Basic Research—Technology Cyclic Fatigue Resistance of RaCe and Mtwo Rotary Files in Continuous Rotation and Reciprocating Motion Sekar Vadhana, BDS, Balasubramanian SaravanaKarthikeyan, MDS, Suresh Nandini, MDS, and Natanasabapathy Velmurugan, MDS Abstract Introduction: The purpose of this study was to evaluate and compare the cyclic fatigue resistance of RaCe (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and Mtwo (VDW, Munich, Germany) rotary files in continuous rotation and reciprocating motion. Methods: A total of 60 new rotary Mtwo and RaCe files (ISO size = 25, taper = 0.06, length = 25 mm) were selected and randomly divided into 4 groups (n = 15 each): Mtc (Mtwo NiTi files in continuous rotation), Rc (RaCe NiTi files in continuous rotation), Mtr (Mtwo NiTi files in reciprocating motion), and Rr (RaCe NiTi files in reciprocating motion). A cyclic fatigue testing device was fabricated with a 60 angle of curvature and a 5-mm radius. All instruments were rotated or reciprocated until fracture occurred. The time taken for each instrument to fracture and the length of the broken fragments were recorded. All the fractured files were analyzed under a scanning electron microscope to detect the mode of fracture. The Kolmogorov-Smirnov test was used to assess the normality of samples distribution, and statistical analysis was performed using the independent sample t test. Results: The time taken for the instruments of the Mtr and Rr groups to fail under cyclic loading was significantly longer compared with the Mtc and Rc groups (P < .001). Scanning electron microscopic observations showed that the instruments of all groups had undergone a ductile mode of fracture. The length of the fractured segments was between 5 and 6 mm, which was not statistically significant among the experimental groups. Conclusions: Mtwo and RaCe rotary instruments showed a significantly higher cyclic fatigue resistance in reciprocating motion compared with continuous rotation motion. (J Endod 2014;40:995–999) Key Words Cyclic fatigue, nickel-titanium files, reciprocation, rotation N ickel-titanium files (NiTi) are commonly used in current endodontic practice. NiTi files 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 of separation (3). One of the reasons for the fracture of NiTi instruments is torsional or cyclic fatigue (4, 5). Torsional fatigue occurs when the tip of the instrument binds in the canal while the shank continues to rotate (3, 5). Cyclic fatigue occurs when the instrument continues to rotate freely in a curvature, and at the point of maximum flexure, tension/compression cycles are generated until fracture occurs (5). Increasing the resistance to file separation has been the main goal for ensuring safety during endodontic instrumentation. Conventional NiTi rotary endodontic files are manufactured by machining starting wire blanks that are in the superelastic austenitic phase (6). Under stress, it changes to the martensitic phase. One of the unique properties of the martensitic phase is that it has excellent resistance to fatigue (7). The stress-induced transformation to the martensitic phase is reversible (8). The temperature at which the martensitic phase gets transformed to the austenitic phase is called the austenitic finish temperature. The higher the austenitic finish temperature, the longer the file remains in the martensitic phase. This phase transformational behavior and microstructure of NiTi can be optimized using thermomechanical processing. This ultimately increases the mechanical and fatigue properties of the file (9). Another method of improving cyclic fatigue resistance is by electropolishing the files. This surface treatment improves the surface smoothness, thereby delaying the initiation of surface cracks (10). An alternative method of increasing cyclic fatigue resistance is the use of rotary NiTi instruments in reciprocating motion (11). Two reciprocating systems are currently available: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland). Reciprocating NiTi files have a better cyclic fatigue resistance when compared with that of continuous rotary NiTi files (12). Studies have been conducted to evaluate the use of various rotary NiTi files including Mtwo (VDW), K3, ProTaper (Dentsply Maillefer, Ballaigues, Switzerland), and Twisted Files (Sybron Endo, Orange, CA) in reciprocating motion, and it has been proved that these files possess better cyclic fatigue resistance (12–14). Mtwo files, which have a cross-section similar to that of Reciproc files (15), have improved cyclic fatigue resistance in both continuous and 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 when compared with that of ProTaper and Helix rotary files (17). However, to date, the effect of reciprocating motion on RaCe files has not been studied. The null hypothesis is that there is no difference in the cyclic fatigue resistance of RaCe and Mtwo rotary files using continuous rotation and reciprocating motion. From the Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research, Maduravoyal, Chennai, India. Address requests for reprints to Dr Suresh Nandini, Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research, Alapakkam Main Road, Maduravoyal, Chennai 600 095, Tamil Nadu, India. E-mail address: [email protected] 0099-2399/$ - 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 Cyclic Fatigue Life of RaCe and Mtwo Files 995 Basic Research—Technology Materials and Methods A 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. All the instruments were previously inspected under an optical stereo microscope (Zoom Stereo Binocular Microscope [ZSM-111], Hicksville, NY); with 20 magnification for any visible signs of deformation. None of the instruments were discarded. All the files were then subjected to cyclic fatigue testing. Cyclic Fatigue Testing Device A static cyclic fatigue testing device was custom fabricated for this study (Fig. 1A and B). It consisted of a main metal frame made of iron to which an artificial canal system and a support for the handpiece were being attached. The canal system, which simulated a root canal, consisted of 2 adjustable metal frames made of brass that can accommodate any instrument to its exact size and taper. It was constructed with a 60 angle of curvature. The curvature started at 5 mm from the tip of the canal. The WaveOne handpiece was mounted over the support, which also ensured the correct positioning and placement of files to the same appropriate depth for all the samples. Cyclic Fatigue Test Sixty samples were randomly divided into 4 groups (n = 15) according to the type of rotary files and rotary motions used. Group Mtc Fifteen Mtwo instruments were allowed to rotate in continuous rotation (CW) motion using a WaveOne motor set at continuous rotation mode with recommended torque control settings and at a constant speed of 300 rpm. Group Mtr Fifteen Mtwo instruments were allowed to rotate in reciprocating motion (counterclockwise [CCW] = 170 and CW = 50) using a WaveOne motor set on the WaveAll mode with recommended torque control settings and at a constant speed of 350 rpm (11). Group Rc Fifteen RaCe instruments were allowed to rotate in continuous rotation motion using a WaveOne motor set on the continuous rotation mode with recommended torque control settings and at a constant speed of 600 rpm. Group Rr Fifteen RaCe instruments were allowed to rotate in reciprocating motion (CCW = 170 and CW = 50) using a WaveOne motor set on the WaveAll mode with recommended torque control settings and at a constant speed of 350 rpm (11). Glycerin (Glycerin Pure; AB Enterprises, Mumbai, India) was used as a lubricant after the use of each file during instrumentation in this study. Instruments were rotated or reciprocated until fracture occurred. To obviate human errors, cyclic fatigue Figure 1. (A and B) The custom fabricated static cyclic fatigue testing device. (C) Stereomicroscopic image of a new RaCe file. (D) Stereomicroscopic image of a fractured RaCe file depicting the fracture at the D5–D6 regions. 996 Vadhana et al. JOE — Volume 40, Number 7, July 2014 Basic Research—Technology testing was performed by viewing the rotating or reciprocating files under a dental operating microscope (Seiler Microscope, St Louis, MO) at a magnification of 25 to precisely determine the time of fracture. All files were tested by 1 operator while the other operator was simultaneously operating the stopwatch. The length of the broken fragments was measured using a vernier caliper under a dental operating microscope (25 magnification). Scanning Electron Microscopic Analysis The fractured fragments of each instrument were collected from all the groups. The fractured surfaces of the files were examined under a scanning electron microscope (FEI Quanta FEG 200; Hillsboro, OR) to determine the modal characteristics of fracture (Fig. 2A–D). Stereomicroscopic Analysis A few samples were randomly selected from each group and observed under an optical stereo microscope (40) before and after the failure of the respective instruments. independent sample t test (SPSS version .17; SPSS, Chicago, IL). A P value of <.05 was used as a criterion for statistical significance. Results Descriptive statistics for the 4 experimental groups are listed in Table 1. It was observed that the cyclic fatigue resistance of the RaCe and Mtwo NiTi rotary files increased significantly when operated in reciprocation compared with the continuous rotation mode. It was also observed that Mtwo files showed significantly more resistance to fracture under cyclic loading than RaCe files. The mean length of the fractured segments was observed as 5–6 mm, which was not statistically significant among the groups. Scanning Electron Microscopic Analysis Results Scanning electron microscopic analysis of fractured surfaces of all the groups revealed craterlike formation along with numerous dimples and microbubbles. These features indicate that the instrument had undergone a ductile mode of fracture, which is predominantly observed in cyclic fatigue failure. Statistical Analysis Discussion The mean and standard deviation of the time to fracture and the fractured fragment length were calculated for all the experimental groups. The Kolmogorov-Smirnov test was used to assess the normality of sample distribution. Both intragroup and intergroup comparisons of cyclic fatigue resistance of the samples were performed using the This study compared the cyclic fatigue resistance of RaCe and Mtwo rotary files in continuous and reciprocating motion and also assessed the mode of fracture under scanning electron microscopy. This study rejects the proposed null hypothesis and has shown that both Mtwo and RaCe rotary files have improved cyclic fatigue resistance in Figure 2. Scanning electron microscopic images of fractured files showing microbubbles, craters, valleys, and dimples. RaCe file used in continuous rotation (A) and in reciprocation motion (B). Mtwo file used in continuous rotation (C) and in reciprocation motion (D). JOE — Volume 40, Number 7, July 2014 Cyclic Fatigue Life of RaCe and Mtwo Files 997 Basic Research—Technology TABLE 1. Intra- and Intergroup Comparison of Mean and Standard Deviation of Time Taken to Fracture of Instruments and Length of Fractured Instruments Continuous Groups n = 15 Mtwo RaCe P value Time ± SD Reciprocating Length ± SD A 283.27 58.346 173.67 49.844A <.001 Time ± SD a 5.533 0.3994 5.467 0.4419a .668 Length ± SD B 815.73 82.893 337.67 39.811B <.001 5.633 0.4419a 5.500 0.4629a .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). reciprocating motion when compared with continuous rotation. Mtwo rotary files were selected because they have a cross-section similar to Reciproc files (12), and RaCe files were chosen because there are no studies using these files in reciprocating motion to date. Cyclic fatigue testing can be performed using a static or dynamic model (15, 18). In a static model, the instrument does not move axially. This creates alternate compressive and tensile stresses in a particular area of the instrument, leading to premature failure. The dynamic model incorporates cyclic axial movement, which provides a better clinical simulation and increases the lifespan of rotary files, but the amplitude, speed of pecking motion, and axial movement are purely subjective in clinical practice. The ability to constrain the files in a precise trajectory is also difficult in dynamic testing (18). There is no evidence for testing rotary files under standardized specifications (3, 19). Hence, a nontooth static model of a standardized artificial canal was used in this study, which minimizes the influence of other variables of file separation other than cyclic fatigue. Earlier cyclic fatigue studies have been conducted using various angles of curvatures such as 30 , 60 , and 90 . A canal of 30 does not constrain the file properly, whereas a curvature of 90 incorporated more stresses to the file (13, 20). The center of curvature in most of the studies 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 curvature with the maximum point of curvature at 5 mm from the tip of the instrument. In this study, fatigue life of the instruments was evaluated by time and not by the number of cycles to fracture (NCF). The time to fracture is easier to record than NCF. In continuous rotation motion, the assessment of the number of cycles to fracture can be obtained by simply multiplying the rotational speed by the time elapsed until fracture occurred. However, in reciprocation motion, the NCF can be determined only by knowing the amplitude of the oscillating motion with a constant time unit, which is not provided by the manufacturers (15). Both Race and Mtwo rotary files showed better resistance to cyclic fatigue in reciprocation compared with continuous rotation motion in this study. The fatigue life is determined by the number of times the crack opens and closes with each cycle. In continuous rotation motion, a 360 rotation is completed in 1 cycle, which leads to increased stress levels and more widening of surface cracks. Tensile stresses are concentrated at a particular point of the instrument in continuous rotation, which culminates to the fracture of the instrument. However, in reciprocating motion using the WaveAll mode (CCW = 170 and CW = 50), it takes 3 cycles to complete one 360 rotation (12). The stresses are distributed to 3 points around the working portion of the file, minimizing the opening of surface cracks (15). In our study, the cyclic fatigue life of RaCe files in reciprocation was 1.9 times more than that of continuous rotation, whereas Mtwo files performed 2.9 times better in reciprocation than in continuous rotation. This increase in the cyclic fatigue life of RaCe files in reciprocation compared with continuous rotation was not proportional compared 998 Vadhana et al. with that of Mtwo files. The reason could be attributed to the difference in the rotational speeds of RaCe files when used in reciprocation (350 rpm) and continuous rotation (600 rpm). Further studies are required to conclude the role of rotational speeds in the cyclic fatigue life of rotary files in reciprocating motion. In our study, Mtwo files performed better than RaCe rotary NiTi files in both continuous and reciprocating motion. Previous studies have shown controversial results regarding the cyclic fatigue life of Mtwo and RaCe in continuous rotation when compared with other files (17, 19–21). In reciprocating motion, Mtwo has been proven to have a similar cyclic fatigue life to that of Twisted Files and Reciproc and superior resistance compared with WaveOne (12). Resistance to cyclic fatigue depends on various factors such as diameter, metal mass, flexibility, cross-sectional shape, and surface finish of rotary files (22). RaCe files have an electropolished surface finish and good flexibility 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 edges in RaCe files. Although the alternating spiraled and nonspiraled segments of RaCe files increase its flexibility, maximum bending occurs at these segment junctions. Because flexural stresses are confined to the fluted portion of the working area and are not distributed evenly throughout the length of the file, fatigue occurs more rapidly in RaCe files (22). The stereomicroscopic analysis of the new RaCe files in our study showed that the junction of the spiraled and nonspiraled segments exists at the D5–D6 region (Fig. 1C). The synergistic effect of the presence of such a junction at 5 mm of the RaCe file (which corresponds to the maximum curvature of the artificial canal) and the static mode 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 instrument and increases the fatigue life (10). However, the role of electropolishing in the cyclic fatigue resistance of the instruments cannot be considered in this study because the cross-sections of both the file systems (RaCe and Mtwo) were different. Another determining factor for the cyclic fatigue life of files is the shape of the file at its circumference. The forces required to disrupt the molecular attraction can be decreased by providing a greater mass of rounded surface following the cutting edge of files at its circumference (22). At the radial extremity, RaCe files have a more angular surface because of their triangular cross-section in comparison with that of Mtwo files. This might lead to increased stresses in a small area, which ultimately resulted in cyclic fatigue failure with less tension. Scanning electron microscopic observations have shown that all specimens of both the groups depicted the presence of microbubbles. Circular abrasion marks, dimples, and craterlike formation confirmed that all instruments had undergone cyclic fatigue. On analyzing the data regarding the length of the fractured fragments, there was no statistical difference in the mean length of all the instruments tested. In this study, almost all the instruments fractured between D5 and D6. The fractured length of the instruments ranged JOE — Volume 40, Number 7, July 2014 Basic Research—Technology from 4.5–5.5 mm, which confirmed the positioning of the files in a precise trajectory. Further studies may be required on the canal centering ability and cutting efficiency of RaCe and Mtwo rotary NiTi files in reciprocating motion. Conclusion Within the limitations of this study, Mtwo and RaCe rotary files were more resistant to cyclic fatigue in reciprocating motion when compared with continuous rotation motion. Mtwo files performed better than RaCe files in both continuous and reciprocating motion. Acknowledgments The authors deny any conflicts of interest related to this study. References 1. Taschieri S, Necchi S, Rosano G, et al. Advantages and limits of nickel-titanium instruments for root canal preparation: a review of the current literature. Schweiz Monatsschr Zahnmed 2005;115:1000–5. 2. Parashos P, Messer HH. Rotary NiTi instrument fracture and its consequences. J Endod 2006;32:1031–43. 3. Plotino G, Grande NM, Cordaro M. A review of cyclic fatigue testing of nickeltitanium rotary instruments. J Endod 2009;35:1469–76. 4. 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