# A New Device to Test the Bending Resistance of Mechanical Endodontic Instruments
## 1 Introduction  
Endodontics have greatly changed in the last years thanks to technological developments and the introduction of nickel-titanium rotary (NTR) instruments. Such improvements have sharply reduced operative times and allowed us to overcome anatomical complexities [1]. Today, NTR instruments come in several shapes to take advantage of the different characteristics of the instruments. Although NTR instruments reduce working time, unexpected separation during instrumentation occurs more often than in stainless steel (SS) files [2]. The two main patterns of fracture investigated in the literature are flexural and torsional failure [3; 4; 5; 6]. These are determined by clinical complexities, for example, severe curvatures and narrow canals. Therefore, various geometric, dimensional, and alloy modifications have been developed to avoid fractures. Different cross-sections have been designed to reduce blade engagement with the canal walls, variable taper was introduced to allow a selective dentin cut, and heat treatments were developed to modify the mechanical properties of nickel-titanium [7; 8]. It is well known that the nickel-titanium alloy exists in two different temperature-dependent structures, austenitic and martensitic, each with its particular features of resistance, flexibility, and elasticity [9]. The austenite phase is a more rigid and aggressive structure, whereas martensite can be easily deformed and is more flexible and ductile. Moreover, the present literature commonly agrees on the influence of the alloy on bending ability, which is the ability of the instrument to bend without any plastic deformation. This allows the file to overcome severecurvatures and to reduce the risk of fracture due to flexural stresses [10]. Therefore, the use of heat treatment during manufacturing processes has made it possible to decide whether the alloy of the NTR instrument would be austenitic or martensitic at clinical temperatures [11]. In addition to this, various brands have proposed different geometric designs to enhance the mechanical features of the instruments and to make NTR instruments able to shape to the most challenging curvatures. The recent literature, especially since the spread of cone beam computed tomography (CBCT), includes a vast variety of articles that have studied root canal curvatures in different ways, considering the angle and the height of curvature, using two vertices to determine curvature, and showing a higher percentage of curvature, mainly due to 3D evaluation [12; 13; 14; 15]. This change in knowledge of the root canal anatomy led to a change in the method of evaluating NTR instruments. Several cyclic fatigue studies have been published, with new devices able to reproduce real anatomical curvatures in vitro [16]. Furthermore, thanks to new devices, it has been possible to evaluate the influence of the pulp chamber opening on the mechanical resistance of NTR files. Despite these improvements being a milestone in cyclic fatigue evaluation, the study of the bending ability of NTR instruments is still obsolete. The most common evaluation device is the bending test assessed by ISO 3630-1, which involves clamping 3 mm of the tip of each instrument in a chuck and applying an angular deflection of 45\({}^{\circ}\)[17]. Starting with the abovementioned premises, a new device has been developed and tested in the present study to test bending ability for other parts of rotary files. Protaper Universal (PTU; Dentsply Sirona, Ballaigues, Switzerland), Protaper Gold (PTG; Dentsply Sirona, Ballaigues, Switzerland), EdgeTaper (ET; Albuquerque, NM, USA), and EdgeTaper Platinum (ETP; Albuquerque, NM, USA) are similar instruments which share a cross-sectional design, tip, and taper dimension and differ only in the heat treatment of the alloy. Previously published studies have already evaluated the torsional resistance and the cyclic fatigue resistance of these instruments but no published studies have yet evaluated the bending ability of the abovementioned rotary files.