## 4 Discussion  
Historically, studies on the configuration of the root canal system of incisors were mainly in vitro, using methodologies such as x-rays [3], clearing technique [6] or, more recently, CBCT [36] and \(\mu\)-CT [8]. However, in vitro methodologies involve tooth extraction, which means that the number of samples is limited. For the purposes of this systematic review, articles were screened to include only human studies using CBCT. This decision was based on two reasons: first, because the marked disparity in results concerning the existence of a second root canal led to the rejection of the use of different methodologies as a potential source of bias; and secondly, because of the high number of samples included in these studies as a consequence of their being extracted from clinical databases created for other diagnostic purposes. After assessing the studies included, we noticed that, even when the same methodology was used, the differences in second root canal percentages (0.4%-45%) remained [14; 23]. These results are in contrast with the findings of other authors [4], who attribute the disparity of results to the methodology used. On the other hand, our findings are consistent with those where the use of a variety of methodologies offered the same precision in the study of the internal morphology of incisors [37; 38]. The clinical application of CBCT has yielded promising results in the identification of morphological relationships among the different teeth of the same patient. The findings of [PERSON] and colleagues [24] show a correlation between the presence of a distolingual root in permanent mandibular molars and the presence of complicated configurations in central incisors. Moreover, [PERSON] and colleagues [39], report that the presence of a second canal in lower incisors could be related to the presence of an additional canal in lower premolars. Likewise, CBCT also confirmed the existence of an anatomical symmetry pattern in the morphology of the root canal system between the right- and left-side incisors of the same patient. Before this, anatomical symmetry had only been anecdotally described in published case reports [40]. Nevertheless, a thorough and careful reading of the analyzed studies revealed that some of them reported high percentages of anatomical symmetry [20; 26; 33]. However, such percentages include symmetry data corresponding to [PERSON]'s type I configuration and, given its high incidence, it is typical for such significant symmetries to be reported. [PERSON] and colleagues [7] claim that findings regarding symmetry in mandibular incisors are not significant for the purpose of making a diagnosis based on contralateral tooth anatomy and insist on the importance of performing an accurate clinical and radiographic diagnosis of the tooth to be treated. In their study, symmetry when there were two root canals (excluding [PERSON]'s classification type I) was of 17.5% and 20.5% in central and lateral incisors, respectively. In conclusion, CBCT is a very useful tool in the study of internal anatomy, allowing clinicians to obtain three-dimensional images of the canal system and contributing to the identification of new anatomical relationships among the different teeth of a given patient and a given population. Another finding of our systematic review is that not all the studies reviewed used the same CBCT devices and that they even used different imaging parameters (Table 2). The reason for this is that none of the CBCT assessed [8; 14; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35] were specifically performed for the study of root canal configuration but were aimed at maintaining minimal exposures to radiation for a particular diagnosis. In this regard, [PERSON] and colleagues [41] reported that the detection of the mesio-buccal canal in maxillary molars was more accurate (93.3%) when CBCT parameters were set at small voxel sizes than when larger sizes were used (60.1%).