ORIGINAL
ARTICLE/ARTICOLO
ORIGINALE
34
8
CONGRESSO
NAZIONALE
BOLOGNA
2015
VINCITORE
PREMIO
FRANCESCO
RIITANO
Wear
analysis
and
cyclic
fatigue
resistance
of
electro
discharge
machined
NiTi
rotary
instruments
HyFlex
EDM:
caratteristiche
superficiali
e
resistenza
alla
fatica
ciclica
di
innovativi
strumenti
rotanti
NiTi
prodotti
per
elettroerosione
F.
Iacono
a,
C.
Pirani
a,*
,
L.
Generali
b,
P.
Sassatelli
c,
C.
Nucci
a,
M.G.
Gandolfi
a,
C.
Prati
aaEndodontic Clinical Section, Master Program for Clinical Endodontics, Department of Biomedical and
NeuromotorSciences(DIBINEM),UniversityofBologna,Bologna,Italy
bDepartmentofSurgery,Medicine,DentistryandMorphologicalScienceswithTransplant Surgery,Oncology
and Regenerative MedicineRelevance (CHIMOMO), School ofDentistry, Endodontic Section, University of ModenaandReggioEmilia,Modena,Italy
cDepartmentofEngineeringEnzoFerrari(DIEF),UniversityofModenaandReggioEmilia,Modena,Italy
Received11March2016;accepted5April2016 Availableonline6May2016
KEYWORDS CM-wire; Electrodischarge machining(EDM); Wear; SEManalysis; Cyclicfatigue. Abstract
Aim: Toevaluatethewearofnewrotaryinstrumentsproducedviaelectrodischargemachining andtotesttheirfatigueresistance.
Methodology: Twenty-oneHyFlexEDM (Colte`ne/Whaledent, Switzerland)fileswereusedfor laboratoryinstrumentationofcurvedrootcanalsofextractedteeth.Superficialcharacteristics wereanalyzedpre-andpostoperativelybyscanningelectronmicroscopy(SEM)(JSM-5200,JEOL, Tokyo,Japan)atsamepointsandwithsameangulationstoidentifythewearfeatures.Numberof fractures,microcracksblunt/disruptionofcuttingedgeandtipdeformationswerereported.
PeerreviewunderresponsibilityofSocieta` ItalianadiEndodonzia.
* Correspondingauthorat:EndodonticClinicalSection,DepartmentofBiomedicalandNeuromotorSciences(DIBINEM),UniversityofBologna, AlmaMaterStudiorum,Bologna,Italy.Tel.:+390512088126;fax:+39051225208.
E-mail:chiara.pirani4@unibo.it(C.Pirani).
Availableonlineatwww.sciencedirect.com
ScienceDirect
jo ur na l h o m ep a ge : w w w.e ls e v i er.c o m / lo c at e /gi e
http://dx.doi.org/10.1016/j.gien.2016.04.006
1121-4171/!2016Societa` ItalianadiEndodonzia.ProductionandhostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction
Introductionofnickeltitanium(NiTi)rotaryinstrumentshas
favored endodontic procedures by minimizing procedural
errors associated with hand instrumentation andreducing
time for chemo-mechanical preparation.1,2 Despite many
advantagesof theNiTiinstruments, unexpectedintracanal
separation dueto torsion orflexural fatigue of the
instru-ments is still a major concern.3,4 In recent years, several
novel thermo-mechanical processing and manufacturing
technologies have been developed to optimize the
micro-structure ofNiTi alloys.5In 2010,CM (controlled memory)
NiTi files have been introduced using a special
thermo-mechanical process ofthe alloythat controlsthe memory
of the material, making the files extremely flexible but
without the shape memory of other NiTi files, as opposed
to whatis found with conventionalNiTi instruments. This
impartstotheHyFlexCM(Coltene/Whaledent,Switzerland)
a highfatigueresistance6,7andthepossibilityto beeasily
bended during use, recovering its original shape heating
abovethetransformationtemperature.8Recentlyapatented
fabricationprocesswasintroducedinthemanufacturingof
newfilesobtainedbythesameCMwire.Infact,HyFlexEDM
(Coltene/Whaledent,Switzerland)instrumentsare
manufac-turedviaelectrodischargemachining(EDM)process.Inthe
EDMprocessanelectricsparkisusedtocuttheworkpiece,
whichtakestheshapeoppositetothatofthecuttingtoolor
electrode.The electrode andtheworkpieceare both
sub-mergedinadielectricfluid,whichisgenerallylight
lubricat-ing oil.9 The electrical sparks causes a local melting and
partially evaporation of small portion of material that is
removed from this local area living a typical crater-like
surface finishing.10 Despite EDM is a common fabrication
process for miniaturized components in medical
technol-ogy,11—13 HyFlex EDM files are the first endodontic
instru-mentsmanufacturedwiththisprocess.
Theaim ofthisstudywas to evaluatethewear ofnew
rotaryinstrumentsproducedviaelectrodischargemachining
andto testtheirfatigueresistanceincomparisonwith the
conventionallymanufacturedHyFlexCM.
PAROLECHIAVE Strumentiendodontici; NichelTitanio; Elettroerosione; Analisisuperficiale; Faticaciclica.
TwentyHyFlexEDMand20HyFlexCM(Colte`ne/Whaledent,Switzerland)weresubjectedto cyclicfatiguetestina708artificialmetalcanal.ResultswerestatisticallyanalyzedusingMann— Whitneytests.
Results: Nofractureswereregisteredduringlaboratorycanalinstrumentation.Nowearandno degradationofthe25.12and25.08fileswerereported.Slightplasticdeformationswereonly observedinthree10.05files.Thecyclicfatiguetestrevealedasignificantincreaseoffatigue resistanceofEDMfilescomparedtoCM(p=0.0001).
Conclusions: ThetypicalirregularsurfaceofHyFlexEDMremainedunaffectedaftermultiple uses,confirmingahighwearresistance.Thenewmanufacturingprocessofelectricaldischarge machininghadasubstantialimpactonfatiguelifetimeofEDMfileswhencomparedwithHyFlex CM.Within limitationsofthepresentinvitroresults,EDMfilesappearedsuitableinshaping severelycurvedcanals.
!2016Societa` ItalianadiEndodonzia.ProductionandhostingbyElsevierB.V.Thisisanopen accessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).
Riassunto
Obiettivo: Esaminarelamorfologiasuperficialeel’usuradistrumentiHyFlexEDMusatiinvitroe testarelalororesistenzainfaticaciclica.
MaterialieMetodi: VentunostrumentiHyFlexEDMsonostatiutilizzatiperlastrumentazionein vitrodicanalicurvi.Lamorfologiadisuperficieeifenomenidiusurasonostatiinvestigatipree postoperativamenteconunMicroscopioElettronicoaScansione(SEM)(JSM-5200,JEOL,Tokyo, Japan).Ulteriori20HyFlexEDMsonostatipoitestatiinfaticaciclicainuncanaleartificialecon angolodicurvaturadi708ecomparaticonstrumentiHyFlexCM.Idatiottenutisonostativalidati coniltestdiMann-Whitney.
Risultati: LeindaginialSEMhannorivelatounaspecificastrutturairregolarecaratterizzatada unasuperficieconmorfologiaa‘‘cratere’’legataallacapacita` erosivageneratadallescariche elettrichedelprocessoproduttivo.Tuttiglistrumenti,dopodiversiutilizzi,hannopreservatoil loropeculiareaspettocrateriformeenonhannomostratoappiattimentodell’angoloditaglio dellelame.Nonsie` verificatanessunafrattura.Itestdifaticaciclicahannoevidenziatouna resistenzastatisticamentesignificativadegliHyFlexEDMrispettoagliHyFlexCM(p=0.0001). Conclusioni: GliHyFlex EDMhanno dimostratounaffidabile comportamentoinvitroquando sottopostiastrumentazionedicanalimoltocurvi.Lasuperficiecrateriformesie` benconservata dopo ripetuti utilizzi. Gli strumenti di diametro minore sono apparsi quelli maggiormente stressati.Laresistenzainfaticaciclicadistrumentidellastessalegasembraesserenotevolmente aumentatadaldifferenteprocessodiproduzione.
!2016Societa` ItalianadiEndodonzia.ProductionandhostingbyElsevierB.V.Cetarticleest publie´ enOpenAccesssouslicenceCCBY-NC-ND( http://creativecommons.org/licenses/by-nc-nd/4.0/)
Materials
and
methods
Twenty-onebrandnewHyFlexEDMweresubjectedtoinvitro
wear tests. Tests were performed using severely curved
canalsselectedfromapoolofextractedhumanmulti-rooted
teethaccordingwithKostietal.14Oncethatcanalpatency
wasverifiedwitha#10K-file,theworkinglengthwas
deter-minedbysubtracting1mm.HyFlexEDMfileswereusedwith
a 16:1 reduction handpiece X-Smart (Dentsply Maillefer,
Baillagues, Switzerland) following the manufacturer’s
recommendation, at 500rpm and 2.5Ncm, in pecking
motion.The operativesequence was:25/12 at 2/3of the
WL,10/05and25/08atWL.Irrigationwasperformedwith
3mlof5%NaOCl(Niclor5,Ogna,Muggio`,Italy)and3mlof
10%EDTA(Tubuliclean,Ogna,Muggio`, Italy).Eachinstrument
wasusedin10curvedcanals,washedinanultrasonicbath
containing detergent for 10min and then autoclaved at
1348C.
Scanning electron microscope (JSM-5200, JEOL, Tokyo,
Japan)wasusedtoevaluatebrandnewHyFlexEDM
instru-ments,andthesamesamplesaftercanalsinstrumentationat
samepointsandwithsameangulationstoidentifythewear
features.
Micrographsweretakenatincreasingmagnificationfrom
35!to5000!,onthetipoftheinstruments,andoncutting
edges at 5mm from the tip. The wear degradation was
evaluated comparing the pre- and post-operative
micro-graphs, in order to verify fractures and the presence of
unwinding,microcracks,bladedisruptionandtip
deforma-tion.
Cyclic fatigue tests were performed on 20 brand new
HyFlexEDM25.08(n=10)and40.04(n=10)usingan
arti-ficialstainlesssteelcanalwith5.0mmradiusofcurvature,
708 angleof curvatureandthe centerof thecurvatureat
approximately5.0mmfromthetipoftheinstrumentandan
overalllengthof18.0mm.Astainless steelapparatus
pro-videdastandardizedplacementoftheNiTiinstrumentsinthe
artificialcanal.15Allthefileswererotateduntilthefracture
andtimewasvisuallyrecordedwithadigitalstop-watch(3M
ESPE,St.Paul,MN,USA).Thenumberofcyclestofracture
(NCF)wascalculatedusingthefollowingformula:NCF=time
(s)failure!rotationalspeed/60.
Thesametestwasalsoperformedon25.04(n=10)and
40.04(n=10)brandnewHyFlexCM,whoseresultshasbeen
takenasabenchmark.
Results
SuperficialinvestigationsconductedbySEMonnewEDMfiles
disclosedthepresenceofthetypicalirregular surface
tex-ture derivedfrom themanufacturing process (Fig. 1a).In
fact,highmagnificationmicrographsdisclosedanon-uniform
structurewerevoidscausedthe‘‘spark-machined’’surface
(Fig. 1b). During rootcanal instrumentation, no files
frac-tured(Table1).Alltheinstruments,aftermultipleuses,well
preserved the ‘‘spark-machined’’ surface without cutting
edge or blunt disruption. All used instruments presented
nosurfacedefectsandonlythree10.05filesshowed
micro-cracksperpendicularlytotheaxisoftheinstrument,nearthe
tip. Spiral distortions and microcracks were absent in
severelystressed25.12and25.08,whenacomparisonwas
madebetweennewandusedfiles(Fig.2).Aspiralunwinding
was observed at high magnification in one 10.05 file.
Theincidence ofdetectedmetaldefectsis representedin
Table1.Onlythreeusedinstrumentspresentedwideareasof
debrisontheinstrumentssurface.
HyFlex EDM files exhibited a statistically higher cyclic
fatigueresistancewhencomparedwithCM(Fig.3).Mann—
Whitneywas performed; a-levelwas a priori set at 0.05.
When40.04EDMwascomparedwith40.04CM,asignificant
increasewasobserved(p=0.0001).
Discussion
ImprovementinthemanufacturingprocessofNiTi
endodon-tic files has been proposed over last years and
thermo-mechanicalprocessingprocedureshave beenadopted
pro-vidingsuperiormechanicalpropertiesto the instruments.3
HyFlex EDM are produced with the well-known controlled
memory(CM)wire butwithadifferent fabricationprocess
thatusetheabilityofelectricalsparkstopartiallymeltand
evaporatesmallportionofmaterialinadielectricmedium.10
Despite EDM being a common fabrication process already
Figure1 NewHyFlex EDM25.12 instrumentat75!(a) showedthetypical irregularsurface texturederived fromtheelectro dischargemachining.(b)Highmagnificationmicrographs(500!)ofthetipofthesameinstrumentdisclosedanon-uniformstructure wherepits,poresandvoidscausethepeculiaraspectofa‘‘spark-machined’’surface.
usedforNiTialloysforsurgicalapplications,12,13HyFlexEDM
arethefirstendodonticinstrumentsmanufacturedwiththis
procedure.ItisknownthatEDMprocesscreatesaroughand
hard surface that could improve the cutting efficiency of
tools16andSEMmicrographsonHyFlexEDMfilesrevealeda
rough‘‘spark-machined’’surface thatis thetypical
super-ficialmorphologyoftheelectrodischargemachined
materi-als. This superficial rough aspect represents a surprising
innovationincomparisonwithconventionalNiTiendodontic
files.Inthepresentstudynofractures,plasticdeformations
ormicroscopicsignsofwearwerereportedinlarge
instru-ments(25.12and25.08)duringinvitroinstrumentationof
multiplecurved canals.Alltheinstruments well-preserved
the‘‘spark-machined’’irregularsurfacewithoutcuttingedge
alterations. Only minor defects were reported on the tip
regionofthreeused10.05files.Thiscouldbeexplainedbya
supposedhighersusceptibilitytotorsionalstressesofsmall
CM-treated files than larger instruments17,18 and caution
would be recommended regarding reuse of small HyFlex
rotaryfiles.
Severalstudiesreportedthatsurfacefinishhasaneffect
onthecrackinitiationprocess19andstructuralirregularities
oftheinstrumentsurfacemaycompromiseitsintegrity
dur-ingclinicaluse,makingthe filesmoresusceptibleto
frac-ture.20 In contrast with these findings, the remarkable
irregularities observed on the surface of the HyFlex EDM
didnotcompromisetheinstrumentsintegrityaftermultiple
Table1 Numberofdefectsafter10rootcurvedcanalsinstrumentation.
HyFlexEDM Fractures Unwinding Microcracks Tipdeformation Bladedisruption
25.12(n=7) 0 0 0 0 0
10.05(n=7) 0 3 3 2 0
25.08(n=7) 0 0 0 0 0
Figure2 HyFlexEDM25.08at75!before(a)andafterusein10canals(b).Micrographsweretakenatthesamemagnification revealingtheabsenceofdeformationsorspiralsunwinding.Athighermagnification(200!),at5mmfromthetip,thecomparison betweennew(c)andusedfile(d)showedthepreservationofthe‘‘spark-machined’’withoutalterationoftheoutlineandnoblade disruptionandmicrocracks.
Figure3 Thechartreportsnumbersofcyclestofailure(NCF), forHyFlexEDMandHyFlexCM.
uses. Surprisingly, the cyclic fatigue tests demonstrated a
significantgreaterresistanceofthe‘‘irregular’’EDM
instru-mentscomparedwiththeconventionallymanufacturedCM
files.ThisisparticularlyunexpectedsincebothEDMandCM
files are manufactured with the same controlled memory
wireandsuchgreatimprovementislikelytoberelatedtothe
electro discharge machining manufacturing process. A
remarkableincreaseincyclicfatigueresistanceofthe
ther-mal treated CM wire instruments compared with
conven-tionalNiTifileshasbeenextensivelydocumented.6,7,21,22It
may therefore be supposed that thermomechanical
treat-mentproceduresperformedontheEDMinstrumentsduring
themanufacturingprocesscausechangesofthephase
com-positionofthewireinfluencingthemechanicalpropertiesof
theinstruments.Thelowdegradationaftermultipleusesand
theimprovedmechanicalproperties,duetotheinnovative
fabrication process, could havean important clinical
rele-vanceinordertominimizetheriskoffractureofthe
instru-ments.
Conclusions
Theelectrodischargemachiningmanufacturingprocess
cre-ates a spark-machined surface unaffected after multiple
uses. Cyclic fatigue tests demonstrated that the
thermo-mechanicaltreatmentprocedureshadanimportantimpact
onfatiguelifetimeofHyFlexEDM.
Conflict
of
interest
Theauthorsdeclaretohavenoconflictofinterestrelatedto
thisstudy.
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