## 4 Discussion
The severe bluntness of the cutting edges strongly reduced the cutting efficiency of the EOF, probably making them unable to face the friction derived from the cutting action against resin, thus decreasing their torsional resistance. Regarding the presence of debris after instrumentation and cleaning procedures, the OR showed the highest incidence, followed by R25 Blue. This is due to the similar cross-section shared by the two instruments, the S-Shaped design. This cross-sectional design significantly enhances the cutting efficiency and ensures a great removal of debris from the canals. Nevertheless, particular attention should be given to the disinfection protocols after their use in order to avoid any cross-contamination between patients. The difference in terms of debris on the surface of the instruments could derive from the depth of the flute of OR that is able to hold more debris. The main limitation of this research is the use of simulated resin teeth instead of ex vivo or in vivo samples. This choice was made in order to standardize as much as possible the external parameters influencing the wear resistance of instruments, such as the hardness of the cut material, the radius and the angle of curvatures, the canal diameter and the canal length since they have a significant influence on the mechanical resistance of NiTi endodontic instruments [21,22]. Selecting resin models, all those factors are considered constants, which is difficult with extracted teeth. Moreover, the resin teeth were fabricated without a roof chamber with a standardized access cavity, even though this parameter was constant among samples, avoiding any influences of different access cavities on the mechanical resistance of NiTi instruments [7,23]. Regarding this, most of the articles in the literature ignore the influence of all the above-mentioned factors on the wear resistance, thus increasing the risk of bias during the methodology. However, the results of this study should not be directly traduced in the clinical practice since the resin of tooth models has a different hardness in comparison to dentine. In fact, the aim of this study was not to assess the maximum number of instrumentable canals with a single instrument but to compare the wear resistance of different instruments. Despite this, the results obtained by [PERSON] et al. regarding the evaluation of wear resistance of Reciproc Blue before and after four uses in extracted teeth with severe curvatures (ranging from 50\({}^{\circ}\) to 70\({}^{\circ}\)) are completely comparable with the results of this research [7]. The authors, evaluating ten samples for each instrument type, found the absence of unwinding, fractures and tip deformations, whilst four used R25 Blue instruments revealed microcracks along the surface perpendicular to the long axis [7]. Since the results of the instrument's surface defects arising from in vitro and ex vivo tests are comparable, it is possible to consider the methodology of this research reliable. Moreover, the use of simulated resin teeth could allow researchers to unconditionally increase the sample size, guaranteeing more reliable results without biological costs, as in the case of ex vivo studies, in which extracted teeth are required, also overcoming the difficulties in the sample selection. Moreover, the simulated resin teeth allow the control of variables such as curvature degrees, radius of curvature, canal diameter, canal length, hardness of the material and design of access cavity, guaranteeing more reliable results. Surely, further in vivo researches are needed to overcome those limitations.