## 4 Discussion
Other authors used extracted teeth and assigned scores to determine density by visualizing digital radiographs [33]. Concerning ET's evaluation, in the present study there were no significant differences regarding the obturation quality over the 40 sequentially prepared canals. Although the reduced number of canals with inadequate obturation was essentially recorded in the first two groups, i.e., amongst the first 20 filled canals, these was no significant impact on the overall obturation quality, between groups. Other authors [33] reported an increasing proportion of adequate density and length of the fillings as the number of treated canals increased. This divergence may be explained by the type of samples, such as natural extracted teeth, with the inherent anatomical variability, whereas, in the present study, the ET was performed in standard simulated curved root canals. Accordingly, [PERSON] et al. [16], in a retrospective evaluation of clinical cases performed by students, attributed the worst result regarding lateral adaptation and obturation length to anatomical variations. The main finding herein was that, as the number of treated canals increased, the total and active treatment and filling times decreased, showing a corresponding profile as a \"learning curve\". Similarly, in a previous study [11] with one senior student, regardless of the instrumentation system used and continuous or reciprocating movement, a significant decrease was reported in the time spent in active instrumentation between sequential groups of 10 teeth. In fact, in both studies, the relationship between training and learning showed a significant initial improvement (i.e., a time decrease with relevance after the first 20 canals) that stabilized along the remaining ones. This minimum number of 20 canals prepared to achieve the required competency for ET, performed by graduating students, has also been corroborated in a very controlled study with four fourth-year undergraduate dental students [33]. Using naturally extracted teeth, the results reinforced that machine-driven files result in an overall better quality of root canal treatments, independent of the Ni-Ti system used. In addition, the total treatment time was clearly reduced with increasing number of ETs, due to shorter time spent not only on preparation, but also on filling procedures. Similar to our findings, the reduction in the operation time was primarily seen from the first to the second session, i.e., from the first 20 extracted teeth treated. Additionally, before the experiment, operators participated in only one calibration session concerning the manuals of the treatment systems. Results from ex-vivo studies have also already emphasized the shorter treatment time associated with rotary instrumentation compared to manual instrumentation, particularly when using systems with fewer files (e.g., WaveOne, Reciproc) [2; 25; 33]. It must be emphasized that the senior student herein had a previous short training of root canal treatments in his pre-clinical curricular path, similar to other studies where they are referred to as novice [4]. Although it is advisable to expose students in their pre-clinical stage to new developments in Endodontics, such as rotary instrumentation, due to several factors (e.g., high number of students, difficulty in natural extracted teeth selection or COVID-19 impact), senior students actually are left with a short pre-clinical experience, anxious to have the opportunity to experience all sorts of advancements in Endodontics. They realize that they will need these tools in the short term, as future professionals. In that sense, it can be assumed that the findings of the present pilot study are in line with the literature and should be considered for implementing mechanized instrumentation in undergraduate curricula in Endodontics. Contrary to the present study, in the referred study by [33], the proportion of adequate ratings of length and adequate seal of root canal fillings also improved with increasing experience.