|Year : 2022 | Volume
| Issue : 3 | Page : 151-155
Comparative evaluation of cyclic fatigue resistance of file systems based on different manufacturing technologies after exposure to NaOCl and multiple sterilization cycles
Pulkit Malhotra1, Parul Bansal2, Vineeta Nikhil3, Shikha Jaiswal3, Sachin Gupta3
1 Private Practitioner, Institute of Dental Studies and Technologies, Modinagar, Uttar Pradesh, India
2 Department of Conservative and Endodontics, Institute of Dental Studies and Technologies, Modinagar, Uttar Pradesh, India
3 Department of Conservative Dentistry and Endodontics, Subharti Dental College, Meerut, Uttar Pradesh, India
|Date of Submission||30-Dec-2019|
|Date of Decision||20-Dec-2021|
|Date of Acceptance||30-Jan-2022|
|Date of Web Publication||30-Sep-2022|
Dr. Parul Bansal
Department of Conservative Dentistry and Endodontics, DJ College of Dental Science and Research, Modinagar, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Aim: To comparatively evaluate in vitro the cyclic fatigue resistance of different file systems based on different manufacturing technologies after exposure to NaOCl and multiple sterilization cycles.
Materials and Methods: Sixty new Nickel − Titanium (NiTi) rotary files were selected and divided into five groups (n = 12) based on different manufacturing technology. These groups were: Protaper Universal (GPT) (DentsplyMaillefer, Ballaigues, Switzerland) Protaper Next (GPTN) (Dentsply, Tulsa, OK, USA), Twisted files (GTF) (Sybron Endo, Orange, CA, USA), Neoniti (GNL) (Neolix, France), and Hyflex CM (GHCM) (Coltene/whaledent inc., 235 Ascort Parkway, Cuyahoga falls, OH, USA). Files were sterilized in an autoclave at 121°C at 15 psi for 15 min and exposed to 5.0% NaOCl solution for 30 s before cyclic fatigue testing till instrument fracture using a standardized grooved block assembly with the artificial canal at 90° angle of curvature using a 16:1 reduction hand-piece powered by a torque-controlled motor. This whole procedure of autoclaving, immersion in sodium hypochlorite solution and rotating in a stainless steel assembly was repeated again and again until the file fractured.
Results: The highest mean for the number of cycles before fracture was observed with Neoniti (GNL) and lowest for Hyflex CM (GHCM). The difference between GPT vs GTF, GPTvs GNL, GNLvs GHCM; GPTNvs GTF, GPTNvs GNL, GPTNvs GHCM; GTFvs GNL, GTFvs GHCM and GNL VS GHCM was statistically significant, whereas the difference between GPTvs GPTN was statistically nonsignificant.
Conclusion: Neoniti and twisted file systems resisted the maximum number of cycles to fracture at 90° angle of curvature after exposure to 5% sodium hypochlorite solution and multiple autoclaving cycles. Hyflex CM showed the least resistance in the similar conditions.
Keywords: Cyclic fatigue resistance, manufacturing technologies, multiple autoclaving, NaOCl, rotary Nickel − Titanium
|How to cite this article:|
Malhotra P, Bansal P, Nikhil V, Jaiswal S, Gupta S. Comparative evaluation of cyclic fatigue resistance of file systems based on different manufacturing technologies after exposure to NaOCl and multiple sterilization cycles. Endodontology 2022;34:151-5
|How to cite this URL:|
Malhotra P, Bansal P, Nikhil V, Jaiswal S, Gupta S. Comparative evaluation of cyclic fatigue resistance of file systems based on different manufacturing technologies after exposure to NaOCl and multiple sterilization cycles. Endodontology [serial online] 2022 [cited 2022 Nov 26];34:151-5. Available from: https://www.endodontologyonweb.org/text.asp?2022/34/3/151/357706
| Introduction|| |
Despite the outstanding properties of Nickel − Titanium (NiTi) instruments, unexpected fracture of instruments occurs during clinical use without any previous signs of permanent deformation. Sattapan et al. classified the separation of NiTi rotary instruments into “torsional failure” and “flexural fatigue.” Torsional fracture occurs when an instrument tip or another part of the instrument is locked in the canal while the shank continues to rotate, whereas flexural fatigue occurs due to repeated compression and tension at the point of maximum flexure.
Various attempts have been made by different manufacturers to overcome instrument separation by not only improving the instrument design but also by improving the mechanical properties of NiTi instruments. Conventional superelastic (SE) wire tends to get fractured. Hence, various systems based on R-phase, M-wire technology, CM technology, EDM technology, etc., have been introduced to improve the properties. File systems based on different manufacturing technologies have different fatigue resistance to separation.
Although these rotary NiTi files based on improved technology are supposed to be single use as suggested by the manufacturers, owing to their high cost and increased longevity due to their improved properties, they are used multiple times after sterilization, especially, in developing countries where finances are limited and cost is a major factor.
For asepsis and to prevent cross-infection, the use of sterilized file is mandatory. Various methods have been used for sterilization. However, autoclaving is the most advocated and commonly used method of sterilization. NaOCl is the most commonly used irrigant and may cause corrosion of endodontic files which may ultimately affect the cyclic fatigue resistance.
In such a scenario, it is very important to be aware of the effect caused by NaOCl and multiple sterilization cycles on the alteration in the properties of these files. Although various studies have shown the effect of multiple sterilization cycles on cyclic fatigue of rotary NiTi files, none of the studies has comparatively evaluated the effect of multiple sterilization cycles and exposure to NaOCl on cyclic fatigue of rotary NiTi files based on different manufacturing technology such as conventional super-elastic M-wire, controlled memory, R-phase and EDM to the best of our knowledge.
Hence, the present study was conducted to comparatively evaluate the effect of multiple sterilization cycles and exposure to 5.0% NaOCl on the cyclic fatigue resistance of rotary NiTi files based on different manufacturing technologies.
| Materials and Methods|| |
Sixty new NiTi rotary files were selected and divided into five groups (n = 12) based on different manufacturing technology.
These groups were:
- Rotary Protaper Universal NiTi (DentsplyMaillefer, Ballaigues, Switzerland) conventional superelastic wire-based instruments
- Rotary Protaper Next NiTi files (Dentsply Tulsa Dental Specialities, Tulsa, OK, USA) M-wire technology-based instruments
- Rotary Twisted NiTi files (Sybron Endo, Orange, CA, USA) R-phase technology based instruments
- Rotary Hyflex CM NiTi (ColteneWhaledent, Cuyahoga Falls, Ohio) controlled memory technology-based instruments
- Rotary Neoniti NiTi files (NEOLIX SAS, Chatres-La-Foret, FranceEDM technology-based instruments.
All instruments with ISO tip size of no. 25, 6% taper (except Neoniti with 8% taper) and instruments length 25 mm (except twisted files available in 23 mm) were selected. All the files were examined for any surface defect under stereomicroscope at ×20 magnification (Olympus, Shinjuku, Tokyo, Japan).
Each file was immersed and rotated in 5.0% sodium hypochlorite solution (Dentpro, Mohali, India) using an endomotor (NeolixEndoman-E, Orikam, Korea) in a beaker till 18 mm of instrument length for 30 s, thereby simulating each canal of a molar having three canals being irrigated with sodium hypochlorite for 10 s each. Each file was then tested for instrument fracture using a standardized grooved block assembly and 16:1 reduction hand-piece powered by a torque-controlled endomotor.
The groove block assembly comprised of a cold worked stainless steel sheet. This sheet acted as a base on which screw-retained plastic attachments were made to hold the endomotor in a fixed position, thereby eliminating the tension on the hand piece by the operator during the test. Another stainless steel block with a groove simulating an artificial canal at 90° of angle of curvature was constructed and retained in a fixed position with the help of screws. A tempered glass was then covered on this grooved steel block to prevent the instruments from slipping out and to allow for the observation of the instrument [Figure 1].
The torque controlled gear reduction endodontic hand piece was then mounted into the plastic attachments of this assembly. Thereafter, each file was inserted in the hand piece and after accurate positioning to 18 mm, each file was allowed to rotate for 10 s simulating an average time required for a rotary NiTi file for cleaning and shaping of a curved canal in a molar.
The speed was set for each group of file according to manufacturer's recommendation and a torque of 1.8 N. cm was set. The files were then rinsed with distilled water and a long handle brush to remove the debris accumulated on it. The files were allowed to air dry. The files were then placed in a sterilization pouch (Crosstex International, USA) with a color changing indicator depicting complete sterilization and placed in an autoclave at 121°C at 15 psi for 15 min.
Autoclaved files were then removed and again immersed in 5.0% sodium hypochlorite solution for 30 s as done before. They were again mounted on the handpiece with similar torque and speed setting and allowed to rotate for 10 s again. The files were then removed from the handpiece and autoclaved again.
This whole procedure of autoclaving, immersion in sodium hypochlorite solution and rotating in a stainless steel assembly was repeated again and again until the file fractured simulating clinical condition for a molar tooth with a curved canal. Fracture of the file was easily detectable because the instruments were visible through the glass window. The time to fracture for each file was visually recorded with a stopwatch to the nearest whole second, and the number of cycles to fracture (NCF) was calculated by the following formula:
No. of cycles to fracture
The values so obtained were tabulated and then statistically analysed.
The NCF data obtained were expressed as mean and standard deviation. Descriptive statistics were calculated using unpaired t-test and SPSS (IBM, SSP Version 21, India) software version 21.0 to find the significant difference between the different groups.
The level of significance was chosen as P ≤ 0.05.
| Results|| |
The highest mean number of cycles before fracture was observed with Neoniti (GNL) and lowest mean number of cycles before fracture was observed with Hyflex CM (GHCM).
The various values of mean for cyclic fatigue for cyclic fatigue in different groups were observed and tabulated in [Table 1]:
|Table 1: Mean of number of cycles to fracture, standard deviation, standard error, t and P for different groups of rotary Nickel Titanium instruments in presence of repeated autoclaving cycles and 5.0% sodium hypochlorite (1.8 N.cm torque and manufacturer recommended speed)|
Click here to view
From the above table, it was inferred that:
No significant difference exists between GPTvs GPTN.
| Discussion|| |
Despite the great development in the design and technology of NiTi instruments, failure of instruments during the root canal treatment is still a great concern, as these instruments can undergo fracture without any visible sign of previous permanent deformation. Even after the development of new generations of endodontic instruments based on different manufacturing technologies, fracture of instruments may occur.
For infection control every file system need to be sterilized before next use, also instruments are exposed to commonly used irrigants such sd NaOCl during function as the canals are filled with the irrigant at the time of instrumentation. Although various studies have been done to compare cyclic fatigue resistance of various file systems, autoclaving and exposure of file system during use may affect the properties of different file systems differently. Hence, in the present study, various file systems based on different manufacturing technology have been compared for cyclic fatigue resistance after exposure to NaOCl and multiple sterilization cycles.
Sterilization may increase, decrease, or may have no effect on cyclic fatigue resistance of a file systems developed from a series of thermomechanical processing. In our study, Protaper universal was taken as the control group as it is madeup of conventional NiTi alloy. Hayashi et al. have shown that additional heat treatment due to autoclaving of NiTi rotary instruments may increase the flexibility of NiTi rotary instruments. Plotino et al. suggested that autoclave sterilization might improve the mechanical properties of instruments (K3XF, Sybron Endo). A study by Alapati et al. suggested that repeated autoclaving of NiTi instruments negatively affected the mechanical properties and leads to premature instrument failure of Mtwo and Protaper NiTi instruments. Hilfer et al. found that repeated autoclaving significantly reduced the mean NCF of 25/.06 Twisted files. Viana et al. have shown that multiple autoclaving increases the depth of NiTi file surface irregularities, causing fatigue propagation. Canalda-Sahli et al. showed that flexibility of NiTi files greatly decreased after autoclave sterilization. In the present study, autoclaving affected file systems based on different manufacturing technologies in different ways.
The use of NaOCl as an irrigant is currently the gold standard to achieve tissue dissolution and disinfection. However, it is highly corrosive and can cause corrosion of endodontic files., This might ultimately reduce the cyclic fatigue. Accordingly to a study by Berutti et al., NiTi instruments immersed for 5 min in 5% NaOCl at 50°C had significantly reduced resistance to cyclic fatigue. In our study, immersion time was 30 s considering 10 s in each canal and 3 canals in a molar tooth.
Different researchers have suggested different methods for testing cyclic fatigue resistance. Although differences in testing devices used for the evaluation can lead to different results, but so far there has been no device or method available for fatigue testing incorporated into international standards for endodontic instruments.
Although the simulated canal does not duplicate the in vivo situation, to avoid bias and for better control of variables, in vitro study was done using the grooved block assembly at 90° angle of curvature as abrupt apical curvatures may be encountered clinically, and they may not be visible on the radiograph. Although in various studies, cyclic fatigue has been performed at 30°, 45°, 60°, and 90° angles of curvature which may affect the results. A study by Cheng et al. concluded that Protaper Next files tended to be more resistant than Protaper Universal files when operated in 45°and 60° canals, although this finding did not hold for operation in 90° canals.
Different file systems work best at different speed recommended by the manufacturer. Hence, files were rotated at manufacturer recommended speed and at low torque of 1.8 N. cm to eliminate fracture due to torsional failure.
The results of the present study revealed that Neoniti instruments resisted fatigue fracture for the longest time demonstrating that Neoniti exhibited highest cyclic fatigue resistance than twisted file, Protaper Next, Protaper Universal, and Hyflex CM.
The order of cyclic fatigue resistance in this study was: GNL > GTF > GPTN > GPT > GHCM. In accordance to the present study, Aminsobhani et al. evaluated the resistance to fatigue failure of five different NiTi rotary files in three different curved trajectories and observed that Neoniti exhibited the highest fatigue resistance compared to RaCe, Twisted file, Protaper Next and Mtwo files and showed significant difference. Neoniti file (Neolix, France) is a unique newly introduced rotary file system. Better performance of Neoniti may be related to EDM technology, martensitic phase, and also the absence of grinding procedure for manufacturing it.
Twisted files, second most resistant file to cyclic fatigue is produced by a special heat treatment followed by twisting rather grinding process. In accordance to our study, Gambarini et al. found twisted files to be significantly more resistant to fatigue than instruments produced with the traditional NiTi grinding process.
Traditional grinding of the NiTi wire limits the overall strength of the instrument, due to formation of micro-cracks and defects along the surface of the instruments and within the internal structure. These defects can cause stress concentration points that weaken the instruments. A study by Pongione et al. showed that instruments made from traditional grinding process (Endosequence) exhibited less cyclic fatigue resistance than Hyflex CM and Profile Vortex.
Thus, Neoniti and Twisted file in the present study had an advantage over other files as the crack propagation is limited in these due to the elimination of the grinding process.
Protaper Next based on M wire technology performed better compared to Protaper universal, although the results were not significant (P ≥ 0.05). This is in agreement with the study conducted by Gambarini et al. who also concluded that instruments made of M-wire alloy did not show significantly higher resistance to fatigue when compared to instruments produced by the traditional NiTi.
Manufacturers claim that M-wire and CM wire technology-based instruments have cyclic fatigue 300%−400% higher than conventional SE wire-based instruments. The present study revealed that M-wire technology-based instrument (Protaper Next) had greater cyclic fatigue resistance as compared to conventional SE-based instrument (Protaper), although the results were not statistically significant (P ≥ 0.05). However, in contrast to many studies, the results of the present study for Hyflex CM instruments based on CM wire technology showed the least cyclic fatigue resistance among all other groups. Although most of cyclic fatigue experiments have been conducted at 45° and 60° curvatures, in this study, we have evaluated the cyclic fatigue in severe abrupt curvatures of 90° was evaluated. In our study, ProTaper Universal showed low mean NCF than all other files except Hyflex CM. This can be attributed to the traditional grinding process.
Within the limitations of this study, these findings do not support the superior cyclic fatigue reports of Hyflex CM rotary NiTi files discussed in earlier studies. This study showed that Hyflex CM has poor cyclic fatigue resistance in 90° curvature compared to other files beside its thermo-mechanical processing and controlled memory.
| Conclusion|| |
Within the limitation of the present study, Neoniti file exhibited significantly high cyclic fatigue resistance than twisted file, Protaper Next, Protaper Universal and Hyflex CM. On comparison, Protaper Universal and Protaper Next groups did not show any significant difference in cyclic fatigue resistance. Hyflex CM exhibited least resistance to cyclic fatigue among all tested groups.
However, the results of the present study are bound to be variable under different circumstances and testing methods. Therefore, further research is needed to further substantiate the findings of the present study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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