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Basic Research—Technology

Influence of Different Manufacturing Methodson the Cyclic Fatigue of Rotary Nickel-TitaniumEndodontic InstrumentsRenata C.V. Rodrigues, DDS,* H�elio P. Lopes, LD,

†Carlos N. Elias, PhD,

‡Georgiana Amaral, PhD,

§

Victor T.L. Vieira, DDS,‡and Alexandre S. De Martin, PhD*

Abstract
Introduction: The aim of this study was to evaluate, bystatic and dynamic cyclic fatigue tests, the number ofcycles to fracture (NCF) 2 types of rotary NiTi instru-ments: Twisted File (SybronEndo, Orange, CA), whichis manufactured by a proprietary twisting process, andRaCe files (FKG Dentaire, La Chaux-de-Fonds,Switzerland), which are manufactured by grinding.Methods: Twenty Twisted Files (TFs) and 20 RaCe files#25/.006 taper instruments were allowed to rotatefreely in an artificial curved canal at 310 rpm in a staticor a dynamic model until fracture occurred. Results:Measurements of the fractured fragments showed thatfracture occurred at the point of maximum flexure inthe midpoint of the curved segment. The NCF was signif-icantly lower for RaCe instruments compared with TFs.The NCF was also lower for instruments subjected tothe static test compared with the dynamic model inboth groups. Scanning electron microscopic analysis re-vealed ductile morphologic characteristics on the frac-tured surfaces of all instruments and no plasticdeformation in their helical shafts. Conclusions: RotaryNiTi endodontic instruments manufactured by twistingpresent greater resistance to cyclic fatigue comparedwith instruments manufactured by grinding. The frac-ture mode observed in all instruments was of the ductiletype. (J Endod 2011;37:1553–1557)
Key WordsCyclic fatigue, endodontic instruments, nickel-titanium,Twisted File, RaCe

From the *Department of Endodontics, S~ao Leopoldo Man-dic, Dental Research Center, Campinas, S~ao Paulo;†Department of Endodontics, Faculty of Dentistry, Est�acio deS�a University, Rio de Janeiro, Rio de Janeiro; ‡Department ofMaterials Science, Military Institute of Engineering, Rio deJaneiro, Rio de Janeiro; and §Department of Endodontics, S~aoLeopoldo Mandic, Dental Research Center, Rio de Janeiro, Riode Janeiro, Brazil.

Address requests for reprints to Dr Alexandre S. De Martin,Av Julio de Mesquita, 983/92, Campinas, SP, Brazil CEP 13025-063. E-mail address: [email protected]/$ - see front matter

Copyright ª 2011 American Association of Endodontists.doi:10.1016/j.joen.2011.08.011

JOE — Volume 37, Number 11, November 2011

Most nickel-titanium (NiTi) rotary endodontic instruments are machined bygrinding although some are produced by twisting the alloy after heat treatment

(1). Low-cycle fatigue fracture is a concern during the clinical use of rotary NiTi instru-ments (2–4). Fracture is defined as low cycle when it occurs in less than 104 cycles. Thistype of failure may be induced by rotating bending stresses when instrumenting curvedcanals (5). Resistance to fracture is determined by the number of cycles an instrumentcan endure under a specific loading condition before fracture occurs (6–8).

Cyclic fatigue tests can be static or dynamic (5, 9, 10). In static tests, the instrumentrotates at a fixed length (ie, with no axial oscillation) (5, 6, 10), whereas in the dynamicmodel the instrument is moved back and forth within the canal (7, 11, 12). The aim ofthis study was to assess the influence of the manufacturing process (grinding ortwisting) on the number of cycles to fracture (NCF) of rotary NiTi instrumentsthrough static and dynamic fatigue tests.

Materials and MethodsForty-four rotary NiTi instruments were used in this study: 22 Twisted files (TFs)

(SybronEndo, Orange, CA), which are machined by twisting, and 22 RaCe files (FKGDentaire, La Chaux-de-Fonds, Switzerland), which are manufactured by grinding.Both sets of files had a nominal size of 0.25 mm at D0, a taper of 0.06 mm/mm, anda triangular cross-section. The RaCe files had a total length of 25 mm, and the TFshad a total length of 27 mm.

Instrument Geometry (Design Features)For standardization of the instruments tested, 10 files of each brand were exam-

ined under a stereomicroscope (Pantec-Panambra, Cambuci, SP, Brazil) to determinetheir diameters at D3 and D13; the number of spirals in the working portion; and theirhelical angle at D3, D6, and D13. The taper of the working portion was calculated bysubtracting the diameters at D3 and D13 as described by Stenman and Spangberg (13)using the following equation:

Taper ðTÞ ¼ D13� D3=10

The diameter at D0 was calculated based on the values of D3 and T using thefollowing equation:

D0 ¼ D3� T� 3

The helical angle is the acute angle formed by the spiral and the long axis of theinstrument. It was obtained by tracing a line tangent to the spirals; this line formedan acute angle with the plane containing the instrument axis. The number of spiralsper millimeter was obtained by dividing the number of spirals by the length of theworking portion. Two instruments of each brand were embedded in acrylic resinand prepared for scanning electron microscopic (SEM) analysis of their cross-sections (JSM 5800; JEOL, Tokyo, Japan).

Cyclic Fatigue of 2 Types of NiTi Rotary Instruments 1553

mailto:[email protected]

http://dx.doi.org/10.1016/j.joen.2011.08.011

Figure 1. A schematic representation of the artificial canal used in the cyclicfatigue tests.

Figure 2. An apparatus used for the cyclic fatigue test.


Bending Resistance TestsThe bending resistance was evaluated using a universal testing

machine (DL 10.000; Emic, S~ao Jos�e dos Pinhais, Brazil) asdescribed in previous studies (14, 15). A 20-N load was appliedat 15 mm/min by means of a flexible stainless steel wire with 1end fastened to the testing machine head and the other end attached3 mm from the instrument tip until it displayed a 45� deflection.The maximum load to bend each file was recorded, and datawere statistically analyzed by the Student t test, with the significancelevel set at 5%. Bending resistance was tested in 10 instruments ofeach brand.

Cyclic Fatigue TestsFor these tests, an artificial canal measuring 1.4 mm in diameter

and 19 mm in total length was fabricated from a stainless steel tube. A 9-mm-long curved segment with a 6-mm radius (measured at the internalconcave surface of the tube) was created between 2 straight segmentsthat measured 7 mm and 3 mm (Fig. 1).

Static TestA stainless steel apparatus with a square base and a vertical axis

was constructed. The vertical axis allowed for the fixture and move-ment of a handpiece. At the base, a bench vise held the artificialcanal. A gap at the base of the apparatus allowed the bench vise

TABLE 1. The Mean Values for the Working Portion Length (WP); the Diameter at D3the Diameter at D0; and the Number of Spirals per Millimeter in the Working Port

Instruments No.Taper

(mm/mm)

Diameter (mm)

D0 D3 D13

RaCe 10 0.06 0.30 0.48 1.09TF 10 0.06 0.22 0.40 0.98

1554 Rodrigues et al.

to move horizontally while maintaining the axis of the instrumentaligned with the straight segment of the artificial canal createdbetween 2 straight segments that measured 7 mm and 3 mm.(Fig. 2). The canal was filled with glycerin to reduce friction, mini-mizing the release of heat. Each file was attached to a contra-angle/micromotor handpiece with 10:1 gear reduction (TC–Motor 3000;Nouvag AG/AS/LTD, Goldach, Switzerland) and introduced into thecanal until the file tip touched a shield positioned at the simulatedapical foramen. Ten instruments of each brand were rotated clock-wise at 310 rpm until fracture. The time of fracture was recordedby the same operator using a digital stopwatch (Leroy) and estab-lished by visual observation of instrument separation. The NCF wasobtained by multiplying the rotational speed by the time (in seconds)when fracture occurred.

Dynamic TestAnother set of 10 instruments of each brand was used for the

dynamic test. The instruments were subjected to the same protocoldescribed in the static test, but for these experiments a mechanicaldevice promoted back and forth axial movements while the filesrotated inside the canal. The amplitude of the axial movements was3 mm, with approximately 2 seconds between oscillations. Data ob-tained from the static and dynamic tests for both brands of fileswere statistically analyzed by the Student t test, with the significancelevel set at 5%. The fractured surfaces and the helical shaft of theseparated instruments were analyzed under SEM (JSM 5800) todetermine the type of fracture and the presence of plastic deformationin the shaft.

and D13; the Helical Angle at D3, D6, and D13; the Number of Spirals; the Taper;ion of the Files

WP (mm)

Helical angleNo. ofspirals Per mmD3 D6 D13

17.61 14.55 17.73 19.56 7 0.415.36 20.64 24.97 31.16 11 0.7


TABLE 2. Means (� standard deviation) of the Maximum Load (g) to BendRaCe and TF Instruments

InstrumentsNo. of

instruments Maximum load (g)

RaCe 10 333.4 (16.5)TF 10 218.2 (15.26)

TABLE 3. Mean (� standard deviation) of the Time(s) and Numbers of Cyclesto Fracture (NCF) for the Instruments Tested

Test

Time NCF

RaCe TF RaCe TF

Static 25.2(5.43)

80.4(8.57)

130.03(28.03)

414.86(44.27)

Dynamic 45.4(14.41)

153.25(36.53)

234.26(26.33)

790.77(188.5)


ResultsInstrument Geometry (Design Features)

The mean length of the working portion; the diameter at D3 andD13; the helical angle at D3, D6, and D13; the number of spirals inthe working portion; the mean taper; the diameter at D0; and thenumber of spirals per millimeter in the working portion of the filesare shown in Table 1. SEM analyses of fractured surface showed thatTFs and RaCe files had a triangular cross-section.

Bending ResistanceThe mean bending resistance, represented by the maximum load

(in grams) to bend the instruments, is shown in Table 2. A significantdifference was observed between the 2 groups (P = 0). Statisticallyless force was required for TFs with respect to RaCe files in thebending test.

Cyclic FractureThe means and standard deviation for the time (in seconds)

and NCF are shown in Table 3. TFs presented a significantly higherNCF compared with RaCe files (P = 0). SEM analysis revealed thatboth brands of files displayed ductile morphologic characteristicson the fracture surfaces. No plastic deformation occurred in thehelical shaft of the instruments (Figs. 3 and 4). The cyclicfatigue testing model (static or dynamic) had no influence on theSEM results.

Figure 3. The fractured surface of TFs subjected to static (A to B) and dynamic (CTF (100� magnification). (B and D) Cracks following machining grooves are obs


DiscussionThe NCF of rotary NiTi endodontic files is affected by their shape,

dimensions, and bending resistance. Therefore, these were the param-eters evaluated in the present study. Slight variations in design may havea significant impact on the behavior of endodontic instruments (13).The greater diameter at D0 and the lower flexibility of RaCe filescompared with TFs may partly explain their lower resistance to cyclicfatigue (5, 6, 16–19).

The performance of rotary instruments in cyclic fatigue assays isdirectly related to their bending resistance (6, 20, 21). In ourexperiments, RaCe files required significantly greater loads than TF todisplay 45� deflection. Therefore, it can be inferred that RaCe filesare less flexible than TFs. Rigid instruments present lower a NCFbecause of the buildup of tensions at the point of maximum flexure,as observed in the present work and in previous studies (19, 22–24).

According to Yao et al (7), the use of standardized artificial canalsin cyclic fatigue experiments minimizes the influence of other variables.In the present study, a metallic tube was used to standardize the entirelength of the canal, the length of the curvature radius, and the length ofthe arc. However, one should bear in mind that the actual lengths of thearc and the radius of the cylindric curved canal are not the same as theinstrument positioned inside the tube (25). It is also important to pointout that because the inner diameter of the tube was larger than that ofthe endodontic instrument and a lubricant was used throughout theexperiments, instruments were allowed to rotate within the canalwithout significant resistance during the cyclic fatigue tests. Friction

to D) tests. (A and C) The absence of plastic deformation in the helical shaft oferved near the fractured surface of TF (1,000�).


Figure 4. RaCe instruments fractured in the static (A to B) and dynamic (C to D) tests. (A and C) The absence of plastic deformation in the helical shaft (100�magnification). (B and D) No cracks were observed on RaCe instruments (1,600� and 1,000� magnification, respectively).


was further reduced by using a lubricant throughout the assays (5, 6,14, 21, 25).

The TF instruments displayed significantly higher NCF values thanRaCe files in both fatigue assays (static and dynamic), as observed inprevious studies with similar methodologies (26–28). Ourobservations suggest that the new manufacturing process involvingtwisting coupled with heat treatment along with the uniquelongitudinal features of TF files (ie, the helical angle, arrangementand number of spirals/mm in the fluted portion, and longitudinallyoriented surface texture) may have positively affected theirperformance (26). The number of spirals in the working portionmay have favored the higher flexibility and fatigue resistance of TFscompared with RaCe files. The morphological features of NiTi rotaryinstruments and their effect on cyclic fatigue resistance have been theobject of several studies (17, 18, 26–28).

According to the manufacturer, Twisted File instruments areproduced by a proprietary process of heating and cooling ofNiTi that leads to a molecular structure known as the R phase.In this state, NiTi can be twisted, resulting in instruments with opti-mized properties (29). The alloy in the R phase displays superelasticity and shape memory, allowing the production of more flex-ible instruments compared with their ground counterparts(26, 29).

Regardless of the instrument brand and manufacturing process, inthe present work, the NCF was significantly higher during dynamic versusstatic fatigue testing; our findings, which are similar to those from otherstudies (7–11), indicate that a concentration of stresses in the same areaof the instrument shaft significantly reduces the NCF. Because in the statictest the file does not move axially, alternating compressive and tensilestresses are concentrated in one area of the instrument. Thesecumulative stresses induce microstructural changes in the alloy. Incontrast, in the dynamic model, the file moves axially within the canal,allowing stresses to be distributed along the instrument shaft. Bypreventing stress concentration in the same area, resistance to fracture

1556 Rodrigues et al.

is enhanced. Our results along with those from Li et al (9) and Lopeset al (10) suggest that this principle may hold true for other types ofinstruments.

SEM analysis of the instruments showed that both file types hada triangular cross-section. Analysis of the fractured surfaces did not revealmorphologic differences between the 2 types of instruments or betweeninstruments fractured during static versus dynamic tests. Moreover, noevidence of plastic deformation was detected in the helical shafts ofany fractured instruments. All fracture surfaces displayed ductilemorphologic characteristics as observed by other authors (5, 21, 26).

In conclusion, TF endodontic instruments, which are manufac-tured by a twisting technology, display higher NCF values comparedwith RaCe instruments, which are manufactured by grinding. The NCFfor both instruments was higher in the dynamic fatigue test than inthe static model. Therefore, it can be inferred that TFs are more flexiblethan RaCe files. These results highlight the importance of applyinga continuous pecking motion during rotary instrumentation of curvedroot canals in order to avoid concentration of stresses in the samearea of the instrument shaft.

AcknowledgmentsThe authors thank FKG Dentaire for providing the instruments

used in this study.The authors deny any conflicts of interest related to this study.

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