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ORIGINAL ARTICLE
Year : 2019  |  Volume : 9  |  Issue : 3  |  Page : 186-191

Cyclic fatigue comparison of different manufactured endodontic files


1 Ministry of Health, Jeddah Specialty Dental Center, Jeddah, KSA
2 Department of Endodontic, Faculty of Dentistry, Umm Al-Qura University, Mecca, KSA
3 Department of Endodontic, Faculty of Dentistry, King Abdulaziz University, Jeddah, KSA
4 King Abdullah University for Science and Technology Health, Fakeeh Care, Jeddah, KSA

Date of Web Publication16-Aug-2019

Correspondence Address:
Dr. Khaled Ali Bukhari
North Obhur, P.O Box 23817-8678, Jeddah
KSA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_137_18

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  Abstract 

Aim: The aim of this study was to compare the cyclic fatigue resistance of endodontic files of different manufacturers operated in disparate motions.
Materials and Methods: Six different brands of nickel–titanium instruments, such as ProFile Vortex™, Vortex Blue™, Twisted File™, HyFlex™, WaveOne™, and S1™ all with a tip size ISO 25 with 0.06 taper, except for WaveOne™ tip size ISO 25 with 0.08 taper, were included in the study. Six groups of 20 rotary files from each system were tested for cyclic fatigue resistance. All files were rotated in a simulated root canal with a certain diameter, angle of curvature, and a radius of curvature of a specific cyclic fatigue testing device until fracture occurred. Time to fracture was recorded for each instrument in each group in seconds. The mean values and standard deviation were then calculated. Data were compared using repeated measures ANOVA for individual comparisons followed by Bonferroni's correction for multiple comparisons. Significance was set at the 95% confidence level.
Results: Time of fracture had statistically significant differences among all groups tested (P < 0.5) except between Group 1 (PV) and Group 4 (HF) and between Group 2 (VB) and Group 3 (TF); there was no statistically significant difference (P > 0.5). Thus, the S1 file offered a long life span, followed by WaveOne™, ProFile Vortex™, HyFlex™, Twisted Files™, and then Vortex Blue™.
Conclusion: The reciprocating instruments (S1 and WaveOne™) had a higher cyclic fatigue resistance than all rotary files used in this study. However, the Profile Vortex (M-wire) and HyFlex (CM) showed better cyclic fatigue resistance than other rotating files in this study. Hence, the blue and R-phase heat treatments did not enhance the cyclic fatigue resistance for rotating instruments.

Keywords: CM wires, cyclic fatigue, M-wire, reciprocation, rotation, thermomechanical treatment


How to cite this article:
Bukhari KA, Almalki MA, Daghestani MH, Bogari DF, Aljifan MK, Alharbi YM, Marqoshi RR. Cyclic fatigue comparison of different manufactured endodontic files. Saudi Endod J 2019;9:186-91

How to cite this URL:
Bukhari KA, Almalki MA, Daghestani MH, Bogari DF, Aljifan MK, Alharbi YM, Marqoshi RR. Cyclic fatigue comparison of different manufactured endodontic files. Saudi Endod J [serial online] 2019 [cited 2019 Nov 13];9:186-91. Available from: http://www.saudiendodj.com/text.asp?2019/9/3/186/264645


  Introduction Top


Root canal preparation has witnessed significant changes following the establishment of nickel–titanium (NiTi) in rotary endodontic instruments.[1] This innovation offers some key benefits when compared with more conventional stainless steel hand files, such as greater flexibility,[2] enhanced geometry in terms of instrumented canals,[3],[4],[5] shorter root canal preparation times,[6],[7] and a lower degree of debris extrusion.[8] However, instrument separation continues to be recognized as a major disadvantage of the clinical use of rotary NiTi files.[9]

The fracture of these files in rotary motion arises in two different ways: flexural (cyclic) and torsional fatigues.[10],[11] The cyclic fatigue fracture arises as a result of metal fatigue, in which there is no binding of the file in the canal but rather free rotation in a curvature, which induces compression/tension cycles at the maximum flexure point until the occurrence of the fracture.[12],[13] While the torsional fatigue occurs when part of the file, such as the tip, is locked in a canal, continued rotation of the canal causes the elastic limit of the metal to be exceeded as a result of handpiece-induced torque, giving rise to tip fractures.[14]

To improve instrument separation resistance during root canal treatment, manufacturers have tried different approaches such as modification to the design, including a number of different cross-sectional designs,[15],[16] the utilization of new alloys,[17] innovative manufacturing techniques,[18] and various actions of motion.[19],[20]

One of these innovative methods was the heat treatment involving twisting NiTi to create the Twisted File™ introduced by SybronEndo (Orange, CA, USA). The manufacturer refers to its approach as “R-phase,” emphasizing that it seeks to enhance the degree of cyclic fatigue resistance in addition to the superelasticity.[18] Dentsply Sirona Dental Specialties (York, PA, USA) devised another method to create M-wire NiTi. Such an approach utilizes thermal processing to enhance the cyclic fatigue resistance and improve the flexibility of the ProFile Vortex™ file (Dentsply Sirona Dental Specialties) (York, PA, USA).[21] Recently, a new method was devised by Dentsply Sirona with the aim of producing NiTi treated with a visible titanium oxide layer to enhance its cyclic fatigue resistance over that of regular M-wire NiTi introducing Vortex Blue™ files.[22] One further development is in the context of the generation of files offering shape memory, which can regain their shape following sterilization such as Coltene HyFlex™ CM (Coltene Whaledent, Cuyahoga Falls, OH, USA). The manufacturer claims that such files are recognized to be up to 300% more fatigue resistant and have no rebound (ColteneEndo 2012c). Furthermore, Dentsply Sirona took a different path, focused on the factor of motion and created reciprocating files, referred to as WaveOne™ files (Dentsply Sirona Dental Specialties) (York, PA, USA), which have been thermally treated to enhance cyclic fatigue resistance as well as flexibility. A new file called the S1 produced by Sendoline (AB, Taby, Sweden) file relies on its reciprocating motion and is made from standard NiTi alloy with no special treatment. It is of interest to compare the cyclic fatigue of these instruments. Therefore, the aim of this study was to compare the cyclic fatigue resistance of endodontic files of different manufacturers operated in disparate motions.


  Materials and Methods Top


This study comprised six groups: Group 1: ProFile Vortex™ files (PF) (tip size ISO 25 with 0.06 taper), Group 2: Vortex Blue™ files (VB) (tip size ISO 25 with 0.06 taper), Group 3: Twisted Files™ (TF) (tip size ISO 25 with 0.06 taper), Group 4: HyFlex™ CM files (HF) (tip size ISO 25 with 0.06 taper), Group 5: WaveOne™ files (tip size ISO 25 with 0.08 taper), and Group 6: S1™ (tip size ISO 25 with 0.06 taper). All files were 25 mm in length. Each group contained 20 instruments. All instruments had been inspected previously using an optical stereomicroscope with ×20 magnification to conduct a morphologic analysis and look for any sign of visible deformation. All defective instruments would have been discarded, even if none of the tested instruments showed such defects. The instruments were randomly distributed in groups of six pieces.

The appliance comprises a handpiece holder connected to a steel base in such a way that the handpiece is accurately placed in relation to the cyclic fatigue testing tube. Simulated canals were made by bending a stainless steel tube 1.5 mm in diameter. Canal curvature was simulated by bending the tube with angles of curvature 60° and radii of curvature of 5 mm [Figure 1]. The center of the radius in the curved section of the canal was 5 mm from the tip of the tube. The total length of the tube was 20 mm, with the attached files inserted into the artificial tube from the top and extruding approximately 2 mm from the distal end [Figure 2]. Three-in-one oil (WD-40 Company, San Diego, CA, USA) was used to lubricate the tube during the testing of the files. The files were moved at a fixed speed and torque according to the manufacturer's instructions for each file. Files were driven by the WaveOne™ endodontic motor. The time to the separation of the instrument was monitored and recorded using a video camera (Nikon 1 v1) with a macro lens attached to a LED screen. Data were collected and transferred to Excel (Microsoft Corporation, Redmond, WA) for further analysis. Statistical analysis was completed with IBM SPSS statistical software (version 22.0) (IBM Corporation, Armonk, NY, USA). The mean values and standard deviation were then calculated. Data were compared using repeated measures ANOVA for individual comparisons followed by Bonferroni's correction for multiple comparisons. Significance was set at the 95% confidence level.
Figure 1: Illustration describing canal geometry using two parameters: radius of curvature and angle of curvature

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Figure 2: The cyclic fatigue device (a). The handpiece aligned with the custom-made artificial canal (b). The tip of the file extended approximately 2 mm from the distal end of the artificial canal (c). A video camera with a light source used to record the time to fracture of each file (d)

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  Results Top


[Table 1] presents the descriptive statistics for the samples from the different types of files, including the mean, minimum, median, maximum, and standard deviation of the fracture time for each file sample. A box plot of time to fracture (seconds) grouped by type of files is shown in [Figure 3]. The one-way ANOVA test revealed statistically significant differences between groups (P < 0.001) (signifying that the null hypothesis should be rejected). The Bonferroni test was performed to identify differences among groups. The time of fracture was a statistically significant difference among all groups tested except between Group 1 (PV) and Group 4 (HF) and also between Group 2 (VB) and Group 3 (TF). Therefore, the S1 file offered a long life span, followed by WaveOne™, ProFile Vortex™, HyFlex™, Twisted Files™, and then Vortex Blue™.
Table 1: Standard deviation and mean values of fracture time (seconds) for each instrument

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Figure 3: Box plot of time to fracture (seconds) grouped by types of files

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  Discussion Top


When rotary endodontic files are rotating in a curved root canal, they are subjected to tensile and compressive forces. The NiTi instruments experience compressive forces on the inside of the curve and tensile forces on the outside of the curve. The file rotating under those forces is at high risk of fracture or of what is referred to as scientifically cyclic fatigue failure, which leads to instrument separation inside the root canal with the associated clinical complications. With the implementation of NiTi material in the manufacturing of endodontic files, cyclic fatigue resistance has been greatly improved. However, the risk of instrument fracture has not diminished. Considerable efforts have been made to analyze the factors contributing to this type of failure, including file design, metallurgy, and file motion. File design, such as the cross-section, showed extremely variable and controversial effects on the cyclic fatigue resistance of the endodontic files.[23],[24],[25],[26] Several of these studies found that differences in cross-sectional design have an enormous influence on the cyclic fatigue resistance of different endodontic files.[27],[28],[29],[30] The negative influence of cross-sectional geometry remains unexplainable and unclear. However, Grande et al.[28] suggested that the core material at the point of greatest stress is the most important determinant of cyclic fatigue resistance. On the other hand, some researchers have not found evidence of such an effect.[16],[31] The researcher's orientation has recently changed toward exploring and testing different metallurgy and different file motions to enhance the cyclic fatigue resistance of different endodontic files.

Our testing apparatus is a modified version of a previously used model.[9],[31],[32],[33],[34],[35] The design of the testing device used in a current study allows a precise, flexible, and reproducible placement of each instrument inside the artificial canal [Figure 1]. This ensured three-dimensional alignment and positioning of the instruments to the same depth. To provide the tested file with greater stability while simultaneously providing a low friction surface during file motion, the tube diameter was made 1.5 mm, which is only 0.2–0.3 mm wider than the diameter of all the files at the distance of 16 mm from the tip of the file (D16). In addition, the digital video camera recording for tested files gave a more precise time measurement and provided the author with the ability to observe the file retrospectively to ensure stability. However, the center of the curvature was located at 7 mm from the apical end of the instrument; at this level, the diameter of the file will increase and may lead to a short fatigue time. Hence, the results from this study should not be directly correlated to clinical conditions.

The effect of rotational speed on the cyclic fatigue of rotary files is tremendously controversial. Several researchers have found rotational speed to not affect cyclic fatigue,[9],[36],[37],[38] while other studies have found cyclic fatigue failure to be faster with higher speeds.[39] In this study, we followed the manufacturer's instructions for each instrument to simulate the clinical situation.

In our study, the S1™ file and WaveOne™ file had the best results and were statistically different from all other rotary files. These results cannot be compared with those of previous studies, as the S1 file has never been tested previously. However, according to all previous studies that have compared reciprocating files and rotating files, reciprocating files showed a large increase in cyclic fatigue resistance, which is in accordance with our result.[19],[40] On the other hand, the WaveOne™ file showed inferior results when compared to the S1™ file. The explanation why the S1™ file showed a higher resistance to cyclic fatigue than the WaveOne™ file, which is made of M-wire and works in nearly the same manner, may be related to differences in the cross-section geometry, taper, and amplitude of the oscillating angles.[41] Theoretically, the S1 reversed file has nearly the same “S” cross-section as the Reciproc™ file from VDW (Munich, Germany) which showed higher resistance to cyclic fatigue than the WaveOne™ file in all previous studies.[42],[43],[44]

The ProFile Vortex file was statistically different from all other rotary files except for the HyFlex™ file. Studies have demonstrated that files made from M-wire had superior cyclic fatigue resistance and showed 400% greater cyclic fatigue resistance than files made from conventional NiTi wire.[17] Although there was no significant difference between the ProFile Vortex™ and HyFlex™, the ProFile Vortex™ showed a slightly higher resistance, which is inconsistent with previous studies.[45],[46] Another inconsistency is the inferior result of Vortex Blue™ when compared to the ProFile Vortex™. Although the ProFile Vortex™ and Vortex Blue™ files have the same features, aside from the special thermomechanical treatment for the Vortex Blue™ file, the ProFile Vortex™ file showed a higher cyclic fatigue resistance when compared to Vortex Blue™ files. This result is inconsistent with all previously published results.[22],[47] Our findings indicate that the special thermomechanical treatment for ProFile Vortex file decreased the resistance to cyclic fatigue failure. It cannot be related to the testing model since they are identical files; on the other hand, if we compare the Vortex Blue™ file to the Twisted files™, which are also subjected to a special thermomechanical treatment, they demonstrate very similar results with no statistical difference between them, which indicates that, under the circumstances of this study, files subjected to thermomechanical treatment showed a decreased resistance to cyclic fatigue failure.


  Conclusion Top


Within the limitations of the present study, we conclude that reciprocating file movement drastically enhanced cyclic fatigue resistance when compared to rotational files. Regarding the comparison of rotational files, the ProFile Vortex (M-wire) and HyFlex (CM) had a better cyclic fatigue resistance than Vortex Blue (blue wire) and Twisted File (R-phase). Hence, the blue and R-phase heat treatment did not enhance the cyclic fatigue resistance.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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[PUBMED]  [Full text]  
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