoftemperaturemakesitespeciallyimportanttotestcyclicfatigueofinstrumentsatbodytemperatureinsteadofatroomtemperature,sinceirrigantatroomtemperaturequicklyequilibratestobodytemperaturewhenusedintherootcanalspace.Thisexperimentalmodelusedametalblocktosimulatearootcanal.Theeffectsofconductionandinsulationofheatarelikelydifferentinthismodelthanthoseofrootdentin,andmaybeafactorforinvivoresults.

Notably,thisstudyonlyexaminedcyclicfatigue;torsionalfatigueandcuttingeffectivenessoffilesarealsoimportantaspectsofafile’sperformance.Futurestudiesshouldexaminetheeffectsoftemperatureandfilemetallurgyonthesecharacteristics. Inconclusion,whenplacedinawaterbath,ESXfilesshowedadecreaseinNCFwhenheatedfrom3˚Cupto37˚C,butnosignificantdifferencewasseenfrom37˚Cto60˚C;VortexBluefilesshowedthesame,excepttheycontinuedtohaveareductioninNCFto60˚C.EdgeFilefilesshowedanincreaseinNCFfrom3˚Cto22˚C,andamarkeddecreaseinNCFfrom22˚Cto37˚C.TherewasnostatisticallysignificantdifferenceinNCFfrom37˚Cto60˚C.Atalltemperatures,EdgeFilefiles

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werefoundtohavesignificantlyhigherNCFthanVortexBluefiles,whichhadsignificantlyhigherNCFthanESXfiles.Sincethecyclicfatigueofvariousfiletypeswasfoundtobesignificantlyaffectedbytemperature,futurecyclicfatiguestudiesarerecommendedtobeconductedatbodytemperature.Considerationshouldbetakenininterpretationofstudiesconductedatroomtemperature.References

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