Root canal instruments manufactured from nickel-titanium (NiTi) alloy were introduced

Cyclic Fatigue and Fracture Characteristics of Ground and Twisted Nickel-Titanium Rotary Files Hyeon-Cheol Kim, DDS, MS, PhD,* Jiwan Yum, DDS, MS,* Bock Hur, DDS, MS, PhD,* and Gary Shun-Pan Cheung, MDS, MSc, PhD Abstract Introduction: The purposes of this study were to compare the fatigue resistance of traditional, ground nickel-titanium rotary instruments with the Twisted File and to examine the fracture characteristics of the fatigued fragment. Methods: Size #25, 0.06 tapered, TF (SybronEndo), RaCe (FKG Dentaire), Helix (DiaDent), and ProTaper F1 (Dentsply Maillefer) were examined with scanning electron microscope for surface characteristics before subjected to a cyclic (rotational bending) fatigue test. The time until fracture was recorded to calculate the number of revolutions for each instrument. The data were compared for differences by using analysis of variance and post hoc Scheffé test. The fragments were examined with scanning electron microscope both in lateral view and fractographically. Results: TF showed a significantly higher resistance to cyclic fatigue than other nickel-titanium files that were manufactured with a grinding process (P <.05). The path of crack propagation appeared to be different for electropolished (TF and RaCe) versus non-electropolished (Helix and ProTaper) instruments. Conclusions: Although all specimens showed similar fractographic appearance, which indicated a similar fracture mechanism, instruments with abundant machining grooves seemed to have a higher risk of fatigue. (J Endod 2010;36:147 152) Key Words Cyclic fatigue, electropolishing, fracture, machining grooves, nickel-titanium, rotary instrument, surface defect, Twisted File From the *Department of Conservative Dentistry, School of Dentistry, Pusan National University, Busan, Korea; and Area of Endodontics, Faculty of Dentistry, University of Hong Kong, Hong Kong SAR, China. Address requests for reprints to Gary Shun-Pan Cheung, MDS, MSc, PhD, Area of Endodontics, Faculty of Dentistry, 3/F, Prince Philip Dental Hospital, 34 Hospital Road, Saiyingpun, Hong Kong. E-mail address: spcheung@hkusua.hku.hk. 0099-2399/$0 - see front matter Copyright ª 2010 by the American Association of Endodontists. All rights reserved. doi:10.1016/j.joen.2009.09.037 Root canal instruments manufactured from nickel-titanium (NiTi) alloy were introduced in 1988 to overcome the rigidity (high modulus of elasticity) of stainless steel material (1). The superelasticity of the material allows the NiTi rotary instruments to be used in continuous rotation, even in curved root canals, to produce a desirable tapered root canal form, with a low risk of transporting the original canal lumen (2 5). However, there is a general perception that NiTi instruments have a high risk of fracture in use. Clinically, there is a real potential for rotary NiTi instruments to separate in the canal; even new instruments might demonstrate unexpected breakage on first use (6). Because NiTi engine-files might not show any visible signs of permanent deformation during clinical uses, instrument fracture appears to occur suddenly (7 9). Increasing the resistance to fracture has been a focus in the design of new NiTi rotary systems (10). The design can affect the mechanical behavior (11, 12) and hence the tendency of the instrument to fracture. Although excess torsion and cyclic fatigue have both been implicated as a reason for file fracture, the latter is probably the more prevalent cause of unexpected breakages (9, 13). Initiation of fatigue crack usually occurs at the surface of a working part. It is especially vulnerable if the area with the highest stress should coincide with the machining marks or miniature grooves (14). Thus, some manufacturers tried to remove the machining scratch marks to enhance the (fatigue) fracture resistance through such process as electropolishing. Examples of electropolished NiTi files are RaCe (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and Twisted File (TF; SybronEndo, Orange, CA). TF is a recently introduced NiTi engine-file manufactured with a twisting method. It was reported to have a higher fracture resistance than ground files (15, 16). The manufacturer claimed that TF has a different surface texture (natural grain structure) that runs in the longitudinal direction and that the instrument is made of the R-phase of NiTi alloy (although no transition temperature data are presented). It was further claimed that these features serve to raise the flexibility and the fracture resistance of the instrument. There is also an absence of transverse-running machining marks (as a result of electropolishing) that would result in slower crack initiation and propagation. To date, only very few reports of the fatigue behavior of this new twisted NiTi file are available (15, 16). Little effort has been made to examine the surface machining grooves and their association with fracture. The purposes of this study were to compare the cyclic bending fracture resistance between traditionally ground NiTi rotary instruments and the TF and to verify the fracture topography of the fatigued fragment. Material and Methods Four brands of NiTi instruments with similar cross-sectional geometries were tested. The following instrument of each brand was examined: TF #25/0.06 taper (SybronEndo), RaCe #25/0.06 taper (FKG Dentaire), ProTaper F1 (Dentsply Maillefer, Ballaigues, Switzerland), and Helix #25/0.06 taper (DiaDent, Chongju, Korea). TF and RaCe both have an equilateral triangular cross section, whereas ProTaper and Helix have similar cross section of a convex triangle. The ProTaper F1 (tip size #20 with 7% taper for the apical few millimeters) was selected for having the same diameter (0.55 mm) as for the other brands at D5 (ie, 5 mm from file tip), thus a similar cross-sectional area at that length. JOE Volume 36, Number 1, January 2010 Cyclic Fatigue of Ground versus Twisted NiTi Files 147

Figure 1. Left column: surface characteristics of the instrument before cyclic loading (original magnification, 500). TF instrument showed a grain structure along the axis of the file (A); RaCe showed a smooth surface with limited presence of the manufacturing marks (B); ProTaper (C) and Helix (D) both demonstrated prominent machining marks that ran transversely. Right column: SEM evaluation (original magnification, 1000) of the separated fragment after fatigue test showing the presence of microcracks near the fracture site. Fine cracks that assumed an irregular path (arrows) were noted in TF (a) and RaCe (b), whereas ProTaper (c) and Helix (d) showed cracks running along the machining grooves. Two new files of each brand were examined for machining grooves and surface texture under a scanning electron microscope (SEM) (Hitachi S-4800 II; Hitachi High Technologies, Pleasanton, CA) before cyclic fatigue testing. The fatigue test was conducted in a custommade device similar to that of Gambarini (17), which allowed a reproducible simulation of rotary instrumentation within a curved canal. In the present study, a simulated canal was used in an attempt to eliminate variations of canal anatomy as a source for torque and force differences. The artificial canal (guiding curvature) is a tempered steel rod-andblock assembly, with a radius of 6 mm and a 40-degree curve. 148 Kim et al. JOE Volume 36, Number 1, January 2010

The instrument was rotated at 300 rpm by using an electric motor (X-smart; Dentsply Maillefer) at maximum torque (5.2 Ncm). To reduce friction between the instrument and the (metal) canal walls, a synthetic oil (WD-40; WD-40 Company, San Diego, CA) was sprayed into the simulated canal. The instrument tip was aligned to a specific mark (to allow for repositioning to the same location), and then the file was set to rotate, synchronized with timing by a stopwatch. The instrument was allowed to rotate freely until fracture. Timing was stopped as fracture was detected visually and audibly. The time was then converted into number of revolutions to failure. The broken fragments were ultrasonically cleaned in absolute alcohol for approximately 120 seconds before examination with the SEM. They were examined in lateral view at various magnifications. Then a random selection of the fragments was remounted on the microscope stage, with the fracture end facing upward, for fractographic examination. The number of rotations to failure for various groups was analyzed by using the one-way analysis of variance in software (SPSS for Windows version 12.0; SPSS, Chicago, IL). Scheffé post hoc test was applied to identify the group(s) that were significantly different from others. Statistical significance was set at a confidence level of 95%. Results The surface features of each brand of instrument under the SEM are shown in Fig. 1. Despite having been electropolished by the manufacturer, TF s surface was not perfectly smooth but showed a unique surface texture that seemed to be a kind of machining grooves (or traces from rolling, shaping, or injection molding) running along the length of the file with multiple pits (Fig. 1A). In contrast, obvious machining grooves, which ran almost perpendicular to the long axis of the instrument, were seen in ProTaper and Helix (Fig. 1C, D). RaCe showed reduced surface irregularities as a result of electropolishing, with remnants of the transverse-running machining grooves vaguely discernible (Fig. 1B). TF was significantly more resistant to cyclic fatigue at the specific curvature setting than the other files (P <.05). RaCe with the same cross-sectional configuration and an electropolished surface was the next resistant, followed by ProTaper and Helix in that order (Table 1). The fracture appeared to be of a brittle character macroscopically; no plastic deformation occurred in the spiraling shaft of any instruments tested. Cracks (incomplete microfractures) near the fracture site were found in a majority of specimens of each brand. The crack lines were not always coincident with the machining grooves in TF and RaCe, whereas ProTaper and Helix exhibited cracking along the machining grooves (Fig. 1a d). The fracture cross sections of all brands showed similar fractographic features, with the presence of crack initiation origin(s), crack propagation region, and overload (fast fracture) zone. There seemed to be some fracture defects along the convex border (ie, the flute) of the convex-triangular cross section of ProTaper and Helix instruments (Fig. 2). Discussion File fracture is a major concern during endodontic treatment. Although multiple factors are responsible for instrument separation in use, cyclic fatigue has been shown to be an important cause because the rotary instrument might be used in curved root canals (7 9). The shorter the radius of curvature, the greater is the chance of fatigue breakage (7, 13). Cyclic fatigue occurs when a metal is subjected to repeated cycles of tension and compression that cause its structure to break down (as a result of concentration of stress at the propagating crack front) and ultimately fracture (13, 18). TABLE 1. Number of Rotations to Failure for Each Instrument Type Mean length (mm) of Group Mean (SD) broken fragment (SD) TF 731 (97) 5.94 (0.41) RaCe 514 (89) 5.31 (0.38) ProTaper 410 (62) 5.47 (0.47) Helix 235 (55) 5.02 (0.54) All experimental groups were significantly different from each other (P <.05). SD, standard deviation; TF, Twisted File. The structural characteristics and geometric designs have a definite influence on the susceptibility of NiTi instruments to fracture mechanically (14, 18 21), whereas the clinician s handling is the main human factor governing the potential for NiTi file to fracture (22). Besides the geometric configuration, the importance of surface texture, such as machining marks and scratches from manufacturing procedures, has been implicated by a number of researchers (18, 23 27). These machining scratches on the instrument surface could act as local stress raisers or even crack-like features that might become the origin of a fatigue crack, whereas a smooth surface is less prone to fatigue-crack initiation (25, 28). This study compared the cyclic fatigue resistance of the TF instrument that is formed by twisting a triangular blank (which shape is imparted to a NiTi wire by either a single or a combination of drawing, grinding, or polishing process in the longitudinal direction) with that of those that are produced by a grinding process (RaCe, ProTaper, and Helix; with the machining being almost perpendicular to the axis of the instrument). Both TF and RaCe are claimed by their manufacturers to have been electropolished to reduce the surface irregularities. The number of rotations (at a fixed curvature setting here) to failure for TF was significantly greater than for other brands tested. A similar result has been demonstrated in 2 previous reports (15, 16). RaCe, in turn, exhibited a better resistance to cyclic fatigue than ProTaper and Helix, probably because of a lower flexural rigidity for its cross section and the near absence of machining marks on the RaCe instrument after electropolishing. The direction of microcrack lines on its surface is rather tortuous and is not related to the underlying, vaguely discernible machining marks (Fig. 1, right column). It seemed plausible that the machining marks, as was observed under the SEM in lateral view, contribute to fatigue failure by facilitating not only crack initiation but also the crack propagation process. SEM observations of the surface characteristics and features of new unused instruments (Fig. 1, left column) did not reveal any microcracks on the cutting edge, at which the fatigue crack was found to be initiated on the post-test SEM photomicrographs. After fatigue testing, the majority of specimens showed an increased amount of surface microcracks (in lateral view) near the location of fracture. The crack initiation was found at or near the cutting edge, and its propagation seemed to be situated amid the high density of microscopic defects (as a result of the manufacturing process) found on the surface of non-electropolished instruments, ie, ProTaper and Helix. On the other hand, it was not possible to find a crack propagation path that followed the course of any (vaguely discernible) machining marks on TF or RaCe instruments, as opposed to the ProTaper and Helix with many crack lines that coincided with the machining grooves. Crack propagation showed a tortuous, nonlinear path that was not related to the surface texture of the electropolished instruments, TF and RaCe. In contrast, ProTaper and Helix instruments showed conspicuous machining marks (grooves) along the faces of the flutes; both of them exhibited a significantly lower resistance to cyclic fatigue than the 2 electropolished brands. The presence of cracking along the machining marks and JOE Volume 36, Number 1, January 2010 Cyclic Fatigue of Ground versus Twisted NiTi Files 149

Figure 2. Scanning electron micrographs of the fracture surface of separated fragment after cyclic rotational-bending test. Notice the crack origins and the regions of fatigue-crack propagations. ProTaper (C) and Helix (D) showed more defects than TF (A) and RaCe (B) along the triangular border (arrows). the lower number of rotations for ProTaper and Helix suggested a relationship between the two. From the results of this study, one might encounter a higher risk of fracture for instruments with a machined surface in the clinic (29). The fracture cross sections of all brands showed quite similar surface features with discernible crack origins and an overload (fast fracture) zone after fatigue-crack progression (13, 30, 31). There were frequent fracture defects in which fast-growing crack met the triangular border (ie, bottom of the flute), especially for ProTaper and Helix. The photomicrographic appearance suggested that initiation of fatigue-crack occurs readily at irregular surface characteristics such as those caused by machining, which act as local stress concentrators. Once the cracks are nucleated, their growth progresses slowly and seemingly along these machining grooves, at least initially, until the 150 Kim et al. JOE Volume 36, Number 1, January 2010

sudden, final fracture occurs (28, 30). Finite element analyses of the stresses on the instrument have indicated that the convex-triangular border of ProTaper (and probably Helix) might result in a higher stress concentration on the periphery under flexure than other crosssectional configurations (14, 32). The fact that TF showed a better fatigue resistance than RaCe (which has almost identical cross section and arguably better surface smoothness) might be a result of the different longitudinal design features including the helical angle, pitch number and arrangement of the spirals in the fluted part, and the longitudinally oriented surface texture. Further study is required to verify this. It is important for clinicians to realize that preexisting conditions associated with the manufacturing process might contribute to the propagation of instrument fractures during use (33, 34). Kuhn et al (18) described the crack nucleation stage being facilitated by a high density of surface defects, and then fatigue failure is largely a crack propagation process. In the clinical situation, the canal curvature causes the endodontic instruments to curve (ie, bend); cyclic fatigue is then a result of repeated tensile-compressive stresses. The maximum tensile stress is situated at the convex side of the curve (outer curve), and crack nucleation and propagation seem to occur when the instrument is under tension (18, 28). Thus, it is possible that the multitude of machining marks on the surface of a ground instrument would lead to crack initiation at multiple locations in which the resolved shear stress is greater than that required for crystallographic slip to occur. The observation here is in agreement with the prevailing idea of an improved resistance of a smooth surface structure to fatigue-crack initiation (15, 16). Electropolishing is one method to achieve a smooth surface for NiTi instruments (27, 35, 36). Several studies have shown that electropolishing might improve the instrument s working properties (36, 37), whereas Herold et al (38) showed that electropolishing did not prevent the development of microfractures. For the cyclic fatigue resistance, residual stresses have been considered as an important factor (14, 21) not only after (simulated) clinical uses but also for a brand-new instrument. When an instrument is machined (ie, being ground), plastic deformation occurs at the surface of the metal, resulting in residual stresses that remain at the surface (39). Although the exact nature and extent of such residual stress after manufacture are unknown, any internal stresses might act as a negative factor to the martensite substructures (40). The effect of residual stresses on material fracture depends largely on its nature, compressive or tensile. Although residual compressive stresses might have a beneficial effect on the fatigue life because they tend to delay crack initiation and growth, residual tensile stresses might significantly reduce the fatigue life of materials because they might accelerate the crack initiation and propagation process (41, 42). Under repeated cycles of clinical loading, residual stresses (whether the beneficial compressive or the detrimental tensile stresses) play a significant role in determining the fatigue life of the material. By removing the surface layer of the material (at sharp edges or peaks of an irregular surface in which the current density is greater than for those relatively smooth or recessed areas) (35), electropolishing has been shown to produce a smooth, amorphous oxide layer that is free of most crystalline defects (43). Residual stresses on the material surface are also removed in the process. According to the manufacturer, TF instruments are created by taking a raw NiTi wire in the austenitic crystalline structure and transforming it into the rhombohedral (R-) phase by a heat treatment process (44). It is important to have knowledge of the relationships between austenite, R-phase, and martensite of NiTi alloys and how they are formed on cooling and/or heating. The transformation from austenite to R-phase takes place on cooling to the R-phase transition temperature; martensite begins to form on further cooling to and below the martensite-start temperature M s. The rhomobohedral structure (with the material confined to a particular shape) can be stabilized by applying a suitable holding temperature. R-phase shows good superelasticity (within about 1% strain) and shape memory effect; its Young s modulus typically is lower than that of austenite. Thus, an instrument made up of the R-phase would be more flexible (45, 46). The manufacturer claims that this R-phase technology represents an advancement in the manufacture of NiTi instruments over previous NiTi (grinding) manufacturing technique, and that this proprietary twisting process with concurrent heat treatment imparts superior mechanical characteristics to their product. Kuhn and Jordan (29) proposed some suggestions to improve the longevity of endodontic files, which include the following: (1) thermal treatments before machining to decrease the work-hardening of the alloy; (2) choosing machining conditions adapted to the NiTi alloy; and (3) electropolishing to reduce the machining damage on the surface. The twisting method for manufacturing NiTi instruments would incorporate the first two of these suggestions. The resultant instrument, which is also electropolished (according to SybronEndo, although surface irregularities seem not completely removed), appears to be significantly more resistant to cyclic fatigue than the other brands (including RaCe with a very smooth, electropolished surface) at one particular curvature setting in this study. Thus, within the limitations of this study, it is concluded that the new method of manufacturing NiTi rotary files by using a twisting process appears to improve the resistance to cyclic fatigue over the traditional NiTi instruments produced by grinding. However, new technologies must be tested against a benchmark and verified in independent studies to give confidence for clinicians to make their choice. References 1. Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod 1988;14:346 51. 2. Schäfer E, Schulz-Bongert U, Tulus G. 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