Design, Metallurgy, Mechanical Properties , and Shaping Ability of 3 Heat‑Treated Reciprocating Systems: A Multimethod Investigation

Abstract

Objective. This study aimed to compare 3 reciprocating systems regarding design, metallurgy, mechanical properties, and shaping ability. And further details about endodontic treatment protocols are accessible for you to learn on our website in Endodontic section.

Materials and methods. New Reciproc Blue R25, WaveOne Gold Primary, and REX 25 instruments (n=41 per group) were analyzed regarding design, metallurgy, and mechanical performance, while shaping ability (untouched canal walls, volume of removed dentin, and hard tissue debris) was tested in 36 anatomically matched root canals of mandibular molars. Results were compared using one-way ANOVA post hoc Tukey and Kruskal–Wallis tests with a signifcant level set at 5%.

Results. All instruments showed symmetrical cross sections with asymmetrical blades, no radial lands, no major defects, and an almost equiatomic nickel and titanium ratio. The highest R-phase start temperatures were observed with WaveOne Gold (46.1°C) and REX (44.8°C), while Reciproc Blue had the lowest R-phase start (34.5°C) and fnish (20°C) temperatures. WaveOne Gold had the lowest time to fracture (169 s) and the highest maximum load (301.6 gf) (P <0.05). The maximum torque of Reciproc Blue (2.2 N.cm) and WaveOne Gold (2.1 N.cm) were similar (P >0.05), but lower than REX (2.6 N.cm) (P <0.05). No statistical diferences were observed among instruments in the angle of rotation (P >0.05) and in the shaping ability in both mesial and distal canals (P >0.05).

Conclusion. Although the overall design, temperature transition phases and mechanical behavior parameters were diferent among tested instruments, they were similar in terms of shaping ability.

Clinical relevance. All tested heat-treated NiTi reciprocating systems showed similar shaping ability, without clinically signifcant errors.

Introduction

In the last years, eforts to reduce the fracture occurrence [1, 2] of NiTi instruments resulted in two major technological advancements: the asymmetric oscillatory kinematic — commonly known as reciprocating motion — and the heat treatment of the NiTi alloy. The reciprocating motion relieves stress on the instrument by a special counterclockwise motion rotation to cut dentine and a short clockwise rotation to relieve the instrument [3]. In comparison to continuous rotation, this kinematic extends the instrument lifespan by increasing its resistance to fatigue [4] and reducing the occurrence of plastic deformation [5–7]. The heat treatment, in its turn, allowed the development of NiTi instruments with the crystalline structure in intermediate stages between austenitic and martensitic phases, but with substantial stable martensite phase under body temperature [8, 9]. The alteration in the crystalline microstructure of the NiTi alloy has a signifcant infuence in its mechanical properties once the martensitic phase has greater elasticity and can reach higher deformation with relatively low stress compared to the austenitic one [10].

Reciproc Blue (VDW, Munich, Germany) and WaveOne Gold (Dentsply Sirona Endodontics, Baillagues, Switzerland) are examples of reciprocating instruments composed of substantial amounts of martensite obtained by proprietary heat treatments of the NiTi alloy. Several research studies have confrmed the increased fatigue resistance and fexibility of these heat-treated systems compared to conventional NiTi instruments [11–15].

Recently, the REX reciprocating system (Medidenta, Las Vegas, NV, USA) was launched into the market with the proposal of having NiTi instruments made with diferent heat treatments, making fexibility and resistance to be consistently balanced depending on the metal mass of each instrument in the series. This system includes instruments for mechanical glidepath [REX Glide Path (17/.05v)], with the alloy in purplish color, and instruments presenting diferent yellowish tonalities for shaping [REX 25 (25/.08v) and REX 40 (40/.06v)]. To date, there is no scientifc evidence to support the efcacy or safety of these new instruments. Therefore, this study aimed to use a multimethod approach to compare the design characteristics, metallurgical features, mechanical performance, and shaping ability of REX instruments with the well-known Reciproc Blue and WaveOne Gold systems. The null hypothesis tested was that there would be no diferences amongst the tested instruments regarding the evaluated properties.

Material and methods

A total of 123 new 25-mm NiTi instruments (41 per group) from 3 reciprocating systems [Reciproc Blue R25 (25/.08v), WaveOne Gold Primary (25/.07v), and REX 25 (25/.08v)] were analyzed regarding design, metallurgical characteristics, and mechanical performance. In addition, twenty-four instruments (8 per group) were employed for testing the shaping ability of Reciproc Blue [4 R25 and 4 R40 (40/.06v)], WaveOne Gold [4 Primary and 4 Large (45/.06v)], and REX [4 REX 25 and 4 REX 40 (40/.06v)] systems in root canals of extracted mandibular molars. Before their use, the selected instruments were examined under a stereomicroscope (×13.6 magnifcation; Opmi Pico, Carl Zeiss Surgical, Germany) looking for defects that would exclude them from being tested, but none was excluded.

Instrument Design

The number of active blades (in units) and the helical angles (in degrees) at the 6 most coronal futes of 6 randomly selected instruments from each system were assessed under a stereomicroscope (×13.6 magnifcation; Opmi Pico) using the ImageJ v1.50e software (Laboratory for Optical and Computational Instrumentation, Madison, WI, USA). These same instruments were further evaluated under scanning electron microscopy (SEM) at ×100 and ×500 magnifcations (Hitachi S-2400, Hitachi, Tokyo, Japan) regarding their active blade design (radial lands and symmetry), crosssectional shape, tip geometry (active or non-active), surface fnishing, deformations, and defects.

Metallurgical Characterization

The semi-quantitative elemental analysis of 3 instruments from each tested system was carried out to evaluate the proportions of nickel, titanium, or any other relevant element, using a scanning electron microscope (S-2400; Hitachi) equipped with an energy-dispersive X-ray spectroscopy (EDS) (Bruker Quantax; Bruker Corporation, Billerica, MA, USA) set at 20 kV and 3.1 A. The analysis was performed at a 25-mm distance from the surface (400 μm2) of each instrument using a dedicated software with ZAF correction (Systat Software Inc., San Jose, CA, USA). Diferential scanning calorimetry (DSC) method (DSC 204 F1 Phoenix; Netzsch-Gerätebau GmbH, Selb, Germany) was used to determine the phase transition temperatures of instruments’ alloy following the guidelines of the American Society for Testing and Materials [16] and a previously documented protocol [13]. Phase transformation temperatures were analyzed by the Netzsch Proteus Thermal Analysis software (Netzsch-Gerätebau GmbH). In each group, DSC test was performed twice to confrm the results. Tested instruments include Reciproc Blue R25, WaveOne Gold Primary, REX Glide Path, REX 25, and REX 40. Unlike Reciproc Blue and WaveOne Gold systems, all set of REX instruments were tested due to diferences in their heat treatment, as claimed by the manufacturer.

Mechanical Tests

The mechanical performance of the selected systems was evaluated through cyclic fatigue, torsional resistance, and bending tests. The sample size calculation was based on the highest diference of 2 of the tested systems after 6 initial measurements considering an alpha-type error of 0.05 and a power of 80%. For the time to fracture, maximum torque and angle of rotation (WaveOne Gold vs. REX), fnal sample sizes of 6, 10 and 70 instruments were determined based on efect sizes of 111.8 (± 62.2), 0.6 (± 0.5) and 31.3 (± 47.2), respectively, while, for the maximum load in the bending test (WaveOne Gold vs. Reciproc Blue), an efect size of 59.6 (± 36.7) resulted in a fnal sample size of 8 instruments. Although sample size calculation determined that 70 instruments would be needed to evaluate the angle of rotation, this high value can be considered of low clinical meaning and therefore the sample size was set at 10 for all parameters.

The cyclic fatigue test was conducted on a non-tapered stainless steel curved tube apparatus (radius of 6 mm and 86° angle) according to a previous reported methodology [13, 17], using glycerin as lubricant. The tested instruments were adapted to a 6:1 reduction handpiece (Sirona Dental Systems GmbH, Bensheim, Germany) powered by a torque-controlled motor (VDW Silver; VDW GmbH) set at RECIPROC ALL (Reciproc Blue and REX) or WaveOne ALL (WaveOne Gold) modes and activated at a static position. The test was performed at room temperature (20 °C) following the guidelines of the American Society for Testing and Materials applied to superelastic NiTi materials [18]. Fracture was detected by visual and auditory inspection. The time to fracture (in seconds) was recorded using a digital chronometer, and the fragment size (in mm) was measured with a digital caliper for experimental control.

Torsional and bending resistance tests were performed according to international standards [19, 20]. In the torsional test, instruments were clamped at their apical 3 mm and rotated counterclockwise at a constant pace of 2 rotations per minute to assess the maximum torque (in N.cm) and the angle of rotation (in degrees) prior to fracture. In the bending test, each instrument was mounted in the fle holder of the motor and positioned at 45° in relation to the foor, while its apical 3 mm was attached to a wire connected to a universal testing machine (Instron 3400; Instron Corporation, Canton, MA, USA). The maximum load needed for a 45° displacement of the instrument, using a load of 20 N and 15 mm/min of constant speed, was recorded in gram/force (gf).

Shaping Ability

After approval of the local Ethics Committee (Protocol CE-FMDUL 13/10/20), ninety-four two-rooted mandibular molars with fully formed apices were randomly selected from a pool of extracted teeth and scanned at pixel size of 11.93 μm in a micro-computed tomographic device (micro-CT) (SkyScan 1173; Bruker-microCT, Kontich, Belgium) set at 70 kV, 114 mA, rotation of 360° with steps of 0.7°, using a 1-mm-thick aluminum flter. The acquired projections were reconstructed into axial cross-sections using standardized parameters of smoothing (1), attenuation coef-fcient (0.05–0.007), beam hardening (20%), and ring artifact (5) corrections (NRecon v.1.6.9; Bruker-microCT). A three-dimensional (3D) model of the internal anatomy of each tooth was created (CTAn v.1.14.4; Bruker-microCT) and qualitatively evaluated (CTVol v.2.2.1; Bruker-microCT) regarding root canal confguration. Then, volume and surface area the mesial and distal canals were calculated from the cementoenamel junction to the apex. Based on these parameters, specimens were anatomically matched to create 3 groups of 4 teeth (n = 12 canals). Then, each set of teeth was randomly assigned to an experimental group according to the preparation system: Reciproc Blue, WaveOne Gold, and REX.

After conventional access cavity preparation, apical patency was confrmed using a size 10 K-fle (Dentsply Sirona Endodontics). Glide path was then performed with a size 15 K-fle (Dentsply Sirona Endodontics) up to the working length (WL), established 1 mm from the apical foramen. All canals were initially prepared with instruments size 25, according to each group (Reciproc Blue R25, WaveOne Gold Primary, and REX 25) and then distal canals were further enlarged with instruments size 40 (Reciproc Blue R40 and REX 40) or size 45 (WaveOne Gold Large). Instruments were activated in reciprocating motion powered by an electric motor (VDW Silver; VDW) set at “RECIPROC ALL” (Reciproc Blue and REX) or “WAVEONE ALL” (WaveOne Gold) modes. Each instrument was moved to the apical direction using a slow in-and-out pecking motion of about 3-mm amplitude with light pressure. After 3 pecking motions, the instrument was removed from the canal and cleaned. The WL was reached after 3 waves of instrumentation. Each instrument was used in one tooth and discarded. Irrigation was performed with a total of 15 mL of 2.5% NaOCl per canal, followed by a fnal rinse with 5 mL of 17% EDTA (3 min) and 5 mL of distilled water using a syringe ftted with a 30-G NaviTip needle (Ultradent, South Jordan, UT, USA) positioned 2 mm from the WL. All procedures were performed by an operator with a large experience using reciprocating systems.

After slightly drying the root canals with paper points (VDW), a fnal scan and reconstruction were performed using the previously mentioned parameters followed by the co-registration of datasets acquired before and after preparation (3D Slicer 4.3.1 software). Shaping ability was assessed by measuring 3 parameters: (i) the volume (in mm3) of dentin removed after preparation, (ii) the volume (in mm3) of hard tissue debris created by the preparation protocols, and (iii) the percentage of unprepared canal walls, according to methodologies published in previous studies [21, 22]. All analyses were done by an examiner blinded to the shaping protocols. Canal interconnections and accessory anatomies were excluded for the analyses.

Statistical Analysis

The Shapiro-Wilk and Lilliefors tests were used to verify the normality of the data. One-way ANOVA and post hoc Tukey tests were carried out to compare the helical angle, time to fracture, angle of rotation, maximum bending load, root canal volume and surface area, volume of removed dentine, hard tissue debris in the mesial canals, and untouched canal walls, while Kruskal–Wallis test was used to evaluate the maximum torque to fracture and the volume of hard tissue debris in the distal canals, with a signifcance level set at 5% (SPSS v25.0 for Windows; SPSS Inc., Chicago, IL, USA). Depending on data distribution, results were summarized as mean (standard deviation) or median (interquartile range) values.

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Results

Instrument Design

The stereomicroscopic assessment revealed similar number of blades and helical angles in the REX and WaveOne Gold instruments (Table 1). SEM analysis (Fig. 1) showed that all instruments had symmetrical cross sections with asymmetrical blades and no radial lands. WaveOne Gold instrument had an ofset parallelogram-shaped cross section, whilst REX and Reciproc Blue had an inverted S-shaped profle. None of the tips could be identifed as active, and the overall geometry and transition angles to the blade varied instruments. While the tip of Reciproc Blue and WaveOne Gold was fat at its end, it showed a bullet-like shape in the REX instrument. In higher magnifcation all instruments showed similar surface fnishing with a pattern of parallel horizontal marks created by the grinding manufacturing process. In the REX instruments, it was also possible to observe some metal rollovers on the blades.

Table 1. Stereomicroscopic assessment of instruments expressed as mean (standard deviation) or median [interquartile range]

Fig. 1. SEM representative images of the tested instruments depicting that all instruments have symmetrical cross sections with asymmetrical blades and no radial lands. REX and Reciproc Blue showed an inverted S-shaped profle, while WaveOne Gold had an ofset parallelogram-shaped cross section. Tips were non-active with diferences in the overall geometry and transition angles to the blade. All surfaces had parallel manufacturing marks with few irregularities. In the REX instruments, it is possible to observed metal rollovers on the blades

Metallurgical Characteristics

EDS/SEM analysis revealed a nearly equiatomic composition of nickel and titanium elements in all instruments (Ni/Ti Ratio 1.016, 1.032, and 1.028 for Reciproc Blue, WaveOne Gold, and REX instruments, respectively), without any other traceable metal element. Cooling and heating curves of tested instruments obtained by the DCS analyses are depicted in Fig. 2. Comparison among systems (Fig. 2a) showed distinct transformation temperature curves suggesting the presence of R-phase in all of them at testing temperature (20 °C). The highest R-phase start temperatures were observed in WaveOne Gold Primary (46.1 °C) and REX 25 (44.8 °C) instruments. Reciproc Blue R25 had the lowest R-phase start (34.5 °C) and R-phase fnish (20 °C) temperatures (Figure 2a). The lowest (8.5 °C) and highest (51.3 °C) austenitic start and fnish temperatures were observed in the WaveOne Gold Primary instrument. DSC test of REX instruments (Fig. 2b) demonstrated similar heat treatment between REX Glide Path and REX 25 with minor diferences in R-phase transformation temperatures, in the cooling transformation of martensitic B19’, and in the austenitic transformation during heating curves. On the other hand, REX 40 showed major diferences mostly on cooling (R-phase to martensite B19’ transformation) and on heating, with an almost perfectly overlapped martensitic B19’ and R-phase transformations to austenite-B2 (Fig. 2b).

Fig. 2. DSC charts showing the phase transformation temperatures of the assessed fles, with the cooling curves on top (reads from right to left) showing the R-phase start (Rs) and fnish (Rf) temperatures and heating curves on bottom (reads from left to right) detailing the austenitic start (As) and fnish (Af) temperatures. a The highest R-phase start temperatures were observed with WaveOne Gold Primary (46.1 °C) and REX 25 (44.8 °C), while Reciproc Blue R25 had the lowest R-phase start (34.5 °C) and R-phase fnish (20 °C) temperatures. The lowest (8.5 °C) and highest (51.3 °C) austenitic start and fnish temperatures were observed in the WaveOne Gold Primary instrument. REX 25 showed DSC curves completely diferent from both others, while all of them appear to be at R-phase at test temperature (20° C). b Transformation temperatures of the REX system instruments. REX 17 (Glide Path) and REX 25 had similar transformation temperatures with minor diferences in R-phase transformation temperatures and in the cooling transformation of martensitic B19’. The REX 40 had the most visible diferences with an almost perfectly overlapped martensitic B19’ and R-phase transformations to austenite-B2

Mechanical Performance

WaveOne Gold had the lowest time to fracture and the highest maximum load (P < 0.05), while no statistical diferences were observed in these parameters between Reciproc Blue and REX instruments (P > 0.05). The maximum torque values of Reciproc Blue and WaveOne Gold were similar (P > 0.05), but lower than REX instrument (P < 0.05). No diference amongst instruments was observed in the angle of rotation (P > 0.05) (Table 1).

Shaping Ability

The homogeneity of groups regarding morphometric parameters of volume and surface area in mesial and distal root canals was confrmed (P > 0.05) (Table 2). No statistical diferences were observed among the tested systems regarding the volume of hard tissue debris (P > 0.05), the dentin removed after preparation (P > 0.05), and the percentage of untouched canal walls in both mesial and distal canals (P > 0.05).

None of the preparation protocols was able to prepare all root canal surfaces (Fig. 3) or render root canals free from hard tissue debris (Table 2). The mean percentages of unprepared canal walls were 21.8% (Reciproc Blue), 17.4% (REX), and 21.5% (WaveOne Gold) in the mesial canals (Table 2), and 16.8% (Reciproc Blue), 13.6% (REX) and 17.0% (WaveOne Gold) in the distal canals (Table 2).

Table 2. Pre- and post-operative parameters (mean, standard deviation, and range interval) evaluated in mesial (n=24) and distal (n=12) root canals of mandibular molars after preparation protocols with 3 reciprocating instruments

Fig. 3. Representative micro-CT 3D models of mesial and distal canals mandibular molars showing the root canals before (green color) and after (red color) preparation with Reciproc, WaveOne Gold and REX systems. Mesial canals were prepared with instruments size 25, while the enlargement of distal canals were done with instruments size 40 (Reciproc Blue R40 and REX 40) or size 45 (WaveOne Gold Large). None of the shaping protocols were able to prepare the entire surface area of the root canal walls

Discussion

The present investigation provides answers to a series of questions about the mechanical behavior of 3 reciprocating systems through the use of a multimethod research analysis. The main advantage of this approach is the possibility to ofset the weaknesses of each test providing more information, better understanding, and superior internal and external validation [23]. In addition, this approach avoids the phenomenon of “knowledge compartmentalization”, i.e., the knowledge about a specifc domain composed of several separate, not intertwined parts, usually obtained in single or double assessment methods [24]. In this study, overall design, manufacturing quality, elemental composition, and phase transformation temperatures of Reciproc Blue, WaveOne Gold, and REX reciprocating NiTi systems were assessed in order to achieve a better comprehension of the results obtained in the cyclic fatigue, torsional resistance, bending load, and shaping ability tests. Notwithstanding similarities regarding the helical angles (Table 1), elemental constitution, and shaping ability (Table 2, Fig. 3), signifcant diferences were observed in the overall design (Fig. 1), mechanical properties (Table 1), and temperature transition phases (Fig. 2), and the null hypothesis was partially rejected.

The analysis of the mechanical performance of NiTi preparation systems must be done taken into account several factors. Since the alloys of tested instruments in this study were similar in terms of elemental constitution, information regarding their phase transformation temperatures (austenitic and martensitic crystallographic arrangements) and design are of utmost importance to explain their mechanical behavior [9, 10]. Considering diferences on the dimensions of instruments available in each tested system, the frst DSC analysis was performed only in instruments with a tip size 25 and revealed the presence of R-phase alloy in all of them at test temperature (20 °C) (Fig. 2a). The R-phase alloy is characterized as an intermediate crystalline phase that occurs along a very narrow temperature range on the heating or cooling curve between martensitic and austenitic forms. This phase change in the crystal structure of the alloy results in lower resistance to elastic deformation (high fexibility and low rigidity), increasing its resistance to cyclic fatigue, while reducing its torsional resistance when compared to conventional austenitic alloys [25]. The intermediate R-phase has specifc temperatures for its formation represented by Rs, for the beginning of phase formation, and Rf for the end [10, 26]. In the present study, REX had the highest Rf temperature (34.2 °C), followed by WaveOne Gold (28.8 °C) and Reciproc Blue (20 °C) (Fig. 2a).

Considering that the mechanical tests were conducted in accordance to an international standard for testing transformation temperature of nickel-titanium alloys at room temperature (20 °C) [18], it would be expected that all instruments had martensitic characteristics during the test. In contrast, at body temperature (36 °C), the instrument that more quickly approached the austenitic crystallographic arrangement would be the Reciproc Blue. Therefore, depending on test temperature, instruments may present changes in their behavior. Since this is the frst study evaluating the REX system, the second DSC analysis was done in its set of instruments (Fig. 2b) and confrmed the manufacturer claim that these instruments are made with diferent customized heat treatments. However, DSC results suggest only minor diferences in the R-phase and martensitic B19’ transformation temperatures on both cooling and heating.

Notwithstanding DSC analysis revealed that REX instrument had higher martensitic composition than Reciproc Blue at room temperature (20 °C) (Fig. 2a), no diferences were observed between them in the cyclic fatigue, angle of rotation (torsional test), and bending resistance tests (Table 1), findings that can be explained by the larger metal core (Fig. 1) and greater number of blades (Table 1) of REX instruments. Diferences in the design also help to explain the highest maximum torque to fracture observed during torsional resistance test of REX instruments (Table 1). On the other hand, even though WaveOne Gold also had a high martensitic composition (Fig. 2a), it showed lower time to fracture (cyclic fatigue) and fexibility (bending resistance) than REX and Reciproc Blue (Table 1). Again, the design of the WaveOne Gold with its large cross-sectional design and taper (Fig. 1) may explain the results. Although only minor diferences were observed in the heat treatments of REX instruments (Fig. 2b), they might infuence their clinical behavior. For instance, at the test temperature (20° C), the lower As of REX Glide Path indicates a more austenitic composition compared to other instruments, which may be translated as a better resistance to torsion. In its turn, REX 40 had the highest As amongst REX instruments. It means that this large-tapered heat-treated instrument may present a high torque strength and fexibility during shaping procedures, an important aspect that may prevent fracture by torsional stress.

In the last years, there seems to be a tendency of industry to develop proprietary heat treatments of the NiTi alloy in order to create ultrafexible instruments with superior amount of martensitic crystallographic arrangement at temperatures above 30°C [15, 17] and/or by changing the design with an increased number of spirals and reduced metal core [14]. In the laboratory, these changes usually improve some mechanical properties of the instrument including cyclic fatigue resistance, angle of rotation, and fexibility (low bending load resistance), but, on the other hand, they may compromise its torsional strength [14]. Besides, in the clinical setting, ultra fexible instruments usually needs to apply more apical pressure to reach the working length [17], which may lead to early plastic deformation or fracture [14]. Therefore, considering the unfeasibility of creating a single instrument that combines all of the best metallurgic and mechanical features with the available technology, the latest generations of rotary systems are including, in the same set, instruments with diferent designs and crystallographic arrangements. In theory, it allows to customize an instrument in order to improve its fracture resistance and/or fexibility depending on the canal morphology or treatment phase. This is, for example, the proposal of some recently launched systems including the EdgeSequel Sapphire (EdgeEndo, Albuquerque, NM), the ProTaper Ultimate (Dentsply Sirona Endodontics, Baillagues, Switzerland), the Genius Profex (Medidenta, Las Vegas, NV), the One Endo File (NanoEndo LCC, Chattanooga, TN), and the REX system evaluated in this study. Although the DSC analysis demonstrated diferences in the heat treatment among REX instruments, it did not translate into a better shaping performance in extracted teeth when compared to the other tested systems (Table 2). The multimethod research applied to this study included not only the evaluation of metallurgical and mechanical properties of instruments, but also the assessment of several shaping ability parameters obtained from the root canal preparation of extracted molars using micro-CT imaging, an analytical tool that allows the longitudinal track of a sample at diferent time points. Preliminary eforts were made to ensure the anatomical matching of the specimens in each group according to some morphometric parameters in order to create a reliable baseline and enhance the internal validity of the study [27]. Although diferences could be observed in the overall design of the instruments (Fig. 1) and in their mechanical behavior (Table 1), no signifcant diferences were observed among them regarding the volume of hard tissue debris, the dentin removed after preparation, and the percentage of untouched canal walls in both mesial and distal canals (Table 2). Besides, no instrument fracture or signifcant deviation of the original canal was observed. These fndings can be explained by the use of instruments with similar dimensions, preparation protocols, and kinematics in anatomically balanced specimens, corroborating recent micro-CT studies [13, 14, 17, 28–31]. None of the preparation protocols was able to prepare all root canal surfaces or render root canals free from hard tissue debris, which is also in accordance with previous publications [32, 33]. Besides, this outcome agrees with other studies that also showed no diference in the percentage of untouched canal areas in extracted teeth after using Reciproc Blue and WaveOne Gold [17, 34]. No comparison could be made with REX system as this is the frst study that assessed its shaping ability.

The strengths of the present study relies on the multimethod assessment of diferent reciprocating instruments using methodologies validated by international standards [16, 18–20] or previously methods with high internal validity [13, 27, 31] which allowed a robust and trustworthy understanding of their mechanical performance. The limitations include the lack of other tests such as cutting efciency, microhardness, buckling resistance, and measurements of instruments’ dimensions. Therefore, future studies should include additional methodologies to evaluate other rotary or reciprocating NiTi systems with diferent designs and crystallographic arrangements.

Conclusions

Under the conditions of this multimethod study, Reciproc Blue, WaveOne Gold, and REX reciprocating systems were similar regarding elemental composition and shaping ability, but showed signifcant diferences in their overall design, temperature transition phases, and mechanical behavior.

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Declarations

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

List of authors:

Emmanuel J. N. L. Silva, Jorge N. R. Martins, Natasha C. Ajuz, Henrique dos Santos Antunes, Victor Talarico Leal Vieira, Francisco Manuel Braz‑Fernandes, Felipe Gonçalves Belladonna, Marco Aurélio Versiani

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