## 4 Discussion
The most frequent procedural error found in this study was apical transportation, followed by blockage, ledge, and canal perforation. Other studies found the most frequent complication to be the presence of ledges [17; 40] although, in our case, we did not find statistically significant differences, as did the study by [PERSON] [41]. In view of the results, it can be affirmed that PTG removed more resin than BS in the coronal and apical third (especially on the external face of the curvature), while BS removed more in the middle third, on the internal face. These results could be due to several causes. At the coronal level, PTG has a higher value for the maximum flute diameter (1.22 mm), so it could remove more material in the coronal third and would be less conservative with the pericervical dentin. Several studies have reported the importance of preserving this dentin and the ways that the greater conicity of files would produce a greater loss of dentin, with a consequent decrease in the resistance of the tooth, and an increase in the appearance of fractures [42; 43; 44; 45]. The higher resin removal produced by BS in the middle third could be due to the fact that this system is better adapted to the canal curvature. The blue alloy, together with the glide path achieved by Z1, coupled with a smaller jump between the diameter of the posterior files, could result in hardly any deformation of the outer part of the curvature and thus avoid the straightening of the canal. The correct shaping and cleaning of the apical third has a direct relationship with the success of the treatment [46]. Apical transport compromises the cleanliness and correct sealing of the canals allowing tissue debris as well as microorganisms to remain [31]. In this study, a reference was taken to affirm the presence of apical transport if there were an apical deformation >0.30 mm [31]. The apical transport was mainly towards the external part of the curve. This is in agreement with other studies [47; 48]. The higher incidence of the apical transport of ProTaper Gold could be due to the fact that, although the F2 file has the same cross-section as the Z4 (0.25 mm), its taper is 2% greater (8% vs. 6%). This greater taper could provide greater rigidity to the instrument and, therefore, less efficiency in adapting to the curvature with greater possibility of straightening [49] since bending is inversely proportional to the size of the file section [50] as well as to the taper of the file [51]. This greater deformation could also be explained by the type of alloy used. The Blue alloy has a higher amount of martensite and R-phase in clinical conditions than does the Gold alloy [52], and this would cause it to better adapt to the curvature, producing less deformation at this level. The fact that PTG produces more apical transport than BS could be explained not only in terms of geometry and alloy, but also in terms of the clinical sequence during the instrumentation process. The diameter jump between files in the PTG system is greater than that in the BS system, and, as the passage from a smaller file to a larger one is less progressive, it could lead to the production of greater deformation in the canals due to a greater contact area, which, by generating greater torque force, would lead to the operator having to tighten more for the file to advance to the working length [53]. It is possible that BS, by having less of a diameter jump between files, makes it possible for these inexperienced students to instrument the canals with greater respect for their anatomy. The BS system starts with a gauge of 0.14 mm (Z1), followed by 0.17 mm (Z2), 0.19 mm (Z3), and finally 0.25 mm (Z4). PTG starts with 0.17 mm (S1), followed by 0.20 mm (S2 and F1), and finally 0.25 mm (F2).