The advent of nickel-titanium (NiTi) rotary instruments has revolutionized root canal

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1 Effect of Environment on Fatigue Failure of Controlled Memory Wire Nickel-Titanium Rotary Instruments Ya Shen, DDS, PhD,* Wei Qian, DDS, PhD,* Houman Abtin, BDS,* Yuan Gao, DDS, PhD, and Markus Haapasalo, DDS, PhD* Abstract Introduction: This study examined the fatigue behavior of 2 types of nickel-titanium (NiTi) instruments made from a novel controlled memory NiTi wire (CM wire) under various environment conditions. Methods: Three conventional superelastic NiTi instruments of ProFile (Dentsply Maillefer, Ballaigues, Switzerland), Typhoon (Clinician s Choice Dental Products, New Milford, CT), and DS-SS NEYY (Clinician s Choice Dental Products) and 2 new CM wire instruments of Typhoon CM and DS-SS NEYY CM were subjected to rotational bending at the curvature of 35 in air, deionized water, 17% EDTA, or deionized water after immersion in 6% sodium hypochlorite for 25 minutes, and the number of revolutions of fracture (N f ) was recorded. The fracture surface of all fragments was examined by a scanning electron microscope. The crack-initiation sites and the percentage of dimple area to the whole fracture cross-section were noted. Results: Two new CM Wire instruments yielded an improvement of >4 to 9 times in N f than conventional NiTi files with the same design under various environments (P <.05). The fatigue life of 3 conventional superelastic NiTi instruments was similar under various environments, whereas the N f of 2 new CM Wire instruments was significantly longer in liquid media than in air (P <.05). The vast majority of CM instruments showed multiple, whereas most instruments made from conventional NiTi wire had one crack. The values of the area fraction occupied by the dimple region were significantly smaller on CM NiTi instruments than in conventional NiTi instruments under various environments (P <.05). Conclusions: Within the limitations of this study, the type of NiTi metal alloy (CM files vs conventional superelastic NiTi files) influences the cyclic fatigue resistance under various environments. The fatigue life of CM instruments is longer in liquid media than in air. (J Endod 2012;38: ) Key Words Controlled memory, EDTA, fatigue, nickel-titanium instrument, sodium hypochlorite The advent of nickel-titanium (NiTi) rotary instruments has revolutionized root canal treatment by reducing operator fatigue and the time required to finish the preparation and minimizing procedural errors associated with hand instrumentation (1). Since their introduction (2), an increasing number of NiTi rotary systems have been marketed by various manufacturers. These systems differ from one another in the design of the cutting blades, body taper, and configuration of the file tip. Despite their increasing popularity, a major concern with the use of NiTi rotary instruments is the possibility of unexpected separation in use (3, 4). Two mechanisms that may lead to NiTi rotary fracture, namely cyclic fatigue and torsional overloading, have been described (5). Cyclic fatigue is a result of rotation around a curve with the consequence of repeated extension and compression of metal and, finally, work hardening followed by fracture (3, 6). The combination of chelating agents and sodium hypochlorite (NaOCl) has been advocated as an effective irrigation regimen to remove the organic and inorganic matter during the instrumentation phase (7 9). The solution of 17% EDTA was thought to chemically soften the root canal dentin and dissolve the smear layer (8, 9). Additionally, manufacturers of NiTi instruments recommend paste-type EDTA use as a lubricant during rotary root canal preparation, presumably to reduce the risk of instrument separation. The corrosion of endodontic instruments can occur during chemomechanical preparation, cleaning procedures, chemical disinfection, or sterilization (10 13). The fatigue process begins with crack initiation at the material surface. It is well known that environmental conditions can modify both the crack initiation and propagation processes. The resistance to corrosion of root canal instruments could influence their clinical behavior. It is likely that pitting or crevice corrosion might occur initially and then increase the probability of fatigue failure, thereby altering the fracture mechanism from conventional fatigue failure to corrosion failure. Although the corrosion behavior of NaOCl on NiTi instruments has been studied (12 14), the effect of EDTA on the fatigue behavior of superelastic NiTi instruments has not been reported hitherto. NiTi alloy belongs to the family of intermetallic alloys. This means that alloy can exist in the following various crystallographic forms, with distinct phases and different mechanical properties: austenitic, transformation and martensitic (15). To improve the fracture resistance of NiTi files, manufacturers have developed new manufacturing processes (16 19). Recently, thermal treatment of NiTi alloy is used to optimize the mechanical properties of this alloy (16, 18 23). NiTi rotary instruments (Typhoon From the *Division of Endodontics, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, The University of British Columbia, Vancouver, British Columbia, Canada; and State Key Laboratory of Oral Diseases, West China College and Hospital of Stomatology, Sichuan University, Sichuan, Chengdu, China. Address requests for reprints to Dr Markus Haapasalo, Division of Endodontics, UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3. address: markush@dentistry.ubc.ca /$ - see front matter Copyright ª 2012 American Association of Endodontists. doi: /j.joen Shen et al. JOE Volume 38, Number 3, March 2012

2 CM; Clinician s Choice Dental Products, New Milford, CT) made from a NiTi wire subjected to proprietary thermomechanical processing (CM wire) have been introduced into the market. The CM wire technology makes the NiTi instruments more resistant to cyclic fatigue than non CM wire NiTi instruments (23). The purpose of this study to examine the fatigue behavior of NiTi instruments with 2 different designs made from a novel controlled memory NiTi alloy and conventional NiTi alloy when immersed in various media. Materials and Methods The fatigue testing protocol has been described previously and was reproduced throughout the experimental period (13, 14, 23). Briefly, each NiTi instrument was constrained to a curvature by 3 rigid, stainless-steel pins; a calibrated digital photograph of the curvature was taken. The instrument was then allowed to rotate at 300 rpm (as recommended by the manufacturers) until fracture. The fatigue life, or the total number of revolutions to failure, N f, was recorded. NiTi rotary instruments of.04 taper and size 25 ProFile (PF; Dentsply Maillefer, Ballaigues, Switzerland), Typhoon (TYP; Clinician s Choice Dental Products, New Milford, CT), Typhoon CM (TYP CM; Clinician s Choice Dental Products), DS-SS NEYY (NEYY, Clinician s Choice Dental Products), and DS-SS NEYY CM (NEYY CM, Clinician s Choice Dental Products) were subjected to rotational bending at the curvature of 35 with an 8-mm radius in air (relative humidity, 65%), deionized water, 17% EDTA with a ph of 7.0 (Fisher Scientific, Ottawa, Canada), or deionized water at the temperature of 23 2 C after the working part (16 mm) of the instrument was immersed in 6% NaOCl (Ultra bleach; Shoppers Drug Mart Pharmaprix, Toronto, Canada) at room temperature for 25 minutes. Only a 16-mm length from the tip of the instrument was immersed in the liquid medium during the test to avoid galvanic action between the instrument and its handle. All tested CM wire instruments from Clinician s Choice Dental Products were prototypes, but according to the manufacturer they are identical with Typhoon instruments that will be available soon commercially. Each group included 12 instruments. After the test, the detached fragment was collected, rinsed briefly with deionized water, and mounted so that the fracture surface was parallel to the microscope stage for detailed examination under scanning electron microscopy (SEM; Stereoscan 260; Cambridge Instruments, Cambridge, UK) operating at 5 to 8 kv. The fracture surface of all detached fragments was examined under SEM at various magnifications to identify the crack (s) and to measure the area fraction occupied by the dimple region. A detailed description has been provided elsewhere (23, 24). Data of the number of crack (s) for each specimen were examined by using the Fisher exact test for differences between different alloys and file designs. The region in which the dimple area could be found was outlined on the photomicrograph for each specimen, which was measured with ImageJ 1.4 g software (National Institutes of Health, Bethesda, MD) on each photomicrograph. Pearson correlation coefficients were calculated between N f and the area occupied by the dimple region. The results were analyzed by using univariate analysis or post hoc analysis in software (SPSS for Windows 11.0, SPSS, Chicago, IL) when necessary at a significance level of P <.05. Results A total of 240 NiTi rotary instruments were tested in air, deionized water, 17% EDTA, or deionized water after immersion in 6% NaOCl for 25 minutes (Table 1). The fatigue life of 3 conventional superelastic NiTi instruments (ie, PF, TYP, and NEYY) was similar under various environments (univariate analysis, P >.05), whereas the N f of 2 new CM wire instruments (ie, TYP CM and NEYY CM) was significantly longer in aqueous media than in air (univariate analysis, P <.05). Two CM wire instruments yielded an improvement of >4 to 9 times in N f. For instruments manufactured from conventional NiTi wire, there was no change in N f (post hoc analysis, P <.05). The N f of 2 CM instruments was similar in the different liquid media (ie, water, water after immersion in 6% NaOCl for 25 minutes, and under 17% EDTA; P >.05). The square (NEYY CM) versus triangular (TYP CM) configuration showed a significantly different lifetime on CM Wire under all environments (univariate analysis, P <.05). NEYY instruments had a significantly longer N f than the TYP and ProFile instruments under all environments (univariate analysis, P <.05). Features of the fracture surface were very similar for both CM wire instruments. The vast majority of CM wire instruments showed multiple, whereas most instruments made from conventional NiTi wire had one crack (Table 2; Fisher exact test, P <.05). The values of the area occupied by the dimple region as part of the total surface area of the fractured cross-section were significantly smaller on CM wire NiTi instruments compared with conventional NiTi instruments with the same design under all conditions (Fig. 1; post hoc analysis, P <.05). The N f was highly correlated with the values of the area occupied by the dimple region (r 2 = 0.86). None of the tested materials was susceptible to pitting or crevice corrosion in aqueous media (water, 17% EDTA, or water after immersion in 6% NaOCl for 25 minutes) as evaluated under SEM. Discussion The main aim of this study was to assess the resistance to cyclic fatigue of the novel CM instruments in various solutions. Clinically, irrigant solutions are stored at room temperature and are used in teeth that have been isolated by rubber dam. Only the working part (16 mm from the tip) of instruments was immersed in solution to avoid galvanic corrosion phenomena that are induced when 2 dissimilar metals are TABLE 1. The N f and the Area/Total Cross-section Area on the Fractured Instrument (%) for Each Brand in Various Environments Water after immersion Air Water 17% EDTA in 6% NaOCl N f area (%) ProFile TYP NEYY TYP CM NEYY CM There was a significant difference in the N f and the dimple area/total cross-section between TYP and TYP CM at 4 conditions (P <.05) and between NEYY and NEYY CM at 4 conditions (P <.05). There was a significant difference in the N f of TYP CM between air with aqueous media (P <.05). There was a significant difference in the N f of NEYY CM between air with liquid media (P <.05). JOE Volume 38, Number 3, March 2012 Fatigue of CM NiTi Instrument in Various Media 377

3 TABLE 2. The Number of Crack Origins (% of specimens with at least initiation site) as Observed on the Fracture of Various Instruments Fatigued at Various Environments Air Water 17% EDTA Water after immersion in 6% NaOCl ProFile 7 (58) 5 (42) 10 (83) 2 (17) 8 (67) 4 (33) 10 (83) 2 (17) TYP 11 (92) 1 (8) 12 (100) 0 11 (92) 1 (8) 12 (100) 0 NEYY 9 (75) 3 (25) 10 (83) 2 (17) 9 (75) 3 (25) 10 (83) 2 (17) TYP CM 6 (50) 6 (50) 0 12 (100) 0 12 (100) 0 12 (100) NEYY CM 1 (8) 11 (92) 0 12 (100) 0 12 (100) 0 12 (100) coupled in a corrosive electrolyte (25). EDTA was chosen as the medium in which the test was performed to simulate the clinical situation. NaOCl is the most common irrigant used in root canal treatment. NiTi instruments come into contact with NaOCl during disinfection or when the solution is present in the pulp chamber and root canal during instrumentation. NaOCl was found to be highly corrosive to the stainless-steel pins in a pilot study, necessitating very frequent changes of these pins if the test were to be performed in such conditions. During this study and pilot experiments, it was noticed that the fatigue life of CM instruments was around 25 minutes (7,500 cycles) in water. Therefore, the contact time of all NiTi instruments with higher concentrations (6%) was 25 minutes for static immersion before fatigue testing. The different crystal structures observed in NiTi alloys impart different properties. For instance, the martensite phase is less stiff and more pliable, possessing an elastic modulus of 31 to 35 GPa compared with 84 to 98 GPa for austenite (26). Differential scanning calorimetric analyses have found that the conventional superelastic NiTi wire has the austenite structure at room temperature, whereas CM wire is a mixture of martensite and austenite structure (27). The results of the present study indicated that 2 NiTi instruments made from CM wire were nearly 400% to 600% more resistant to fatigue failure than instruments made from conventional NiTi wire with the same design under various envionments. It is consistent with the view that the martensitic form of NiTi has remarkable fatigue resistance; the crack growth in martensite is slower than in austensite and that stress-induced formation of martensite reduces crack growth rate (28). Both the cross-section factor and material property had a substantial impact on fatigue lifetime (23, 29). This was confirmed by the present findings that square-configuration (ie, NEYY and NEYY CM) Figure 1. A photomicrograph of a fracture surface of (A) NEYY, (B) NEYY CM, (C) Typhoon, and (D) Typhoon CM instruments with the region of fatigue crack propagation and dimple area outlined (dotted line) with (arrows) in deionized water after immersion in 6% NaOCl for 25 minutes. 378 Shen et al. JOE Volume 38, Number 3, March 2012

4 instruments had a significantly longer N f than triangular-configuration (ie, TYP and TYP CM) instruments under all environments. The fatigue of NiTi alloys is sensitive to temperature, both locally and environmentally. A function of the aqueous media in metal fatigue behavior is to carry the heat away from the metal-to-metal contact in order to reduce their operating temperature. On repeated loading, the latent heat of the stress-induced martensitic transformation released can elevate the local temperature, leading to a shortened fatigue life (30). The fatigue behavior of conventional superelastic NiTi instruments testing in air produced similar results as those in aqueous media. The short fatigue life (<1,000 cycles) on conventional superelastic NiTi instruments makes the material withstand few load cycles and is not long enough for heat attack. Hence, the absolute number of revolutions survived in aqueous media does not differ much from that in the air for conventional NiTi instruments. On the other hand, an aqueous medium, water or EDTA, would serve as an effective heat sink for the long fatigue life of the CM instrument; the low-cycle fatigue life therein was distinguishable from that in air. Therefore, it may not be surprising that the fatigue life of CM wire instruments in media (from 5,100 to 7,800 cycles [17 26 minutes]) was more than twice as long as that in air (from 2,400 to 3,600 cycles [8 12 minutes]) according to this study. Notice that when NiTi instruments were tested in a dry condition, there was no coolant or lubricant present. The results of the present study indicated that NiTi instruments made from CM wire were nearly 300% to 800% more resistant to fatigue failure than instruments made from conventional NiTi wire in a dry condition. The sources of heat in a dry condition may include friction between the instrument and pins and internal friction from movement of the interface between phases inside the material. This may partly explain that the standard deviations of the fatigue life of CM wire instruments were higher when tested in air than those in liquid media. Also, for the same reason, the mean length of the fractured segment of all instruments in liquid media was more consistent than in air (data not shown). NiTi alloy has proved to be one of the most biocompatible materials, and it is extremely resistant to corrosion (31). Tobushi et al (32) found that the influence of corrosion fatigue of NiTi wire in water did not appear in the region of low-cycle fatigue. This was confirmed by the present findings. Additionally, a neutral EDTA solution reduces the mineral and noncollagenous protein component of dentin, leading to surface softening but not to erosion of the surface dentin layer during the irrigation (8, 9). The results here indicated that the fatigue lives of all NiTi instruments cycled either under water or 17% EDTA were comparable. These findings are in agreement with previous data (33, 34); the authors attributed the resistance of NiTi in EDTA solution to the ability of EDTA to form the passive film on the metal surface (35). The influence of NaOCl on the corrosion of NiTi instruments has been studied (12 14); despite minor signs of corrosion, they did not appear to involve any significant alteration of the mechanical properties and performance of the instruments. Immersion in 5% NaOCl at a high temperature (37 C and 50 C) for 5 minutes did not reduce the cyclic fatigue resistance of conventional NiTi rotary instruments significantly (36, 37). Such findings lend support to our study here; immersed CM NiTi and conventional NiTi instruments in 6% NaOCl solution for 25 minutes did not reduce the cyclic fatigue fracture resistance. The fractographic appearance of instruments fatigued in NaOCl was very similar to those fatigued in air and water. In addition, no pitting corrosion was formed on the surface (ie, the periphery of the fracture cross-section). The data obtained here cannot be directly extrapolated to clinical conditions. Therefore, conclusions from the present study must be drawn with caution. The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to critical size. With continued cyclic loading, the growth of the dominant crack or cracks will continue until the remaining uncracked section of the component no longer can support the load. At this point, the fracture toughness is exceeded, and the remaining cross-section of the material experiences rapid fracture. This rapid overload fracture is the third stage of fatigue failure, which is occupied by the dimple region. In the present study, the area occupied by the dimple region as part of the total surface area of the fracture cross-section was measured for each specimen. The values of this area were significantly smaller on instruments made from CM wire than on instruments made from conventional NiTi wire. Hence, it is not surprising that CM files had fatigue resistance superior to files made from conventional NiTi alloy. To our knowledge, this study is the first to focus on the evaluation of fatigue behavior of the novel CM instruments under various environments. Microstructure often plays an important role in controlling the corrosion behavior of materials. The austenitic NiTi was electrochemically more active than the mixture of austenite and martensite (38). However, both NiTi instruments with austenite structure (ie, PF, TYP, and NEYY) and NiTi instruments with a mixture of martensite and austenite structures (ie, TYP CM and NEYY CM) were not susceptible to corrosion in deionized water, deionized water after immersion in 6% NaOCl for 25 minutes, or 17% EDTA at room temperature. Within the limitations of this study, the type of NiTi metal alloy (CM instruments vs conventional superelastic NiTi instruments) influences cyclic fatigue resistance under various environments. The fatigue life of CM instruments is longer in liquid media than in air. Acknowledgments The authors thank Dentsply Tulsa Dental Specialties and Clinician s Choice Dental Products for donating the files used in this study. The authors deny any conflicts of interest related to this study. References 1. Peters OA. Current challenges and concepts in the preparation of root canal systems: a review. J Endod 2004;35: Walia HM, Brantley WA, Gerstein H. 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5 14. Cheung GS, Shen Y, Darvell BW. Effect of environment on low-cycle fatigue of a nickel-titanium instrument. J Endod 2007;33: Wang FE, Pickart SJ, Alperia HA. Mechanism of the NiTi martensitic transformation and the crystal structures of NiTi-II and NiTi-III phases. J Appl Phys 1972;43: Gambarini G, Grande N, Plotino G, et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008;34: Alapati SB, Brantley WA, Iijima M, et al. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009;35: Condorelli GG, Bonaccorso A, Smecca E, Sch afer E, Cantatore G, Tripi TR. Improvement of the fatigue resistance of NiTi endodontic files by surface and bulk modifications. Int Endod J 2010;43: Gambarini G, Plotino G, Grande NM, Al-Sudani D, De Luca M, Testarelli L. Mechanical properties of nickel-titanium rotary instruments produced with a new manufacturing technique. Int Endod J 2011;44: Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickeltitanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/.04 rotary instruments. J Endod 2008;34: Kramkowski TR, Bahcall J. An in vitro comparison of torsional stress and cyclic fatigue resistance of ProFile GT and ProFile GT Series X rotary nickel-titanium files. J Endod 2009;35: Gao Y, Shotton V, Wilkinson K, Phillips G, Johnson WB. Effects of raw material and rotational speed on the cyclic fatigue of ProFile Vortex rotary instruments. J Endod 2010;36: Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Fatigue testing of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37: Cheung GS, Darvell BW. Fatigue testing of a NiTi rotary instrument. Part 2: fractographic analysis. Int Endod J 2007;40: Angelini E, Zucchi F, Caputo A. Degradation processes on metallic surfaces. In: Barbucci R, ed. Integrated Biomaterial Science. Dordrecht: Kluwer Academic Plenum Publishers; 2002: Kusy RP. A review of contemporary archwires: their properties and characteristics. Angle Orthod 1997;67: Shen Y, Zhou H, Zheng Y, Campbell L, Peng B, Markus H. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37: Mckelvey AL, Ritchie RO. Fatigue-crack growth behavior in the superelastic and shape-memory alloy Nitinol. Metall Mater Trans A 2001;32A: Zhang EW, Cheung GSP, Zheng YF. A mathematical model for describing the mechanical behaviour of root canal instruments. Int Endod 2011;44: Prymak O, Klocke A, Kahl-Nieke B, Epple M. Fatigue of orthodontic nickel-titanium (NiTi) wires in different fluids under constant mechanical stress. Mater Sci Eng A 2004;378: Hodgson DE, Wu MH, Biermann RJ. Shape memory alloys. In: Davis JR, Allen P, Lampman SR, et al., eds. Properties and Selection: Nonferrous Alloys and Special- Purpose Materials. 10th edn. Materials Park, OH: ASM International; 1990: Tobushi H, Nakahara T, Shimeno Y, Hashimoto T. Low-cycle fatigue of TiNi shape memory alloy and formulation of fatigue life. J Eng Mater Technol 2000;122: Darabara M, Bourithis L, Zinelis S, Papadimitriou GD. Susceptibility to localized corrosion of stainless steel and NiTi endodontic instruments in irrigating solutions. Int Endod J 2004;37: Bonaccorso A, Tripi TR, Rondelli G, Condorelli GG, Cantatore G, Sch afer E. Pitting corrosion resistance of nickel-titanium rotary instruments with different surface treatments in seventeen percent ethylenediaminetetraacetic Acid and sodium chloride solutions. J Endod 2008;34: Reinhard G, Radtke M, Rammelt U. The role of the salts of weak acids in the chemical passivation of iron and steel in aqueous solutions. Corrosion Science 1992;33: Berutti E, Angelini E, Rigolone M, Migliaretti G, Pasqualini D. Influence of sodium hypochlorite on fracture properties and corrosion of ProTaper Rotary instruments. Int Endod J 2006;39: Pedulla E, Grande NM, Plotino G, Pappalardo A, Rapisarda E. Cyclic fatigue resistance of three different nickel-titanium instruments after immersion in sodium hypochlorite. J Endod 2010;37: Wang J, Li N, Han EH, Ke W. Effect of ph, temperature and Cl- concentration on electrochemical behavior of NiTi shape memory alloy in artificial saliva. J Mater Sci Mater Med 2006;17: Shen et al. JOE Volume 38, Number 3, March 2012