Cyclic fatigue of ProTaper Ultimate and AF-F ONE rotary endodontic nickel-titanium systems in single and double curvature simulated canals at body temperature: a comparative study

Introduction

The advent of nickel-titanium (NiTi) rotary files in the 1980s has led into a new era in endodontics [1]. Since then, the adoption of NiTi rotary files for root canal preparations has been steadily increasing. This popula­rity is attributed to the superior flexibility of NiTi files compared with stainless steel instruments, even though they may not be more durable [2, 3]. Using stainless steel instruments is associated with various issues, including ledges, perforations, zips, and straightening of root canals. Such complications appear less frequently when NiTi rotary files are employed [4].
However, it is worth noting that numerous reports have indicated that NiTi endodontic files are notably more prone to fractures, especially during the shaping of curved or narrow root canals, which could potentially impact the success of root canal treatments. NiTi files are reported to have up to seven times higher risk of fracturing compared with stainless steel files [5, 6].
Several underlying factors contribute to these fractures, and the most common is torsional or cyclic fatigue. Cyclic fatigue results from the deterioration of the file due to continuous cycles of tension and compression during bending, leading to structural breakdown and fractures in the instrument. In clinical situations, this typically occurs while shaping severely curved canals. In narrow but straight canals, instrument fractures are primarily due to torsional stress, while in curved and narrow canals, instrument fractures occur due to both torsional and flexural stress [4, 7].
In recent years, dental market has witnessed the emergence of numerous brands offering NiTi rotary instruments. These advancements have been made possible due to innovations in metallurgy and manufacturing processes, resulting in instruments which are both more flexible and resistant to fracture, owing to their innovative designs and heat treatments [8].
Among these innovative instruments is the ProTaper Ultimate rotary system, developed by Dentsply Sirona. This system represents the latest iteration of the ProTaper family. It is one of the first systems to incorporate specific heat treatment technology for achieving instruments with varying mechanical properties, striking a balance between flexibility and strength. It comprise eight distinct instruments, including Slider (16/.02v), SX (20/.03v), Shaper (20/.04v), F1 (20/.07v), F2 (25/.08v), F3 (30/.09v), FX (35/.12v), and FXL (50/.10v), each manufactured using one of three different heat-treated alloys, such as M-wire (Slider), Gold-wire (SX, Shaper, F1, F2, and F3), and Blue heat-treated wire (FX and FXL). ProTaper Ultimate instruments exhibit a parallelogram cross-section design with varying acute angles at different sections and an alternating off-set machining process, where the center of mass of the instrument does not align with the center of rotation [9].
In 2020, another noteworthy entrant into the market was the AF-F ONE file manufactured by Fanta Dental in Shanghai, China. The AF-F ONE file features a flat-designed and S-shaped cross-section, offering advantages, such as reduced blade engagement, extended fatigue lifespan, and efficient debris removal from the canal, with two active cutting points, AF-R wire, and a non-cutting tip [10].
On the other hand, the ProTaper Gold file system (PTG) produced by Dentsply, Tulsa Dental Specialties, USA, has been on the market since 2014. This system’s metallurgy is enhanced through special heat treatment technology. ProTaper Gold consists of three shaping files and five finishing files. With its convex triangular cross-section and progressively tapered design, PTG incorporates a non-cutting tip that allows each file to safely navigate the secured portion of the canal [11, 12].

Objectives

Given the paucity of knowledge concerning the mechanical characteristics of these systems, i.e., ProTaper Ultimate and AF-F ONE, the objective of the current study was to evaluate the resistance of cyclic fatigue and fragment length of ProTaper Ultimate, AF-F ONE, and ProTaper Gold rotary nickel-titanium instruments. All tests were conducted at 37 ± 1°C in single- and double- curved canals. In this study, the null hypothesis was that there is no difference in cyclic fatigue resistance and the length of fractured segments between single- and double-curved canals for the tested instruments at 37 ± 1°C temperature.

Material and methods

Canal manufacture and design

AutoCAD 2018 was employed to design three- dimensional canal drawings to create artificial canals, which were then converted into DWG and DXF 2005 file formats. These drawings accurately depicted single- and double-curved canals, specifying dimensions and angles. The canals were constructed using an electrical discharge wire-cutting machine. The drawings accurately depicted both single- and double-curved canals, as follows: the double-curved canal featured two curves: curved with a 60-degree angle and a 5 mm radius of curvature in the coronal region, along with an apical curve featuring a 70-degree angle and a 2 mm curvature radius. The single-curved canal had a consistent 60-degree angle with a 5 mm curvature radius. Artificial canals had a working length of 18 mm and an inner diameter of 1.5 mm, and they were built using two slices of stainless steel, with the first slice being a 1.5 mm thick sheet milled to create the canals, and the second slice was added to achieve the desired thickness. These two parts were joined with screws, and a tempered glass cover was used to protect the artificial canal and enhance safety and visi­bility [1, 13, 14].

Methods

The study was performed after obtaining an appro­val of the Ethical Committee of the College of Denti­stry (REC reference 130, on May 1, 2023). Unused rotary instruments were arranged into three groups (each, n = 30), as follows; Group A: ProTaper Ultimate #25/.08 (tip size/taper); Group B: AF-F ONE #25/.06 (tip size/taper); and Group C: ProTaper Gold #25/.08 (tip size/taper). Sub-categories were then divided into two sub-groups (each, n = 15), based on canals’ curvature (single- or double-curved).
Each endodontic rotary file underwent a thorough examination under a stereomicroscope at 20X magnification to identify and exclude any manufacturing defects; no instruments were discarded. A dental surveyor was used to attach X-Smart Plus endo motor (Dentsply Sirona, Ballaigues, Switzerland) with handpiece 6 : 1 to ensure each file’s precise and repeatable positioning in the simulated canal [15]. Additionally, pipe clamps and specially customized rubber semi-circular pieces were designed to hold the dental handpiece securely within the surveyor. Stainless steel blocks were secured in a water bath with screws, filled with distilled water, and maintained at a constant temperature of 37 ± 1°C for all groups [16, 17]. A mini spirit levelling tool was employed to check block’s parallel and handpiece’s head to water bath’s base. To minimize friction, a synthetic lubricant (glycerin) was applied to the entire artificial canal wall. Files in each group were operated inside the artificial canals without any pecking motion until a fracture (Figure 1). Speed/torque settings for each group were adjusted according to the manufacturer’s instructions as follows: Group A: 400 rpm/4-5 Ncm; Group B: 500 rpm/2.6 Ncm; and Group C: 300 rpm/3.10 Ncm.

Measurement of the number of cycles until fracture

Time of fracture was meticulously recorded in seconds using a video time recording method, capturing the duration from the initiation of rotation inside a canal until a fracture. This approach was employed in addition to visual observation to enhance the precision of assessing fractures’ incidences [18]. Subsequently, the recorded time was multiplied by the recommended rotational speed (in rpm) of each file to calculate the number of cycles until fracture (NCF) for each instrument using the following equation [19, 20]: NCF = rpm × time to fracture (seconds)/60

Measurement of fragment length

Length of the fractured fragment of each instrument was assessed using a digital vernier caliper (Dentirak, Germany), with this length being subtracted from the original length of each instrument without a fracture, which was 25 mm [21] according to the following equation: Fracture fragment = Original length – Length of instrument after fracture

Scanning electron microscopy

Two files, which experienced fracture due to cyclic fatigue were chosen randomly from each group to undergo scanning electron microscopy (SEM) analysis to examine the fractured files’ surface characteristics.

Statistical analysis

Statistical Package for Social Sciences (SPSS) for Windows software, version 24 (IBM SPSS Inc., Chicago, IL, USA) was used for statistical analysis. Standard descriptive methods, including mean, standard deviation, and minimum and maximum values were employed to assess the number of cycles until fracture and the length of fractured fragment for each rotary instrument. Shapiro-Wilk test was utilized to assess the normality of dataset, demonstrating that data were parametric. NCF and FL data were subjected to One-way analysis of variance (ANOVA) to identify any statistical variances among all groups. Post-hoc Tukey tests were performed to compare the instruments further, and to identify any statistical differences among groups. All analyses were conducted using SPSS, with a significant level set at 5%.

Results

In this study, the statistical analysis of the number of cycles until fracture (NCF) within each group (ProTaper Ultimate, AF-F ONE, and ProTaper Gold) in both single- and double-curved artificial canals revealed that in both single and double curvature, the highest mean of cycles to fracture was observed in AF-F ONE group, followed by ProTaper Gold group and ProTaper Ulti­mate (as shown in Table 1). In single-curved canals, post- hoc Tukey test demonstrated a significant difference in NCF between the AF-F ONE group and other groups (p ≤ 0.05). In contrast, no statistically significant differences were identified between the ProTaper Ultimate and ProTaper Gold groups (p > 0.05), as shown in Table 2. Similar results were observed in double-curved canals (Table 3). Furthermore, an independent t-test revealed a significant difference in cyclic fatigue (NCF) means between single- and double-curved simulated canals across all the groups (p ≤ 0.05).
Regarding the fragment length, the results of One-way ANOVA test for fragment length indicated that there were no statistically significant differences observed among the groups regarding canal curvature, whether single- or double-curved (p > 0.05), as presented in Table 4. However, an independent t-test revealed that the length of fractured fragments was significantly greater in the single-curved canals than in the double-curved canals across all the groups (p ≤ 0.05) (Table 5).
Examination of the fractured file surfaces under a scanning electron microscope revealed evident signs of cyclic failure. In Figures 2 and 3, the red arrow identify specific characteristics linked to ductile fractures, including dimples, micro-voids, and small surfaces where cracks initiated.

Discussion

Instrument separation during endodontic procedu­res can pose a significant risk and undermine the overall success of treatment. Consequently, it becomes impera­tive to assess the mechanical characteristics and cyclic fatigue resistance of newly introduced NiTi rotary files [22]. Environmental or intra-canal temperature is a crucial factor that affects cyclic fatigue [23]. This study evaluated the endurance to cyclic fatigue of NiTi instruments under body temperature conditions, which has been reported to significantly impact their lifetime [24-27]. Stainless steel is commonly used to construct simulated canals in non-clinical tests to standardize testing procedures [7, 11]. A static model was employed in this investigation because during the static test, the instrument revolved inside a curved canal at a constant length, meaning that there is no axial oscillation.
In contrast, in a dynamic model, the endodontic file goes up and down inside a curved canal [28]. Even though the dynamic model aims to simulate how files work in root canals, the model cannot reproduce clinical conditions. Procedural errors are more likely to occur, and keeping the file in a precise trajectory is more difficult [29, 30].
This study found that the AF-F ONE file exhibited significantly higher cyclic fatigue resistance than the ProTaper Ultimate and ProTaper Gold instruments in simulated canals with both single and double curvature configurations, leading to the rejection of null hypothesis. Several factors are more likely to contribute to the AF-F ONE file’s superior performance. First was the metallurgy of the AF-F ONE file, which was manu­factured with a new heat treatment process known as AF-R wire. The manufacturers claim this file incorporates a novel heat treatment termed AF-R wire, and enhanced cutting efficiency with higher torsional and cyclic fatigue resistance. In contrast, the ProTaper Ultimate system (Shaper and Finishers F1, F2, and F3) exhibits a martensitic crystallographic arrangement at room temperature after manufacturing. However, the system tend to adopt characteristics of austenitic plus R phase alloy when exposed to body temperature [9]. A behavior similar to ProTaper Gold instruments is reported by Oh et al. [31]. This transformation implies that these instruments may become less flexible when subjected to elevated temperatures during root canal procedures.
The second reason is that the F-ONE design, characterized by its distinctive S-shaped, flat-sided cross- section of the AF-F ONE file, offer advantages, such as reduced blade engagement and extended fatigue life­span [10]. In contrast, the ProTaper Gold and ProTaper Ultimate files possess symmetrical blades without flat sides [9]. Additionally, the smaller taper of the AF-F ONE file may contribute to its enhanced cyclic fatigue resistance. Previous research has highlighted that instruments with a narrower taper exhibit an increased resilience to cyclic fatigue [32].
The findings of this study align with prior research concerning the cyclic fatigue resistance of both new and used 2Shape and AF-F ONE rotary instruments. Notably, the research demonstrated that NCF of the new AF-F ONE instruments was significantly higher than the equivalent file group of 2Shape instruments. All instruments were tested in artificial stainless steel of 1.5 mm inner diameter, with a curvature radius of 5 mm and 50-degree angle curvature, providing a valid reason for similar outcomes of this study [33].
In a relevant research conducted by Ghahramani et al. [34] in 2022, the authors examined cyclic fatigue by comparing AF-F ONE, One Curve files, and Hyflex EDM One file. The results of their research indicated that the Hyflex EDM One file exhibited the highest cyclic fatigue resilience, followed by the One Curve and AF-F ONE files. This study highlighted the superior performance of the Hyflex EDM One file attributed to its manufacturing process involving electrical discharge machining (EDM) that produces C-wires with optimal cross-section and diameter. It is worth noting that these findings contrast with those of the current study due to differences in environmental conditions. Ghahramani et al. [34] conducted their study at room temperature, while the present study was done at body temperature. It is important to recognize that temperature can potentially influence a study’s outcomes.
Furthermore, this research revealed that the ProTaper Ultimate demonstrated the lowest mean number of cycles before failure in both single- and double- curved canals, and there was no significant disparity in NCF among ProTaper Ultimate and ProTaper Gold. This may be because they have the same taper tip size, similar surface finishing, and near-equiatomic nickel-titanium ratios [9]. These results are in line with a study by Rubio et al. [35] 2022 that compared cyclic fatigue across six endodontic systems, including ESP Files Thermoflex, ProTaper Ultimate, ProTaper Next, Blue Shaper, One Curve, and 2Shape in an artificial canal made of stainless steel with the following characteristics: 3.5 mm radius of curvature, 60º curvature, 2 mm width, and 21 mm length. In this context, the ESP Files Thermoflex instrument outperformed other systems due to its metallurgical design, manufacturing process, taper, size, cross-section, helix angle, file kinematics, core diameter, and heat treatment of alloy, all of which can influence resistance to fracture. At the same time, ProTaper Ultimate had the least resistance to cyclic fatigue due to its high torque.
Moreover, this study’s data highlight a significant observation: rotary files exhibited lower cyclic fatigue resistance in double-curved canals than single-curved ones. Notably, S-shaped canals present one of the most challenging conditions in clinical situations during root canal instrumentation with NiTi rotary instruments [15]. The phenomenon observed in double curvature canals suggests that these instruments tend to break first within the apical curvature, followed by the coronal curvature. This behavior can be attributed to the sharp bend in the apical region, characterized by a 2 mm radius, as opposed to the more gradual curvature found in the coronal area, with a radius of 5 mm [15, 36].
In evaluating instrument performance, the length of a fractured portion of each file is utilized at the middle or just below the point of maximum curvature level, verifying the instruments’ precise position within the canal. There was considerable stress on the instrument at this point, so there was no significant difference between the tested files in single- or double-curved canals [28, 37]. However, the fractured portion was notably shorter in double-curved canals due to specific canal geometry factors. In single-curve canals, the curvature’s center was situated 5.25 mm from the canal’s end, representing a curve length. In double-curved canals, the midpoint of the second apical curvature was situated 2 mm from the canal’s end, and this particular curve had a length of 2.4 mm. This result aligns with previous studies evaluating cyclic fatigue in ProTaper Gold and Edge Evolve files in canals with both single and double curvatures. These earlier studies found significantly shorter fractured segments in canals with double curvature than those with a single curvature [38].

Conclusions

The AF-F ONE file demonstrates superior cyclic fatigue resistance compared with ProTaper Gold and ProTaper Ultimate in single- and double-curved canal configurations. Notably, it was observed that the number of cycles until fracture consistently showed lower values in double-curved canals when compared with single-curved ones for all tested files. Additionally, while there was no significant disparity observed in the fragment length among the groups within single- or double-curved canal, the fragment length was shorter in double-curved canals. These findings underscore the impact of canal curvature on cyclic fatigue resistance across these instruments.

Disclosures

1. Institutional review board statement: The study was approved by the the Ethical Committee of the College of Dentistry, Mustansiriyah University, REC reference 130, 1/May/2023.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References