Coronal flaring promotes more excellent safety during endodontic treatment, reducing the amount of debris extruded through apical foramen and the fracture of preparation instruments [7, 8]. It also facilitates the action of chemical solutions, the establishment of patency, the determination of root canal length, and apical preparation diameter [9, 10]. However, such instruments are subject to torsional tension during their use that can lead to file fracture. This occurrence can compromise the endodontic procedure. Therefore, the study of its mechanical properties is essential.
In this study, the record of torsional fatigue resistance and angular deflection were provided by a specific program (MicroTorque; Analógica) until the moment of fracture. The first 3 mm of the tip of the instrument were clamped for testing because this portion is the most susceptible to fracture [15]. Regarding the torsional test, the results showed that the MK Sequence 17.12 file (P < .05) showed the highest torsional strength. Probably, these results could be related with the greater taper of this instrument, which probably favored a larger cross-sectional area when compared to the other groups.
In a complementary evaluation, the cross-section of each instrument in D3 was evaluated under SEM, and the area was measured through software (AutoCAD; Autodesk Inc, San Rafael, CA, USA) [5]. ProTaper SX presented the smallest area (0.055mm3), followed by Mtwo 25.07 (0.095mm3), W-XN (0.110mm3), MK Orifice Shapper (0.145mm3), being the most extensive area verified in MK Sequence 17.12 (0.185mm3). Previous studies have shown that the mechanical properties of the instruments are influenced by the cross-section, taper, helical angle, and pitch length. Also, the lower the radial distance between the periphery and the center of the core, the higher will be the stress supported by the instrument in torsion. So, files with larger cross-sections present higher torsional resistance [3, 16]. Although this instrument has a heat-treated alloy, a study that verified the properties of a coronal flaring file, with and without heat treatment, concluded that this is not a factor that influences this outcome [4]. There were no differences between the instruments Mtwo 25.07, W-XN, and Orifice Shapper (P > .05). ProTaper SX presented the lower torsional strength (P < .05) probably due to the smaller cross-sectional area among the tested instruments.
Angular deflection is associated with how much the instrument tolerates elastic and plastic deformation before breaking in torsional motion. This characteristic may be a safety factor in clinical practice because plastic deformation can be verified before a fracture occurs when the instrument is removed from the canal [6]. High values of angular deflection mean considerable strain before the moment of failure [25]. The highest angular deflection values were presented by Mtwo 25.07. This result may be due to the characteristics of the NiTi alloy and the instrument geometry. MK Sequence 17.12 showed the lowest values, probably associated with the large diameter of the instrument. Other studies reported that instruments with larger metal mass volume tend to present lower flexibility and lower angular deflection to fracture, which could explain the results of this study [2, 3]. Thus, the first null hypothesis was rejected.
Bending stiffness is related to instrument performance when used in curved canals [6]. However, in cervical flaring instruments, this property is related to the flexibility to perform brushing movements against dentinal projections [4, 17]. The results showed that there were differences among all the groups compared, rejecting the second null hypothesis of the study. MK Sequence 17.12 needed the highest strength to bend and ProTaper SX the lowest. Again, taper and the cross-sectional area are factors related to such property [24]. Although Ataya et al. have shown that coronal flaring heat-treated instruments presented lower bending stiffness when compared to the same file without heat treatment [4], our results were different probably because the tip diameter, taper, and cross-section of the heat-treated instruments used in this study were different from those previously tested.
The highest torsional strength and bending stiffness of MK Sequence 17.12 indicates that these instruments would probably suffer less risks regarding torsional fracture in narrow and constricted canals and would be able to remove dentin projections more efficiently. On the other hand, the greater angular deflection presented by Mtwo 25.07 could be beneficial to clinicians, indicating that there was plastic/permanent deformation and imminent fracture before it occurs [6]. Future studies should be performed comparing the results of these mechanical tests with the clinical performance of these instruments.