Evaluation of torsional strength and bending stiffness of Edge Endo file systems using different heat treatments

Introduction

Using NiTi files in root canal preparation has many advantages than stainless steel files. One of the most important advantages is that their higher flexibility and centering ability reduce unwanted alterations of the root canal shape [1, 2]. However, NiTi rotary files have a higher fracture incidence [3], particularly in curved root canals, leading to compromised root canal treatment [4]. During treatment of curved canals, an endodontic file is exposed to high stresses, which may cause instrument’s fracture and canal blockage [5]. The most common issue during root canal preparation is the intra-canal fracture of instruments caused by torsional stress, cyclic fatigue, or both [6]. Clinically, during instrumentation of severely curved canals, files are subjected to severe bending, which causes alternative tension and compression cycles, leading to file fracture [7]. Bending stiffness indicates the instrument’s flexibility and ability to follow the canal’s original path without deterioration to the canal shape [7]. In straight and narrow canals, fractures usually occur due to torsional stress, while in curved and narrow canals, they occur due to flexural and torsional stresses [8]. The torsional fatigue arises when the end of the file becomes locked apically, while the shaft is still rotating [2].
To improve instruments properties and fracture resistance, the manufacturers use different heat-treated alloys with various features [9]. There is a wide range of NiTi files with many types of heat treatments, each of them having unique metallurgic features. FireWire Blaze is a novel heat treatment created by Edge Endo in new rotary NiTi files systems. However, there is no information about this new heat treatment, and how it affects the behavior of new files.
EX7 (Edge Endo, Albuquerque, New Mexico, United States) is a NiTi file instrument manufactured by FireWire™ heat treatment that increases its flexibility and fatigue resistance, and minimizes the shape memory [10]. EX7 Utopia™ (Edge Endo, Switzerland) is a new NiTi file system, with a remarkably gold blade combined with FireWire Blaze heat treatment. EX7 Utopia provides all benefits of the original X7 blade design, with more cutting efficiency and fracture resistance, as claimed by the manufacturer [11]. EOB Utopia™ (Edge Endo, Albuquerque, NM, United States) is another new reciprocating single-file system that features FireWire Blaze heat treatment [12].

Objectives

This research aimed to examine the torsional resistance and bending stiffness in the new heat-treated edge NiTi files (EX7 Utopia and EOB Utopia), and compare them with the traditional EX7 file at body temperature. The null hypothesis was that there is no difference in torsional resistance and bending stiffness in the tested edge NiTi files.

Material and methods

A total number of 90 files, with 45 files in each test group (n = 15 file per sub-group) were used: EX7 file (25 mm length, # 25 tip size, and 0.06 constant taper); EX7 Utopia file (25 mm length, # 25 tip size, and 0.06 constant taper); EOB Utopia file (25 mm length, # 25 tip size, and 0.08 variable taper).
The sample size was determined using G*Power 3.1 software.

Torsional resistance test

Torsional resistance was tested according to the repe­titive torsional loads method described by Park et al. [13]. A customized stainless steel plate with a hole of 5 × 5 × 5 mm, stabilized with screws in a water path containing distilled water at 37℃ temperature (n 15/file) was used. The tip of the file was fixed in the hole by light-cured composite resin (SHOFU, Kyoto, Japan), while the other side of the file was connected to an endomotor with auto-stop property (X-Smart Plus, Dentsply). Files were used with 2 Ncm maximum torque and 300 rpm maximum speed in clockwise (CW) direction. Load was applied until maximum preset torque was reached, and the endo­motor stopped automatically. Cycles were repeated until file fracture, and the number of cycles until fracture of each file was recorded. From each group, two fractured files were cleaned with alcohol, and examined under a scanning electron microscope at 200× and 2K× magnification for fractographic analysis of the fracture surface.

Bending stiffness test

Bending stiffness was evaluated with a cantilever-bending test using a microcomputer-controlled electronic universal testing machine (Tinius Olsen, H50KT Tensile Tester, United States) by determining the force of deflection (n 15/file). Each file was fixed from its handle by a mini-bench vise submerged in distilled water at 37 °C temperature inside a digital water bath. Load was applied at 3 mm from the tip with a 10 N loading cell; when the loading cell moved 3 mm vertically downward, the force of deflection was recorded to measure the bending stiffness.

Statistical analysis

Torsional and bending tests results (Tables 1 and 2) were recorded and analyzed using one-way ANOVA and Tukey HSD at p < 0.05 level of significance. SPSS version 22.0 (Chicago, Illinois, USA) was employed for statistical analysis.

Results

The results of the torsional test are shown in Table 3. The EX7 file demonstrated the highest number of torsional loads until fracture (56.867), followed by the EX7 Utopia file (43.600), while the EOB Utopia file demonstrated the lowest number of torsional loads until fracture (34.133). The differences between the EX7 and EX7 Utopia, and between the EX7 Utopia and EOB Utopia files were not statistically significant (p > 0.05) (Figure 1). Fractographic analysis of the fractured surfaces resulted in torsional fracture topography showing the presence of concentric abrasion marks with fibrous dimples (Figure 2).
The results of the bending test are described in Table 4. The deflection of 3 mm in files at 3 mm from the tip was within the elastic limit for all files. The results showed that the EX7 Utopia file presented the highest resistance to bending forces (0.460 N), followed by the EOB Utopia file (0.444 N), while the EX7 file showed the lowest bending resistance (0.410 N) (p < 0.05). The differences between the EX7 Utopia and EOB Utopia, and between the EOB Utopia and EX7 were not significant (p > 0.05) (Figure 3).

Discussion

Type of heat treatment of NiTi alloys affects the mechanical behavior of files with the same design features and dimensions. Different types of thermo-mechanical treatments are produced to enhance mechanical features of NiTi files by increasing their flexibility, fracture resistance, and centering capacity [14-16]. This study investigated the impact of a new heat treatment, FireWire Blaze, on the torsional fatigue resistance and bending stiffness of new Edge Endo NiTi instruments.
Characteristic features of NiTi files affect their beha­vior during bending and torsional stresses [17]. Nume­rous factors affect the torsional strength of NiTi rotary instruments, including the file’s taper, flute depth, number of threads, pitch length, core diameter, cross-sectio­nal geometry, and alloy heat treatment [18].
EX7 has a fire wire heat treatment and a triangular cross-section with a variable helical angle that equips the file with high strength and flexibility [19]. EX7 Utopia has the same features as EX7, but with the FireWire Blaze heat treatment [11]. The EOB Utopia file is a reci­procating file with an S-shaped cross-section and mini­mal number of threads with the FireWire Blaze heat treatment [12, 20].
In previous studies, the repetitive torsional load (RTL) method showed the same results as the static torsional load method (STL), except that the RTL method simulated clinical conditions; therefore, in this study, the RTL method was used [21, 22]. A 300 rpm and 2 Ncm speed and torque were selected, as higher speeds and torques resulted in premature fracture of the tested files, thereby compromising the reliability of the results for comparison. To estimate the flexibility of endodontic NiTi files, ISO 3630-1 devised a standardized bending test that includes securing 3 mm of the file’s tip and subjecting it to a 45° angular deflection. Nevertheless, this test was initially designed for stainless steel instruments; this study assessed the bending stiffness of NiTi rotary files using a cantilever-bending test [23].
To simulate clinical conditions, and because the metallurgy of NiTi instrument is affected by the ambient temperature [24], this study was carried out at simulated 37℃ body temperature using digital water bath. The temperature was monitored throughout the study with a thermometer.
The EX7 file showed the highest flexibility and resistance to torsional loads. This may be related to the type of heat treatment compared with the EX7 Utopia file, as these two files have the same design features and differ only in the type of heat treatment. The parabolic cross-section of the EX7 Utopia file is equipped with more core diameter compared with a S-shaped cross-section of the EOB Utopia file [25, 26]. Furthermore, the EX7 Utopia file has a higher number of threads and shallower flutes, which provide more resistance to torsional loads [23]. The S-shaped cross-section, deep, wide flutes, and minimum number of threads contribute to the EOB Utopia file’s higher flexibility compared with the EX7 Utopia file. Although the EOB Utopia file exhibits a higher taper, this does not result in a significant effect on its torsional strength and bending stiffness properties.
The new file systems exhibited lower flexibility and torsional resistance when compared with the traditional EX7 file, which could be related to the new heat treatment. From the behavior of these files, it can be concluded that they underwent more austenite rather than martensite phases compared with the EX7 file. The predominance of the stiff austenite phase made them stiffer and more vulnerable to fatigue crack propagation [19]. However, no prior studies have investigated or compared the mechanical behavior of the new files. Therefore, the findings of the current study cannot be directly correlated with a previous research.
A limitation of this study is the lack of standardization of the taper of the tested files. The EOB Utopia primary file has a 25 ISO tip size with 8% of regressive apical taper, which is present in most canals. The EOB Utopia small file, with a 21 ISO tip size and 6% constant taper, is utilized in small canals. While, the EOB Utopia large file, with a 40 ISO tip size and 8% of regressive taper, is usually employed in large canals. Standardizing the taper by using the EOB Utopia small file would compromise the standardization of tip sizes. Another limitation of this study is that the cutting direction of the EOB Utopia file was counterclockwise (CCW), whereas the other files cuts were done in a clockwise (CW) direction. Since there is no endomotor with an auto-stop function in the CCW direction, all tested files were used in the CW direction during the torsional strength test, following the method described by Varghese et al. [26]. This could lead to an underestimation or overestimation of the torsional strength of the EOB Utopia reciprocating file.

Conclusions

The new FireWire Blaze heat treatment significantly affects the torsional and bending resistance of the studied edge NiTi files. The EX7 file is significantly more flexible and demonstrates more resistance to torsional loads than the other tested new files systems.

DISCLOSURES

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References