## 1 Introduction  
Good knowledge of the root canal anatomy is essential for the success of any endodontic treatment. The development of new materials and techniques has contributed to increasing the chances of ensuring good results. Nevertheless, anatomical knowledge is still the most valuable tool when it comes to addressing each separate case and deciding which materials and tools to use [1]. In 1984, [PERSON] drew up a classification based on the different root canal morphologies [2]. Figure 1 shows [PERSON]'s configuration for the lower incisor. The purpose of this figure, created by the authors, is to provide a better understanding of the classification. Over time, the study of internal root canal morphology has been approached using different methodologies: traditional radiography, radiography using radiopaque contrast agents, sectioning, scanning electron microscopy, clearing technique, Cone-Beam Computed Tomography (CBCT) and micro-Cone-Beam Computed Tomography (\(\mu\)-CT) [3; 4; 5; 6; 7; 8]. Besides, in recent years, there has been an increase in the number of published studies on the morphology of the root canal system (Figure 2). Figure 1: [PERSON]’s configuration adapted to the permanent mandibular incisor. Figure 2: Studies according to the U.S. National Library of Medicine database, using “root canals” and “anatomy morphology” as the keywords. Source: U.S. National Library of Medicine. US National Library of Medicine. [[https://www.nlm.nih.gov](https://www.nlm.nih.gov)]([https://www.nlm.nih.gov](https://www.nlm.nih.gov)) (accessed on 28 February 2020). All of them have contributed to creating a detailed anatomical map of permanent mandibular incisors, conveying the fact that the internal morphology of such tooth is not simple and that it is necessary to invoke variations of Vertucci's classification [9]. CBCT offers the advantage of being a highly accurate technique that provides three-dimensional images in a non-invasive way, exposing patients to lower doses of radiation than other techniques such as traditional computed tomography. Its use in the assessment of root canal morphology has been endorsed by the European Society of Endodontology and provides clinicians with evidence-based criteria. The greatest disadvantage of CBCT in the study of dental anatomy is the presence of image artifacts caused by the presence of highly radiopaque materials from the filling and restoration of the tooth or resulting from other factors such as the patient's movements [10]. Likewise, in in vivo studies, the strong impact of the use of CBCT as a method to study the root canal system is reflected in the high number of samples obtained in some clinical trials, allowing adequate statistical analysis in prevalence studies [11]. Failure of endodontic treatment is usually a cause for concern for both clinicians and patients. [PERSON] identified the presence of isthmuses or untreated canals as the main cause of endodontic therapy failure [12]. Lack of knowledge of anatomy and, consequently, missing untreated canals is highly associated with the presence of periapical lesions. Specifically, as regards such lesions, the prevalence rate of missed canals ranges between 12.2% and 17.4% in central and lateral mandibular incisors, respectively [13]. These data should encourage clinicians to search for a more complex internal anatomy and reveal the importance of chemo-mechanical preparation of the root canal as the main strategy for its disinfection [14]. On the other hand, there are widely differing data on the existence of a second root canal. Reported percentages range from 0.4% [15] to 70% [16] when none of the methodologies were excluded. This renders it necessary to eliminate the heterogeneity as regards the methodology used. The purpose of this study was to carry out a systematic review on the morphology of the root canal system in permanent mandibular incisors, assessed using CBCT in human clinical studies, with voxel sizes of up to 0.3 mm.