## 4 Discussion
Another limitation of the use of SEM is the risk of sample destruction caused by the preparation process [15]; however, the scanning electron microscope used in this research allowed the samples' examination without any preparation thanks to its functioning at high vacuum pressure. As mentioned before, the knowledge of the wear resistance of endodontic instruments is fundamental to having an in-depth comprehension of their mechanical limits, in order to reduce as much as possible, the likelihood of intracanal fracture and using them as effective as possible, reducing the waste and discard of several instruments. According to this, to date, there are no data available regarding the comparison between the tested instruments. The OR is a recently introduced single-file reciprocating instrument characterized by two different cross-section designs; a convex triangular for the apical 4 mm and an S-shaped for the rest of the instrument with a patented asymmetric cross-section. This manufacturing choice is probably intended to enhance the mechanical properties of the instrument, increasing the torsional resistance of the tip by increasing its polar moment of inertia [16] and increasing the cyclic fatigue resistance and the flexibility of the coronal part by reducing the cross-sectional mass with the S-shaped design [17]. Moreover, while the tip diameter is 0.25 mm, the taper is 0.06 variable and it is drastically reduced in the coronal part of the instrument thanks to the use of 1 mm wire diameter for its manufacturing, to preserve peri-cerical dentin during the instrumentation procedures [18]. The proprietary heat treatment of the alloy is the C-Wire. The EOF is a recently introduced single-file reciprocating instrument conceived as a replica-like system of WOG with a different heat treatment, which, according to the manufacturer, should guarantee enhanced flexibility, cyclic fatigue and torsional resistance, despite the similar morphological characteristics such as cross-sectional design, tip diameter, taper, helix and pitch angle etc. [19]. However, [PERSON] et al. comparing the morphological characteristics of EdgeTaper Platinum (replica-like systematic of PTG systematic) (EdgeEndo, Albuquerque, New Mexico) and PTG through a Micro-CT analysis found that, despite the manufacturer's claims, the metal mass volume (mm\({}^{3}\)) and cross-section area (\(\upmu\)m\({}^{2}\)) were different with a statistically significant difference [20]. Those premises, in addition to the different heat treatments of the two instruments, could explain the differences in terms of mechanical and wear resistance between EOF and WOG. According to the results, there were no statistically significant differences in the preoperative surface analysis among the tested brand instruments, except for the incidence of metal defects and residual debris on EOF instruments, which probably arose from their manufacturing process. Despite this, the metal defects on the tips of the three EOF instruments did not interfere with their wear resistance after four canal uses since any microfracture or microcracks were found in those points in the post-operative analysis. The debris found on the EOF instruments before instrumentation are probably metal debris from wire blanks machining since their radiolucency in the SEM acquisition. After the instrumentation of 4 resin canals, the EOF showed the worst surface integrity in terms of unwinding, blunting of cutting edges and tip flattening. This denoted less wear resistance in comparison to other brands' instruments since they did not report statistically significant differences among them. The reason behind those findings could be the softness of the alloy arising from the different heat treatments that make the instruments more vulnerable to torsional stresses and friction against dental walls.