DTIC ADA555771: The Effect of Autoclaving on Torsional Moment of Two Nickel-Titanium Endodontic Files

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30-01-2012 Scientific journal publication
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The effect of autoclaving on torsional moment of two nickel–titanium endodontic files
5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER
J. B. King, H. W. Roberts, B. E. Bergeron & M. J. Mayerchak
5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT
Tinker Air Force Base, Oklahoma City, OK NUMBER

AKNeDe sAlDeDrR ESASi(rE SF) orce Base,
Biloxi, MS

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
International Endodontic Journal, doi:10.1111/j.1365-2591.2011.01958.x
Wiley Blackwell Publishing,
doi:10.1111/j.1365-

111111 RRiivveerr SSttrreeeett Hoboken, NJ 07030-74 11. SPONSOR/MONITOR’S REPORT
2H 5ob9o1k.e2n0, 1N1J. 0071093508-5.x77 445 , 156–161, NUMBER(S)
2012

12. DISTRIBUTION / AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited

13. SUPPLEMENTARY NOTES

14. ABSTRACT
Aim To evaluate the effects of repeated autoclaving on torsional strength of two nickel–titanium (NiTi) rotary endodontic files: Twisted Files
(SybronEndo, Orange, CA, USA) and GT Series X files (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA). Methodology Four groups of 20 GT Series
X (size 20, .06 taper) and four groups of 20 Twisted Files (size 25, .06 taper) were subjected to 0, 1, 3 or 7 autoclave cycles. The nonautoclaved files
served as controls. Failure in torsion was recorded using a torsiometer according to ANSI/ADA 58 and ISO 3630-1 standards. Mean results were analysed
using analysis of variance (anova) at a 95% confidence level. Dunnett’s procedure was used for post hoc analysis. Results There was no significant
difference in torsional moment between the number of autoclave cycles for Twisted Files. However, the mean torque at failure was significantly lower for
GT Series X files after three (P < 0.001) and seven (P < 0.001) autoclave cycles. For Twisted Files, there was a significant increase (P < 0.001) in mean
number of degrees of rotation to failure with more autoclave cycles compared to nonautoclaved files. For GT Series X files, there was no significant
association (P = 0.527) between the number of autoclave cycles and the degrees of rotation to failure. Conclusions Repeat autoclaving of unused GT
Series X files between three and seven times resulted in a significant reduction in torsional strength, whilst there was no effect observed for Twisted Files.
. International Endodontic Journal, 45, 156–161, 2012.

15. SUBJECT TERMS
autoclave, nickel–titanium, rotary endodontic, torsion.
16. SECURITY CLASSIFICATION OF: 17. LIMITATION 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON
unclassified OF ABSTRACT OF PAGES Howard W Roberts, Col
a. REPORT b. ABSTRACT c. THIS PAGE 19b. TELEPHONE NUMBER (include area
unclassified unclassified unclassified 5 code)
[phone]

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39.18

doi:10.1111/j.1365-2591.2011.01958.x
The effect of autoclaving on torsional moment of
two nickel–titanium endodontic files
J. B. King1, H. W. Roberts2, B. E. Bergeron2 & M. J. Mayerchak3
1DepartmentofEndodontics,TinkerAirForceBase,OklahomaCity,OK;2KeeslerAirForceBase,Biloxi,MS;and3PrivatePractice,
Roanoke,VA,USA
Abstract Results There was no significant difference in tor-
sionalmomentbetweenthenumberofautoclavecycles
KingJB,RobertsHW,BergeronBE,MayerchakMJ.The
for Twisted Files. However, the mean torque at failure
effect of autoclaving on torsional moment of two nickel–
was significantly lower for GT Series X files after three
titanium endodontic files. International Endodontic Journal,
(P<0.001) and seven (P<0.001) autoclave cycles.
45,156–161,2012.
For Twisted Files, there was a significant increase
Aim Toevaluatetheeffectsofrepeatedautoclavingon (P<0.001) in mean number of degrees of rotation to
torsionalstrength oftwo nickel–titanium (NiTi)rotary failure with more autoclave cycles compared to non-
endodontic files: Twisted Files (SybronEndo, Orange, autoclaved files. For GT Series X files, there was no
CA, USA) and GT Series X files (Dentsply Tulsa Dental significant association (P=0.527) between the num-
Specialties, Tulsa,OK, USA). ber of autoclave cycles and the degrees of rotation to
Methodology Four groups of 20 GT Series X (size failure.
20,.06taper)andfourgroupsof20TwistedFiles(size Conclusions Repeat autoclaving of unused GT
25,.06taper)weresubjectedto0,1,3or7autoclave Series X files between three and seven times resulted
cycles. The nonautoclaved files served as controls. in a significant reduction in torsional strength, whilst
Failure in torsion was recorded using a torsiometer there wasno effectobserved forTwisted Files.
accordingtoANSI/ADA58andISO3630-1standards.
Keywords:autoclave, nickel–titanium, rotary end-
Meanresults were analysed using analysisof variance
odontic, torsion.
(anova)ata95%confidencelevel.Dunnett’sprocedure
was usedfor posthocanalysis. Received15May2011;accepted28August2011
preparation is complex; however, cyclic fatigue and
Introduction
torsionalfailurehavebeenfrequentlyimplicated(Gam-
Nickel–titanium (NiTi) is an alloy with increased barini 2001, Xuet al. 2006). Cyclic fatigue conditions
flexibility and shape memory capabilities, and contin- involveforces impartedon the rotating endodontic file
ued research into different NiTi manufacturing meth- as it repeatedly bends around a curved canal, often
odshasbeenhallmarkedwiththegoalofimprovingthe causing failure independent of torsional (twisting)
physical and mechanical properties of the NiTi rotary forces acting on the file. Additionally, friction encoun-
endodontic file (Thompson 2000). These fabrication teredduringthenegotiationofnarrowrootcanalscan
techniquesareusuallytargetedtowardsincreasingfile cause files to bind, which can lead to fracture if the
flexibilityandstrength,whicharepromotedtoenhance mechanical properties of the NiTi alloy are exceeded
treatment efficiency and reduce the rate of iatrogenic (Xuetal. 2006).
errors (Gambarini 2001, Gambarini etal. 2010). The ArecentlymarketedNiTifilesystem,theTwistedFile
aetiology of instrument fracture during root canal (SybronEndo,Orange,CA,USA),issaidtobefabricated
using a unique, proprietary process involving heating,
coolingandtwistingofaNiTiwireblank(SybronEndo
Correspondence: HowardW. Roberts, 81st Dental Squadron,
Literature2010).Thisprocessissaidtoinducechanges
606 Fisher Street, Keesler AFB, MS 39534, USA (e-mail:
[email protected]). intheNiTimolecularphasestructure,whichinturnis
156 InternationalEndodonticJournal,45,156–161,2012 ª2011InternationalEndodonticJournal
Kingetal. AutoclaveeffectonNiTifiletorsion
advertised to optimize the Twisted File’s physical failure (Eggert etal. 1999, Rapisarda et al. 1999,
properties (SybronEndo Literature 2010). According Scha¨fer 2002, Alapatietal. 2005, Valois etal. 2008).
to manufacturer’s promotional literature and nonsci- Thepurposeofthisstudywastoobservetheeffectof
entificpublications,theTwistedFileisallegedtopossess 1, 3 and 7 autoclave cycles on the torsional failure of
threetofourtimestheresistancetotheeffectsoftorsion TwistedFilesandGTSeriesXfiles.Thehypothesiswas
and cyclic fatigue, and in most root canal configura- formed that autoclave conditions would not have an
tions,asinglefileisadvertisedtobeusedtoprepareand effect oneither NiTifile system (nullhypothesis).
shapethe entire tooth(Sybron EndoLiterature 2010).
TheGTSeriesXfile(TulsaDentalSpecialties, Tulsa,
Materials and methods
OK,USA)isbasedonM-wiretechnologyandissaidto
be based on a proprietary NiTi composition combined Eighty GT Series X (size 20, .06 taper) files and eighty
with a reduced core mass design (GT Series X TwistedFiles(size25,.06taper)wereeachdividedinto
Brochure 2009). Accordingly, the reduced core is four groups (n=20). The control group was not
described as achieved by reducing the number of subjected to autoclaving. Groups 1, 2 and 3 were
cutting flutes combined with wider and increased subjected to autoclave sterilization (STATIM 5000;
blade angle to produce a slimmer instrument that is Sciscan USA, Canonsburg, PA, USA) one, three and
advertised to create a more flexible, yet strong design seven times, respectively. Each autoclave cycle con-
for increased cutting efficiency (GT Series X Brochure sisted of a peak temperature of 132(cid:2)C for 6min, and
2009). all cycles were performed according to manufacturer’s
Manufacturer’sinstructionsforGTSeriesXfilesstate instructions. For Groups 2 and 3, instruments were
theyareintendedexclusivelyforsingle-patientuse(GT cooled to room temperature, removed from the steril-
SeriesXBrochure2009),whilsttheuseofTwistedFile ization packaging and repackaged prior to subsequent
is dependent on canal conditions, clinical demands autoclave cycles. After the annotated autoclave cycles
placed upon the file and critical file inspection before were complete, groups were subjected to torsional
andaftereachuse(SybronEndoLiterature2010).The testing.
TwistedFilemanufacturer’sinstructionsalsostatethat Torsional testing was accomplished in accordance
thefileshouldnotbeusedifthefilebecomesdeformed, with ANSI 58 and ISO 3630-1 standards using a
looses cutting efficiency or fails to function normally microprocessor-controlled torsiometer (Torsiometer/
(Sybron Endo Literature 2010). Whilst physical evi- Memocouple;SabriDentalEnterprises,Downer’sGrove,
dence of file deformation is evident, assessment of IL, USA). Prior to respective file placement in the
rotary file’s cutting efficiency and function is more torsiometer, the handle was removed at the point of
subjective in nature and may tempt clinicians to use shaftattachment withwirecutters,withtheshaftend
the Twisted File to prepare and shape more than one secured in a chuck connected to the torsiometer’s
treatment case. This creates a situation in which a rotating motor. The file’s distal three millimetres was
rotary endodontic file may be subject to multiple thensecuredintothetorsiometer’storquesensor.Each
autoclave sterilization cycles. Furthermore, inaddition filewasthenrotatedinaclockwisedirectionasviewed
to autoclaving before prior use, practitioners also may from the shank end at a speed of two rotations per
prefer using prearranged packages of selected files of minute until file tip fracture (Best etal. 2004) with
which not all may be used during a single treatment. torsional moment and rotational degrees to failure
This could involve unused rotary files that might be recorded.Meanresultswereevaluatedusinganalysisof
subjected to multiple autoclave cycles for subsequent variance (anova) at a 95% level of confidence. Dun-
cases. nett’s procedure was used forposthoc analyses.
The effects of autoclaving NiTi files have generated
contrastingreports:somestudiessuggestthatsteriliza-
Results
tion may increase NiTi torsional strength (Silvaggio &
Hicks1997),whilstotherinvestigationshavereported MeantorsionaltestingresultsareshowninTable1.A
increased susceptibility to cyclic fatigue and decreased significant difference (P <0.001) in file failure torque
torsional moment (Canalda-Sahili etal. 1998, Viana was identified within the GT Series X files, with files
etal.2006).AdditionalresearchsuggestsrepeatedNiTi undergoing three and seven autoclave cycles demon-
autoclave sterilization negatively affects the mechani- strating significantly lower failure torque as compared
cal properties and leads to premature instrument to control (P<0.001). In contrast, no significant
ª2011InternationalEndodonticJournal InternationalEndodonticJournal,45,156–161,2012 157
AutoclaveeffectonNiTifiletorsion Kingetal.
Table1 Mean(SD)maximumtorqueatfailure(Nm).n=20. surfaceconcentrations decreasedwithautoclaveexpo-
sure time, which led the authors to propose that
Autoclavecycles Twistedfile GTseriesX
oxidationwastakingplace(Shabalovskaya&Anderegg
0 51.40(6.46)A 97.95(11.04)A
1995). Auger spectroscopy analysis identified increas-
1 46.32(5.84)A 94.75(8.84)A
3 43.25(7.28)A 85.67(9.59)B ing parallel amounts of both surface oxygen and
7 48.70(4.73)A 78.10(7.36)B titanium deposits on autoclaved endodontic NiTi files,
which suggested the presence of titanium oxides
Valueswithincolumnsannotatedwithsameletterarestatisti-
callysimilar(P=0.05). (Rapisarda etal. 1999). Additionally, atomic force
microscopy found that rotary NiTi files demonstrated
increasesinfilesurfaceirregularitiesafterautoclaving,
Table2 Mean(SD)rotationaldegreestofailure(degrees).
whichwereattributedtocorrosionprocesses(Canalda-
n=20.
Sahili etal. 1998).
Autoclavecycles Twistedfile GTseriesX
An evaluation of NiTi rotary files that failed during
0 586.55(64.18)A 445.65(62.39)A clinical use identified that the predominant failure
1 670.63(59.74)B 441.80(47.30)A mode was file fracture owing to crack propagation
3 672.65(103.16)B 483.11(66.57)A
through grain boundaries that initiated at oxide
7 702.25(80.64)B 430.75(66.49)A
formations. Even though the rotary files evaluated
Valueswithincolumnsannotatedwithsameletterarestatisti-
had undergone an average of six to eight clinical uses
callysimilar(P=0.05).
(inferring repeat autoclaving conditions), the authors
ascribedtheidentifiedoxideswereattributablemoreto
differencewasidentifiedwithintheTwistedFilegroups NiTi file manufacturing and preparation process (Ala-
(P=0.563). patiet al.2005).
Results of rotational degrees to failure are listed in However,anotherstudyreportedthatfiveautoclave
Table2. A significant difference (P<0.001) in rota- cyclesdidnotnegativelyaffectthefatiguepropertiesof
tional degrees to failure was found within the Twisted Profile size 25 and size 30, .06 taper files (Viana etal.
Fileswithallautoclavedsamplesdemonstratingsignif- 2006), and this somewhat corroborated an earlier
icantlyhigherrotationaldegreestofailurecomparedto investigationwhichfoundthatautoclavecyclesof1,5
control. A significant difference was not identified and 10 times increased both torsional flexibility and
(P=0.527) within the GTSeries X filegroups. rotationalflexibilityforProfileSeries29,.04taperNiTi
files (Silvaggio & Hicks 1997). Contrastingly, another
investigation indentified that 10 autoclave cycles
Discussion
resulted in an overall decreasing trend of torque
The torsional properties of NiTi files have been attrib- resistance with Nitiflex (Dentsply Maillefer, Ballaigues,
uted to file design (Turpin etal. 2000, Kim etal. Switzerland) and Naviflex (Brasseler, Savannah, GA,
2009a,b),alloycomposition,aswellasmanufacturing USA) files (Canalda-Sahili etal. 1998), and a recent
thermomechanical processes (Kell etal. 2009). Fur- report found that repeat autoclaving significantly
thermore, other reports ascribe the most decisive reduced cyclic fatigue resistance of size 25, .06 taper
influence concerning NiTi file stress distribution and TwistedFilesbutnotthesize25,.04taperTwistedFiles
torsional behaviour to be the cross-sectional configu- andProfile GTSeries X files(Hilfer etal. 2011).
ration of a particular file (Turpin etal. 2000, Berutti The torsional values of baseline GT Series X files
etal. 2003, Xu etal. 2006, Kim etal. 2009b, Zhang found during this evaluation corroborate well with
etal. 2010). resultspublishedintworecentstudies(Kelletal.2009,
The effects of autoclave conditions on the surface Kramkowski & Bahcall 2009). Interestingly, the con-
characteristicsandmechanicalpropertiesofNiTialloys ditionsofthisstudyresultedinasignificantdecreasein
andendodonticinstrumentshavebeenreported.Ithas torsional moment resistance in the GT Series X files
beenidentifiedthatNiTialloysdisplayhighercorrosion after three and seven autoclave cycles as compared to
rateswithincreasedtemperature(Pun&Berzins2008) control.Therewasnosignificantdifferencenotedafter
whilstthatrepeatedautoclavingconditionsalteredthe one autoclave cycle. However, repeat autoclaving did
surfaceconcentrationsofnickel,titanium,oxygenand not induce any change in the rotational degrees to
carbon in NiTi alloys (Shabalovskaya & Anderegg failureofGTSeriesXfile.Thesefindingsarecontrasted
1995).Furthermore,thelatterreportstatedthatnickel by those of Kell et al. (2009) who found a transient
158 InternationalEndodonticJournal,45,156–161,2012 ª2011InternationalEndodonticJournal
Kingetal. AutoclaveeffectonNiTifiletorsion
increase in torque at failure and a decrease in mechanicalloadingwilleventuallyleadtodeformation
rotational degrees to failure for GT Series X files. and file survival could be largely dependent on cross-
However,theirdissimilarresultsweretheproductofa sectionalconfiguration(Kimetal.2008,2009a,Ounsi
differing protocol, as Kell etal. (2009) used an inno- etal. 2008, Arbab-Chirani etal. 2011) as well as the
vative method that included controlled functional use inherentfracturetoughness(McKelvey&Ritchie1999,
of the rotaryfiles instandardized resin blocks followed Plotino etal. 2009).
byautoclaving.Becauseoftheseaddedconditions,the Unfortunately, crack propagation resistance of NiTi
authorssuggestedthattheinitialincreaseintorqueat alloyhasbeenreported tobelowercomparedtothose
failure could be possibly the outcome of work harden- of other alloys of similar strength; therefore, once a
ing. microcrack is initiated, it may quickly propagate to
The results of the Twisted File testing are worthy of result in catastrophic failure (McKelvey & Ritchie
note.IncontrasttotheGTSeriesXresults,thenumber 1999,Plotinoetal.2009).Investigationsintoclinically
of autoclave cycles did not significantly affect the fracturedNiTifilessuggestthatapredominantnumber
torsionalmomentoftheTwistedFile,whilstrotational of failures are attributable to crack propagation initi-
degreestofailuresignificantlyincreased.However,the ated from grain boundaries, oxide particles and/or
meantorquevaluesatfailurefortheTwistedFileunder surface imperfections (Alapati et al. 2005, Alexandrou
the conditions of this study consistently accord with etal.2006,Shenetal.2009).Asautoclaveconditions
those recently reported by Gambarini etal. (2010) for have been suggested to induce corrosion with some
Twisted Files without a surface deoxidation finishing NiTi endodontic files (Rapisarda etal. 1999, Valois
process. This recent study is the first to the author’s etal.2008),perhapsincreasedsurfacecorrosioncould
knowledge that describes in detail the Twisted File potentiallyexplainthetorsionalresultsoftheGTSeries
surface treatment process (Deox) as the ageing for an Xfiles.Unfortunately,ultrastructuralevaluation offile
extended period of time the newly twisted NiTi rotary surfaces after autoclavecycles was not possible.
files in a heated salt bath. Furthermore, according to Theresultsofthisstudycorroboratetherecommen-
Gambarini etal. (2010), the Deox process is not dations of the Twisted File manufacturer, as Sybron-
equivalent to a surface electropolishing finishing tech- Endo allows clinician discretion in terms of the
nique but is designed to enhance file durability via instrument use/reuse. Furthermore, this study should
stabilizingthemetalliccrystallinestructureandreduc- provide sufficient data to allow clinicians to autoclave
ing surface defects (Gambarini etal. 2010). Addition- an unused GT Series X file before clinical use, as
ally,Gambariniet al.(2010)confirmedthevalueofthis recommendedbythemanufacturer.Directcomparison
Deox treatment by reporting that Twisted Files with of the results of this study with those of previous NiTi
Deox treatment had 90% greater torque failure values torsional study reports may be only accomplished in
as compared to Twisted Files with no deoxidation general as both files evaluated in this study involve
treatment. The results of Gambarini etal. (2010) newer manufacturing techniques. When feasible,
compared with the results of this study reasonably future research should include functional forces as
allow the present authors to assume that the Deox well as file failure analysis involving ultrastructural
procedure is a relatively new processing technique examinations.
introduced bythe manufacturer.
Studies based on clinically fractured NiTi instru-
Conclusion
ments suggest that fracture usually occurs owing to a
single overloading event under torsion, tensile or Under the conditions of this study, repeat autoclaving
bending–loading forces (Alapati et al. 2005, Parashos of unused Twisted Files did not affect their torsional
& Messer 2006, Spanali-Voreadi et al. 2006). The strength. However, the same conditions werefound to
causeoffailureowingtotheseforceseitherindividually significantlyreducethefracturestrengthofGTSeriesX
or in combination continues to be a source of debate filesafter threeautoclave cycles.
(Parashos et al. 2004). Some maintain that metal
fatigueisthepredominantfailuremechanism(Cheung
Acknowledgements
etal. 2005, Peng etal. 2005), whilst other investiga-
tors report that torsional failure accounts for the None of the authors involved in this study have any
majority of clinical failures (Sattapan etal. 2000, financial interest in any materials used in this study.
Alapati etal. 2005). Regardless of the mechanism, This research was supported by the American Associ-
ª2011InternationalEndodonticJournal InternationalEndodonticJournal,45,156–161,2012 159
AutoclaveeffectonNiTifiletorsion Kingetal.
ation of Endodontics Foundation. Additionally, the Hilfer PB, Bergerone BE, Mayerchak MJ, Roberts HW,
authorsgratefullyappreciatethemanufacturer’sdona- Jeansonne BG (2011) Multiple autoclave cycle effect on
tion of the files used in this study. The opinions cyclicfatigueofnickel-titaniumrotaryfilesproducedbynew
expressedinthisworkarethoseoftheauthorsonlyand manufacturingmethods.JournalofEndodontics37,72–4.
KellT,AzarpazhoohA,PetersOA,El-MowafyO,TompsonB,
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BasraniB(2009)Torsionalprofilesofnewandused20/.06
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