Effect of autoclaving on the cyclic fatigue resistance of nickel-titanium rotary instruments: An in vitro study

Abstract

Aim:

The aim of the study is to determine the effect of autoclaving on the cyclic fatigue resistance (CFR) in respect to three different rotary nickel-titanium (NiTi) systems.

Methods:

Three rotary NiTi files with different manufacturing process were selected and arranged into three groups. Group 1: Neolix Neoniti files (n = 15), Group 2: Edge Files (n = 15), Group 3: NeoendoFlex files (n = 15). Each group was then subdivided into three subgroups containing 5 samples each based on the number of cycles of autoclaving (0, 1, 5 cycles). The instruments were subjected to autoclave without usage. The files of size 25, taper 6%, and 25 mm length were taken. After the autoclaving cycles, the files were rotated in cyclic fatigue testing device with simulated canal curvature of 60° until file separation. Time taken for the instrument separation (in minutes) was multiplied by the number of rotations per minute to attain the mean number of cycles to failure for every file. The obtained results were subjected to statistical analysis using two-way ANOVA and Newman–Keuls multiple post hoc procedures.

Results:

Neolix Neoniti files exhibited the maximum CFR (98.80) than EdgeFiles (80.47) and Neoendo Flex files (41.80) (P = 0.0001). Reduction in CFR was observed with increase in the number of cycles of autoclaving (i.e., 0, 1, 5 cycles).

Conclusion:

Autoclaving has a negative influence on the CFR of the rotary NiTi files used. Neolix Neoniti files exhibited greater CFR than EdgeFiles and Neoendo Flex files.

INTRODUCTION

In 1988, Walia suggested the use of nickel-titanium (NiTi) alloy named as nitinol in the manufacturing of rotary instruments owing to its superior mechanical properties such as shape memory and superelasticity.[1,2] These properties of NiTi alloys are responsible for the austenitic to the martensitic phase transformation. Either the process of cooling or the application of stress causes phase transformation in the NiTi alloy and is thermoelastic in nature. The mechanical properties and the transformation temperatures of Ni–Ti alloys are strongly dependent on their chemical composition and thermomechanical processing. A thorough knowledge of the physical properties and limitations of endodontic files can reduce undesirable procedural errors, like file separation in the canal, transportation, and strip perforation.[3]

Cyclic fatigue is due to numerous cycles of tension and compression that eventuate when a file rotates in the curved canals. The rotary files tend to fracture at a certain point when it reaches its flexural limit.[3] Most rotary NiTi endodontic instruments are produced by grinding. Some of the rotary NiTi files are produced by twisting the alloy after it had been subjected to heat treatment. Literature had revealed many studies regarding the effects of operating rate, instrument design, surface treatments, root canal irrigants upon the cyclic fatigue resistance (CFR) of rotary NiTi files. Possible strategies to improve the CFR comprises either improving the process of manufacturing or utilize novel alloys having excellent properties. Thermal processing methods optimize the microstructure of the NiTi alloy, thus affecting the CFR of the instruments.[4,5,6,7]

Accurate determination of CFR requires utilization of natural root canals. However, after usage the root canal anatomy will change and the samples cannot be reused. Thus, it becomes difficult to standardize the experimental setting. Consequently, several equipments such as glass and metal tubes were used for determining CFR. Nonetheless, there is no international laboratory standard for determining the CFR of rotary NiTi files.[8,9]

According to the manufacturers, rotary NiTi files are single use. However, in clinical settings, these are frequently reused after sterilization. After multiple uses, surface defects such as pitting, strip formation, microfractures/microcrack formation, and disruption of cutting edges are apparent on the instrument surface. Upon further use, the instrument may fracture.[10]

Zinelis et al. stated that heat treatment of the apical 5-mm portion of a NiTi rotary instrument can extend its fatigue life by reducing its residual strain. Furthermore, NiTi files have more CFR when subjected to heat treatment at 430°C which results in superior elastic behavior and provides better shaping ability. When the instrument is heat treated at 600°C, the NiTi alloy undergoes martensitic and R-phase transformation. At this temperature, due to the recrystallization of NiTi alloy, decrease in the surface hardness will be evident. Hence, the heating temperature is an essential aspect closely related to the performance of NiTi alloy.[11]

Literature search showed that autoclaving has effect on the behavior of rotary NiTi files with varying conclusions. Some researchers concluded that autoclaving negatively influences the physical properties of rotary NiTi files and causes files to fail prematurely. Other researchers have reported that autoclaving for 10 times did not impact the fracture prediliction of rotary NiTi files.[3,12] The unique properties of NiTi alloy such as superelasticity and shape memory are greatly influenced by the manufacturing procedure of the file.[8,11] Yahata et al. had stated that the flexibility of the rotary NiTi instruments might improve when they are subjected to heat retreatment during autoclaving.[13] Sundaram et al. have suggested that sterilization temperatures above 125°C could reverse the NiTi deformation caused during clinical use.[10]

Three different NiTi file systems were selected based on the criteria that they were produced by different manufacturing process and of different cross-sections. Neolix Neoniti files have nonhomogenous rectangular cross-section and are manufactured using wire-cut electric discharge machining (WEDM) process.[14] The EdgeFile system has parabolic cross section and is manufactured using proprietary annealed heat treatment forming a branded fire wire.[15] Neoendo Flex files have triangular cross section and are subjected to gold thermal treatment.[16]

The main objective of the present research is to estimate whether autoclaving process has any influence on the CFR of newly introduced heat-treated rotary files, i.e., Neolix Neoniti, EdgeFiles, and Neoendo Flex file systems.

METHODS

Sample size calculation

The sample size was estimated using the G*power, version 3.1.9.4 (Franz Fauluniversitat, Kiel, Germany). Sample size was estimated with α–error is 5% and power of the test is 80% or 20% β-error with effect size of 0.6. Hence, it was calculated as 15 subjects in each group.

Three groups of rotary NiTi instruments of identical tip size of 0.25 mm and 6% taper were taken: Group 1-NeolixNeoniti files (Chatres-la-Foret, France), Group 2-EdgeFile X3 (EdgeFile, Albuquerque, New Mexico, USA), Group 3-Neoendo Flex files (Orikam, India). Each file group consists of 15 instruments. It was further divided into three subgroups containing five instruments, each based on the number of autoclaving cycles, i.e., Subgroup A-no autoclaving, Subgroup B-one cycle of autoclaving, and Subgroup C-five autoclaving cycles. Autoclaving process was carried out at a temperature of 121°C for 15 min duration, 15lb pressure, which is followed by 15 min of drying. Finally, all the files were tested for CFR. A customized equipment was used for determining CFR. This equipment incorporates a stainless steel block with simulated canals [Figure 1]. The mounted handpiece enables correct positioning of every file into the simulated canal. Each file was tested in the customized artificial canal having 60° curvature angle and 5 mm of curvature radius. Each file was used at a rate of 300 rpm in a 16:1 contraangle reduction handpiece activated by a torque modulated endodontic motor (X Smart Endomotor, Dentsply Maillefer, Switzerland) till file separation had taken place. The time taken for the file separation was noted utilizing the stopwatch and video camera.

Figure 1.

Cyclic fatigue testing machine with mounted handpiece

Number of cycles to failure (NCF) = Time taken for file separation (In minutes) × Number of rotations per minute.

Statistical analysis

Mean values of NCF and its standard deviations were evaluated for the samples. Two-way ANOVA test was used to calculate significant differences among the groups. Once the analysis revealed a significant difference, Neuman–Keuls post hoc test was done to calculate the mean differences (with P < 0.05). The data analysis was carried out utilizing SPSS 20.0 version software (IBM, Bangalore, Karnataka, India).

RESULTS

Higher NCF is observed in Group 1 (98.80) when compared to others. Least NCF is observed in Group 3 (41.80) (Graph 1). Mean NCF in Group 2 is 80.47. Two-way ANOVA comparison [Table 1] showed that the mean NCF is dissimilar in different groups which is statistically significant (P = 0.0001). The mean NCF of three subgroups (A, B, C) was also dissimilar which is statistically significant (P = 0.0002). However, there is no statistically significant difference in the interaction effect of three main groups (1, 2, 3) and three subgroups (A, B, C) with respect to NCF.

Table 1.

Comparison of three main groups (1, 2, 3) and three subgroups (A, B, C) with mean number of cycles to failure by two-way ANOVA

Sources of variation	Degrees of freedom	Sum of squares	Mean sum of squares	F	P
Main effects
Main groups	2	25,401.11	12,700.56	41.6124	0.0001 (S)
Sub groups	2	6574.71	3287.36	10.7708	0.0002 (S)
Two-way interaction effects
Groups x subgroups	4	846.22	211.56	0.6931	0.6016 (NS)
Error	36	10,987.60	305.21
Total	44	43,809.64

S: Significant, NS: Nonsignificant

Pairwise comparisons of groups [Table 2] revealed a significant difference between Groups 1 and 2 (P = 0.0211), Groups 1 and 3 (P = 0.0001), Groups 2 and 3 (P = 0.0001). The mean NCF is in the order of Group 1 >2>3. Pairwise comparison of subgroups revealed a significant difference only between Subgroup A and C (P = 0.0262). A nonsignificant difference was observed between Subgroups A and B (P = 0.1105) and Subgroups B and C (P = 0.2895).

Table 2.

Comparison between three main groups (1, 2, 3) and three subgroups (A, B, C) with mean number of cycles to failures by Newman–Keuls multiple post hoc procedures

Groups	Group 1	Group 2	Group 3	Sub Group A	Sub Group B	Sub Group C
Mean	98.80	80.47	41.80	89.40	71.67	60.00
SD	30.45	16.54	10.68	36.02	25.73	26.46
Group 1	-
Group 2 (P)	0.0211*	-
Group 3 (P)	0.0001*	0.0001*	-
Sub Group A				-
Sub Group B (P)				0.1105	-
Sub Group C (P)				0.0262*	0.2895	-

*Significant. SD: Standard deviation

DISCUSSION

The main purpose of autoclaving rotary files is to eliminate the microbial contamination, thereby increasing the chance for the success of root canal therapy.[12] Several clinicians choose having fixed sets of particular files. However, all these files might not be used during the treatment. Therefore, the rotary files that have not been utilized are also exposed to multiple cycles of autoclaving. Majority of the studies in the literature had analyzed the influence of frequent autoclaving on the rotary NiTi file systems.[2,17,18]

Autoclaving has a negligible influence on the physical properties of stainless steel instruments. Silvaggio and Hicks concluded that autoclaving of rotary NiTi files did not raise the probability of file fracture.[10,19] Dissimilarities in the CFR among rotary NiTi files are owing to the differences in many factors such as the process of manufacturing, operational features, file design, characteristics, canal curvature, and the method of fatigue failure testing. Methods to improve the longevity of the NiTi instruments include the following: (i) thermal treatments before machining; (ii) appropriate machining conditions of the NiTi alloy; and (iii) electro-polishing.[11]

According to Aminsobhani M et al., the Austenite start temperatures of Neolix Neoniti file were in a manner that at the mouth temperature, the austenite phase existed. Neoniti reveals R-phase transformation.[20] The austenite-finish temperatures of Neoniti were higher than human body temperature, therefore, material was in the phase transformation from martensite to austenite, gives the instruments more flexibility when used in the clinical situation.[21] The EdgeFile files and Neoendo files are predominantly in austenite phase at room temperature.

In the present study, Neolix Neoniti files showed the highest CFR than EdgeFiles and Neoendo Flex files. It could be due to differences in the manufacturing process of the three file groups. Neolix Neoniti files have a nonhomogenous rectangular cross-section, and it has multiple tapers in a single instrument. It is manufactured using the WEDM process. This manufacturing process improves the flexibility and shape memory of this system.

These observations are in compliance with previous studies.[14,22] Neolix Neoniti files showed highest CFR than RaCe, Mtwo, Twisted file, and ProTaper Next X2 files.[22] This may be due to the unique manufacturing process and heat treatment of this file. WEDM process creates an electrical discharge which produces sharp cutting ends, changing variable profiles, and flexibility in the rotary NiTi file systems.

Rubio et al. compared the cyclic fatigue resistance of 10 different types of endodontic files and concluded that the instruments manufactured with CM wire technology, i.e., Hyflex EDM and Neoniti were superior to other endodontic files, and there was no statistically significant difference between Hyflex EDM and Neolix files.[23]

EdgeFiles have better CFR when compared to Neoendo Flex files. This can also be imparted to the manufacturing difference between the two file systems. The EdgeFile system has a parabolic cross-section. This system undergoes proprietary annealed heat treatment, forming a branded firewire that increases NiTi instruments' flexural strength and flexibility.

Adıgüzel et al. evaluated the CFR of EdgeFile X3 files with One Curve, One Shape, and 2Shape files in vitro. They concluded that EdgeFile system showed highest CFR than the other groups tested. EdgeFile system was manufactured using Firewire technology that is responsible for its improved CFR over the other file systems.[24]

According to Tanomaru-Filho et al., the heat treatment CM that is applied to the ProDesign and HyFlex CM systems contributed positively to the best results presented in relation to instruments without heat treatment (MTwo) or with FireWire heat treatment (EdgeFile).[15] Neoendo Flex files exhibited minimum CFR in the present study. The current results are in accordance with the research done by Mahajan et al.[16] They stated that Neoendo Flex files exhibited less CFR than One curve files. Other reason for better CFR of Neolix Neoniti files could be attributed to its rectangular cross section which contributes to bulk of the instrument when compared to parabolic cross section of edge file and triangular cross section of Neoendo files.[14]

It is also observed that, after every autoclaving cycle, there was a marked reduction in CFR of rotary NiTi files. Hilfer et al. inferred that frequent autoclaving markedly reduced the NCF of size 25, taper 6% Twisted Files.[12] Nair et al. conducted an atomic force microscopy and proposed that the roughness on the file surface became more pronounced with numerous autoclave cycles.[25] Limitation of the present study is that the endodontic files were rotated in a stainless steel simulated canal. However, the files may behave differently when instrumented in a natural tooth. Hence, the results may slightly vary in the in vivo settings. Lubricating agent was not used in the present study. Hence, the effect of the presence of lubricating agents on the CFR of these files also needs to be evaluated.

CONCLUSION

This study concluded that Neolix Neoniti files, which were manufactured using EDM technology, exhibited highest CFR compared to EdgeFiles and Neoendo Flex files. Reduction of CFR in the rotary NiTi files was observed with an increase in number of cycles of autoclaving.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.