## 4 Discussion
[PERSON] and colleagues [42] performed a systematic review on the influence of demographic factors on the prevalence of a second root canal in the mandibular incisors. Their results are consistent with ours, confirming a lower prevalence in the Asian population. However, one of their inclusion criteria was the use of an equal to or lower than 0.2 mm voxel size. In our study, we included larger voxel sizes in order to assess whether they also identify the presence of a second canal. This has an important clinical applicationin guiding the clinician in the decision to expose the patient to radiation as low as reasonably achievable (A.L.A.R.A.) [43]. According to our assessment of the CBCT parameters used in the studies (Table 2), there is no direct relationship between voxel size and the detection of a second root canal (when voxel sizes between 0.125 mm and 0.3 mm were used). The most relevant result was that using the same voxel size (0.2 mm) the presence of a second root canal varied between 2.7% [14] and 44% [23]. A reasonable explanation for this would be that the first and second studies were conducted in different geographical areas (China and Italy). [PERSON] and colleagues, in a study based on Chinese patients [29], also reported a low prevalence of second root canals (7.2%), despite using a smaller voxel size [44]. Thus, this finding would be consistent with those of other authors [45; 46; 47] who claim that the morphology of the root canal system is directly related to the geographical area where the study is conducted. Except for the study by [PERSON] and colleagues [14], who clearly specify the inclusion of a particular ethnic group, in all the remaining studies CBCT was performed in local healthcare facilities without clarifying whether other ethnic groups had been excluded. Therefore, our systematic review mentions a geographical area variable rather than an ethnic variable. In addition, studies using a 0.3 mm voxel size [8; 31] revealed higher second canal incidence rates than others where smaller voxel sizes were used [14; 20; 23; 25; 28]. Hence, as regards clinical application, our study proves the lack of justification for overexposing a patient to radiation if the purpose of the CBCT is to confirm the presence of a root canal. However, this systematic review cannot clarify which particular voxel size is appropriate in the study of internal anatomy. In the future, it would be necessary to carry out new studies that can confirm it. Another of the aspects assessed in the reviewed studies was the influence of tooth position. Out of a total of 19 studies, 2 made no differentiation between central and lateral incisors; of the remaining studies, 14 reported a greater prevalence of second root canals in lateral incisors. With the purpose of illustrating the existing trend in the number of canals, we performed an analysis of the weighted average. The results showed a higher prevalence of a second canal in the lateral incisors (23.7%) than in the central incisors (17.2%). The literature revealed other studies with consistent results [48; 49], although most of the in vitro studies conducted do not consider said variable [7; 9], possibly because of the difficulties that are inherent to this technique. Accordingly, this is an extremely important finding because of its clinical applications. As for age, very few studies addressed this variable, which leads to confusion and contradictory results concerning its influence [24; 29] in root canal system morphology. Regarding the influence of gender, there was no unanimity as to the results, finding studies that reported a higher prevalence of a second canal in men [19; 20; 24; 31], women [27; 29], and also some where no significant differences were found [8; 22; 25; 30; 34; 35]. The studies reviewed reveal broad consensus in that the most frequent configuration is Vertucci's type I, followed by type III, II, V, IV, VII and VI, respectively (Table 1). However, there are certain studies that do not report type III configuration as the most frequent.