Cyclic fatigue resistance evaluation of heat-treated rotary files after immersion in NaOCl and EDTA: an in vitro study

Introduction

Nickel-titanium (NiTi) instruments have revolutionized the field of dentistry when they were initially introduced into the market in 1988 [1, 2]. Since then, NiTi rotary files became an essential part of endodontic treatment due to their properties and ability to reduce the time and number of files required to shape a root canal [3].
Manufacturers aim for continuous development and improvement of alloys used in rotary files, as these rotary files experience a considerable problem, i.e., unexpected fracture inside a canal, which can lead to additional time and effort to extract separation parts or bypass it to complete the treatment [4, 5]. During the process of removing the file from a canal, residual dentin is also removed [6], which can weaken the tooth or cause other complications, such as perforation.
File separation can occur in two ways. First is called “tension fracture”, and occurs when the tip of the file becomes stuck in the canal, and the shank continuously rotates [7]. Second way is when the file is subjected to repeated cycles of tension and stress, leading to mental fatigue and eventual fracture, and it is known as “fatigue fracture” [7]. Studies show that fractures usually occur due to fatigue fracture [8].
To improve cyclic fatigue resistance (CFR), NiTi instruments have been enhanced in many ways, such as design modification, developing alloy, and creating new alloys with better mechanical properties [9, 10]. Many treatments are performed using traditional NiTi alloys, including thermo-mechanical techniques, electron polishing, and heat treatment [11, 12].
Plex-V file (Orodeka, China) is a new file system manufactured from CM wire using special heat treatment and triangular convex cross-section. This file provides very active cutting efficiency and have good pro­perties, such as shape memory, high resistance to fatigue, and high flexibility.
AF rotary file (Fanta Dental Materials Co., Shanghai, China) is another new file system introduced into the market, made of AF-H wire, a special alloy subjected to special heat treatment. This system is characterized by good mechanical properties and triangular convex section.
During endodontic treatment, files come into contact with irrigation solutions inside the pulp chamber and root canal. Studies have shown that there is an effect of irrigation solutions on endo files, which decreases their fatigue resistance, especially hydrochloride sodium, a strong alkaline that selectively removes nickel from the file’s surface [13-15].
To date, there are no studies on the effect of any irri­gation solutions on cyclic fatigue resistance (CFR) of Plex-V and AF rotary files. For new instruments, it is important to study their mechanical properties.

Objectives

The aim of this study was to evaluate the effect of NaOCl and EDTA on CFR values of AF and Plex-V files, and to compare them for highest resistance. The null hypothesis of the study was that there is no effect of the studied solutions on the fatigue resistance of AF and Plex-V files, and there are no differences in the cyclic fatigue resistance between the two instruments.

Material and methods

In total, 60 new rotary files (30 files per system) with ISO of 25.4% and 25 mm long were included. At first, all files were examined by stereo-microscope for any defects. Then, they were immersed into solutions (5.25% NaOCl and 17% EDTA) for 15 min at room temperature; 16 mm of file length was immersed (a part that comes into contact with irrigation solutions) to avoid galvanic corrosion due to two types of metals. Each file system was randomly divided into three different groups:

• Group A: control, no files immersed (n = 10);
• s immersed in NaOCl solution (n = 10);
• Group C: files immersed in EDTA solution (n = 10).

After immersion, the files were tested with a cyclic fatigue test device using three different stainless-steal artificial canals. For this study, a canal with a 120-degree angle and 2.5 mm radius of curvature was selected (Figures 1 and 2). Files rotated freely and continuously inside the canal with an addition of synthetic oil to reduce friction. AF files rotated at 350 rpm and Plex-V files at 500 rpm, as recommended by the manufacturers. To maintain a staple, insert position of the file’s hand-piece that the file contacts with, it was linked with the device. In this study, an endodontic rotary motor hand-piece 6 : 1 (Sirona Dental Systems GmbH, Bensheim, Germany) with VDW motor was used. The files were rotating inside the canal until a fracture occurred, and time to fracture (TF) was recorded in seconds and number of cycles to fracture (NCF) was calculated as TF multiplied by rpm/ 60. In order to protect from separation of the files, a glass bar was utilized to cover the canal.

Statistical analysis

SPSS, version 25.0 was employed in data analysis. Continuous data were presented as median and quartiles, while categorical data were shown as frequency and percentage. Data distribution was determined using one-sample Kolmogorov-Smirnov test. To examine statistical differences between two independent variables, Mann-Whitney test was applied. Kruskal-Wallis H test was utilized to determine statistical differences between more than two independent variables. A p-value of less than 0.05 was considered statistically significant. Sample size was calculated using G*Power, version 3.1.9.2, and the power of the study was considered 80%.

Results

Table 1 describes the NCFs of the three groups of both types of files. The mean NCFs of all Plex-V file groups were significantly greater than these of AF file groups (p < 0.001). The inferential analysis revealed a statistically significant difference between the groups of the same brand. In NFC evaluation (p = 0.006) of AF file groups (control, NaOCl, and EDTA), the mean values were 20.65, 8.50, and 17.35, respectively (Figure 3). Pair-wise comparison showed a significant difference in NFC when comparing the control and NaOCl groups (p = 0.006). However, pair-wise comparison did not find a statistical difference in NFC between the control and EDTA groups, nor NaOCl and EDTA groups (p = 0.078 and p = 1.000, respectively). Regarding NFC of Plex-V file groups (control, NaOCl, and EDTA), a significant difference (p = 0.021) was found, and the mean values were 21.75, 13.0, and 11.75, respectively (Figure 4). Pair-wise comparison showed that there was a statistically significant difference in NFC between the control and NaOCl groups as well as control and EDTA groups (p = 0.021 and p = 0.016, respectively). No statistically significant difference was observed in NFC between the EDTA and NaOCl groups (p = 0.919).
In this study, only the time and number of cycles completed by a file until a fracture occurred were considered, and the location of file’s fracture was not taken into account.

Discussion

Corrosion is one of the reasons that lead to mental fatigue. Micro-cracks not visible with the naked eye occur on the instrument surface and with every use, they become wider and deeper until fracture [16]. Corrosion can occur when the file is exposed to sterilization cycles, disinfection, or irrigation solutions [17].
Manufacturers recommend single use of files [18], and the incidence of fracture indeed decreases, but this may be difficult to achieve because of the costs of the files. In order to overcome unexpected breakage, rotary files are constantly evolving, and one of the ways to improve mechanical properties of the instruments is the development of an alloy they are made from [12, 19].
The current study aimed to evaluate the effect of the most common irrigation solutions (NaOCl and EDTA) on the CFR of two types of heat-treated NiTi rotary files (Plex-V and AF). For this study, in order to unify all of the experimental conditions, an artificial canal was used with natural teeth that differ from each other regarding hardness of dentin and canal geometry. In addition, a test device was allowed to maintain one entry position for all the studied files within the canal. Synthetic oil was applied inside the canal as a lubricant to reduce friction of the file with walls of the canal during rotation, which also diffused heat caused by rubbing [3]. The CFR test method utilized in this study is similar to that used previous studies [20, 21]. The immersion duration chosen (15 min) is similar to clinical situation for a single-canal tooth preparation [22]. Files from each brand were selected. To make the study as close as possible to clinical situation, Plex-V file was rotating inside the canal at 500 rpm, and AF file was rotating at 350 rpm (based on recommendations provided by the manufacturers). The difference between the two files was the alloy that it was made from.
To the best of our knowledge, there are no studies about the effect of NaOCl and EDTA on CFR of Plex-V and AF files. For Plex-V files, the results can be compared with studies done on files manufactured by typical cm wire.
In the present study, the RFC values of Plex-V files reduced after immersion in EDTA solution, unlike files immersed in NaOCl solution. This outcome agrees with previous studies, such as Nair’s et al. study [23] investigating HyFlex cm, HyFlex EDM, and twisted files, in which EdgeFile X3, HyFlex cm, and twisted files were not greatly affected by immersion in 2.5% NaOCl for 3 min. In Palma et al. study [24], files with cm wire tested in an artificial canal with 45-degree angle and 5 mm radius demonstrated resistance to fatigue after immersion in 3% NaOCl for 1 and 5 minutes. The same result was obtained by Cai et al. study [25] on cyclic fatigue resistance of two brands of files produced from cm wire. After immersion in NaOCl and EDTA solutions for 15 min, the files showed no effect on CFR. The authors related this result to the location of corrosion: if the corrosion occur far from the maximal curvature point, CFR will not be affected.
In the contrary, Rohen’s study compared CFRs of different types of rotary files, including ProTaper Gold, Kedo-S, Kedo-SG Blue, and Pro AF Baby Gold after immersion in 2.5% NaOCl and 2.5% NaOCl plus 17% EDTA (both for 10 min). As a result, AF Baby Gold (cm wire) were the most affected files by NaOCl and EDTA solutions [26]. The reason for different results may be due to the difference in NaOCl concertation and immersion time.
For AF files, it was observed that they are affected by NaOCl and not affected by EDTA. The reason is that hypochlorite sodium, a strong alkaline, has the ability to selectively remove nickel from an instrument’s surface, which causes a chipper that works as a pressure to concentration area; therefore, a fracture occurs [13]. The null hypothesis of the current study was rejected, because the Plex-V file (Orodeka, China) was more resistant to breakage than the AF file (Fanta Dental Materials Co., Shanghai, China). However, this result could be because the cm wire of Plex-V file provides high flexi­bility. The more flexible the instrument is, the more resistance to fatigue [3].
Another reason for this result is the alloy of Plex-V’s special cm wire, which allows for the instrument’s high flexibility, high fatigue resistance, and better mechanical properties compared with the AF file’s AF-H wire. However, more studies are needed to evaluate mechanical properties of AF-H wire alloy.

Conclusions

Within the limitation of this study, a reduction in CFR values of AF and Plex-V files after immersion for 15 minutes in 5.25% sodium hypochlorite and 17% EDTA was observed. However, the Plex-V file showed more resistance compared with the AF file.

Disclosures

1. Institutional review board statement: The study was approved by the Research Ethics Committee and Scien­tific Research and Postgraduate Board of Damascus University with approval No.: 2764, dated May 22, 2022.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References