The impact of clinical use on the torsional behavior of Reciproc and WaveOne instruments

ABSTRACT Torsional overload is a fracture representative parameter for instruments in single-file techniques. Objective The aim of this study was to assess the influence of clinical use, in vivo, on the torsional behavior of Reciproc and WaveOne instruments considering the possibility that they degraded with use. Material and Methods Diameter at each millimeter, pitch length, and area at 3 mm from the tip were determined for both types of instruments. Twenty-four instruments, size 25, 0.08 taper, of each system were divided into two groups (n=12 each): Control Group (CG), in which new Reciproc (RC) and WaveOne Primary (WO) instruments were tested in torsion until rupture based on ISO 3630-1; and Experimental Group (EG), in which each new instrument was clinically used to clean and shape the root canals of one molar. After clinical use, the instruments were analyzed using optical and scanning electron microscopy and subsequently tested in torsion until fracture. Data were analyzed using one-way analysis of variance at a=.05. Results WO instruments showed significantly higher mean values of cross-sectional area A3 (P=0.000) and smaller pitch lengths than RC instruments with no statistically significant differences in the diameter at D3 (P=0.521). No significant differences in torsional resistance between the RC and WO new instruments (P=0.134) were found. The clinical use resulted in a tendency of reduction in the maximum torque of the analyzed instruments but no statistically significant difference was observed between them (P=0.327). During the preparation of the root canals, two fractured RC instruments and longitudinal and transversal cracks in RC and WO instruments were observed through SEM analysis. Conclusion After clinical use, no statistically significant reduction in the torsional resistance was observed.


INTRODUCTION
Nickel-Titanium (NiTi) rotary instruments of the root canal system; however, they may experience premature failure caused by fatigue 17,24 or by torsional overload. Torsional fracture is a result of the binding of the instrument tip inside the root canal, which is typically smaller than the instrument tip diameter 20 . The rupture occurs when the torsional stress locks the instrument in dentine walls and becomes higher than the maximum torque; the instrument can withstand at that point 15 .
This risk may be reduced by performing coronal enlargement and by creating a glide path before using any NiTi rotary instrument 3 .
Two brands of NiTi instruments, adopting the concept, were recently introduced into the market: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply-Maillefer, Ballaigues, Switzerland). These is produced by an innovative thermomechanical treatment process 12, 14,18,19 . They employ a reciprocating motion, rather than a continuous rotary motion; the former reduces the torsional stress by periodically reversing the rotation, and ultimately increases the lifespan of the instrument 14,27 . In the reciprocating motion, the values of clockwise and counterclockwise rotations are different for each instrument. A large rotating angle, counterclockwise (CCW) to the cutting direction, determines that the instrument advances in the canal to engage and cut the dentin, whereas a smaller angle, clockwise immediately disengaged and safely progresses along the canal path, while reducing the screwing 22 . Previous studies showed that the reciprocating instruments promoted minimal apical transportation, maintaining the original canal curvature better than rotary instruments, although they are associated with producing higher debris extrusion 4,6 .
Several studies assumed that the reciprocating  10 (2015) found no difference in torsional resistance comparing Reciproc and WaveOne instruments. However, very limited information is available on the torsional behavior of instruments used in time a NiTi instrument encounters resistance it undergoes torsional loading. The load is greater whenever the dentin is hard or the canal diameter is small. This torsional load exerted on the surface of the instrument can prevent its rotation to a greater or lesser extent. In extreme cases, the instrument may fracture when the resistance is so high that it constrains the movement of the instrument 2 .
To evaluate the magnitude of the torsional load to which the instruments are subjected when used in Considering that the instrument progress in the canal without pre-enlargement and being aware that the amount of torque generated during the shaping of root canals clearly depends on the size of the contact areas between the instruments and the canal walls 20 , the purpose of this research was to analyze the effect of the clinical use on torsional resistance of Reciproc and WaveOne instruments.
Before the mechanical testing, all the brand new instruments received from the manufacturers were photographed using a high-resolution digital camera (20D; Canon, Tokyo, Japan). Their dimensional characteristics were assessed against the American National Standards Institute/ American Dental Association Specification No. 101. The measurements (n=12 for each type) were obtained using Image Pro Plus 6.0 (Media Cybernetics; Silver Spring, MD). Lines were outermost diameters at each millimeter from the tip were measured. The same method was used to determine the pitch length. To visualize and measure the cross-sectional area at 3 mm from the tip (A3), three instruments of each type were randomly selected and cut to approximately 2.7 mm from the tip using a metallographic cutter (Isomet 1000; Buehler, Illinois, USA). The cross-sectional surfaces were polished with sandpaper to 3.0 mm from the tip and then imaged using a scanning electron microscope (SEM) (JSM 6360; Jeol, Tokyo, areas for each instrument were determined using the same software described above. Such dimensional and geometrical characteristics are on torsional behavior 16 . Three instruments of each system were examined by SEM to assess their surface characteristics verifying if there were surface defects, such as microcracks, even in new instruments. The instruments were then divided into two groups, each one containing twelve new instruments of each type: (i) Control Group (CG), in which a torsion test was performed to establish the mean at fracture, and (ii) Experimental Group (EG), in which the instruments were employed clinically, in vivo, by an experienced endodontist to shape the root canals (3 or 4 per tooth) of one upper or lower molar randomly distributed between the two groups #10 and #15) (Dentsply-Maillefer, Ballaigues, Switzerland), patency was established. Direct and angled radiographs of each tooth were obtained using a paralleling technique to evaluate anatomy, as well as to determine the canal radius and angle 23 (1997), and its approximate length. These parameters were measured by projecting the radiographic images was measured along the outer canal wall.
Then, the canals were cleaned and shaped in accordance with the manufacturer's instructions.

The impact of clinical use on the torsional behavior of Reciproc and WaveOne instruments
The instrumentation was completed gently with an "in-and-out" motion, and each canal was prepared until the working length was reached (0.5 mm of the canal patency length), when the instrument was immediately withdrawn. The instruments were operated using the manufacturer's pre-programmed settings for a Silver Reciproc engine (VDW GmbH, Munich, Germany): "Reciproc all" mode for RC size 25 and "WaveOne all" mode for WO Primary. The instrument was removed from the canal after three pecks or when resistance was encountered, and it was used in a lateral brushing motion. A 5.25% sodium hypochlorite solution was used for irrigation and RC-Prep (Premier Dental Products, Norristown, instrument were cleaned after each group of the three "in-and-out" movements.
After use in each patient, the instruments were washed and ultrasonically cleaned for 5 minutes in acetone. The RC and WO instruments of the EG were observed by optical microscopy (Mitutoyo TM, the presence of distortion, unwinding defects, and any macroscopic deformation. Before the torsion test, the same three instruments of each system, previously examined by SEM, were reexamined in the same position and area to assess the effect of the clinical use on their surface characteristics.
The torsion tests were performed based on ISO 11 using a torsion machine described in detail elsewhere 1 . Torque values were assessed by measuring the force exerted on a small load cell by a lever arm linked to the torsion axis. A resistive angular transducer connected to a process controller measured and controlled the rotation angle. The rotation speed was set clockwise to 2 rpm. The end of the shaft was clamped into a chuck that was connected to a reversible geared motor. Three millimeters of the instrument's tip was clamped into another chuck with brass jaws to prevent it from sliding. Continuous recordings of torque and angular deflection, as well as measurements of the maximum torque and angular designed computer program. To determine the parameters amongst the different groups, data obtained were subjected to a one-way analysis of

RESULTS
The mean values (± standard deviations) of diameter at 3 mm from the tip (D3) for RC and WO were 0.504±0.010 mm and 0.499±0.020 mm, respectively ( Figure 1a). Moreover, the pitch length increased along the active part of both instruments, with a steeper increase recorded in RC instruments. In general, RC presented larger pitch lengths than WO instruments, as illustrated in Figure 1b.
In this study, both file systems used had different cross-sections, with RC presenting an S-shaped cross-section and WO presenting a concave triangular-shaped cross-section. Similarly, the mean values of area at 3 mm from the tip, A3, for RC and WO were 0.112±0.005 mm 2 and 0.123±0.005 mm 2 , respectively. However, although the D3 (P=0.521) were present, WO instruments RC (P=0.000).
T h e m e a n va l u e s o f m a x i m u m t o r q u e determined for RC and WO new instruments (CG) were 1.763±0.226 Ncm and 1.852±0.293 Ncm, respectively (Figure 2). Although new WO instruments showed higher values for maximum torque than RC instruments, no statistically (P 295±35° and 241±28° for RC and WO instruments, (P=0.000).
Radius and angle of curvature mean values (± standard deviations) characterizing the geometry of the root canals of RC and WO instruments were 5.3±1.9 mm; 5.5±2.2 mm and 23±11°; 24±15°, difference (P>0.05) in root canal geometry among the groups.
The mean values of torsional resistance after clinical use (EG) are also summarized in Figure  2 As illustrated in Figure 3, the lateral surfaces of all RC and WO instruments submitted to root canal shaping exhibited longitudinal microcracks, i.e., cracks parallel to the longitudinal axis of the instrument. In addition, transversal microcracks along the cutting edge were observed in the instruments after clinical use. From these images, it is noteworthy that both instruments presented similar crack patterns. A predominance of larger and wider cracks in the region between 1.0 and 2.5 mm from the instrument tip was observed. Figure 3 reveals a high concentration of typical transversal and longitudinal microcracks on the surface of a RC instrument that fractured during clinical use. In this investigation, two RC instruments experienced intracanal failure during

DISCUSSION
Several factors, such as anatomy of the root canals, size, taper, design, alloy chemical composition, and thermomechanical process applied during manufacturing might affect the torsional behavior of NiTi instruments 15 . No significant difference in radius and angle of curvature could of geometrical factors on the torsional behavior was avoided.
It is important to emphasize the dimensions of the instruments at 3 mm from the tip because this is the point of apprehension of the instruments for torsional tests (according to ISO 3630-1), and where the maximum curvature is found in most root canals in clinical practice with the greatest mechanical solicitations 26 . Although similar diameters at D3 higher (P=0.000) than the one observed for RC instruments. As discussed in the literature, the maximum torque values increased proportionally with A3 15 . It is also generally accepted that the larger the pitch length, the smaller is the torsional resistance of an endodontic instrument 7 . WO instruments presented smaller pitch length than RC instruments. These characteristics of smaller pitch length and higher A3 lead to a tendency of a higher torsional strength for WO instruments. These results corroborate a recent clinical trial that accessed the fracture incidence of the reciprocating posterior teeth. The overall instrument separation incidence concerning the number of canals shaped was found to be as low as 0.13% 5 .
Although these instruments are less susceptible to the "taper lock" effect due to the clockwise motion of detachment, once used as a singleoperate under higher torsional loads owing to increased contact area with dentin walls, which may exceed its torsional strength and eventually lead to fracture 2,20 . The clinical application caused a tendency of decreasing the maximum torque but consumed less than 10% of the torsional resistance of the RC and WO instruments. Compared with not observed in this study and may be explained by the reduced clinical use (only 1 molar) or by the reciprocating motion of these instruments. The (295°) and WO (241°) instruments were far beyond the amplitude of rotation employed in the CCW movement (170°/150°), which may also explain the low incidence of failure during the clinical use. In a recent study, a very low incidence of fracture in Reciproc instruments and deformation after clinical use was also observed 21 . According to the literature, this low frequency of fracture can be possibly understood as an improvement in fatigue resistance associated with the reciprocation kinematics 16 , the instrument cross-sectional design 22 , and the superelastic M-wire alloy 12, 18,19 .
In accordance with the manufacturer and during the clinical use, these instruments were used under apical pressure. Longitudinal cracks in RC and orientation on their surfaces under torsional load were observed through SEM analysis. According to Bahia, et al. 2 (2008), during cyclic torsion, planes with a maximum shear stress are either perpendicular and/or parallel to the longitudinal axis indicating that instrument fracture may occur as a result of tri-axial stresses, which could be the case in fractured RC instruments that presented both types of cracks ( Figure 4). Nevertheless, the fracture that occurred in both RC instruments may be explained by the lower mean A3 values and the larger pitch length in comparison with WO instruments, thus promoting a smaller mass at the tip of the RC instruments.

CONCLUSIONS
Within the limitations of this study, no statistically significant differences in torsional resistance between Reciproc and WaveOne instruments were observed, although data suggest a tendency to higher torsional strength for WaveOne instruments. Geometric factors such as the pitch length, the diameter, and area at 3 mm from the tip, in addition to the manufacturing process, may have an impact on the clinical behavior of these instruments. Finally, we observed that the clinical use in one molar torsional resistance for the analyzed instruments.