# A New Device to Test the Bending Resistance of Mechanical Endodontic Instruments
## 4 Discussion  
Since one of the two reasons of instrument failure is flexural behavior, bending ability has been demonstrated to be one of the main features of NTR instruments. For this reason, the flexural ability of a rotary instrument has been thoroughly investigated using different methods of evaluation [4; 17]. Protaper Universal (PTU; Dentsply Sirona, Ballaigues, Switzerland) is a sequence of six instruments with a triangular convex cross-section that does not undergo any thermal treatment. A second version of these files, the Protaper Gold (PTG), was produced using the same sequence and the same cross-section but it was improved with a proprietary thermal treatment, the Gold Wire (Dentsply Sirona, Ballaigues, Switzerland). This proprietary thermal treatment has been widely evaluated in the literature and is well known to have enhanced features such as bending and flexural behavior. EdgeEndo (Albuquerque, NM, USA) released several types of instrument with different designs and shapes to the marketplace. EdgeTaper (ET) is a system similar to PTU, sharing the cross-sectional design and sequence and no heat-treated alloy. Moreover, EdgeEndo modified EdgeTaper with a proprietary thermal treatment to improve its bending behavior and cyclic fatigue resistance, creating the EdgeTaper Platinum (ETP). From the results of the present study, it is possible to witness different bending behaviors both among the different instruments and along the same instrument, focusing on three precise points: 3, 6, and 9 mm from the tip. In Table 1, the bending resistance of a single instrument increases along its cutting surface (3, 6, and 9 mm). The bending behavior of an NTR instrument is mainly influenced by features as its core and cross-section diameter: the smaller the core diameter or the cross-sectional design, the higher the flexibility [18]. The statistical difference between these two brands might be justified by the different NiTi alloys used to manufacture the different instruments. The information given by the results cited above could be underestimated because it is logical to think that the bigger the core of the file, the greater the stiffness of the instrument. However, such information, assessed by this new testing machine, could drive clinicians' choice of instrument used in daily practice, considering the anatomical complexities of the case regarding root canal curvatures. Table 2 shows the comparison between ETP and PTG. These two files are quite similar since they share most features, such as cross-sectional design and tip and taper dimension, except for the heat treatment. Each brand has its own proprietary thermal treatment, Gold and Platinum Wire [19]. From the results of the present study, both proprietary thermal treatments enhance the flexibility of the file and this is in accordance with most current literature [20]. The most relevant result is the significant difference between ETP and PTG. The present study shows that ETP's bending resistance is higher than that of PTG at all the tested portions and for every instrument of the sequence. These findings confirm and complete the results of previously published articles on cyclic fatigue resistance comparisons of these two files. This increased flexibility should depend on the Platinum Wire treatment, the only difference between the two instruments. Thanks to this new device, it is possible to witness the progressively higher flexural resistance of NTR instruments from the apical part of the instrument toward the coronal part. Since the current prevalent test for bending evaluation is the bending test assessed by ISO 3630-1, the new testing device proposed by the present article could be a viable solution to overcome the limitation of the former bending test. The innovation relies on evaluating bending at three different points, which would provide a more representative description of the bending ability of an NTR instrument. The ISO 3630-1 test was first developed to analyze the bending ability of SS manual instruments.