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Table of Contents   
ORIGINAL ARTICLE  
Year : 2023  |  Volume : 26  |  Issue : 1  |  Page : 26-30
Effect of autoclave sterilization on the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-one nickel–titanium endodontic instruments


1 Alkarkh Health Directorate Baghdad, Ministry of Health, Baghdad, Iraq
2 Department of Conservative Dentistry, College of Dentistry, Mustansiriyah University, Baghdad, Iraq

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Date of Submission15-Dec-2021
Date of Decision23-Jan-2022
Date of Acceptance30-Jan-2022
Date of Web Publication17-Aug-2022
 

   Abstract 

Background: Rotary nickel-titanium files have become the most commonly used engine-powered endodontic instruments due to their improvement in fatigue resistance and flexibility. The autoclaving is the most commonly used method for sterilization. However, the risk of instrument fracture, mainly due to cyclic fatigue during rotation within a curved canal, is still a matter of critical concern.
Aims: This study aimed to measure and compares the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-One files in a single curved simulated canal before and after autoclave sterilization.
Materials and Methods: Twenty-four files each of EdgeFile X7, 2Shape, and F-One files were used in this study. They were grouped as group A: EdgeFile X7, group B: 2Shape, and group C: F-One files. Then, each group was subdivided into two, sterilized and nonsterilized, subgroups. The files were tested using a custom-made artificial canal. The number of cycles to fracture (NCF) was calculated.
Statistical Analysis Used: One-way analysis of variance, post hoc Tukey's test, and independent t-test were used.
Results: In nonsterilized instruments, the NCF of the EdgeFile X7 group was significantly higher than that in the other groups (P < 0.05). In sterilized instruments, there was a nonsignificant difference in NCF between EdgeFile X7 and F-One files (P > 0.05). The NCF of 2Shape was significantly lower than other files (P ≤ 0.05) in sterilized and nonsterilized groups. The autoclaving significantly decreased (P ≤ 0.05) the cyclic fatigue resistance of the tested files.
Conclusions: EdgeFile X7 was the most fatigue resistant. Autoclaving reduced the cyclic fatigue resistance of the tested files.

Keywords: 2Shape; autoclave sterilization; cyclic fatigue; EdgeFile X7; F-One

How to cite this article:
Al-Amidi AH, Al-Gharrawi HA. Effect of autoclave sterilization on the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-one nickel–titanium endodontic instruments. J Conserv Dent 2023;26:26-30

How to cite this URL:
Al-Amidi AH, Al-Gharrawi HA. Effect of autoclave sterilization on the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-one nickel–titanium endodontic instruments. J Conserv Dent [serial online] 2023 [cited 2023 Jan 27];26:26-30. Available from: https://www.jcd.org.in/text.asp?2023/26/1/26/353906

   Introduction Top


The use of nickel–titanium (NiTi) rotational endodontic files improved the predictability of chemomechanical cleaning and shaping; nonetheless, instrument separation is a possibility and has been mentioned as a complication in the endodontic literature. While contemporary NiTi instruments have excellent characteristics, they can nevertheless be broken owing to the cyclic (flexural) fatigue. Cyclic fatigue occurs when instruments are bent repeatedly in curved canals, causing stress and distortion inside the instruments and eventually, fracture as a result of complete tension-compression cycles.[1]

A meta-analysis and systematic review conducted in 2020 revealed that autoclaving of endodontic instruments was the most efficient approach for sterilizing them.[2] Sterilization of endodontic files before use is suggested by the vast majority of suppliers. Furthermore, a clinician may not use all the prearranged packages of files during a single appointment so that the unused new NiTi files may be subjected to many rounds of autoclave sterilization.[3] On the other hand, rotary files are usually reused many times for financial reasons that lead to their recurrent sterilization. The single use of files is advised but rarely practiced. For example, when it comes to endodontic files, just 5% of dentists in Germany claimed that they utilize them as single-use files.[4] Thus, the NiTi endodontic files, and to avoid cross-contamination, usually autoclaved before the use of a new file or before reuse of the used one.[3]

A variety of research looks at the sterilizing effects of autoclaves on the physical qualities of NiTi endodontic files, with varying results. Several studies have shown that using an autoclave repeatedly damages files and produces fractures.[2],[5] On the other hand, other investigations demonstrated that repeated sterilization cycles did not lead to the failure of NiTi files.[6] A study by Plotino et al. in 2012 found that repeated autoclaving cycles increased the number of cycles to fracture (NCF) of the K3XF file while its characteristics were unaffected.[7] In addition, Zhao et al. in 2016 found that K3XF and HyFlex CM had a longer cyclic fatigue life than Twisted Files, K3, and Race files after autoclave sterilization.[3] It appears that studies have shown varied outcomes in different NiTi file types.

To increase the fatigue resistance and hence avoid instrument fracture, manufacturers developed thermomechanical processing using NiTi alloys with different transformation temperatures.[8] A wide range of heat-treated NiTi rotary instruments is available in the market, each with its unique metallurgical attributes. The EdgeFile X7 (Edge Endo; Albuquerque, New Mexico, United States) is NiTi files produced by a FireWire process that combines heat treatment with cryogenic applications to create a unique crystalline matrix of the heat-treated FireWire alloy, reducing the effect of shape memory inherent in NiTi instruments while increasing flexibility and resistance to cyclic fatigue.[9] The 2Shape system (Micro Mega, Besancon, France) has been subjected to a proprietary thermomechanical technique known as heat-treated T-Wire technology.[10] The F-One file (Fanta Dental, Shanghai, China) introduces a novel heat treatment termed AF-R Wire in a single-file system. The instrument has two active cutting points and noncutting tip.[11] Each newly developed instrument should be investigated for cyclic fatigue resistance as one of the essential properties of the NiTi files. There are no published studies in the literature that compared the effect of autoclave sterilization on the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-One NiTi instruments.

This study aims to measure and compare the cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-One files in a single-curved simulated canal before and after autoclave sterilization. The null hypothesis was that no significant difference in cyclic fatigue resistance among the three types of endodontic instruments and autoclave sterilization does not affect their fatigue resistance.


   Materials And Methods Top


Seventy-two NiTi rotary files of the same size (tip diameter 0.25 mm and constant taper 0.04) were used in this study. These files were distributed into three groups (n = 24 for each), group A: EdgeFile X7; group B: 2Shape; and group C: F-One files. Then, each group was subdivided into two, sterilized and nonsterilized, subgroups (n = 12 for each).

For the sterilized instruments, three sterilization cycles were used using an autoclave (Stern Weber 22 Plus; Italy). Each cycle lasted 35 min at a temperature of 134°C.[12] A custom-made artificial canal used for cyclic fatigue testing had an angle of curvature of 60° with a radius of 5 mm, a curvature center at 5 mm from the canal end, and the length of the curve of 5.25 mm.[13] Stainless steel was used to mill the artificial canal. It had a working length of 18 mm and an inner diameter of 1.5 mm[14] [Figure 1].
Figure 1: Cyclic fatigue testing device

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Stereomicroscope (CARL COLB, Germany) was used to look for any deformation in all of the instruments for exclusion. For an operating instrument, it was determined that the speed and torque of the electric motor (X-Smart Plus, Dentsply Maillefer, Switzerland) should be used following the manufacturer's instructions. The electric motor's handpiece was secured to a surveyor to keep the handpiece in its position during operation. It allows for the perfect placement of each rotary file in the artificial canal and ensures optimum orientation and placement of the files at the same working length[15] [Figure 2]. The files were rotated until separation was observed. A fracture was discovered, and the time spent from starting file rotation to file separation was recorded in minutes. Video recording was employed for cross-checking the timing of file separation to prevent human error and standardize the investigation.[16] When the recorded time was multiplied by the motor speed, it was possible to calculate the NCF of each tested instrument. Each file's broken fragment length (FL) was measured with a Vernier.
Figure 2: File position inside the block

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Various statistical tests, including the Shapiro–Wilk test, one-way analysis of variance (ANOVA), post hoc Tukey's test, and independent t-test, were used to analyze the NCF and FL data. Software of SPSS version 26 (IBM-SPSS Inc., Chicago, LI, USA) had been used for statistical analyses at a significance level of 0.05.


   Results Top


Shapiro–Wilk test revealed that the data were normally distributed. The mean and standard deviation of the NCF for each file were calculated [Table 1]. The ANOVA test showed a significant difference (P < 0.05) in cyclic fatigue resistance among the three groups of investigated instruments. In nonsterilized instruments, the post hoc Tukey's test showed that the NCF of the EdgeFile X7 group was significantly greater than that of other groups (P < 0.05). Sterilized instruments showed that the NCF of EdgeFile X7 and F-One files was significantly greater than the 2Shape instrument (P < 0.05). The NCF of EdgeFile X7 and F-One files was not significantly different (P > 0.05). Independent t-test demonstrated that the NCF mean significantly lower in the sterilized instruments than in the nonsterilized instruments in all examined groups (P < 0.05).
Table 1: The mean and standard deviation for the number of cycles to fracture of the sterilized and nonsterilized instruments

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   Discussion Top


During instrumentation and when the rotary file was used in curved root canals, cyclic fatigue was one of the leading causes of fracture due to repeated cycles of tensions and compressions at the point of maximum curvature. Changing the design, metallurgy, and kinematics of the files and heat treating them are some of the methods the manufacturers plan to use to increase the resistance to cyclic fatigue of endodontic files.[17] Although the extracted tooth model more strictly resembled the clinical condition and represents the best model to assess the cyclic fatigue resistance, such tests would be destructive for the sample (the tooth). Furthermore, no two root canals were entirely identical, and it is impracticable to regulate the amount and point of stress applied on the rotating instrument because of variations in canal curvature among the extracted teeth. Moreover, it is impossible to standardize experimental conditions.[18]

There are two models of stainless-steel block: dynamic and static models. In a dynamic model, stress is distributed along the file shaft during the insertion and removal in the vertical direction. In contrast, stress will be concentrated on a specific part of the file in a static model.[19] Although a dynamic model may replicate the clinical situations, it was not easy to control precisely the pecking motion practically.[20] Dynamic fatigue tests use more oscillations in the instruments during root canal preparation. The only additional information a dynamic model can supply is the real-time an instrument spends working in a canal under the conditions examined,[21] so a static model was used in this study. In this investigation, the cyclic fatigue resistance has been tested after three cycles[12] of autoclaving to simulate situations practiced by some clinicians when they reuse instruments for economic reasons, although these instruments are recommended for single use. Other investigators[7] tested instruments after ten cycles; in their study, they investigated instruments recommended to be used more than once. According to the manufacturer's recommendations, EdgeFile X7 and F-One files rotated at 500 revolutions per minute (rpm) while 2Shape instruments rotated at 400 rpm. The torque was set at 2.5 N/cm for all instruments.

The cyclic fatigue resistance of EdgeFile X7, 2Shape, and F-One files was examined before and after autoclave sterilization. For the nonsterilized instruments, the EdgeFile X7 displayed a higher value of NCF than other evaluated instruments. This could be attributed to its manufacturing procedure by a proprietary process called FireWire[9] compared to F-One files manufactured with a new heat treatment called AF-R Wire.[11] In comparison, the 2Shape instruments were fabricated using heat-treated T-Wire technology.[10] Up to date, the specifics of the metallurgy of the EdgeFile X7 and F-One files remain, as yet, unpublished, and therefore, our assumptions remain unverified.

The EdgeFile X7 has a triangular cross-section and variable helix angle that provides sufficient strength with high flexibility to increase the flexural fatigue resistance.[9] The cross-section may contribute to the increased resistance to cyclic fatigue of EdgeFile X7 compared to F-One files manufactured with flat designed and S-shaped cross-sections.[11] On the other hand, a triple helix of two primary cutting edges and one secondary cutting edge is used to manufacture 2Shape instruments with an asymmetrical cross-section.[10] Zhang et al. indicated that the cross-sectional design of the instrument had a more significant impact on the development of stress than its size or taper.[22]

For the sterilized instruments, the NCF of EdgeFile X7 and F-One files was higher than 2Shape files. This may be attributed to the specialized heat treatment and the cross-section design of these endodontic rotary instruments. In comparison, there was no difference in resistance to cyclic fatigue between EdgeFile X7 and F-One rotary files, although the difference in cyclic fatigue resistance between these files was significant before the sterilization. This may be related to the type of metallurgy that resulted in less effect of autoclaving on the F-One file.

The sterilized instruments showed a lower cyclic fatigue resistance than that of the nonsterilized instruments. This indicated that autoclaving causes a significant reduction in the cyclic fatigue resistance since the high temperature and the pressure of autoclaving might cause a change in the metallurgical properties, affecting the integrity and flexibility of the file. This could be attributed to the transition of the alloy from the martensite phase to the stiffer austenite phase, which was more susceptible to fatigue crack propagation.[23] Furthermore, autoclaving cycles increased the depth of irregularities on the file surface, which act as areas for stress application and crack origination. This would make the instruments more prone to fatigue crack propagation with lower cutting efficiency and decreased resistance to instrument wear. The crack length increased as the cyclic fatigue test progressed.[5]

The results of this study agreed with different studies that showed a similar effect of autoclave sterilization that causes a reduction in the cyclic fatigue resistance of many NiTi files.[2],[5] On the other hand, the results from this study were in contrast to the result of another recently published article in which the EdgeFile X7 demonstrated a higher cyclic fatigue resistance after autoclave sterilization. This may be related to the difference in methodology since the mentioned study investigated the cyclic fatigue resistance of size 0.30/0.4 files after ten cycles of autoclave sterilization compared to this study that examined the cyclic fatigue resistance of size 0.25/0.4 files after three sterilization cycles.[24]


   Conclusions Top


EdgeFile X7 is more resistant to cyclic fatigue than 2Shape and F-One files. Autoclave sterilization of the tested files reduced their cyclic fatigue resistance. Single use of these files is recommended to reduce the risk of separation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

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Correspondence Address:
Dr. Arkan H Al-Amidi
Alkarkh Health Directorate Baghdad, Ministry of Health, Baghdad
Iraq
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcd.jcd_611_21

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