|Year : 2022 | Volume
| Issue : 1 | Page : 13-16
Comparison of cyclic fatigue resistance of three different rotary NiTi systems – An in-vitro study
Deepika Geeth1, Subbiya Arunajatesan2, Rathakrishnan Mensudar2, Yamini Rajachandrasekaran3, Jonathan Vasantharajan4
1 Department of Conservative Dentistry and Endodontics, CSI College of Dental Sciences and Research, Chennai, Tamil Nadu, India
2 Department of Conservative Dentistry and Endodontics, Sree Balaji Dental College and Hospital, Chennai, Tamil Nadu, India
3 Department of Periodontics and Implantology, Sathyabama Dental College, Chennai, Tamil Nadu, India
4 Private Practitioner, Pedodontist, Madurai, Tamil Nadu, India
|Date of Submission||28-Jun-2021|
|Date of Decision||23-Jan-2022|
|Date of Acceptance||26-Jan-2022|
|Date of Web Publication||14-Mar-2022|
Dr. Deepika Geeth
714-C, Lilly Vila, Anna Nagar, Batlagundu, Madurai - 624 002, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Background: Nickel–titanium (NiTi) alloys created a revolution in the instrumentation of endodontics. However, they undergo unexpected fracture during the instrumentation of curved root canals due to cyclic fatigue. Hence, innumerable scientific and manufacturing researches are focused on fabricating NiTi rotary instruments with greater cutting efficiency and increased fatigue resistance. Aim: The aim of this study is to compare and evaluate the cyclic fatigue resistance of three different NiTi rotary file systems: Hyflex CM, Endostar E3, and Mani Silk. Materials and Methods: In this in-vitro study, 30 Hyflex CM (25/0.06), 30 Mani Silk (25/0.06), and 30 Endostar E3 (25/0.06) instruments were included. The static cyclic fatigue test was performed using a custom-made jig. The artificial canal was made of stainless-steel jig with an inner diameter of 1.5 mm, 45° angle of curvature, and radii of curvature of 5 mm. All the instruments were rotated according to the manufacturer's recommendations until failure occurred. The time taken to failure was recorded in seconds for each group using a digital chronometer. The data were analyzed statistically using the one-way analysis of variance, Post hoc test of Bonferroni was performed to identify pair-wise significance through SPSS 21.0 software (SPSS Inc, Chicago, IL, USA). The statistical significance was set at 5%. Results: Hyflex CM had the statistically highest fatigue resistance followed by Mani Silk and Endostar E3 (P < 0.05). Conclusion: Within the limitations of the present in vitro study, it was found that cyclic fatigue resistance of Hyflex CM files was higher than the cyclic fatigue resistance of Mani Silk and Endostar E3 files.
Keywords: Cutting efficiency, cyclic fatigue, NiTi rotary systems, root canal instrumentation
|How to cite this article:|
Geeth D, Arunajatesan S, Mensudar R, Rajachandrasekaran Y, Vasantharajan J. Comparison of cyclic fatigue resistance of three different rotary NiTi systems – An in-vitro study. SRM J Res Dent Sci 2022;13:13-6
|How to cite this URL:|
Geeth D, Arunajatesan S, Mensudar R, Rajachandrasekaran Y, Vasantharajan J. Comparison of cyclic fatigue resistance of three different rotary NiTi systems – An in-vitro study. SRM J Res Dent Sci [serial online] 2022 [cited 2022 Dec 5];13:13-6. Available from: https://www.srmjrds.in/text.asp?2022/13/1/13/339636
| Introduction|| |
A new era began in endodontics with the establishment of Nickel–itanium (NiTi) alloys and the successive industrialization of biomechanical preparation (hand to rotary instruments) of the root canal system. Despite its diverse advantages, NiTi rotary instruments undergo unexpected failure due to cyclic fatigue, especially in curved root canals. Over the past many years, NiTi alloys were industrialized through traditional machining with few transitions with regard to its production. The alloys machined through this process exist in a superelastic austenitic phase on stress, it transforms to an unstable martensite which has an excellent resistance to cyclic fatigue but this phase is reversible. Hence, innumerable scientific and manufacturing researches were focused on fabricating NiTi rotary instruments with greater cutting efficiency and increased fatigue resistance. In recent years, thermomechanical processing and surface treatment of NiTi instruments were found to enhance the microstructure and phase transformation behavior between martensite and austenite which endure the files to be in the martensite phase for a long time, thereby increasing the fatigue resistance.
Hyflex CM (Coltene Whaledent, Cuyahoga Falls, OH) is manufactured by a patented heat treatment (CM wire) which improves the cyclic fatigue resistance of instruments. It utilizes a special process that controls the shape memory which makes the files extremely flexible without rebound effect, thereby providing superior canal tracking.
Endostar E3 instruments (Poldent Co., Warsaw, Poland) are manufactured by conventional machining with S-shaped cross-sectional design having greater efficiency to cut dentin and its noncutting tip are claimed by the manufacturer to reduce the risk of perforation and ledging.
Mani Silk instruments (Mani Prime Dental products, Japan) are electropolished and are proprietary heat treated at the apical end which improves the flexibility and fatigue resistance.
Although the development of new NiTi rotary instruments is a fast production process with new versions of files rapidly emerging in the market, the clinician may find it difficult to choose the correct instrumentation system. Consequently, the probability of separation of NiTi rotary instruments inside the root canal has increased dramatically during instrumentation. The aim of this study was to evaluate and compare the cyclic fatigue resistance of Hyflex CM NiTi rotary files with the recently marketed Endostar E3 and Mani Silk NiTi rotary files. The null hypothesis tested was that there are no differences in the cyclic fatigue resistance between the tested three NiTi rotary systems.
| Materials and Methods|| |
A total of new 90 rotary Hyflex CM, Endostar E3, and Mani Silk (n = 30/system), size 25/.06, are randomly divided into three groups. The sample size (n = 90) was calculated using G * power statistics. As there were no human participants, ethical approval was not required for this study. All the instruments were inspected under an optical stereomicroscope with ×20 magnification. None of the instruments that were subjected to test was devoid of any manufacturing defects.
A cyclic fatigue testing device was milled in stainless steel block and custom fabricated for this study. The artificial canals were constructed with a 45° angle of curvature having a total length of 20 mm with an inner diameter of 1.5 mm and the center of the curvature was located with a radius of 5 mm from the tip of the file. The dental handpiece (16: 1 reduction handpiece) was attached to the descending crosshead of the Instron (8874) testing machine (Boston, US). A V-shaped notch was constructed for guiding the file [Figure 1].
|Figure 1: (a) Lateral view of customized artificial canal fabricated from stainless steel block (b) Occlusal view (c) Tested files in position inside the canal (d) Cyclic fatigue testing device (e) Fractured segments of files due to cyclic failure|
Click here to view
The files were rotated (X-Smart; Dentsply Sirona, York, PA, USA) according to the manufacturer's recommended speed and torque for each system as follows: Hyflex CM files at 500 rpm/2.5 Ncm, Endostar E3 files at 250 rpm/2.5Ncm, and Mani Silk files at 500 rpm/3Ncm. The time to fracture was recorded and stopped as soon as the fracture of the instrument was visually detected. The number of cycles to failure was calculated by the following formula,
NCF = Revolutions per minute × Time to failure (seconds)/60
| Results|| |
The mean, standard deviation, and standard error of the time to fracture were calculated using SPSS software 21.0 version. The one-way analysis of variance was used to study the significant differences among the NiTi rotary systems. The mean number of cycles to failure for Hyflex CM, Mani Silk, and Endostar E3 are 1165.06, 626.29, and 506.14, respectively [Table 1]. Hyflex CM files have the highest fatigue resistance than Mani Silk and Endostar E3 file groups (P < 0.05). Post-hoc test of Bonferroni (t-test) was performed to analyze the pair-wise significance of each pair of groups. In the intergroup comparison, statistics between three files groups revealed that Hyflex CM had greater fatigue resistance than Mani Silk and Endostar E3 (P < 0.05). In comparison of Mani Silk and Endostar E3 files, the results showed that Mani Silk files performed better than Endostar E3 files [Table 2]. The Endostar E3 file group showed the least fatigue resistance among the three file groups (P < 0.05).
|Table 1: Mean and standard deviation of time taken to fracture of instruments|
Click here to view
| Discussion|| |
This study was conducted to compare the cyclic fatigue resistance of three different NiTi rotary systems Hyflex CM, Mani Silk, and Endostar E3. The results showed that Hyflex CM rotary files exhibited greater fatigue resistance than Mani Silk and Endostar E3 files; hence, the null hypothesis was rejected. Studies on cyclic fatigue failure of rotary NiTi instruments are performed either in a static or dynamic mode. Among these two modes/models, dynamic mode or the pecking motion/movements proved to prevent the breakage of files inside a curved canal due to its equal distribution of stress onto the walls than the files tested in a static mode where there is greater stress concentration at a single point. In spite of testing the files in dynamic mode, in this study, the rotary files were tested in a static mode to overcome the confounding variables such as subjectivity of pecking movements (speed, amplitude, and axial movement) which greatly varies among the operators in clinical practice. Moreover, the skill to restrict the files in a predetermined path is also found to be perplexed in dynamic testing.
Numerous innovations and in-vitro studies are being conducted to rule out the drawback of NiTi rotary systems, i.e., cyclic fatigue. This study tested and compared the fatigue resistance of Mani Silk and Endostar E3 NiTi files as there no studies with regard to these instrumentation systems to date. These files were tested for cyclic failure in a custom-fabricated artificial stainless-steel canal with a 45°C angle and 5 mm radius of curvature. All three instrumentation systems were standardized and rotated at different speeds according to the manufacturer's instructions. Upon statistical analysis the results showed that the Hyflex CM files exhibited greater resistance than the other two tested NiTi rotary files, Mani Silk and Endostar E3. The results were in accordance to the previous study by Koçak et al. where Hyflex CM showed increased cycles to failure than the other tested NiTi rotary files. The reason for the increased resistance to failure of Hyflex CM files is attributed to its low percentage of Ni (nickel) content, stable martensitic phase structure, greater austenitic finish temperature (Af), and its proprietary process during manufacturing.
Endostar E3 files showed that the least cyclic fatigue resistance in this study as these files was manufactured from conventional NiTi alloy which has a stable austenite phase making the files less flexible and more prone to fracture inside the curved canal. In addition to it, the S-shaped cross-section design with two cutting edges has increased efficiency to cut dentin but it is more prone to fracture due to greater stress concentration, whereas Hyflex CM files due to crack propagation mechanism a large number of branched cracks propagates faster at a slower rate compared to the conventional NiTi alloy.
Mani Silk files showed increased resistance to fracture next to Hyflex CM files. The probable reason for this could be that the files have undergone thermomechanical treatment (i.e., heat treated) as well as surface treatment (i.e., electropolishing) which makes the files to be in the martensitic phase increasing the flexibility of files, but the drawback of Mani Silk file is that the alterations are concentrated only at the apical region of the file, thereby an uneven stress concentration is created inside the curved canal leading to cyclic failure.
The other variable that could have increased the fatigue resistance is its variations in cross-sectional design. Hyflex CM files have symmetrical triangular cross-section with reduced metal mass and do not generate stress on the walls of the dentin, thereby reducing the risk of fracture. Mani Silk files had a unique teardrop cross-section which reduces the screw-in effect within the walls of the dentin improving the fatigue resistance of files. Although both Hyflex CM and Mani Silk were heat treated, there was statistically difference between the files. The reason could be the undisclosed property of its manufacturing. Although we had tested the recently developed heat-treated NiTi rotary files which undergo phase transformation when subjected to enthalpy changes, this study failed to replicate the oral environment. Hence, further in-vivo studies are necessary to investigate and validate the outcomes of this study.
| Conclusion|| |
This study aid in observing the differences in the fatigue resistance of Hyflex CM, Endostar E3, and Mani Silk rotary NiTi files which helps the clinician to thoroughly understand the mechanical properties and its clinical performance of the files. There is no study in the literature to date which tested the fatigue resistance of Mani Silk and Endostar E3 files marketed during the same era. In this study, Hyflex CM instruments exhibited greater fatigue resistance than the newly marketed Mani Silk and Endostar E3 instruments. Thus, the fatigue resistance of heat-treated NiTi alloy was superior to conventional NiTi alloy.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]