Torsional Fracture Resistance of Niti Rotary Glide Path Files under Flexural Stress

Abstract

This study aimed to evaluate the effects of flexural stress on the torsional fracture resistance of NiTi glide path files. PathFile #16/02, RaCe #15/04, RaCe Evo #15/04, HyFlex EDM #15/03, TruNatomy Glider #17/02, and V Taper 2H #17/04 were examined by scanning electron microscopy (SEM) (n = 3/brand) and subjected to differential scanning calorimetry (n = 2/brand). Torsional fracture resistance testing was performed in straight (ISO 3630-1) and flexural modes (n = 15/brand/mode). Flexural mode testing involved instruments rotating within a stainless-steel artificial double-curved canal. Ultimate strength and distortion angle until failure were recorded, and fractured instruments were examined by SEM. Statistical analyses involved independent sample t-test and one-way analysis of variance with Games–Howell pots hoc test. Austenitic transformation- finishing temperatures of heat-treated files were above body temperature. For RaCe Evo, HyFlex EDM, TruNatomy Glider, and V Taper 2H, the flexural mode resulted in a significantly higher distortion angle compared to the straight mode (p < 0.05). The maximum torque of RaCe Evo, HyFlex EDM increased with the flexural stress (p < 0.05). V taper 2H showed the highest distortion angle and ultimate strength. SEM showed typical patterns of torsional fracture for all tested files. The flexural stress positively affected distortion angle of heat-treated NiTi glide path files.

1. Introduction

Nickel-titanium (NiTi) rotary files enable faster and more efficient root canal shaping [1] and maintain original canal anatomy better than manual preparation, even when obtaining glide path [2,3]. NiTi rotary glide path file usage prior to canal shaping preserves original canal anatomy and reduces risks of transportation and stresses generated by NiTi shaping files [2,4,5].

The NiTi rotary files initially available for glide path preparation, such as PathFile (Dentsply Sirona, Ballaigues, Switzerland), have a small diameter and taper to reach the apical canal. Then, manufacturers developed NiTi glide path files with heat treatments that increase flexibility and resistance to fracture, and files with various designs. A recently released heat-treated file system, TruNatomy (Dentsply Sirona), is suitable for minimally invasive root canal preparation. TruNatomy Glider has a variable taper that averages 2%, with tip size of #17 [6].

The NiTi alloy is composed of nickel and titanium with an equi-atomic ratio [7]. The NiTi alloy can exist in different crystal structures depending on temperature, i.e., austenite and martensite [7,8]. The martensitic phase of the NiTi alloy has lower Young’s modulus and yield strength compared to those of austenitic phase of the NiTi alloy [7]. Namely, martensite is easily deformed and is soft and ductile [7,8]. Conventional NiTi files mainly consist of austenite and exhibit superelasticity, which allows deformation of 8% strain to be recoverable [7,8].

NiTi files made from a thermo-mechanically treated alloy, Controlled Memory (CM) wire, do not possess superelastic properties. CM wire files contain some martensite at room temperature [8], which have demonstrated remarkable flexibility [9,10]. These include V Taper 2H (SS White, Lakewood, CA, USA) and HyFlex EDM (Coltene-Whaledent, Altstätten, Switzerland) [10]. HyFlex EDM files produced by electrical discharge machining of CM wire exhibited superior resistance to cyclic fatigue fracture [11]. HyFlex EDM glide path files are available in sizes #10/05 and #15/03, and the latter released more recently has had limited assessment of its mechanical properties.

Heat treatments with electropolishing during manufacturing produce blue surfaces with enhanced properties, as in RaCe Evo (FKG Dentaire, La Chaux-de-Fonds, Switzerland) [12]. However, the torsional fracture resistance of RaCe Evo glide path files has not been compared to original RaCe (FKG Dentaire).

When the instrument’s tip becomes tightly bound within a canal and the handpiece continues to rotate beyond the maximum strain it can withstand, torsional fracture occurs [13]. Testing torsional resistance in vitro according to ISO 3630-1 involves immobilizing 3 mm of the instrument’s tip, while rotating the shaft continuously until fracture [14]. Although these tests are performed on straight instruments, actual canals are curved, and instruments resist both torsional and bending stresses simultaneously.

This study aimed to compare NiTi rotary glide path file’s design, phase transformation behavior, and torsional fracture resistance under flexural stress for six brands.

2. Materials and Methods

PathFile (tip size #16, 2% taper), RaCe (tip size #15, 4% taper), RaCe Evo (tip size #15, 4% taper), HyFlex EDM (tip size #15, 3% taper), TruNatomy Glider (tip size #17, variable 2% taper), and V Taper 2H (tip size #17, variable 4% taper) instruments (n = 35/brand) were obtained. PathFile, RaCe, RaCe Evo, and HyFlex EDM have a constant taper, whereas TruNatomy Glider and V Taper 2H possess a variable taper. PathFile, HyFlex EDM, and TruNatomy Glider have square cross-sectional shape, while RaCe, RaCe Evo, and V Taper 2H demonstrate triangular cross-sectional shape.

2.1. Instrument Design

New unused instruments (n = 3/brand) were carefully examined by scanning electron microscopy (SEM; S-4700, Hitachi, Tokyo, Japan, ×30 magnification). Surface features were observed, and the pitch length and helix angles were measured with Image J software (NIH, Bethesda, MD, USA).

2.2. Phase Transformation Behavior

Unused instruments (n = 2/brand) were subjected to differential scanning calorimetry (DSC), which demonstrated their phase transformation behavior. Each instrument tip (4 mm) was sealed in an aluminum cell and DSC analysis performed (DSC250, TA instruments, New Castle, DE, USA). Sample was heated from 25 °C to 90 °C, then cooled to −90 °C, and heated again to 90 °C at 10 °C/min. The DSC curve was used to determine phase transformation temperatures.

2.3. Torsional Fracture Resistance

Instruments (n = 30/brand) were subjected to torsional fracture tests in either straight (n = 15/brand) or flexural mode (n = 15/brand) using a universal testing machine (UTM; Universal Mechanics Analyzer, IB systems, Seoul, Korea). This consisted of a torque sensor, an upper chuck connected to a reversible geared motor, and a lower chuck with a staging platform (Figure 1A,B).

Torsional testing in the straight mode was performed according to ISO3630-1. Each instrument’s tip (3 mm) was clamped into a lower chuck with brass jaws that prevented sliding (Figure 1A). The instrument’s shaft was clamped into the upper chuck and rotated (2 rpm) clockwise continuously until fracture.

Torsional testing in flexural mode was performed with the UTM using a stainless-steel jig that simulated root canal with a double curvature (135°) (Figure 1B). The jig contained two stainless-steel components with prepared grooves on their inner surfaces that created an artificial root canal when joined with screws (Figure 1B,C). The apical curve of the canal was positioned 6 mm from each instrument’s tip (3 mm in the metal jig + 3 mm in the lower chuck), and the coronal curve was located 3.3 mm vertically from the apical curvature (Figure 1C,D). The internal diameter of the artificial canal (1 mm) allowed unimpeded rotation of the instrument without binding. Each instrument’s tip (3 mm) was clamped in the lower chuck, and the staging platform was repositioned laterally such that the longitudinal axis of the instrument was perpendicular to the plane. The instrument’s shaft was clamped into the upper chuck and rotated (2 rpm) clockwise continuously until fracture. Rotation angles and torque values were measured in real-time with a torque sensor contained in the UTM and recorded automatically by a software program installed in a notebook connected to the UTM. The ultimate strength and angular distortion at failure were recorded.

After torsional resistance tests, the fractured and lateral surfaces of the instruments were examined under SEM (Teneo VS, FEI, Hillsboro, OR, USA) to identify patterns (×100, ×1000 and ×5000 magnifications).

2.4. Statistical Analysis

For torsional fracture resistance testing, a priori sample size calculation was performed to determine effect size, using results from a previous study [15]. Analysis of variance was selected from the F test family using an alpha type error of 0.05 and a power beta of 0.95 (G*Power for Windows v3.1; Heinrich Heine, Universität Düsseldorf, Düsseldorf, Germany). A minimum of 12 samples/group were required to observe the same effect (3.07) [15]. Therefore, sample size was set to 15/group.

Torsional fracture resistance testing data were verified for normality by the Shapiro–Wilk test. Homogeneity of variances between instrument brands was not satisfied by Levene’s test. Therefore, one-way analysis of variance was followed by Games–Howell post hoc tests. Differences in the torsional fracture resistance between straight and flexural test modes for each instrument brand were evaluated by independent sample t-test. Statistical analyses were performed with SPSS version 25 (IBM, Armonk, NY, USA), and the p-value set at 0.05.

3. Results

3.1. Instrument Design

SEM revealed surface features on unused instruments (Figure 2). RaCe and RaCe Evo had markedly longer pitch length, and alternating long and short pitch length, which distinguishes them from the other instruments (Figure 2B,C). PathFile, HyFlex EDM, and V Taper 2H had similar pitch length in their apical 4 mm regions (Figure 2A,D,F). HyFlex EDM showed longer pitch length, followed by V Taper 2H and PathFile in the middle 4 mm region (Figure 2A,D,F). TruNatomy Glider had the shortest pitch length in both middle and apical regions (Figure 2E). All instruments except PathFile showed longer pitch length in the middle region compared to the apical region. Helix angles were measured at the cutting edge located around 3 mm from instrument tip. Helix angles of PathFile, HyFlex EDM, TruNatomy Glider and V Taper 2H were 16.9°, 15.9°, 20.9° and 20.5°, respectively. Helix angles of RaCe and RaCe Evo were alternately 11.5° and 16.0° for both instruments. The lateral surface of PathFile showed milling mark and metal rollover (Figure 2A), TruNatomy Glider showed milling mark (Figure 2E), and HyFlex EDM showed characteristic crater-like pattern without milling mark (Figure 2D). Whereas RaCe, RaCe Evo, and V Taper 2H demonstrated smooth, flawless surfaces (Figure 2B,C,F).

3.2. Phase Transformation Behavior

DSC plot of PathFile showed a single peak for the heating curve, and no distinct peak during cooling (Figure 3A). DSC plots of RaCe and HyFlex EDM showed single peaks in the heating and cooling curves, respectively (Figure 3B,D). RaCe Evo, TruNatomy Glider, and V Taper 2H had a single peak in heating, and double peaks during cooling, as austenite transformed to R-phase, then transformed to martensite (Figure 3C,E,F). A_s and A_f in the heating curves and M_s, M_f, R_s and R_f in cooling curves were measured (

Table 1. Phase transformation temperatures from the DSC plots.

	Heating Curve		Cooling Curve
	As	Af	Rs	Rf	Ms	Mf
PathFile	−16.63	0
Race	3.85	27.88			21.92	−4.04
Race Evo	12.04	45.37	37.50	15.74	−27.5	−65.87
HyFlex EDM	26.54	50.1			42.31	17.31
TruNatomy Glider	10.58	37.02	30.96	8.65	−34.23	−64.04
V Taper 2H	15.77	44.33	40.19	12.21	−20.38	−45.87

A_s and A_f, Austenitic transformation starting and finishing temperatures; R_s and R_f, R-phase transformation starting and finishing temperatures; M_s and M_f, Martensitic transformation starting and finishing temperatures.

).

3.3. Torsional Fracture Resistance

Ultimate strengths (N cm) were significantly higher (p < 0.001) under flexural stress than in straight mode for RaCe Evo and HyFlex EDM (

Table 2. Instrument’s ultimate strength (N cm) from torsional fracture resistance testing in straight and flexural mode shown as mean (standard deviation).

	PathFile	RaCe	RaCe Evo	HyFlex EDM	TruNatomy Glider	V Taper 2H
Straight	0.213 (0.030) a	0.225 (0.039) a	0.250 (0.060) a,b	0.293 (0.051) b	0.285 (0.063) b	0.404 (0.079) c
Flexural	0.227 (0.024) a	0.233 (0.056) a	0.348 (0.055) b,c,*	0.348 (0.019) b,c,*	0.307 (0.061) b	0.397 (0.081) c
pvalue	0.188	0.706	<0.001	<0.001	0.355	0.827

p value calculated from independent sample t-test. Asterisk indicates significant difference between straight and flexural modes (p < 0.05). Different superscript letters indicate significant difference between instruments (p < 0.05).

). Distortion angles increased significantly (p < 0.001) under flexural stress for RaCe Evo, HyFlex EDM, TruNatomy Glider, and V Taper 2H but were not significantly different than in straight mode for PathFile and RaCe (Table 2).

When comparing between files, V Taper 2H showed the highest ultimate strength, and PathFile had the lowest in both test modes (Table 2). The ultimate strength of TruNatomy Glider was higher than those of PathFile and RaCe and lower than that of V Taper 2H.

The distortion angle was highest for V Taper 2H, followed by RaCe Evo and TruNatomy Glider in both test modes (

Table 3. Instrument’s distortion angle (degrees) from torsional fracture resistance testing in straight and flexural mode shown as mean (standard deviation).

	PathFile	RaCe	RaCe Evo	HyFlex EDM	TruNatomy Glider	V Taper 2H
Straight	748.22 (82.68) b	574.42 (85.29) a	743.92 (122.61) b	627.17 (47.32) a	687.25 (45.08) b	2036.76 (517.97) c
Flexural	772.37 (68.98) b	572.84 (99.6) a	1003.34 (117.76) c,*	755.04 (83.67) b,*	938.55 (103.75) c,*	2969.86 (831.34) d,*
pvalue	0.263	0.971	<0.001	<0.001	<0.001	<0.001

p value calculated from independent sample t-test. Asterisk indicates significant difference between straight and flexural modes (p < 0.05). Different superscript letters indicate significant difference between instruments (p < 0.05).

). RaCe showed the lowest distortion angle in both test modes.

Graphs of the torque and distortion angle were plotted for representative samples in each group (Figure 4). As soon as the file was rotated, the torque increased in proportion to the distortion angle, which is regarded as elastic deformation, and the elastic limit of each file is marked in the graph. After that, the torque was found to increase unevenly, the decrease and increase were repeated, and the deformation between the elastic limit and failure point was regarded as plastic deformation. The amount of plastic deformation increased with the flexural stress (Figure 4).

SEM images of the lateral surface of the fractured files showed a typical ductile fracture pattern for the tested instruments (Figure 5). Circular abrasion marks were observed in the central area of the fractured surfaces for all the samples (Figure 6). There was no difference in the fractographic patterns in the microscopic images according to the test mode.

4. Discussion

In this study, the effect of flexural stress on the torsional resistance of NiTi glide path files was material dependent. PathFile and RaCe, which are manufactured without heat treatment, were not affected by flexural stress. In contrast, heat-treated NiTi files showed increased torsional resistance during flexural stress. RaCe Evo and HyFlex EDM showed significantly higher distortion angle and ultimate strength in flexural than straight modes. Similarly, TruNatomy Glider and V Taper 2H showed higher distortion angle under flexure. Although there was no significant difference, the mean ultimate strength of V Taper 2H was higher at straight mode compared to that of flexural mode (Table 2). The smooth, flawless surface of V Taper 2H might favorably act upon the ultimate strength in straight mode, yet further study is needed.

Larger distortion angles under flexural stress in heat-treated instruments can be attributed to their phase composition. While PathFile and RaCe consist of austenite, HyFlex EDM contain austenite and martensite (Figure 3A,B,D). RaCe Evo, TruNatomy Glider, and V Taper 2H are a mixed state of austenite, martensite, and R-phase (Figure 3C,E,F). When torsional fracture resistance testing was performed in flexural mode, V Taper 2H, RaCe Evo, and TruNatomy Glider, which possess some martensite and R-phase at room temperature, showed larger distortion angles than other files. The elastic modulus of R-phase is lower than martensite, which is lower than that of austenite [10]. Therefore, instruments containing martensite and R-phase can deform more readily under flexural stress. Indeed, charting of torque with distortion angle (Figure 4) showed that increased distortion angles from flexural stress involved plastic deformation, which are a warning sign for torsional fracture of the instrument. The mean distortion angle of RaCe was higher for straight mode compared to that of flexural mode, although the difference was not statistically significant (Table 3). The smooth, electropolished surface and alternating pitch length of RaCe might have affected the distortion angle in straight mode, yet further research is needed.

In this study V Taper 2H showed the highest distortion angle and ultimate strength. This instrument’s large diameter at 3 mm from the tip (D3), relatively short pitch length, and metallurgical properties contributed to the highest torsional fracture resistance. Estimated D3 size was largest for V Taper 2H (0.29 mm), followed by RaCe and RaCe Evo (0.27 mm), HyFlex EDM (0.24 mm), TruNatomy Glider (0.23 mm), and PathFile (0.22 mm). PathFile and RaCe, manufactured from conventional NiTi alloy, showed lower ultimate strength than those of heat-treated instruments, which agrees with previous reports that manufacturing method and metal mass affect torsional strength [14,15].

The ultimate strength of TruNatomy Glider was higher than PathFile, and comparable to HyFlex EDM or RaCe Evo, which have larger D3 than TruNatomy glider. However, TruNatomy Glider had the shortest pitch length (Figure 2E), and thereby a greater number of threads than other instruments. More threads as well as square cross-sectional design ensured that TruNatomy Glider withstood torsional load despite its small diameter. This agrees with previous reports that NiTi files with more threads [16,17,18,19], and instruments with larger core mass and square cross sections had higher torsional resistance [15,20].

In contrast to flexural mode, for torsional resistance in straight mode, there were no significant differences in the distortion angles between PathFile, made from conventional NiTi alloy, and HyFlex EDM or TruNatomy Glider that are heat-treated instruments. PathFile exhibited superior flexibility in the 3–6 mm portion of the instrument due to smaller taper (2%), and more threads in the middle portion (Figure 2). Indeed, multiple factors including diameter, pitch length, and heat treatment affect distortion angles of NiTi glide path files [14,15,21,22].

This study’s limitation is that although instruments have different size and taper, the same artificial double-curved root canal was utilized. PathFile and TruNatomy Glider with small taper had less severe instrument bending within the canal. Furthermore, these torsional fracture resistance tests were performed at room temperature, and previous studies reported decreased cyclic fatigue resistance of conventional NiTi files that had A_f below body temperature [23,24]. These limitations need to be addressed in further studies.

In conclusion, the NiTi rotary glide path file ultimate strength was unaffected, and the heat-treated instruments’ angular distortion actually increased under flexural stress. V Taper 2H that are manufactured from heat-treated CM-wire demonstrated the highest resistance to torsional fracture.