Basic Research—Technology

Cyclic Fatigue Resistance of RaCe and Mtwo Rotary Filesin Continuous Rotation and Reciprocating MotionSekar Vadhana, BDS, Balasubramanian SaravanaKarthikeyan, MDS, Suresh Nandini, MDS,and Natanasabapathy Velmurugan, MDS

Abstract

Introduction: The purpose of this study was to evaluateand compare the cyclic fatigue resistance of RaCe (FKGDentaire, La Chaux-de-Fonds, Switzerland) and Mtwo(VDW, Munich, Germany) rotary files in continuous rota-tion and reciprocating motion. Methods: A total of 60new rotary Mtwo and RaCe files (ISO size = 25, taper= 0.06, length = 25 mm) were selected and randomlydivided into 4 groups (n = 15 each): Mtc (Mtwo NiTi filesin continuous rotation), Rc (RaCe NiTi files in continuousrotation), Mtr (Mtwo NiTi files in reciprocating motion),and Rr (RaCe NiTi files in reciprocating motion). A cyclicfatigue testing device was fabricated with a 60� angle ofcurvature and a 5-mm radius. All instruments wererotated or reciprocated until fracture occurred. Thetime taken for each instrument to fracture and thelength of the broken fragments were recorded. All thefractured files were analyzed under a scanning electronmicroscope to detect the mode of fracture. TheKolmogorov-Smirnov test was used to assess thenormality of samples distribution, and statistical anal-ysis was performed using the independent sample ttest. Results: The time taken for the instruments ofthe Mtr and Rr groups to fail under cyclic loading wassignificantly longer compared with the Mtc and Rcgroups (P < .001). Scanning electron microscopic obser-vations showed that the instruments of all groups hadundergone a ductile mode of fracture. The length ofthe fractured segments was between 5 and 6 mm, whichwas not statistically significant among the experimentalgroups. Conclusions: Mtwo and RaCe rotary instru-ments showed a significantly higher cyclic fatigue resis-tance in reciprocating motion compared with continuousrotation motion. (J Endod 2014;40:995–999)

Key WordsCyclic fatigue, nickel-titanium files, reciprocation, rotation

From the Department of Conservative Dentistry and EndodonticResearch, Maduravoyal, Chennai, India.

Address requests for reprints to Dr SureshNandini, Department ofAcademy of Higher Education and Research, Alapakkam Main Road0099-2399/$ - see front matter

Copyright ª 2014 American Association of Endodontists.http://dx.doi.org/10.1016/j.joen.2013.12.010

JOE — Volume 40, Number 7, July 2014

Nickel-titanium files (NiTi) are commonly used in current endodontic practice. NiTifiles offer many advantages over stainless steel files such as flexibility and elasticity

(1, 2). Despite these advantages, NiTi instruments appear to have a high risk ofseparation (3). One of the reasons for the fracture of NiTi instruments is torsionalor cyclic fatigue (4, 5). Torsional fatigue occurs when the tip of the instrumentbinds in the canal while the shank continues to rotate (3, 5). Cyclic fatigue occurswhen the instrument continues to rotate freely in a curvature, and at the point ofmaximum flexure, tension/compression cycles are generated until fracture occurs(5). Increasing the resistance to file separation has been the main goal for ensuringsafety during endodontic instrumentation.

Conventional NiTi rotary endodontic files are manufactured by machining startingwire blanks that are in the superelastic austenitic phase (6). Under stress, it changes tothemartensitic phase. One of the unique properties of the martensitic phase is that it hasexcellent resistance to fatigue (7). The stress-induced transformation to the martensiticphase is reversible (8). The temperature at which the martensitic phase gets trans-formed to the austenitic phase is called the austenitic finish temperature. The higherthe austenitic finish temperature, the longer the file remains in the martensitic phase.This phase transformational behavior and microstructure of NiTi can be optimized us-ing thermomechanical processing. This ultimately increases the mechanical and fatigueproperties of the file (9). Another method of improving cyclic fatigue resistance is byelectropolishing the files. This surface treatment improves the surface smoothness,thereby delaying the initiation of surface cracks (10).

An alternativemethod of increasing cyclic fatigue resistance is the use of rotary NiTiinstruments in reciprocating motion (11). Two reciprocating systems are currentlyavailable: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Bal-laigues, Switzerland). Reciprocating NiTi files have a better cyclic fatigue resistancewhen compared with that of continuous rotary NiTi files (12). Studies have been con-ducted to evaluate the use of various rotary NiTi files including Mtwo (VDW), K3, Pro-Taper (Dentsply Maillefer, Ballaigues, Switzerland), and Twisted Files (Sybron Endo,Orange, CA) in reciprocating motion, and it has been proved that these files possessbetter cyclic fatigue resistance (12–14). Mtwo files, which have a cross-section similarto that of Reciproc files (15), have improved cyclic fatigue resistance in both continuousand reciprocating motion (12). RaCe files (FKG Dentaire, La Chaux-de-Fonds,Switzerland) have a triangular cross-section with distinct positive cutting angles(16). RaCe files have improved cyclic fatigue resistance in continuous rotation whencompared with that of ProTaper and Helix rotary files (17). However, to date, the effectof reciprocating motion on RaCe files has not been studied. The null hypothesis is thatthere is no difference in the cyclic fatigue resistance of RaCe and Mtwo rotary files usingcontinuous rotation and reciprocating motion.

s, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and

Conservative Dentistry and Endodontics,Meenakshi Ammal Dental College andHospital,Meenakshi, Maduravoyal, Chennai 600 095, Tamil Nadu, India. E-mail address: nandini_80@hotmail.com

Cyclic Fatigue Life of RaCe and Mtwo Files 995

Basic Research—Technology

Materials and MethodsA total of 30 new rotary Mtwo (VDW, Munich, Germany) and 30

RaCe instruments (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland)(ISO tip size = 25, taper = 0.06, length = 25 mm) were selected. Allthe instruments were previously inspected under an optical stereo mi-croscope (Zoom Stereo Binocular Microscope [ZSM-111], Hicksville,NY); with 20�magnification for any visible signs of deformation. Noneof the instruments were discarded. All the files were then subjected tocyclic fatigue testing.

Cyclic Fatigue Testing DeviceA static cyclic fatigue testing device was custom fabricated for

this study (Fig. 1A and B). It consisted of a main metal framemade of iron to which an artificial canal system and a support forthe handpiece were being attached. The canal system, which simu-lated a root canal, consisted of 2 adjustable metal frames made ofbrass that can accommodate any instrument to its exact size and ta-per. It was constructed with a 60� angle of curvature. The curvaturestarted at 5 mm from the tip of the canal. The WaveOne handpiecewas mounted over the support, which also ensured the correct posi-tioning and placement of files to the same appropriate depth for allthe samples.

Cyclic Fatigue TestSixty samples were randomly divided into 4 groups (n = 15) ac-

cording to the type of rotary files and rotary motions used.

Figure 1. (A and B) The custom fabricated static cyclic fatigue testing device. (C) Sfractured RaCe file depicting the fracture at the D5–D6 regions.

996 Vadhana et al.

Group MtcFifteen Mtwo instruments were allowed to rotate in continuous

rotation (CW)motion using aWaveOnemotor set at continuous rotationmode with recommended torque control settings and at a constantspeed of 300 rpm.

Group MtrFifteen Mtwo instruments were allowed to rotate in reciprocating

motion (counterclockwise [CCW] = 170 and CW = 50) using a Wave-One motor set on the WaveAll mode with recommended torque controlsettings and at a constant speed of 350 rpm (11).

Group RcFifteen RaCe instruments were allowed to rotate in continuous

rotation motion using a WaveOne motor set on the continuous rotationmode with recommended torque control settings and at a constantspeed of 600 rpm.

Group RrFifteen RaCe instruments were allowed to rotate in reciprocating

motion (CCW = 170 and CW = 50) using a WaveOne motor set onthe WaveAll mode with recommended torque control settings and ata constant speed of 350 rpm (11). Glycerin (Glycerin Pure; AB Enter-prises, Mumbai, India) was used as a lubricant after the use of each fileduring instrumentation in this study. Instruments were rotated or recip-rocated until fracture occurred. To obviate human errors, cyclic fatigue

tereomicroscopic image of a new RaCe file. (D) Stereomicroscopic image of a

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Basic Research—Technology

testing was performed by viewing the rotating or reciprocating filesunder a dental operating microscope (Seiler Microscope, St Louis,MO) at a magnification of 25� to precisely determine the time of frac-ture. All files were tested by 1 operator while the other operator wassimultaneously operating the stopwatch. The length of the broken frag-ments was measured using a vernier caliper under a dental operatingmicroscope (25� magnification).

Scanning Electron Microscopic AnalysisThe fractured fragments of each instrument were collected from

all the groups. The fractured surfaces of the files were examined undera scanning electron microscope (FEI Quanta FEG 200; Hillsboro, OR)to determine the modal characteristics of fracture (Fig. 2A–D).

Stereomicroscopic AnalysisA few samples were randomly selected from each group and

observed under an optical stereo microscope (40�) before and afterthe failure of the respective instruments.

Statistical AnalysisThe mean and standard deviation of the time to fracture and the

fractured fragment length were calculated for all the experimentalgroups. The Kolmogorov-Smirnov test was used to assess the normalityof sample distribution. Both intragroup and intergroup comparisonsof cyclic fatigue resistance of the samples were performed using the

Figure 2. Scanning electron microscopic images of fractured files showing microband in reciprocation motion (B). Mtwo file used in continuous rotation (C) and i

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independent sample t test (SPSS version .17; SPSS, Chicago, IL). A Pvalue of <.05 was used as a criterion for statistical significance.

ResultsDescriptive statistics for the 4 experimental groups are listed in

Table 1. It was observed that the cyclic fatigue resistance of the RaCeand Mtwo NiTi rotary files increased significantly when operated inreciprocation compared with the continuous rotation mode. It wasalso observed that Mtwo files showed significantly more resistance tofracture under cyclic loading than RaCe files. The mean length of thefractured segments was observed as 5–6mm, which was not statisticallysignificant among the groups.

Scanning Electron Microscopic Analysis ResultsScanning electron microscopic analysis of fractured surfaces of all

the groups revealed craterlike formation along with numerous dimplesand microbubbles. These features indicate that the instrument had un-dergone a ductile mode of fracture, which is predominantly observed incyclic fatigue failure.

DiscussionThis study compared the cyclic fatigue resistance of RaCe and

Mtwo rotary files in continuous and reciprocating motion and also as-sessed the mode of fracture under scanning electron microscopy. Thisstudy rejects the proposed null hypothesis and has shown that bothMtwo and RaCe rotary files have improved cyclic fatigue resistance in

ubbles, craters, valleys, and dimples. RaCe file used in continuous rotation (A)n reciprocation motion (D).

Cyclic Fatigue Life of RaCe and Mtwo Files 997

TABLE 1. Intra- and Intergroup Comparison of Mean and Standard Deviation of Time Taken to Fracture of Instruments and Length of Fractured Instruments

Groups Continuous Reciprocating

n = 15 Time ± SD Length ± SD Time ± SD Length ± SD

Mtwo 283.27 � 58.346A 5.533 � 0.3994a 815.73 � 82.893B 5.633 � 0.4419a

RaCe 173.67 � 49.844A 5.467 � 0.4419a 337.67 � 39.811B 5.500 � 0.4629a

P value <.001 .668 <.001 .426

A significant difference in P values is marked as A and B (P < .05). An insignificant difference in P values is marked as a and a (P > .05).

Basic Research—Technology

reciprocating motion when compared with continuous rotation. Mtworotary files were selected because they have a cross-section similar toReciproc files (12), and RaCe files were chosen because there are nostudies using these files in reciprocating motion to date. Cyclic fatiguetesting can be performed using a static or dynamic model (15, 18).In a static model, the instrument does not move axially. This createsalternate compressive and tensile stresses in a particular area of theinstrument, leading to premature failure. The dynamic modelincorporates cyclic axial movement, which provides a better clinicalsimulation and increases the lifespan of rotary files, but theamplitude, speed of pecking motion, and axial movement are purelysubjective in clinical practice. The ability to constrain the files in aprecise trajectory is also difficult in dynamic testing (18).

There is no evidence for testing rotary files under standardizedspecifications (3, 19). Hence, a nontooth static model of astandardized artificial canal was used in this study, which minimizesthe influence of other variables of file separation other than cyclicfatigue.

Earlier cyclic fatigue studies have been conducted using variousangles of curvatures such as 30�, 60�, and 90�. A canal of 30� doesnot constrain the file properly, whereas a curvature of 90� incorporatedmore stresses to the file (13, 20). The center of curvature in most of thestudies was between 5 and 7 mm from the tip of the instrument (4).Thus, in this study, an artificial canal was fabricated with a 60� of cur-vature with the maximum point of curvature at 5 mm from the tip of theinstrument.

In this study, fatigue life of the instruments was evaluated by timeand not by the number of cycles to fracture (NCF). The time to fracture iseasier to record than NCF. In continuous rotation motion, the assess-ment of the number of cycles to fracture can be obtained by simplymultiplying the rotational speed by the time elapsed until fractureoccurred. However, in reciprocation motion, the NCF can be deter-mined only by knowing the amplitude of the oscillating motion with aconstant time unit, which is not provided by the manufacturers (15).

Both Race and Mtwo rotary files showed better resistance to cyclicfatigue in reciprocation compared with continuous rotation motion inthis study. The fatigue life is determined by the number of times thecrack opens and closes with each cycle. In continuous rotation motion,a 360� rotation is completed in 1 cycle, which leads to increased stresslevels and more widening of surface cracks.

Tensile stresses are concentrated at a particular point of the instru-ment in continuous rotation, which culminates to the fracture of the in-strument. However, in reciprocating motion using the WaveAll mode(CCW = 170 and CW = 50), it takes 3 cycles to complete one 360� rota-tion (12). The stresses are distributed to 3 points around the workingportion of the file, minimizing the opening of surface cracks (15). Inour study, the cyclic fatigue life of RaCe files in reciprocation was 1.9times more than that of continuous rotation, whereas Mtwo files per-formed 2.9 times better in reciprocation than in continuous rotation.This increase in the cyclic fatigue life of RaCe files in reciprocationcompared with continuous rotation was not proportional compared

998 Vadhana et al.

with that of Mtwo files. The reason could be attributed to the differencein the rotational speeds of RaCe files when used in reciprocation (350rpm) and continuous rotation (600 rpm). Further studies are requiredto conclude the role of rotational speeds in the cyclic fatigue life of rotaryfiles in reciprocating motion.

In our study, Mtwo files performed better than RaCe rotary NiTifiles in both continuous and reciprocating motion. Previous studieshave shown controversial results regarding the cyclic fatigue life ofMtwo and RaCe in continuous rotation when compared with other files(17, 19–21). In reciprocating motion, Mtwo has been proven to have asimilar cyclic fatigue life to that of Twisted Files and Reciproc andsuperior resistance compared with WaveOne (12).

Resistance to cyclic fatigue depends on various factors such as diam-eter, metal mass, flexibility, cross-sectional shape, and surface finish of ro-tary files (22). RaCe files have an electropolished surface finish and goodflexibility and thus are expected to possess a better cyclic fatigue life (23).In our study, it showed a reduced fatigue life in comparison with Mtwo.

This could be caused by the presence of alternating cutting edgesin RaCe files. Although the alternating spiraled and nonspiraled seg-ments of RaCe files increase its flexibility, maximum bending occursat these segment junctions. Because flexural stresses are confined tothe fluted portion of the working area and are not distributed evenlythroughout the length of the file, fatigue occurs more rapidly in RaCefiles (22). The stereomicroscopic analysis of the new RaCe files inour study showed that the junction of the spiraled and nonspiraled seg-ments exists at the D5–D6 region (Fig. 1C). The synergistic effect of thepresence of such a junction at 5 mm of the RaCe file (which corre-sponds to the maximum curvature of the artificial canal) and the staticmode of testing could have resulted in premature failure of RaCe files.Figure 1D shows that the failure occurred at the segment junctions.Electropolishing potentially removes the residual stresses on the instru-ment and increases the fatigue life (10). However, the role of electro-polishing in the cyclic fatigue resistance of the instruments cannot beconsidered in this study because the cross-sections of both the filesystems (RaCe and Mtwo) were different.

Another determining factor for the cyclic fatigue life of files is theshape of the file at its circumference. The forces required to disrupt themolecular attraction can be decreased by providing a greater mass ofrounded surface following the cutting edge of files at its circumference(22). At the radial extremity, RaCe files have a more angular surfacebecause of their triangular cross-section in comparison with that ofMtwo files. This might lead to increased stresses in a small area, whichultimately resulted in cyclic fatigue failure with less tension.

Scanning electron microscopic observations have shown that allspecimens of both the groups depicted the presence of microbubbles.Circular abrasion marks, dimples, and craterlike formation confirmedthat all instruments had undergone cyclic fatigue.

On analyzing the data regarding the length of the fractured frag-ments, there was no statistical difference in the mean length of all theinstruments tested. In this study, almost all the instruments fracturedbetween D5 and D6. The fractured length of the instruments ranged

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Basic Research—Technology

from 4.5–5.5 mm, which confirmed the positioning of the files in a pre-cise trajectory. Further studies may be required on the canal centeringability and cutting efficiency of RaCe and Mtwo rotary NiTi files in recip-rocating motion.

ConclusionWithin the limitations of this study, Mtwo and RaCe rotary files

were more resistant to cyclic fatigue in reciprocating motion whencompared with continuous rotation motion. Mtwo files performed bet-ter than RaCe files in both continuous and reciprocating motion.

AcknowledgmentsThe authors deny any conflicts of interest related to this study.

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