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ORIGINAL ARTICLE
Year : 2022  |  Volume : 12  |  Issue : 1  |  Page : 114-119

Effect of autoclave sterilization on cyclic fatigue of EdgeTaper platinum and ProTaper gold nickel–titanium rotary instruments: An in vitro study


1 Division of Endodontics, Faculty of Dentistry, Thammasat University, Pathumthani, Thailand
2 TUDS Oral Research Centre, Faculty of Dentistry, Thammasat University, Pathumthani, Thailand
3 Division of Operative Dentistry, Faculty of Dentistry, Thammasat University, Pathumthani, Thailand

Date of Submission29-Oct-2020
Date of Decision21-Jan-2021
Date of Acceptance28-Mar-2021
Date of Web Publication8-Jan-2022

Correspondence Address:
Panupat Phumpatrakom
Faculty of Dentistry, Thammasat University, 99 Moo. 18 Paholyothin Road, Klongluang 12130, Pathumthani
Thailand
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_281_20

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  Abstract 


Introduction: The heat during autoclave process affects the fracture resistance of the nickel–titanium rotary file and might exert some effect on its mechanical properties. This study aimed to evaluate the effect of autoclave sterilization on cyclic fatigue of EdgeTaper Platinum® and ProTaper Gold®.
Materials and Methods: Forty new files from EdgeTaper Platinum® and ProTaper Gold® rotary files with a #25 tip size were used in this study. Each system was subdivided into four groups according to the sterilization cycle; Group 1: No sterilization, Group 2: Two cycles of sterilization, Group 3: Five cycles, and Group 4: Ten cycles. Cyclic fatigue was evaluated using an artificial stainless steel canal with 60° of curvature and 5 mm radius of curvature at 37°C. The number of cycles to failure (NCF) was calculated and compared using Kruskal–Wallis test and Mann–Whitney U-test. The surface of the fractured file was evaluated with scanning electron microscopy.
Results: There were significant differences within the two file brand subgroups regarding the NCF (P > 0.05). ProTaper Gold® also revealed no significant differences in NCF (P > 0.05) related to the sterilization conditions. In addition, EdgeTaper Platinum® showed significantly higher cyclic fatigue resistance than ProTaper Gold® in all groups of this experiment (P < 0.05). The fractured plane showed typical features of cyclic fatigue failure for all instruments.
Conclusion: Autoclave sterilization did not affect cyclic fatigue resistance of both EdgeTaper Platinum® and ProTaper Gold® in all groups of the experiment. When compare in each group of this experiment, EdgeTaper Platinum® exhibited higher cyclic fatigue resistance than ProTaper Gold®.

Keywords: Autoclave sterilization, cyclic fatigue, EdgeTaper platinum, ProTaper gold, scanning electron microscope


How to cite this article:
Phumpatrakom P, Klinsontorn N, Jutrakulkeeree T, Thanarojwongsa V, Klaisiri A, Aguilar P. Effect of autoclave sterilization on cyclic fatigue of EdgeTaper platinum and ProTaper gold nickel–titanium rotary instruments: An in vitro study. Saudi Endod J 2022;12:114-9

How to cite this URL:
Phumpatrakom P, Klinsontorn N, Jutrakulkeeree T, Thanarojwongsa V, Klaisiri A, Aguilar P. Effect of autoclave sterilization on cyclic fatigue of EdgeTaper platinum and ProTaper gold nickel–titanium rotary instruments: An in vitro study. Saudi Endod J [serial online] 2022 [cited 2022 Jan 25];12:114-9. Available from: https://www.saudiendodj.com/text.asp?2022/12/1/114/335242




  Introduction Top


Nickel–titanium (NiTi) rotary instrument has been introduced in 1988. It has advantages over the conventional stainless steel files such as superelasticity, shape memory, and more resistance to cyclic fatigue.[1],[2] Over decades, NiTi rotary file manufacturing process was manipulated by design, rotational direction, thermomechanical process and modify phase. ProTaper Gold® has been widely used and shown the improvement of metallurgy resulted in higher fatigue resistance. Another system is EdgeTaper Platinum®. The manufacturer recently claimed that it had 2 times higher cyclic fatigue resistance than ProTaper Gold®.[3]

The proprietary heat treatment of NiTi rotary files revealed a significant impact on increasing the resistance to cyclic fatigue. In contrast, the torsional resistance was not improved by the heat treatments.[4] The autoclave is the standard sterile technique in dentistry. Current literature revealed the controversial results of the effect of heat during the sterilization process on NiTi rotary files. Some studies showed that repeated cycles of autoclave sterilization did not seem to influence the mechanical properties of K3, Mtwo, and Vortex NiTi rotary files.[5] In addition, heat sterilization from autoclave or heat dryer did not affect the torsional strength of Profile NiTi endodontic files.[6]

On the contrary, it was found that 10 cycles of autoclave sterilization increased the number of cycles to failure (NCF) of the EdgeFileX7 and increased the cyclic fatigue resistance of the XF file of the K3 system.[5],[7] However, Ozyurek et al. reported that the heat during the autoclave process affected fracture resistance and results in increased surface roughness of NiTi rotary file under a scanning electron microscope.[8] Another study found that 5 cycles of moist heat sterilization significantly increased in the surface roughness of ProTaper® and Race® NiTi instruments. The presence of defects on the cutting edges of these instruments may be a reason for their susceptible to fracture.[9]

Numerous studies on cyclic fatigue of NiTi rotary systems are available in the literature but there was still lacking of study of the consequences of autoclave sterilization on the cyclic fatigue resistance of EdgeTaper platinum® and ProTaper Gold®. Accordingly, this study aimed to evaluate the effect of autoclave sterilization on cyclic fatigue of EdgeTaper Platinum® and ProTaper Gold®.


  Materials and Methods Top


The research process was conducted at the dental material laboratory, Faculty of Dentistry, Thammasat University, Pathum Thani, Thailand. EdgeTaper Platinum® (Edge Endo®, New Mexico, USA) and ProTaper Gold® (Dentsply Sirona®, Ballaigues, Switzerland) with a #25 tip size were used in this study. Forty new files from each system were subdivided into four groups according to sterilization cycle; Group 1: No sterilization, Group 2: Two cycles of sterilization, Group 3: Five cycles, and Group 4: Ten cycles to mimic the repetition of sterilization in clinical use.

The samples in Groups 2, 3, and 4 from each system were placed in the autoclave (Midmark M11 Ultraclave®, Canada) and steamed at 130°C and 27.1 psi pressure until it finished the cycle of sterilization. The hot cycle and sterilization time were 22 min and the dry time continuing in the autoclave was 30 min. After each cycle of sterilization, the samples were left dry overnight before the next sterilization cycle. Each group was subjected to 2, 5, and 10 cycles of sterilization, respectively. The samples in the control group were not exposed to any autoclave sterilization.

The cyclic fatigue testing device was a made-to-order artificial canal in a stainless steel block. The canal had 2 mm internal diameter with 60° of curvature and 5 mm radius of curvature.

All samples were tested in an artificial canal. The working length inserted in the artificial canal was 18.0 mm. The handpiece for the rotary instrument was stabilized in a specially designed made-to-order handpiece stabilizer. The tests were running in a controlled temperature water bath (Whip mix®, Kentucky, USA) at 37°C. X-smart plus endodontic motor (Dentsply Sirona®, Ballaigues, Switzerland) was used at 300 rpm with 3 N/cm2 torque control as recommended by the manufacturer. The samples were allowed to continuously rotate until fractured. The time required for fracture to occur was recorded for every instrument and used to calculate the number of cycle to failure [Figure 1].
Figure 1: The artificial canal (a) with 2 mm internal diameter with 60° of curvature and 5 mm radius of curvature. A handpiece stabilizer used during cyclic fatigue testing in controlled temperature (b)

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During each rotation, NCF was recorded and calculated for each sample using the following formula:



After the cyclic fatigue testing, a fractured sample from each group was randomized and transferred to scanning electron microscopy (Field Emission Scanning Electron Microscope, JSM-7800F, JEOL, Tokyo, Japan) using ×200 and ×1500 magnification to investigate the pattern of fracture.

Statistical analysis

The different groups of sterilization cycle in each brand were compared using Kruskal–Wallis test. The two different brands with the same sterilization cycles were compared by Mann–Whitney U-test.


  Results Top


The means and standard deviation of NCF for EdgeTaper platinum® and ProTaper Gold® in all groups are shown in [Table 1].
Table 1: Mean±standard deviation number of cycles to failure according to types and sterilization conditions

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Within each brand of rotary files, there were no significant differences among the four different groups of the number of sterilization cycles (P > 0.05). However, EdgeTaper platinum® needed significantly greater NCF (greater resistance to cyclic fatigue) than ProTaper Gold® (P < 0.05).

Scanning electron microscopy photographs (×200 and ×1500) of the fractured plane showed typical features of cyclic fatigue failure. The photographs presented smooth fracture surfaces with microvoids and a mechanical characteristic of ductile fracture [Figure 2] and [Figure 3].
Figure 2: Scanning electron microscopic photographs showing the fractured surface of EdgeTaper platinum® (a-d) and ProTaper Gold® (e-h) after cyclic fatigue testing. All fractured surfaces showed a smooth surface. The arrows indicated initiation of cracks

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Figure 3: Scanning electron microscopic photographs showing the fractured surface of EdgeTaper platinum® (a-d) and ProTaper Gold® (e-h) after cyclic fatigue testing

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  Discussion Top


Fracture of endodontic NiTi rotary instruments could occur under two circumstances: torsional fracture and flexural fatigue.[10] A torsional fracture occurs when the tip or any part of the instrument is locked in a canal while the shaft continues to rotate; the instrument exceeds the elastic limit of the metal and shows plastic deformation followed by fracture. The other type of instrument fracture is caused by work hardening and metal fatigue, resulting in flexural fracture.

Due to economic reasons, NiTi rotary instruments are generally re-used in clinical settings by autoclave sterilization. Therefore, cycles of sterilization are ascertainable.[11] The current study focused on cyclic fatigue testing and the effect of autoclave sterilization on EdgeTaper platinum® and ProTaper Gold® NiTi rotary instruments. In addition, photographic investigation through scanning electron microscopy was used in this experiment.

Numbers of previous experiments, involving cyclic fatigue testing, were carried out using artificial canal aim to minimize canal variation which found in natural teeth to maintain standardization.[12],[13] Consequently, the present study was conducted in an artificial canal but also controlled temperature at 37°C to mimic intracanal temperature in order to enhance these instrument's mechanical properties.

A major drawback of laboratory tests that investigate the mechanical properties of NiTi endodontic files is that confounding factors such as material properties, design, and dimensions of the file, were not eliminated.[14] In addition, these factors are also brand specific, making it difficult to investigate the effect that a single variable has on the mechanical properties of the files. However, EdgeTaper platinum® and ProTaper Gold® are similar in design and undergo a similar thermal treatment process.[4],[15] Therefore, any differences in the mechanical properties can probably be explained by the superiority of one brand over the other.[3] For instance, ProTaper Gold® has been introduced as an improved version of the ProTaper Universal®. It is more resistant to cyclic fatigue than ProTaper Universal®.[16],[17] The explanation was probably by the heat treatment process in ProTaper Gold® which produces files with 2-stage specific transformation behavior and high austenite finish temperature.[18] The high austenite finish temperature means that the file will be in the martensitic phase at body temperature resulting in improved instrument's flexibility and better resistance to cyclic fatigue.[18]

This study revealed that autoclave sterilization had no effect on the cyclic fatigue resistance of both EdgeTaper platinum® and ProTaper Gold® among the four different groups of the number of sterilization cycles. The result was similar to the study by Plotino et al.[5] who reported that the cyclic fatigue resistance of the NiTi instrument “K3, M-two, and Vortex” was not affected after 10 sterilization cycles. The result was also consistent with the study by Bulem et al. which revealed that 5 consecutive cycles of autoclave sterilization had no effect on the cyclic fatigue resistance of NK files.[19] Furthermore, it has been reported that the cyclic fatigue resistance was increased in the XF file of the K3 rotary system.[5]

On the contrary, Alshwaimi noted that autoclave sterilization of Proflexendo reduced their cyclic fatigue resistance.[20] It has been hypothesized that heat generated from autoclave sterilization and heat being generated during cyclic fatigue test and friction between file and walls of the artificial canal may alter some changes of metallurgy of the files. However, other researchers reported that heat generated by the autoclave sterilization was not abundant to alter the atomic structure or modify the metal mechanical properties.[21]

The result of the current study revealed that EdgeTaper Platinum® exhibited significantly higher resistance to cyclic fatigue than ProTaper Gold®. The reason might be the different composition of more martensitic phase in EdgeTaper Platinum®.[22] Moreover, EdgeTaper Platinum® had a 0.06 taper, while ProTaper Gold® had 0.08 taper. Some studies reported that instruments with lower taper tend to present greater flexibility and higher cyclic fatigue resistance.[23],[24],[25] Probably, the lower taper of EdgeTaper platinum® might also contribute to its performance on the cyclic fatigue resistance test.

An artificial canal was used in this study to standardize the anatomical variation of natural teeth. However, parameters including radius of curvature, degree of the curvature, internal canal diameter, and also canal material are based on the most relevant study.[26],[27] An international standard validation was needed since a small variation in parameters could influence the result of cyclic fatigue resistance.

A morphological aspect of ductile fracture was shown in the instruments' surface fractures. There was no difference in the surface appearance of instruments autoclaved in different cycles. The scanning electron microscopy images showed the regular fractographic appearance of cyclic fatigue with crack initiation areas and overload zones in all groups. This is in accordance with the results found in other studies.[28],[29],[30],[31],[32]

Differential scanning calorimetry study revealed that ProTaper Gold® had approximately 50.1°C for austenite finish temperature, indicating that these instruments also mainly contain martensite or R-phase under clinical conditions.[18] EdgeTaper Platinum® has been claimed to improve the mechanical properties of the NiTi file. Because of these factors and improvement, the fractured surface of both ProTaper Gold and EdgeTaper Platinum® established a much smoother surface than the conventional NiTi files.

Nevertheless, the files used in this experiment were sterilized with no prior clinical usage; therefore, this may not truly imitate normal clinical endodontic practice. Further study should be conducted with additional clinical used before the sterilization cycle to imitate the real situation in the clinical setting.


  Conclusion Top


Autoclave sterilization did not significantly affect cyclic fatigue resistance of both EdgeTaper Platinum® and ProTaper Gold® in all groups of the experiment. Moreover, EdgeTaper Platinum® exhibited significantly higher cyclic fatigue resistance than ProTaper Gold®.

Financial support and sponsorship

This study was financially supported by the Intramural Endowment Fund, Faculty of Dentistry, Thammasat University.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Sharroufna R, Mashyakhy M. The effect of multiple autoclave sterilization on the cyclic fatigue of three heat-treated nickel-titanium rotary files: EdgeFile X7, Vortex Blue, and TRUShape. Biomed Res Int 2020;2020:8826069.  Back to cited text no. 7
    
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[PUBMED]  [Full text]  
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