Comparison between a novel nickel-titanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/.04 rotary instruments.

Abstract

ProFile 25/.04 instruments manufactured from three variants of Nitinol (1A, 1B & 2AS) were compared with stock production ProFile 25/.04 instruments and fatigue tested to failure. Cyclic fatigue testing was performed by rotating instruments at 300 RPM in a simulated steel root canal with 5 mm radius and 90 degrees curve until instrument separation. Time to failure was recorded. Torsion testing was undertaken by clamping 3 mm of each instrument tip between brass plates and rotating it at 2 RPM until failure. Data were recorded for torque and angle at fracture. Statistical differences were found with nickel-titanium variant 1B (M-Wire NiTi) nearly 400% more resistant to cyclic fatigue than stock ProFile 25/.04 (P < .001). Torsion testing found differences between all 508 Nitinol groups and M-Wire NiTi (P < .001). ProFile 25/.04 files manufactured from M-Wire NiTi have significantly greater resistance to cyclic fatigue while maintaining comparable torsional properties.

Full text

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Comparison between a Novel Nickel-Titanium Alloy and
508 Nitinol on the Cyclic Fatigue Life of ProFile 25/.04
Rotary Instruments
Eugenia Johnson, DDS, Adam Lloyd, BDS, MS, Sergio Kuttler, DDS, BS,
and Kenneth Namerow, DDS
Abstract
ProFile® 25/.04 instruments manufactured from three
variants of Nitinol (1A, 1B & 2AS) were compared with
stock production ProFile 25/.04 instruments and fatigue
tested to failure. Cyclic fatigue testing was performed
by rotating instruments at 300 RPM in a simulated steel
root canal with 5 mm radius and 90° curve until in-
strument separation. Time to failure was recorded.
Torsion testing was undertaken by clamping 3 mm of
each instrument tip between brass plates and rotating
it at 2 RPM until failure. Data were recorded for torque
and angle at fracture. Statistical differences were found
with nickel-titanium variant 1B (M-Wire™ NiTi) nearly
400% more resistant to cyclic fatigue than stock ProFile
25/.04 (P ? .001). Torsion testing found differences
between all 508 Nitinol groups and M-Wire™ NiTi
(P ? .001). ProFile 25/.04 files manufactured from
M-Wire™ NiTi have significantly greater resistance to
cyclic fatigue while maintaining comparable torsional
properties. (J Endod 2008;34:1406 –1409)
Key Words
Canal preparation, cyclic fatigue, M-Wire NiTi, nickel
titanium, rotary endodontics
R
canal systems. Civjan et al. (1) envisioned the use of nickel-titanium alloys in dentistry,
but Walia et al. (2) are credited with the creation of a K-bladed file from nickel-titanium
orthodontic arch wires machined to produce a fluted instrument. The hand files man-
ufactured from nickel-titanium exhibited significantly greater elasticity and superior
resistance to torsional fracture compared with stainless steel files, along with the mem-
ory effect that returned a Nitinol file from a bending moment of 90° to its original shape
(2). Clinicians rapidly adopted rotary nickel-titanium instruments, with their increased
flexibility and strength, when files with tapers greater than the ISO standard .02 taper
were introduced (ProFile Series 29; Dentsply Tulsa Dental Specialties, Tulsa, OK) (3).
Crown-down preparation techniques, in combination with nickel-titanium rotary files
of greater taper, enabled predictable and expeditious tapered canal preparations while
minimizing transportation and iatrogenic errors (4–8). The commercial success of the
U-blade design of the ProFile rotary instruments led to the introduction of other nickel-
titanium files of different designs and tapers.
Nickel-titanium instruments are considerably more flexible and resistant to cyclic
fatigue than stainless steel, but they possess only comparable torque strength. The
perception of high torque strength comes from the greater mass of metal found in the
larger nickel-titanium tapers, whereas, in contrast, a larger metal volume results in
earlier cyclic fatigue failure (9, 10). For instance, ProFile 25/.02 has the lowest torque
strength, decreased metal mass, and the highest resistance to cyclic fatigue, whereas a
ProFile 25/.06 has highest torque strength, greatest metal mass, and the least resistance
to cyclic fatigue. ProFile 25/.04 instruments are found in a range between these two
tapers (11). Although the advantages over stainless steel instruments are many, sepa-
ration of nickel-titanium instruments still occurs, especially after extended use (12).
Separation of rotary nickel-titanium instruments takes place by static torsional, dynamic
torsional, or cyclic fatigue. Fractographic analysis of discarded instruments shows the
mechanism of instrument fracture (12); cyclic fatigue occurs unexpectedly and without
any visible signs of permanent deformation across transgranular boundaries, and tor-
sional failure results in unwinding of the instrument before separation, seen as a dim-
pled rupture fracture surface, which is a characteristic of ductile fracture (13).
An instrument rotating freely in a curved canal is subjected to compression and elon-
gation simultaneously, resulting in work hardening and metal fatigue and, ultimately, instru-
ment separation. This will occur at the midpoint of the greatest curvature of the canal and is
a function of the number of rotations the file undergoes (14). The factors determining cyclic
fatigue including radius of curve, degree of canal curvature, instrument diameter, taper of
instrument, number of times used, operator experience, and instrument mass have been well
researched (11, 14 –20). Torsional fracture occurs when the tip of the rotating instrument
binds in the canal while the motor continues to rotate. The elastic limit of the file is exceeded
in the absence of torque control settings on the motor, causing plastic deformation and,
subsequently, fracture. Thus, an instrument needs to be resistant to cyclic fatigue to have
sufficient flexibility to permit the preparation of curved systems but also has to have sufficient
torque strength so instrument separation does not occur should the file bind at its tip.
ProTaper (Dentsply Maillefer, Ballaigues, Switzerland) possesses exceptional torque
strength,whichasaresulthasdiminishedresistancetocyclicfatigueandamarkeddecrease
otary endodontic instruments manufactured from the pseudoelastic alloy nickel-
titanium (Nitinol 55) have revolutionized the biomechanical preparation of root
From the Department of Endodontics, Nova Southeastern
University College of Dental Medicine, Health Professions Di-
vision, Fort Lauderdale, FL.
Dr Eugenia Johnson is married to Dr Ben Johnson, founder
of Tulsa Dental Products and inventor of the ProFile nickel-
titanium instrument.
Address requests for reprints to Dr Adam Lloyd, Department
of Endodontics, Nova Southeastern University College of Dental
Medicine, Health Professions Division, 3200 S University Dr, Fort
Lauderdale, FL 33328. E-mail address: lloyda@nova.edu.
0099-2399/$0 - see front matter
Copyright © 2008 American Association of Endodontists.
doi:10.1016/j.joen.2008.07.029
Basic Research—Technology
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Johnson et al.
JOE — Volume 34, Number 11, November 2008

Page 2

inflexibility.ArecentinvestigationfoundthatProTaperinstrumentsaltered
thecanalshapeandrecommendedthatmoreflexibleinstrumentsbeused
to prepare the apical portion in curved canals (21). To increase the flexi-
bility and the resistance to cyclic fatigue and overcome the incidence of
separation,avariantoftheexistingnickel-titaniumalloycouldbeusedinthe
ProTapersystemandforrotaryinstrumentsingeneral.
Arecentstudyusingatomicforcemicroscopygraphicallyshowed
a significant difference in surface irregularities as a consequence of
machining differences between various nickel-titanium instrument
manufacturers(22).Thesamplestestedindicatedthatthefileswiththe
lowest to highest vertical depth profile were NitiFlex (Dentsply
Maillefer), GT hand files and GT rotary files (Dentsply Tulsa Dental
Specialties), and Quantec rotary files (SybronEndo, Orange, CA), re-
spectively. The base of these surface irregularities acts as a stress in-
ducerbecauseanappliedloadwillbeconcentratedatonepointorarea,
instead of being spread over a smooth surface. A scanning electron
microscopicstudyofdiscardednickel-titaniuminstrumentsfoundwid-
ened machining groove cracks containing dentinal debris, postulating
that dentin chips wedged into the surface defect resulted in instrument
fracture (13). The investigators recommended that manufacturers use
processesthatminimizetheproductionofsurfacecracksinadditionto
nickel-titanium alloys that improve fracture toughness (23).
TheProFileseriesofinstrumentsaremanufacturedfromthemost
commercially pure form of Nitinol (Nitinol SE508) by Nitinol Devices
and Components Inc (Fremont, CA) for instruments produced in the
UnitedStatesandEuroFlexGmbH(Pforzheim,Germany)forthosepro-
ducedinEurope.Therawwireundergoesaseriesofcoldannealingto
draw the wire to the correct cross-sectional diameter followed by ther-
mocycling while under strain (24). Once these treatments are com-
plete, the wire blanks are manufactured into rotary endodontic instru-
mentsthroughagrindingprocess.Thenickelcomponentof508Nitinol
is55.8wt%,withtitaniumaccountingforthebalance.Instrumentsand
hardwaremanufacturedfrom508Nitinolareusedinmanybranchesof
medicine,includingorthopedicsforjointreplacementandreconstruc-
tionandascardiovascularstentsthataltertheirsizeonintroductioninto
the higher temperatures of the human body.
Recently,areplacementforthecurrentnickel-titaniumalloyused
in endodontic instruments has been developed (25). The novel variant
nickel-titaniumalloyiscomposedof508Nitinol,whichhasundergone
a proprietary method of treatment, comprised of drawing the raw wire
under specific tension and heat treatments at various temperatures re-
sulting in a material that includes some portion in both the martensitic
andthepremartensiticRphasewhilemaintainingapseudoelasticstate.
The goal of this study was to assess the resistance to cyclic and tor-
sional fatigue of ProFile 25/.04 rotary endodontic instruments made from
508 Nitinol alloy or an experimental alloy (M-Wire NiTi, Dentsply Tulsa
Dental Specialties). The null hypothesis is that both cyclic and torsional
fatigue will occur equally in rotary instruments manufactured from any
alloy.
Materials and Methods
Wire blanks were created from three different lots of variant nickel-
titanium that had been subjected to different thermocycling programs
and strain, representing three different experimental groups (1B, 1A,
and 2AS). Stock production wire that had not undergone proprietary
processing was used as the control because this is currently used to
manufacture commercially available ProFile rotary instruments. The
wire blanks were machined to produce ProFile 25/.04 rotary instru-
ments. The identical file design was machined at the Dentsply Maillefer
facilitybyusingadifferentsourceofrawNitinolalloy(EuroFlexGmbH).
The European samples were divided into two groups, with one group
having undergone an additional surface electropolishing treatment. A
totalof360ProFile25/.04rotaryendodonticinstrumentswereusedfor
testing resistance to cyclic fatigue (n ? 180), and torsional fatigue
(n ? 180).
Experiment A (Cyclic Fatigue, n ? 30/Group)
The cyclic fatigue testing protocol has been described previously
andwasreproducedthroughouttheexperimentalperiod(26).Thefile
rotatedfreelyinagroovedtemperedsteelassemblythatreplicateda90°
curve of a 5-mm radius. Two flutes at the tip of the instrument were
visualized as it rotated just beyond the curve and was monitored on a
magnified display. The handle of each instrument was inserted into an
8:1gearreductionhandpiece(TUL-8M,DentsplyTulsaDentalSpecial-
ties)andattachedtotheassemblymountedonalaboratorybench.The
handpiecewascalibratedwiththeelectricmotorandoiledbeforeeach
group. The tempered steel jig was lubricated with a silicone spray be-
tween groups. The torque control was set to maximum and 300 RPM
(AsepticoEndoDTC;Aseptico,Woodinville,WA)beforeitwasactivated,
and time to instrument separation was recorded to an accuracy of 0.1
seconds. Data were interpreted by using a Welch analysis of variance,
with a Dunnet T3 post hoc test applied to all pair-wise comparisons
between instrument alloys.
Experiment B (Torsional Fatigue, n ? 30/Group)
Reproduction of the testing for torsional fatigue of instruments to
ANSI/ADA specification no. 28 was performed similarly to previous in-
vestigators (27), providing data for torque at fracture (inch ounces)
and angle of rotation at fracture. The handle of each instrument was
removedandtheshaftendfastenedintoachuckcontrolledbyadigital
display showing degrees rotation preprogrammed to rotate at 2 RPM
(Twist Test Unit; Mountain View Specialties, Mountain View, CA). The
apical3mmofthefilewasclampedbetweentwo3-mmthicksoftbrass
inserts that permitted a connection through a conductive plate con-
nectedtoadigitaltorquemetermemocouple(MGT50Z;Mark-10Corp,
Long Island, NY) forming a break detection circuit. Only the apical 3
mmofeachfilewasclampedconsistentlybecausethishasbeenshown
to be a source of variability (27), and the brass inserts were replaced
between each experimental group. Each instrument was rotated until
fractureoccurred,recordingtorqueandrotationstofailure.Datawere
analyzed by using an analysis of covariance in which the degree of
rotation was used as the covariate. All pair-wise comparisons were
conducted by using a Tukey HSD test.
The examiner was blind to the type of alloy being tested in all
experimentalprocedures,anddatawererecordedonacodedscore
sheet, with the code being broken at the end of testing all samples.
Data were entered into a spreadsheet (Excel 2004; Microsoft,
Redmond, WA) and analyzed by using StatView (SAS Institute Inc,
Cary, NC).
Results
The mean and standard deviations for cyclic fatigue are shown in
Table1andillustratedasaboxplot(Figure1).Differencesbetween
the groups were statistically significant (p ? 0.001) except for the
DentsplyMaillefer–producedsamples,wheretherewerenodifferences
betweenthelifespanofelectropolishedandnonelectropolishedinstru-
ments under cyclic fatigue testing.
Adjusting for the effect of rotation, the mean torque at fracture
(Table2)wasfoundtobestatisticallysignificantbetweenDentsplyTulsa
Dental, Maillefer, and variant nickel-titanium samples (p ? 0.001).
Variants 2AS and 1A and 2AS and 1B (M-Wire NiTi) groups were not
significantly different.
Basic Research—Technology
JOE — Volume 34, Number 11, November 2008Fatigue Testing of 508 Nitinol and M-Wire™NiTi
1407

Page 3

Discussion
Theaimofthisstudywastoeliminatefiledesignasavariableand
assessthecyclicandtorsionalfatigueofanewnickel-titaniumalloy,by
using raw sources of wire blanks that were machined to a standard
25/.04 instrument design (ProFile 25/.04, Dentsply Tulsa Dental Spe-
cialties).Thisisthefirststudytocomparerotarynickel-titaniuminstru-
ments manufactured to the same file design but produced from three
different sources of Nitinol (Nitinol Devices and Components Inc,
Dentsply Tulsa Dental Specialties, and EuroFlex GmbH) permitting di-
rectstudyofthedifferencesbetweenalloysandeliminatingfiledesignas
a variable. To determine benchmarks for competing file designs and
performance evaluations of altered nickel-titanium alloys, various in-
vestigatorshavereportedreproduciblemechanismsthatcanbeusedto
compare the modes of instrument separation, cyclic, and torsional fa-
tigue (19, 26). The protocol for this study followed the testing param-
eters of these prior authors.
Instrument separation has been recognized as a disadvantage of
nickel-titanium files. Analyses of the modes of fracture from discarded
instruments have been reported for cyclic and torsional fatigue occur-
ring 44.3% and 55.7%, respectively (28). The incidence of separation
has been reported as high as 14% of the instruments discarded, with
ProTaper being discarded twice as frequently as ProFile instruments
andexhibitingductilefracturewithnoinstrumentdistortionratherthan
cyclic fatigue failure with significant unwinding, which is characteristic
of ProFile (13, 29).
Datafromanearlierstudyclearlyshowthatsignificantimprovements
have been made over time when older alloys are compared with the most
recent derivatives of nickel-titanium alloy used to produce rotary instru-
ments(26).Theauthorsfoundcyclicfatiguemeanvaluesof105seconds?
10usingProFile25/.04ina5-mmradiuscurve.Absolutevaluesforcyclic
fatiguereportedhereareimprovedcomparedwithstaticcyclicfatigueval-
uespreviouslyexaminedwherethemeantimetofailureforProFile15/.04
was 139 seconds ? 18 (19). The present findings show ProFile 25/.04
instrument failure at 670 seconds ? 117 (M-Wire NiTi 100416-1B), 136
seconds?25(Dentsplystock25/.04),and304seconds?68(Maillefer
electropolished,br1a).
Given that the samples obtained from Dentsply Maillefer are ma-
chinedroboticallyandweresuppliedasnew,onewouldnotexpectsuch
a large standard deviation (SD) for the nonelectropolished group re-
sistance to cyclic fatigue when compared with the labor-intensive
DentsplyTulsaDentalSpecialtiessamples,wherefilesareintroducedby
hand at the various stages of production. The large differences in the
SDs of the experimental alloys are not unexpected because they are
roughlyproportionaltotheincreaseinresistancetocyclicfatigueofthe
control (stock Dentsply Tulsa Dental Specialties 25/.04).
Differences between endodontic rotary files produced at the dif-
ferent production plants may be caused by differences in the grinding
processrequiredtotaperandplaceflutesintothenickel-titaniumalloy
blanksortheamountofcoldworkthesuppliersperformwhiledrawing
the nickel-titanium wire. The study performed by Valois et al. (22)
essentiallyisaninvestigationintothequalityofthemachiningprocessof
the manufacturer. Nickel-titanium instruments are typically ground by
ceramic- or diamond-impregnated cutting wheels, the type of which is
proprietary and undisclosed by the manufacturer, leading to different
surfacetextures.Theverticaldepthprofileofinstrumentsmanufactured
by Dentsply Maillefer was found to be less than files manufactured by
Dentsply Tulsa Dental Specialties and statistically different from Tycom
(Irvine,CA)-producedinstrumentsthatreflectedthe“roughestgrind.”
TheauthorsdidnotinvestigatethesourceoftheNitinol,whichmayplay
a role in the creation of microfractures during instrument production.
Investigatorshaveshownthatsurfacedefects,createdduringthegrind-
ing process, can result in crack propagation (30) and have hypothe-
sizedthatductilefractureoccurswhendentinchipsbecomeembedded
in machining defects (23). The authors suggest surface defects intro-
duced during the machining process and greater resistance to fracture
couldbeachievedbyelectropolishingorionimplantation(13).Acom-
parison of electropolished (EndoSequence; Brasseler USA, Savannah,
GA) and nonelectropolished (ProFile, Dentsply Tulsa Dental Special-
ties) instruments with a 6% taper and multiple tip sizes in an in vitro
modelusingextractedmaxillaryandmandibularmolarsfoundtheelec-
tropolished samples exhibit crack formation significantly more often
than nonelectropolished instruments (31). All the EndoSequence 35/
.06 instruments showed microfractures after use in 7 canals, and cu-
mulative results found that fractures occurred 15% of the time and
deformed at 17%. ProFile instruments showed no microfractures and
preparedcanalswithoutasingleinstrumentseparating.Investigationof
Dentsply Maillefer produced electropolished and nonelectropolished
ProFile25/.04filesactuallyreducedresistancetocyclicfatigueanddid
notprovideanybenefitfortorsionalresistance(31).Ourfindingsarein
agreement showing that electropolishing does not increase fracture
resistance in performing cyclic fatigue and torsional testing.
The degree of rotation at fracture was not significant during tor-
sionalfatiguetestingandisinagreementwithYaredandKulkarni(27).
Significantdifferenceswerefoundbetweenthegroupsofvariantnickel-
TABLE 1. Cyclic Fatigue (in Seconds) Mean, SD, and Minimum and Maximum Values
InstrumentMean
670.43
578.38
304.42
299.92
228.84
135.91
SD Minimum
449.02
364.86
279.06
252.56
137.26
95.64
Maximum
903.77
783.66
329.79
347.27
297.34
180.55
Variant NiTi 1B (lot 100416-1B, M-Wire)
Variant NiTi 1A (lot 97670-1A)
Maillefer electropolished
Maillefer brushed
Variant NiTi 2AS (lot 104010-2AS)
Dentsply Tulsa Dental 25/.04
117.27
109.76
67.92
126.81
34.99
24.68
A
B
C
C
D
E
Levels not connected by same letter are significantly different.
0
200
400
600
800
1,000
Cyclic Fatigue ( Sec.)
100416-1B
104010-2AS
97670-1A
Maillefer Electropolished
Maillefer Brushed
Stock 25/.04
Box Plot of Cyclic Fatigue by Instrument
Figure 1. Box plot showing cyclic fatigue by instrument group.
Basic Research—Technology
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Johnson et al.
JOE — Volume 34, Number 11, November 2008

Page 4

titanium,stockalloy,andMailleferalloywiththerangeofmeantorque
values between 0.80 and 1.44 inch ounces.
Conclusion
The results of this study show that ProFile 25/.04 rotary instru-
mentsmanufacturedfromvariant1B(M-WireNiTi)hassuperiorresis-
tance to cyclic fatigue, representing an increase of up to 390% com-
paredwiththesameinstrumentdesignproducedfromstock508Nitinol
(p ? 0.001). The torque at fracture of M-Wire NiTi ProFile 25/.04
instruments was comparable to existing Dentsply Tulsa Dental Special-
ties production and greater than Dentsply Maillefer–sourced alloy.
The proprietary-treated M-Wire NiTi has desirable properties su-
perior to current 508 Nitinol–produced instruments and could be
adopted by other file designs to provide significant advances in rotary
endodontic instruments.
Acknowledgment
TheauthorswouldliketothankPatrickHardigan,PhD,forthe
statistical analysis and with help editing the manuscript. Thanks
also to Marcie Littleton, Manufacturing Projects and Facility Man-
ageratDentsplyTulsaDentalSpecialties,andBenJohnson,DDSfor
providing us with the machined samples.
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TABLE 2. Torsional Fatigue (in Degrees of Rotation to Failure and Peak Torque in Inch Ounces) Mean and SD
Instrument
Mean Torque
at Failure
1.44
1.28
1.20
1.13
0.90
0.80
SD Torque
Mean Degrees Rotation
to Failure
485.55
492.30
456.70
597.90
680.23
662.13
SD Rotation
Dentsply Tulsa Dental 25/.04
Variant NiTi 1A (lot 97670-1A)
Variant NiTi 2AS (lot 104010-2AS)
Variant NiTi 1B (lot 100416-1B, M-Wire)
Maillefer brushed
Maillefer electropolished
0.14
0.10
0.14
0.11
0.08
0.09
65.35
56.82
73.33
59.29
52.96
78.63
A
B
BC
C
D
D
Levels not connected by same letter are significantly different.
Basic Research—Technology
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