Influence of the phase transformation behavior of R-phase nickel-titanium instrument on its cyclic fatigue resistance and shaping ability
Ryoko Gomi1, Masato Izawa1, Yasuhisa Tsujimoto1, 2
1 Department of Endodontics, Nihon University School of Dentistry at Matsudo
2 Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo
1, 2 870-1, Sakaecho, Nishi-2, Matsudo, Chiba 271-8587, Japan
First author: Ryoko Gomi
e-mail address: mary12013@g.nihon-u.ac.jp
1 Abstract
The purpose of this study was to investigate the influence of phase transformation behavior of R-phase Ni-Ti file on its cyclic fatigue resistance and the root canal shaping ability.
The pre-treated NRT (NRT-N) and NRT files (MANI, Tochigi, Japan) were examined. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were performed to analyze the phase- transformation behavior. The numbers of rotations until fracture for the 06/#25 and 06/#40 files were determined by performing cyclic fatigue resistance tests. A curved root canal model was apically enlarged using the 06/#25, 06/#30, 06/#35, and 06/#40 files. Next, pre- and post-operative images of the model were superimposed. The increases in the outer and inner canal widths of the curvature were measured at 0, 1, 2, 3, 4, and 5 mm from the apex, in order to determine the canal centering abilities of the files.
The XRD patterns showed that the predominant phases in NRT-N and NRT at room temperature were the austenite phase and the R-phase, respectively. The DSC results showed that the reverse transformation finish temperature from the R-phase to the austenite phase in the case of NRT was 40.45±2.63°C, while it was 23.20±3.33°C for NRT-N. The NRT files exhibited superior cyclic fatigue resistance regardless of the instrument size. When the #40 files were used for apical enlargement, the increase in the outer canal width at 0 mm in the case of the NRT file was smaller than that in the case of the NRT-N file (p < 0.05). When the #30 and #35 files were used, the NRT file exhibited a higher centering ability at 0 mm than did the NRT-N file variety (p < 0.05).
Thus, it was concluded that Ni-Ti file exhibited R-phase had better cyclic fatigue resistance and shaping ability than Ni-Ti files exhibit austenite phase at clinical temperature.
2 Introduction
To ensure the success of endodontic treatments, it is important to eliminate bacteria from the root canals and to obturate the root canal, so a hermetic seal that prevents the reinfection of the canal can be realized(1). Root canal preparation is an essential step in root canal treatment and involves removing the infected dentin mechanically and spreading a root canal irrigant till the apex (2, 3).
Usually, the infected dentin is removed under a microscope. However, it is difficult to remove the infected dentin at curved canal. Therefore, nickel-titanium (Ni-Ti) files have become indispensable for the root canal treatment of curved canals (4). Ni-Ti files are superelastic and exhibit excellent flexibility, which allows them to better prevent procedural complications, in contrast to previously used instruments (4, 5). However, they have a high risk of sudden fatigue fracture when used in curved canals (6, 7). To solve this problem, the crystalline structures of recently developed Ni-Ti alloys have been modified ( i.e., subjected to a phase transformation). This has led to the development of R-phase Ni-Ti files that exhibit better flexibility and cyclic fatigue resistance than those of conventional Ni-Ti files (8).
The crystalline structure of Ni-Ti alloys switches between austenite and martensite phases in response to the temperature changes and stress (9); when a Ni-Ti alloy in the high-temperature austenite phase is cooled, it undergoes a phase transformation, known as the martensitic transformation, to martensite phase. Conversely when the Ni-Ti alloy in the low-temperature martensite phase is heated, it undergoes a phase transformation back to austenite; this is known as the austenitic transformation (i.e., a reverse transformation). Thus, with changes in the temperature, the base metal currently used in Ni-Ti files undergoes a phase transformation from the austenite phase to the martensite phase and then back to the austenite phase. Further, in R-phase Ni-Ti alloys, an intermediate R-phase forms when the alloy undergoes a transformation from the high-temperature austenite phase to the low- temperature martensite phase, as well as, during the reverse transformation. The Ni-Ti files in which the R-phase is formed, owing to a temperature change, are marketed as R-phase Ni-Ti files. R-phase Ni-Ti files have a higher cyclic fatigue resistance (10, 11) and are softer and suppler (12, 13) than conventional Ni-Ti files. R-phase Ni-Ti files have also been reported to help reduce outward deviations from the root canal apex during root canal shaping to a greater extent than conventional Ni-Ti files (12). The cyclic fatigue resistance and shaping ability of Ni-Ti files are determined not only by the phase transformation undergone by the Ni-Ti files but also by the file design (14, 15). Further, R-phase Ni-Ti files that have experienced different phase-transformation temperatures, exhibit different crystalline structures. When R-phase Ni-Ti files subjected to different phase-transformation temperatures are compared, the differences in
3
their cyclic fatigue resistances and root canal shaping abilities become apparent. However, to our knowledge, there are no studies that have investigated the effects of the phase transformation on R-phase Ni-Ti files subjected to different phase transformations but having the same file design on the cyclic fatigue resistances and root canal shaping abilities of the files.
In this study, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were performed to analyze the phase-transformation behavior of two types of R-phase Ni-Ti files, namely, pre-treated NRT (NRT-N) and NRT (MANI, Tochigi, Japan) files. In addition, the influence of phase-transformation behavior of R-phase Ni-Ti file on its cyclic fatigue resistance and the root canal shaping ability was examined.
Materials and methods
In the present study, NRT-N and NRT files were used for the cyclic fatigue resistance tests and root canal shaping abilities. However, the wires each of NRT-N and NRT files before shaping were used for XRD and DSC.
1. X-ray diffraction
The XRD measurements were performed using a X’pert-pro Galaxy equipped with X’celerator (PANalytical, Almelo, the Netherlands) to identify the crystallographic phases. Five each of NRT-N and NRT test specimens with 10 mm length were cut from the 1.0-mm-diameter wires of the each NRT-N and NRT files by using a slow-speed water-cooled diamond saw. The five segments each of the NRT-N and NRT wires were adhered together on a slide and ground using #1000 SiC sandpaper under running water to obtain a plane surface. The specimens were then washed in absolute ethanol using an ultrasonic cleaner for 3 min and dried at room temperature. The XRD of the NRT-N and NRT wires were performed at 22±2°C in a range from 30° to 80° in 2θ with a step size of 0.0042°
using CuKα radiation under a tube voltage of 45 kV and current of 40 mA. Further, the NRT wires were also analyzed in a range from 35° to 45° in 2θ with a step size of 0.0042°.
2. Differential scanning calorimetry
To determine the phase-transformation temperatures of NRT-N and NRT, DSC measurement was performed using a DSC7020 system (Hitachi High-Tech Science Corporation, Tokyo, Japan).
The wires of NRT-N and NRT were cut by a slow-speed water-cooled diamond saw. Each test specimen weighed 35 mg and consisted of 2 segments, each approximately 3 mm in length. The cut specimens were washed in absolute ethanol using an ultrasonic cleaner for 3 min and dried at room
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temperature. Each test specimen was then placed in an open aluminum pan. α-Al2O3 powder was used as the standard samples and dry nitrogen was flowed into the chamber at a rate of 100 ml/min during analysis. The specimen was heated from room temperature to 120°C and then cooled to -120°C to obtain the cooling DSC curve. Next, it was heated from -120°C to 120°C to obtain the heating DSC curve. Liquid nitrogen was used as the coolant. The heating and cooling rate was 10°C/min. The temperatures to start and finish R-phase transformation from the austenite phase (Rs
and Rf) were determined using DSC curve obtained during cooling process (9). The temperature to start reverse transformation from the martensite phase to R-phase and finish reverse transformation from R-phase to austenite phase (As and ARf) were determined using DSC curve obtained during heating process, respectively (9). DSC data was analyzed by Muse computer software (Hitachi High-Tech Science Corporation, Tokyo, Japan).
3．Cyclic fatigue resistance test
Cyclic fatigue resistance tests were performed according to the method described by Zinelis et al. (16). The test device (Fig.1) used in the present study was constructed by MANI (Tochigi, Japan). For the experiment, the 06/#25 and 06/#40 NRT-N and NRT files were used. A curved canal was assumed, and, each file was held approximately 4 mm from the tip by two supersteel pins to ensure maximum curvature, and while the shank was immobilized with a chuck. The files were rotated at 300 rpm in this state, and the number of rotations until fracture was measured (n=10).
4．Root canal shaping and assessment
< Canal preparation >
Twenty simulated root canals (curvature of 30°) in clear resin blocks (Endo-training-bloc A0177, Dentsply Maillefer, Ballaigues, Switzerland) (Fig. 2A) were used in the present study. #10 stainless steel (SS) K-type files were used for patency. The working length was determined using a #15 SS K-file. The working length was measured up to the opening of the apical foramen of the model. The torque control engine was set to 300 rpm according to the instructions of the manufacturer by using a Dentaport ZX (J Morita, Kyoto, Japan). The canals were randomly divided into two experimental groups (n = 10) and prepared using the NRT-N and NRT files. Each file was replaced with a new one when it had been used three times. The root canal shaping was done by using pure water in the root canal. The root canal shaping protocol is shown in Fig. 2B. All the canals were prepared as follows:
the 12/#40 SS NRT files were used in the region of the canal orifice; a 06/#40 file was used up to 10 mm; a 06/#35 file was used up to 13 mm; and a 06/#30 file was used up to 15 mm. The apical
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enlargement was performed with 6% tapered #25, #30, #35, and #40 NRT-N and NRT files in turn.
The root canal was rinsed with 2 mL pure water after each file had been used.
< Assessment of canal preparation >
Pre- and post-instrumentation root canal models enlarged using #25, #30, #35, and #40 files were photographed at 30× magnification using a digital microscope (VW5000, Keyence, Osaka, Japan) (Fig. 3A). To obtain clear images, 0.1% methylene blue dye (Wako Pure Chemical Industries Ltd., Osaka, Japan) was introduced into the canal before and after instrumentation. The preoperative and postoperative photographs were superimposed using the Adobe Photoshop software (Adobe Systems Incorporated, San Jose, CA) (Fig. 3B). The obtained images were analyzed with the Image J 1.41 software (National Institute of Health, MD, USA). The analyses were performed in accordance with the method developed by Yun et al. (17). First, a line parallel to the major axis was drawn;
perpendicular lines were then drawn from above the line at distances of 1, 2, 3, 4, and 5 mm from the apical foramen (0 mm). Then, the increases in the outer and inner canal widths (distance from the root canal wall before shaping to the root canal wall after shaping) were measured (Fig. 3B). In addition, the centering ability (18) was used as an indicator of deviation from the center of the root canal as a result of the shaping. In other words, all small/large values for the measured increase in the outer and inner canal widths were determined for each measurement level, and their ratio was designated the centering ability of each level (a value close to 1 indicated a small deviation).
Statistical analysis
After confirming the normality and homoscedasticity of the data according to the Kolmogorov-Smirnov test and F-test, respectively, each data set was analyzed using the Student’s t-test at significance levels.
Results
1. X-ray diffraction
The XRD patterns are presented in Fig. 4. NRT-N showed the main diffraction peaks at approximately 42.5° and 77° (Fig. 4A). NRT showed the main diffraction peaks at approximately 42.4°, 42.8°, 77.0°, and 77.9° (Figs. 4B and 4C). Based on the previous study (19, 20), the XRD pattern for NRT-N was assigned to austenite phase and the XRD pattern for NRT was assigned to R-phase.
6 2. Differential scanning calorimetry
The typical DSC curves for NRT-N and NRT files are presented in Fig. 5. The upper DSC curves in the figure represent the cooling process, while the lower curves represent the heating process. The exothermic peak was associated with the transformation from the austenite phase to the R-phase during the cooling process. The two endothermic peaks occurred during the heating process: one associated with transformation from the martensite phase to the R-phase and the other associated with transformation from the R-phase to the austenite phase (Figs. 5A and 5B). The transformation temperatures obtained from the DSC curves are listed in Table 1. The ARf for NRT-N was 23.20± 3.33°C and that for NRT was 40.45 ± 2.63°C.
3．Cyclic fatigue resistance test
The numbers of cycles until fracture for the various files are listed in Table 2. For the #25, NRT-N and NRT files, the values were 998.4±87.9 and 5,560.8±834.9, respectively. For the #40, NRT-N and NRT files, they were 518.6±116.7 and 1,079.5±264.8, respectively. Thus, the cyclic fatigue resistance of #25 and #40 NRT files are both significantly greater than #25 and #40 NRT-N files (p <
0.01).
4．Root canal shaping
< Increase in the canal width >
The increase in the inner canal width at the 5-mm point was 202.5 µm when the #25 NRT-N file was used for apical enlargement. Further the corresponding value for the NRT file was 175.8 µm (Fig.
3C). #25 NRT file was significantly less the increase in the inner canal width at the 5-mm point than
#25 NRT-N file (p < 0.05). When the #30 and #35 files were used, however, the increases in the canal width at the various measurement points were not significantly different (p < 0.05) (Figs. 3D and 3E).
The increase in the outer canal width at the 0-mm point was 213.4 µm when the #40 NRT-N file was used for apical enlargement. Further the corresponding value for the NRT files was 178.6 µm (Fig.
3F). #40 NRT file was significantly less the increase in the outer canal width at the apex than #40 NRT-N file (p < 0.05).
< Centering ability >
Table 3 lists the results of the calculations of the centering ability. The centering abilities were significantly better in the case of the size #30 and #35 NRT files than for the NRT-N files of the same size at the 0-mm point (p < 0.05).
7 Discussion
R-phase Ni-Ti files have been developed recently, as they exhibit greater cyclic fatigue resistances than those of conventional Ni-Ti files. The R-phase is an intermediate phase that forms during the forward transformation from austenite to martensite on cooling as well as the reverse transformation from martensite to austenite on heating. It was reported that some R-phase Ni-Ti files exhibit the austenite phase at room temperature (13, 21).
When a stress is applied to a Ni-Ti alloy at temperatures greater than ARf, the alloy recovers without permanent deformation, even if it undergoes a strain of 8%. This is known as superelasticity.
On the other hand, Ni-Ti alloys in the martensite phase are softer and suppler than those in the austenite phase and undergo plastic deformation when subjected to a stress (22). Since the R-phase is an intermediate phase, it exhibits both the elasticity of the austenite phase and the suppleness of the martensite phase. The present study was performed to examine the effects of the phase-transformation behavior on the cyclic fatigue resistance and canal shaping ability of R-phase Ni-Ti files having the same design but different phase-transformation behaviors.
The XRD patterns indicated that the crystalline structure of NRT-N consisted of the austenite phase while that of NRT consisted of the R-phase at room temperature. Further, the DSC curves of both types of files exhibited two endothermic peaks during the heating process (Figs. 5A and 5B).
The first peak corresponded to the transformation from the martensite phase to the R-phase, while the second peak corresponded to the transformation from the R-phase to the austenite phase. The peak observed during the cooling process corresponded to the transformation from the austenite phase to the R-phase. Thus, the ARf values (Table 1) suggested that the crystalline structure of the NRT-N consisted of the austenite phase, while that of the NRT consisted of the R-phase at room temperature. This conclusion was supported by the XRD patterns.
Metal fatigue can be categorized into torsional fatigue and cyclic fatigue depending on the mechanism of action (6). The cyclic fatigue is a major factor of fractures within root canals, because it is well known that seventy percent of the Ni-Ti file fractures during root canal shaping are caused by cyclic fatigue (23). In the present study, both the #25 and the #40 NRT files were significantly greater cyclic fatigue resistance than #25 and #40 NRT files (Table 2). It has been reported that the higher ARf temperatures in the case of R-phase Ni-Ti alloys led to better flexibility and cyclic fatigue resistance (24). The results obtained in the present study were very similar. It was assumed that the deformation of austenite-phase NRT-N files on application of load was accompanied by change in crystalline structure from the austenite phase to a stress-induced R-phase (25). Further, the deformation of the R-phase NRT files was accompanied by the reorientation of the R-phase variants (25). Because the deformation of NRT does not result in a phase transformation, it was assumed that
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the cyclic fatigue resistance of NRT was higher than NRT-N. The results of the present study were suggested that R-phase Ni-Ti files which appear R-phase have higher cyclic fatigue resistance than those with austenite phase in clinical temperature.
It has reported that, in curved canals, previous Ni-Ti files without R-phase often shape the outer root canal wall at the apex of the curve and the inner root canal wall at the coronal part of the curve (17, 26–28). In the present study, #30 and #35 NRT files were better centering ability than #30 and
#35 NRT at the 0-mm point (Table 3). When the #30 and #35 NRT-N files were used for apical enlargement, the increase in the outer canal width was greater, and the increase in the inner of canal width was smaller than those for the corresponding NRT files at the 0-mm point (Figs. 3D and 3E), indicating that the NRT exhibited significantly greater centering abilities (Table 3). When a Ni-Ti file is used in curved canals, the file will be changed its shape then the file return to its original shape by superelasticity. Thus, the amount of shaping on the outside at the apex was increased (28).
Austenite-phase Ni-Ti alloys exhibit superelasticity; however, R-phase Ni-Ti alloys are not superelastic (25). The NRT files had a better centering ability probably because they were less resilient than NRT-N files. Ebihara et al. (12) compared root canal shapings done using a previous Ni-Ti file without R-phase and an R-phase Ni-Ti file and reported that the outward transportation at the apex was suppressed in the case of the R-phase Ni-Ti file. However, no reports have compared root canal shapings done using R-phase Ni-Ti files with the same design but different phase-transformation temperatures. This study demonstrated that Ni-Ti files exhibited R-phase were less transportation than Ni-Ti files exhibited austenite phase at clinical temperature.
Thus, although both NRT-N and NRT files exhibited an R-phase when subjected to a temperature change, the NRT file showed a higher fatigue resistance and better apical centering ability than NRT-N file because the R-phase exhibits in NRT file at clinical temperature. Therefore, when shaping of curved canal, using Ni-Ti file exhibited R-phase is more effective than Ni-Ti file exhibited austenite phase at clinical temperature in cyclic fatigue resistance and shaping ability.
Acknowledgements
The authors would like to express their deepest appreciation to Professor H. Hosoda of Tokyo Institute of Technology for help with performing XRD and for advice. The authors deny any conflicts of interest related to this study.
9 References
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11 F
Figures
Fig.1 Cyclic fatigue resistance test device used in the present study (MANI, Tochigi, Japan).
A.
B.
Fig. 2 A: Simulated root canal in a clear resin block.
B: Protocol for preparing a transparent root canal model.
12 Fig. 3 Results of the root canal shaping experiments
A: Image of the root canal model before being shaped.
B: Representative superimposed pre- and post-shaping photographs.
Increases in the inner and outer canal widths during root canal shaping with the C: #25 NRT-N and NRT, D: #30 NRT-N and NRT, E: #35 NRT-N and NRT, and F: #40 NRT-N and NRT files.
*:p ＜ 0.05
13 Fig. 5 DSC curves of NRT-N (A) and NRT (B).
Fig. 4 XRD profiles
A: NRT-N ( 2lj of 30°- 80° ) B: NRT ( 2lj of 30°- 80° ) C: NRT ( 2lj of 35°- 45° )
14 Tables
Table 1. Phase transformation points of the two file types, obtained from the DSC curves
Cooling Heating
Rs (°C) Rf (°C) As (°C) ARf (°C) NRT-N 18.85±0.69 4.21±1.31 -11.21±0.48 23.20±3.33 NRT 32.66±3.07 22.30±2.60 3.23±1.54 40.45±2.63 Results connected by lines indicate a significant difference (p < 0.01).
Table 2. Numbers of rotations until fracture
#25 #40 NRT-N 998.4±87.9 518.6±116.7 NRT 5,560.8±834.9 1,079.5±264.8
Results connected by lines indicate a significant difference (p < 0.01).
Table 3. Centering abilities of the various files
Levels from the apex (mm)
0 1 2 3 4 5 NRT-N #25 0.65±0.21 0.68±0.17 0.77±0.14 0.81±0.13 0.67±0.09 0.72±0.15 NRT #25 0.82±0.19 0.53±0.19 0.70±0.18 0.82±0.10 0.70±0.16 0.86±0.18 NRT-N #30 0.58±0.19 0.61±0.19 0.80±0.16 0.74±0.15 0.64±0.14 0.73±0.14 NRT #30 0.78±0.16 0.52±0.15 0.78±0.11 0.77±0.16 0.63±0.14 0.74±0.12 NRT-N #35 0.51±0.16 0.51±0.09 0.85±0.06 0.51±0.10 0.43±0.07 0.55±0.09 NRT #35 0.64±0.10 0.49±0.08 0.80±0.16 0.50±0.10 0.43±0.12 0.57±0.14 NRT-N #40 0.51±0.23 0.55±0.15 0.73±0.10 0.43±0.06 0.40±0.07 0.51±0.09 NRT #40 0.56±0.20 0.62±0.15 0.67±0.13 0.43±0.07 0.38±0.06 0.53±0.06 Results connected by lines indicate a significant difference (p < 0.05).
