Mendez_Gonzalez_et

INTRODUCTION

NiTi rotary instruments have revolutionized the field of endodontics due to their superplasticity and shape-memory properties, providing them with flexibility and resistance to fatigue. This allows them to navigate the intricate and varied geometries of root canals, ultimately reducing treatment time. These benefits extend to both practitioners and patients, as NiTi instruments also reduce the risk of canal transportation, preserve dentin, and decrease the likelihood of deformation or apical compression, especially in curved root canals (1-5).

The NiTi alloy typically comprises approximately 56% nickel and 44% titanium by weight, maintaining an equiatomic ratio of 1:1. Thermal treatments play an essential role in modifying the thermoelastic martensitic transformation of the alloy, which can be induced either thermally or through tension. This transformation occurs in one step (B2↔B19´) or two steps (B2↔R and R↔B19´) (6). The austenite phase (B2) displays a body-centered cubic (BCC) crystalline structure, known for its elastic properties, contributing to its hardness and resistance.

Conversely, the martensite phase (B19') represents a low-temperature phase with a monoclinic crystalline structure, making it softer and more ductile compared to austenite, thereby demonstrating plastic behavior (6, 7). The R or rhombohedral phase represents a second-order transition, induced by tension in the crystalline network caused by Ti3Ni4 precipitates, enhancing resistance to cyclic fatigue (8-10). However, even minor variations in composition, as small as 0.1%, can lead to significant changes in transition temperatures and the mechanical properties of the alloy. Consequently, these variations can influence the behavior of the alloy during root canal procedures.

Despite its superelasticity, the risk of instrument fracture within the root canal remains a significant concern for endodontists. The incidence of fracture of rotary systems ranges from 0.09 to 5%, primarily attributed to cyclic fatigue. This phenomenon occurs when a NiTi endodontic instrument undergoes an excessive number of tension-compression cycles while rotating in a curved root canal, often unexpectedly. Fracture may lead to fragment remnants being retained within the canal, hindering cleaning and shaping procedures and potentially compromising treatment outcomes (11-15).

The objective of this study is to evaluate the cyclic fatigue of Protaper GoldTM, HyflexTM EDM, and ZenflexTM rotary systems while also assessing their physicochemical properties and thermal behavior.

MATERIALS AND METHODS

This research has been conducted in full accordance with the World Medical Association Declaration of Helsinki and has been approved by the Research Ethics Committee of the Faculty of Stomatology, UASLP, with code: CEI-FE-041-021.

A total of 69 new instruments were utilized, comprising three distinct NiTi rotary file systems (n=23 each). Specifically, ProTaper GoldTM 25/.08 (Dentsply Tulsa Dental, Tulsa, OK, USA); HyflexTM EDM 25/.08 (Coltene/Whaledent, Allstätten, Switzerland); and ZenflexTM 25/.06 (ZF; Kerr Corporation, Pomona, CA, USA).

CYCLIC FATIGUE TEST

The cyclic fatigue was assessed using an Automatic Electronic Device (AED) (16) (refer to Figure 1). Each file was inserted to a depth of 18mm, and rotational movements were initiated following the manufacturer's instructions for each respective system: Protaper GoldTM (300 rpm and 3.1 Ncm), ZenFlexTM (500 rpm and 2 Ncm), and HyFlexTM EDM (400 rpm and 2.5 Ncm), until fracture occurred. The duration of each trial was recorded, and the resulting fragments were collected and measured for length using a digital vernier.

DIFFERENTIAL SCANNING CALORIMETRY (DSC)

Phase transitions of the rotary instruments were analyzed using a TA-Instruments DSC Q2000 (TA Instruments, LLC, Waters, USA). Segments were cut from each rotating instrument, and 10 mg of the material was inside an aluminum pan (n=2/rotatory instruments by group). Subsequently, the samples were subjected to 2 heating and 2 cooling thermal cycles within a temperature range between -50 to 120ºC, employing a rate of 5°C/min in a nitrogen atmosphere. Corresponding to the phase transitions were determined utilizing the specialized analysis software TA Universal Analysis (TA Instruments, LLC, Waters, USA).

CHEMICAL ANALYSIS by SPECTROSCOPY OF INDUCTIVELY COUPLED PLASMA (ICP-OES)

The chemical composition analysis of the instruments was conducted using ICP-OES. Segments weighing between 0.05 and 0.1mg were digested with a mixture consisting of 1.75 mL of 36.5-38% hydrochloric acid (HCl) (Macron, Fine Chemicals®, Radnor, PA, USA), 5.75 mL of 64.5-66.5% nitric acid (JT Baker), and 2 mL of 48-51% hydrofluoric acid (HF) (Analytyka®). These segments were placed in EasyPrep Plus CEM™ containers within a microwave (Mars 6 CEM™) programmed to undergo a 15-min ramp process with a power range of 900 to 1050 kW, followed by an isothermal process for 30 min at 210ºC. Subsequently, the resulting solution was neutralized with 5 mL of 4% v/v boric acid (H3BO3) and diluted to a final volume of 50 mL with 2% v/v nitric acid. Finally, the samples were analyzed using ICP-OES at wavelengths of 308.802 nm for titanium (Ti) and 341.476 nm for nickel (Ni) using an iCAP 7400 Duo instrument (Thermo Scientific™, Waltham, MA, USA).

X-RAY DIFFRACTION

The crystalline phases of the instruments were analyzed using a PANalytical X'Pert PRO X-ray diffractometer (PANAlytical, Almelo, NL) operating at a wavelength of 1.54 Å with a 35 kV acceleration voltage and 30 mA current. The obtained diffraction patterns were then processed using the HighScore Plus software (PANAlytical, Almelo, NL) to analyze signals and identify the crystalline phases present in the samples.

STATISTICAL ANALYSIS

The distribution of data was assessed using the Shapiro-Wilk test. A non-parametric Kruskal-Wallis analysis was employed to compare multiple groups, followed by Tukey's post hoc test for pairwise comparisons. All statistical tests were conducted at a significance level of 95% (α=0.05). Statistical analyses were conducted utilizing SAS JMP statistical software (SAS Institute Inc., Cary, NC, USA), version 16.

RESULTS

Table 1 shows the mean values and standard deviations for fracture time (tF), number of cycles (NCF), and length of the fractured segment (FL). Statistically significant differences were observed among the three groups (p<0.05, Kruskal-Wallis test). Post hoc analysis using the Tukey-Kramer method revealed significant differences (p<0.05) between ZenFlexTM and HyFlexTM EDM, ZenFlexTM and Protaper GoldTM, as well as HyFlexTM EDM and Protaper GoldTM for each of the variables assessed. ZenFlex™ files showed the most significant resistance to cyclic fatigue, followed by HyFlex™ EDM and Protaper Gold™. Regarding fracture time, HyFlex™ EDM files showed the longest time to fracture, followed by the ZenFlex™ and Protaper Gold™ groups.

Figure 2 illustrates the SEM micrographs obtained before and after exposing the instruments to cyclic fatigue fracture. In Figure 2.A, the active portion of the ZenFlexTM instrument exhibits defects on the cutting edges, along with artifacts of organic residue and granules of excess material on the finished surface (indicated by arrows) (Figure 2.B). Similarly, defects are observed in the Protaper GoldTM experimental group, including circular, oval, and fused (coalescence) concavities in the non-cutting tip, as well as striations, micro-cracks on the cutting edge, and artifacts of organic residue and excess material granules on the finished surface (Figure 2.E). Manufacturing lines perpendicular to the file’s longitudinal axis are also visible (Figure 2.F). For the HyFlexTM EDM experimental group, the micrograph shows the rectangular shape of the file's tip and defects on the lateral part, along with the characteristic surface finish corresponding to electrical discharge machining (Figure 2.I and Figure 2.J, respectively).

Subsequently, the fractographic study of the experimental groups after cyclic fatigue fracture is presented. In the cross-section of the ZenFlexTM experimental group, a triangular shape with peripheral cracks and crack propagation is evident (arrows) (Figure 2.C), along with patterns of cones and craters characteristic of cyclic fatigue failure (Figure 2.D). For the Protaper GoldTM experimental group, peripheral cracks and crack propagation are observed in the cross-section of the fracture (Figure 2.G), along with patterns of cones and craters and coalescence of micro-cavities (*) in the approaches to the edge of the cross-section (Figure 2.H). Figure 2.K illustrates the trapezoidal cross-section of the HyFlexTM EDM file, showing the initiation of edge cracks (rounded area), peripheral indentations, and crack propagation (arrows), as well as patterns of cones and craters (delimited area), with crack propagation and coalescence of micro-cavities (*) (Figure 2.L).

Thermal scans offer valuable insights into the phase transformations occurring in NiTi alloys as a function of temperature, as well as the energies involved in these transitions (Figure 3). Upon heating, endothermic peaks are observed, indicating the energy absorption required for the instrument to transition from the austenite phase to the martensite phase. For the Protaper GoldTM file, the onset of transformation occurred at a lower temperature compared to ZenflexTM and HyflexTM EDM, commencing around 11.0, 30.26, and 38.4ºC (As-austenitic onset), respectively, and concluding at 37.0, 60.5, and 61.4°C (Af-austenitic finish), respectively, with corresponding enthalpies of 3.42, 2.87, and 8.4 J/g. During the cooling process, the transformation from austenite to martensite was evident.

For the HyflexTM EDM file, two exothermic peaks are observed, indicating a two-stage transformation process and the presence of an intermediate phase known as the R phase. The transformation begins at 65.3ºC (Rs-R-phase onset) and concludes at 32.1ºC (Rf-R-phase finish) with an enthalpy of 2.78 J/g, followed by a martensite phase starting at 5.6ºC (Ms-martensitic onset) and ending at -23.2ºC (Mf-martensitic finish) with 3.54 J/g. Upon cooling, ZenflexTM and Protaper GoldTM exhibit a single-stage transition, starting at 32.0 and 57.0ºC (Ms) and concluding at 11.8 and 26.5ºC (Mf), with enthalpies of 2.47 and 3.45 J/g, respectively.

The atomic ratio of Ni:Ti was determined using ICP-OES to be 1.0:0.94 for ZenflexTM, 1.00:1.07 for Protaper GoldTM, and 1.00:1.06 for HyflexTM EDM. Statistical analysis revealed no significant difference in the nickel content (p=0.119) or titanium content (p=0.079) among the different rotary instruments. However, it is noteworthy that ZenflexTM exhibits a higher atomic content of nickel compared to titanium.

The design parameters of the rotary instruments were assessed by analyzing micrographs using the ImageJ program (Figure 4). In terms of the helical angle, ZenflexTM demonstrates a smaller angle (15.96±1.21°) compared to Protaper GoldTM and HyflexTM EDM, which exhibit more open angulations of 19.77±0.42° and 21.27±0.73°, respectively (Figure 4.A-C).

Regarding the type of point, all evaluated instruments feature a passive tip (Figure 4.G-I). The cross-sectional micrographs (Figure 4.J-L) reveal a negative cutting angle, with Protaper GoldTM and HyflexTM exhibiting a triangular shape, while ZenflexTM displays a polyhedral section.

Figure 5 shows the X-ray diffraction (XRD) pattern of the rotary instrument files, providing insights into the phases present in the instruments at room temperature. In the case of ZenflexTM, the austenite phase (B2) was evident, characterized by an intense peak at 42.614º in 2θ associated with the (110) plane, along with two signals of better intensity around 62 and 78° in 2θ corresponding to the planes (200) and (211), respectively.

Contrastingly, the HyflexTM EDM instrument exhibits a distinct pattern, featuring a double peak around 42 to 43° in 2θ, characteristic of the R phase. Furthermore, the XRD pattern of Protaper GoldTM demonstrates a broadening of the main peak, indicative of a coexistence of phases likely attributable to the temperature at which the analysis was conducted.

DISCUSSION

Cyclic fatigue stands as the primary cause of instrument fracture, accounting for between 50 and 90% of mechanical failures (17). This phenomenon is influenced by various factors, including anatomical aspects of the root canal such as radius and angle of curvature, as well as the physical and chemical properties of the NiTi alloy (e.g., equiatomic ratio, heat treatment, and crystallographic arrangement), alongside the design of the instrument and the kinematics of the drive motor (11, 18, 19). It's crucial to recognize that these factors do not act independently; rather, they collectively contribute to instrument fracture.

The objective of this study was to assess the cyclic fatigue fracture of ZenFlexTM, Protaper GoldTM, and HyFlexTM EDM rotary files in simulated curved canals while also examining their physicochemical properties. The absence of regulations governing the marketing of NiTi rotary files without prior studies can lead to accidents during the endodontic procedure, as operators may be unaware of the mechanical and physical characteristics of the instruments they are working with.

SEM studies have revealed the presence of cracks and defects on the surface of new NiTi instruments, likely associated with manufacturing processes, which can predispose them to fracture and reduce their fatigue resistance. These defects often act as stress concentration points, initiating cracks that propagate to critical lengths, ultimately resulting in fracture (20, 21). This is exemplified by the observation of cracks and defects on the cutting edges of Protaper GoldTM and ZenFlexTM instruments prior to cyclic fatigue evaluation, which coincide with the fracture sites observed after the cyclic fatigue analysis.

The fractographic study of post-fracture micrographs, combined with findings from existing literature, indicates that patterns of cones and craters of ductile rupture are characteristic features of cyclic fatigue failure. These patterns have been extensively documented in previous studies (12, 17, 19, 22, 23). During cyclic fatigue failure, cracks initiate and propagate due to the presence of machining imperfections, such as corners and defects, which act as stress concentration points. These imperfections serve as the starting points for fatigue cracks, which then propagate under the alternating tensile-compressive bending stresses experienced during cyclic loading. Even a few larger particles are sufficient to initiate microcavity growth and coalescence without causing macroscopic plastic deformation. As the cracks propagate and coalesce, they result in the formation of a fracture surface characterized by hemispherical depressions, often referred to as cones. This process underscores the importance of understanding the role of surface imperfections and cyclic loading in the fatigue failure of NiTi rotary instruments, as it provides insights into the mechanisms underlying their fracture behavior. According to the findings of the present study, ZenFlexTM files exhibit greater resistance to cyclic fracture, attributed to their lower metallic mass and triangular cross-section. Previous research has demonstrated that instruments with a triangular cross-section, like ZenFlexTM, possess greater resistance to cyclic fatigue compared to those with a convex triangular or rectangular cross-section of similar diameter (24-26). Furthermore, instruments with a smaller taper, such as ZenFlexTM, demonstrate enhanced resistance to cyclic fatigue compared to larger instruments with greater taper. This is because the size at the point of maximum stress during a test can significantly impact the fatigue life of NiTi rotary instruments (27, 28). However, despite ZenFlexTM's superior performance in terms of cyclic fatigue, HyFlexTM EDM exhibits longer resistance to cyclic fracture. Studies, such as those conducted by Kaval et al. (29), recommend using files like HyFlexTM EDM for safe use in highly curved canals due to their increased resistance to bending fatigue. The prolonged resistance to cyclic fracture observed in HyFlexTM EDM may be attributed to the presence of the R phase in the thermograms, which is characterized by a two-step transformation (austenite to R phase and R phase to martensite) (30). This intermediate phase, positioned between the austenite and artensite phases, contributes to enhancing fatigue life. This enhancement may be associated with dislocation networks and/or coherent precipitations of Ni4Ti3 formed during cooling, which are generated by thermomechanical treatments such as post-deformation annealing (9, 31).

Similarly, it has been observed that the austenitic transformation temperature (As) commences at 38.4°C, indicating that the HyFlexTM EDM instrument undergoes a transition between the R-martensite phases at the onset of treatment, thereby enhancing its resistance to bending (32). Zenflex is in an austenitic phase, rendering it more rigid, which is associated with a slight increase in the nickel content in the equiatomic ratio of the NiTi alloy, leading to a decrease in the starting temperature of the martensitic state transition (Ms) (33). However, ZenflexTM offers two smaller tapers than HyFlexTM EDM, potentially benefiting from cases with more aberrant curvatures. In the case of Protaper GoldTM, the transition temperature range is 30-60°C, indicating that the instrument transitions between the austenite and martensite phases at the beginning of treatment, possibly making Protaper GoldTM more reliable in the treatment of narrow and calcified root canals. Additionally, studies have demonstrated the superiority of the HyFlexTM EDM instrument over Protaper GoldTM, as files manufactured with controlled memory have been reported to exhibit greater resistance to cyclic fatigue (34, 35).

Regarding the design of rotary instruments, HyflexTM EDM has a wider angle (21.271±0.73°), which can induce faster dentin wear, thus presenting a greater risk of the instrument becoming lodged in the walls, which can cause a fracture. Concerning the pitch or thread pitch, the HyflexTM EDM instrument features smaller spaces between spirals or thread pitches, which decreases the tendency to twist and suction tendencies decrease (36).

The analysis of all the obtained results allows us to discern the advantages conferred by the instruments through a combination of factors, including design, chemical composition, crystallographic structure, and thermal treatments, all of which influence the mechanical properties of the instruments, particularly their resistance to cyclic fatigue. By considering these aspects, clinicians can make informed decisions when selecting the most suitable rotary instrument for each specific case, thereby enhancing treatment safety. Also, it is important to note the absence of an international standard for evaluating the fatigue resistance of rotary instruments, with the American Dental Association (ADA) yet to establish a standardized protocol for testing. Cyclic fatigue tests vary widely in design, making it challenging to compare results across studies. Discrepancies in study findings can also be attributed to differences in endodontic motors, where real kinematic values such as torque and rpm may differ from manufacturers' declared values. Additionally, variations in the devices used for evaluating cyclic fatigue contribute to divergent results. Notably, most research employs static conditions, with only a small percentage utilizing dynamic fatigue devices (13). Static models apply flexion forces consistently at the same point on the file, failing to replicate the real conditions encountered in clinical practice, where files are not stationary within the canals during instrumentation (14). In contrast, dynamic models distribute tension along a broader area along the instrument's axis and mimic the pecking movement performed by clinicians during root canal treatment (11). These dynamic models provide a more realistic simulation of clinical scenarios, offering valuable insights into the fatigue resistance of rotary instruments under dynamic conditions.

The AED utilized in our study offers several key advantages over alternative devices. Notably, its ability to record crucial data such as the number of cycles and fracture time, coupled with automation and self-calibration features, sets it apart (37). One of its most significant advantages lies in its capability to mitigate the influence of human error, ensuring consistent and reliable results. By providing precise control over applied force and speed, the AED proved to be highly effective in evaluating cyclic fracture parameters. Although the standardized stainless-steel artificial canal cannot fully replicate clinical conditions, it serves to minimize extraneous variables influencing file fracture beyond cyclic fatigue (37). Consequently, the findings from these in vitro studies hold considerable value for endodontic specialists, offering valuable insights into treatment protocols (29).

Finally, it is important to acknowledge certain limitations inherent in the research. Firstly, the ZenFlexTM files' torsional fracture was not evaluated, nor was their fracture due to cyclic fatigue investigated in an in vitro model using extracted teeth. Additionally, the study did not encompass comparisons involving higher degrees of curvature. Addressing these limitations could provide further depth to the understanding of rotary instrument performance and fracture mechanisms. Future studies could further explore the clinical implications of the differences in cyclic fatigue resistance among these NiTi rotary file systems, particularly in challenging clinical scenarios such as retreatments or curved canal anatomies. Investigating the long-term performance of these instruments under dynamic conditions, including repetitive use and sterilization cycles, could provide valuable insights into their durability. Additionally, advanced characterization techniques, may help elucidate the role of heat treatment and crystalline structure on mechanical properties.

CONCLUSIONS

The ZenFlexTM file demonstrated superior resistance to cyclic fatigue, demonstrating its durability under repetitive stress. Conversely, the HyFlexTM EDM file exhibited prolonged working time prior to fracture, indicating its resilience in withstanding prolonged usage. On the other hand, the Protaper GoldTM file exhibited a larger size of separated fragments upon fracture.

AUTHOR CONTRIBUTION STATEMENT

Conceptualization and design: M.V.M.G. and M.G.S.

Literature review: M.C.B.J., A.I.O., M.V.M.G. and M.G.S.

Methodology and validation: V.A.E.B., A.P.G., C.G.F. and M.G.S.

Formal analysis: V.A.E.B., A.P.G., M.V.M.G. and M.G.S.

Investigation and data collection: M.C.B.J., A.I.O., C.G.F., A.P.G. and M.G.S.

Resources: V.A.E.B., A.P.G. and M.V.M.G.

Data analysis and interpretation: A.P.G., M.V.M.G. and M.G.S.

Writing-original draft preparation: M.V.M.G. and M.G.S.

Writing-review & editing: A.P.G., C.G.F. and M.G.S.

Supervision: M.V.M.G., A.P.G. and M.G.S.

Project administration: C.G.F., A.P.G. and M.V.M.G.

Funding acquisition: A.P.G. and M.V.M.G.

ACKNOWLEDGMENTS

María Concepción Betanzos-Juárez and Alejandra Izquierdo-Orozco thank CONAHCYT for the scholarship granted for the master’s degree in Endodontics at the UASLP.

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