Root Canal Morphology of the Permanent Mandibular Incisors by Cone Beam Computed Tomography: A Systematic Review

applied sciences Review Root Canal Morphology of the Permanent Mandibular Incisors by Cone Beam Computed Tomography: A Systematic Review Silvia Herrero-Hernández 1, Nansi López-Valverde 1,* , Manuel Bravo 2, Óliver Valencia de Pablo 3, Manuel Peix-Sánchez 1, Javier Flores-Fraile 1, Juan Manuel Ramírez 4, Bruno Macedo de Sousa 5and Antonio López-Valverde 1 1Department of Surgery, University of Salamanca, Instituto de Investigación Biomédica de Salamanca (IBSAL), 37007 Salamanca, Spain; silvia_herrer[email protected] (S.H.-H.); [email protected] (M.P.-S.); j.flor[email protected] (J.F.-F.); [email protected] (A.L.-V.) 2Department of Preventive and Community Dentistry, Facultad de Odontología, Campus de Cartuja s/n, 18071 Granada, Spain; [email protected] 3Department of Endodontics, Universidad Europea, 28670 Madrid, Spain; [email protected] 4Department of Morphological Sciences, University of Cordoba, Avenida Menéndez Pidal s/n, 14071 Cordoba, Spain; jmramir[email protected] 5Institute for Occlusion and Orofacial Pain Faculty of Medicine, University of Coimbra, Polo I-Edifício Central Rua Larga, 3004-504 Coimbra, Portugal; br[email protected] *Correspondence: [email protected]; Tel.: +34-656238285 Received: 24 June 2020; Accepted: 13 July 2020; Published: 17 July 2020   Abstract: Knowledge of dental anatomy through the assessment of the anatomic variations of each tooth’s root canal system is essential to undertake endodontic therapy. The aim of this systematic review was to analyze the different studies on the internal morphology of permanent mandibular incisors where Cone-Beam Computed Tomography (CBCT) X-ray imaging is used. Pubmed, CENTRAL, Wiley Library and Web of Science electronic databases were searched for scientific studies included until March 2020. The terms used in the search were: “permanent mandibular incisors”, “root canal morphology” and “cone-beam computed tomography”. The search was limited to studies whose aim was the analysis of the morphology of the root canal system evaluating the parameters of methodology, population, sample, number and configuration. A total of 19 studies met the inclusion criteria. There was a noticeable lack of unanimity in the setting adjustments of each of the CBCT devices used. The presence of two root canals varied from 0.4% to 45%. The most frequent configurations were Vertucci’s Types I, III, II, V, IV, VII and VI. Type VIII configuration was non-existent. CBCT revealed the existence of anatomical symmetry patterns, and there was no unanimity of criteria regarding the presence of a second root canal. Results concerning the presence of a second root canal in the mandibular incisors differ widely, with a possible influence of the geographic area where the study was conducted. The prevalence of a second canal is higher in mandibular lateral incisors than in mandibular central incisors. There was no direct relationship between voxel size (0.125–0.3 mm) and increased prevalence of a second canal. Keywords: permanent mandibular incisors; root canal morphology; cone-beam computed tomography (CBCT) Appl. Sci. 2020,10, 4914; doi:10.3390/app10144914 www.mdpi.com/journal/applsci Appl. Sci. 2020,10, 4914 2 of 15 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, Vertucci drew up a classification based on the different root canal morphologies [ 2 ]. Figure 1 shows Vertucci’s configuration for the lower incisor. The purpose of this figure, created by the authors, is to provide a better understanding of the classification. Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 15 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, Vertucci drew up a classification based on the different root canal morphologies [2]. Figure 1 shows Vertucci’s configuration for the lower incisor. The purpose of this figure, created by the authors, is to provide a better understanding of the classification. Figure 1. Vertucci’s configuration adapted to the permanent mandibular incisor. 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 (µ-CT) [3–8]. 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 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 (accessed on 28 February 2020). Increase in studies until April 2020 Figure 1. Vertucci’s configuration adapted to the permanent mandibular incisor. 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 ( µ -CT) [ 3 – 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). Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 15 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, Vertucci drew up a classification based on the different root canal morphologies [2]. Figure 1 shows Vertucci’s configuration for the lower incisor. The purpose of this figure, created by the authors, is to provide a better understanding of the classification. Figure 1. Vertucci’s configuration adapted to the permanent mandibular incisor. 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 (µ-CT) [3–8]. 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 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 (accessed on 28 February 2020). Increase in studies until April 2020 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 (accessed on 28 February 2020). Appl. Sci. 2020,10, 4914 3 of 15 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. Von Arx 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. We believe that this systematic review will contribute to the understanding of the widely differing results expressed in the literature as regards the presence of a second root canal and to the finding of possible explanations, as well as to the search for the lowest radiation dose possible for the study of internal anatomy. 2. Methods The study was planned and structured according to the PRISMA guidelines (Preferred Reporting Items for Systematic Review and Meta-analysis) [17]. 2.1. Protocols The search strategy was conducted using the condition, context and population framework (CoCoPop), based on the following question: “What is the prevalence of root canal configuration of the permanent mandibular incisors?”. To answer this question, the condition was the morphology of the root canal system in the mandibular incisors. Only studies that used an in vivo CBCT methodology were included. The context included all the in vivo studies carried out using CBCT, without excluding any country in the world. The population consisted of patients who had been subjected to CBCT, regardless of its diagnostic purposes. 2.2. Search Method for the Identification of Studies A bibliographic search of the Pubmed, CENTRAL, Wiley Library and Web of Science electronic databases was conducted in order to identify the most relevant studies available until 28 March 2020. The purpose was to identify in vivo prevalence studies where the canal system of the permanent mandibular Appl. Sci. 2020,10, 4914 4 of 15 incisors was analyzed using CBCT. The last update was made on 1 July 2020. The search terms were “permanent mandibular incisors”, “root canal morphology”, “cone-beam computed tomography” and “CBCT”. The keywords were used individually or in combination, using the boolean operators “AND”, “OR”, “NOT” to independently search for the term “cone-beam computed tomography” or its abbreviation (“CBCT”), include all terms, or exclude “permanent mandibular incisor” because of the possibility of finding studies that evaluated all dental groups. The search was completed manually by reading two endodontic journals: Journal of Endodontics and International Journal of Endodontics. 2.3. Inclusion and Exclusion Criteria The inclusion criteria covered studies that evaluated the configuration of the root system of the permanent mandibular incisors and that were published in English. Furthermore, the evaluation of the canals had to indicate not only the presence of a second canal but also its internal configuration using Vertucci’s classification. Exclusion criteria were studies that did not use CBCT as a diagnostic tool, in vitro studies, systemic reviews, case reports, and duplicate studies. Besides, those that did not indicate the number of samples and patients who participated in the study, or whose samples had been partially analyzed in other included studies, were excluded. 2.4. Data Extraction and Analysis First, two reviewers (SH-H and NL-V) undertook the reading of all the titles and abstracts, and those that did not refer to the research question were removed. Subsequently, the aforementioned reviewers conducted an independent evaluation, following the inclusion/exclusion criteria, until reaching a consensus on the studies to be included in the study. Finally, a total of 19 studies were included and the full texts of the selected studies were obtained for review. The extraction of data from each study was performed using Excel spreadsheets, completing the following categories: year of publication, sample size, country, percentage of a second canal, Vertucci’s configuration, other relevant results, CBCT model, voxel size, FOV, CBCT settings and software visualization. 2.5. Quality of the Reports of the Included Studies We used the STROBE recommendations checklist adapted by Martins et al. for cross-sectional studies on root and root canal anatomy using CBCT, as a proxy indicator of quality [ 18 ]. The objective of the checklist is to assess limitations and risk of bias in the studies, which could lead to an erroneous reading of the results. In addition, it increases the validity and strength of the findings and the reproducibility of the method, and it is an indicator that improves the overall quality of the prevalence studies. Each item was assessed by reviewers S.H.-H. and N.L.-V., who attributed scores of 0 (not reported) or 1 (reported), carrying out a complete count of all the studies included. 3. Results 3.1. Characteristics of the Studies A total of 2290 studies were gathered and analyzed. Of these, 2219 were removed because of their being duplicates or not related to the study. Of the remaining 71 studies, 52 were removed due to lack of relevance, not using CBCT as their diagnostic methodology, or being in vitro trials, case studies or systematic reviews, leaving a total of 19 studies [ 8 , 14 , 19 – 35 ] (Figure 3. Flowchart). Table 1includes the information corresponding to each of the assessed studies including their items: population, sample, second root canal percentage, Vertucci classification (%) and other results. Table 2shows the details of each study regarding the parameters used in the CBCT imaging process. Appl. Sci. 2020,10, 4914 5 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 15 the intra and interrater reliability test). Furthermore, it was interesting to note how all the studies used the word “Cone-beam computed tomography or CBCT” in the title, but none of them mentioned the type of study (Table 3). Figure 3. Flowchart of the study selection process. PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analyses) [17]. Figure 3. Flowchart of the study selection process. PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analyses) [17]. 3.2. Quality of the Reports of the Included Studies The evaluation of “specific preferred reporting items for cross-sectional studies on root and root canal anatomy using cone-beam computed tomography (CBCT)” [ 18 ] demonstrated that 50% of the items were reported in all of the published studies. The items that were most frequently disregarded by the researchers were “future research” (not indicated in 78.9% of the studies), “strengths and limitations” (68.4% of the studies failed to assess limitations) and “reliability” (52.6% did not perform the intra and interrater reliability test). Furthermore, it was interesting to note how all the studies used the word “Cone-beam computed tomography or CBCT” in the title, but none of them mentioned the type of study (Table 3). Appl. Sci. 2020,10, 4914 6 of 15 Table 1. Root canal configuration of permanent mandibular incisors. Study/Year nCountry 2nd Root Canal Vertucci Other Outcomes I II III IV V VI VII Others Baxter et al./2020 [7]604 CI 604 LI Germany 22.6% 24.3% 76.1% 76.6% 22% 21.3% -0.6% 1% 1.1% 1% - - 0.2% 0.1% Symmetry: Type I: 77% (CI), 77% (LI); Rest of types: 17.5% (CI), 20.5% (LI). Sroczyk et al./2019 [19]212 CI 208 LI Poland 34.1% 31.8% 65.4% 67.2% 1% 0.9% 26.4% 25% - 0.5% 5.3% 3.9% - 1% - 0.9% 2.5% Symmetry: Type I: 54.86% (CI), 56.12% (LI). Rest of types: 23.53% (CI), 19,19% (LI). Mashyakhy et al./2019 [20]410 CI 412 LI Saudi Arabia 26.3% 30.8% 73.7% 69.2% -26.3% 29.8% -- 1% - - - Symmetry: 91.2% (CI), 85.8% (LI). Mirhosseini et al./2019 [21]330 CI 351 LI Iran 23.9% 35% 76.1% 65% 0% 0.6% 15.8% 15.7% 0.6% 0.9% 7.6% 17.8% - - - % 2nd root canal: LI >CI Pan et al./2019 [22]408 CI 400 LI Malaysia 5.1% 12.3% 94.9% 87.8% -1% 3.7% 0.3% 4.1% 8.2% - - - - Valenti-Obino et al./2019 [23]487 CI 491 LI Italy 45% 43% 55% 57% 34.3% 35.7% 9.3% 6.9% 0.6% -- - 0.8% 0.4% -Symmetry: 44.6% (CI), 44.8% (LI). Martins et al./2018 [15] 240 CI 240 LI 1203 CI 1234 LI China Portugal 0.4% 5% 27.4% 29.9% 99.6% 95% 72.6% 70.1% - 2.9% 2.4% 6.1% 0.4% 0.8% 24% 23.1% - - 0.1% - - 1.3% 0.3% 0.2% - - - - - - 0.5% 0.2% - - 0.1% 0.3% The Asian group have a higher prevalence of Vertucci type I configuration compared to the white ethnic group. Wu et al./2018 [24]800 CI Taiwan 15.6% 84.4% - 13.5% 2.1% - - - Correlation between complicated root canal CI-DLR in PMFM. Saati et al./2018 [25]207 CI 207 LI Iran 15.5% 21.8% 54.5% 56.5% -34.2% 26.1% -11.3% 17.4% - - - - Shemesh et al./2018 [26]1472 CI 1508 LI Israel 40.5% 37.9% 59.5% 62.1% 4% 4.3% 33.7% 31.9% 0.8% 0.4% 0.5% 0.5% - - 1.5% 0.8% Symmetry: 69.8% (CI), 68.7% (LI). Verma et al./2017 [27]400 CI 400 LI India 31.8% 35% 68.3% 65% 11% 13.2% 15.3% 15.2% 1.7% 3.0% 3.7% 3.6% - - - % 2nd root canal: LI >CI Da Silva et al./2016 [28]200 CI 200 LI Brazil 35.5% 39.5% 64.5% 60.5% - 0.5% 18% 25.5% - - 14.5% 12% 0.5% - 2.5% 1.5% - - Zhengyan et al./2016 [29]3375 CI 3257 LI China 3.8% 10.6% 96.2% 89.4% 0.1% 1% 2.7% 7.7% 0.1% 0.3% 0.7% 1.2% - - 0.2% 0.4% 2nd root canal >LI women Geduk et al./2015 [30]1438 I Turkey 3.6% 64.4% 15.2% 19.4% 0.2% 0.8% - - - 2nd root canal >41–50 years Appl. Sci. 2020,10, 4914 7 of 15 Table 1. Cont. Study/Year nCountry 2nd Root Canal Vertucci Other Outcomes I II III IV V VI VII Others Altunsoy et al./2014 [31]1582 CI 1603 LI Turkey 15.3% 19.2% 84.4% 80.2% 0.4% 1.3% 0.8% 1.0% 4.3% 5.4% 10.1% 12.1% - - - 2nd root canal: men >women Han et al./2014 [32]1286 CI 1294 LI China 15.7% 27.4% 84.3% 72.6% 3.4% 4.0% 6.5% 15.5% 1.2% 2.3% 3.9% 5.1% - 0.2% 0.3% 0.2% 0.4% 0.1% IL >IC. Distance apex-root canal bifurcation: 6–12 mm. Lin et al./2014 [33]706 CI 706 LI China 10.9% 25.5% 89.1% 74.5% 2.4% 3.7% 6.2% 19.3% 1.7% 2.1% 0.6% 0.4% - - - Symmetry: 95.2% (CI), 93.8% (LI).2nd root canal: LI >CI Liu et al./2014 [34]786 CI 785 LI China 8.9% 17.5% 91.1% 82.5% 2.0% 3.9% 5.3% 10.4% 1.3% 2.8% 0.3% 0.3% - - - - Aminsobhani et al./2013 [35]632 CI 614 LI Iran 27.3% 29.4% 72.7% 70.6% 11.3% 7.1% 4.7% 3.7% 7.7% 15.4% 3.6% 3.2% - - - AV: 21.3 ± 0.10 (CI), 21.9 ± 0.13 (LI). No gender difference n(number of incisors); CI (central incisor); LI (lateral incisor); DLR (distolingual root); PMFM (permanent mandibular first molar); I (incisors); AV (average length). Table 2. Cone-beam computed tomography parameter values of each study. Study/Year Country % 2nd Root Canal CBCT Model Voxel Size FOV Settings CBCT Software Visualization Baxter et al./2020 [7]Germany 23.45% Galaxis Galileo (Sirona, Bensheim, Germany) 0.3 mm 15 cm385 Kv/5–7 mA/- Sroczyk et al./2019 [19]Poland 32.9% Cranex 3D (Soredex, Tuusula, Finland) - - - Horos Mashyakhy/2019 [20]Saudi Arabia 28.55% 3D Accuitomo 170 (Morita, Kyoto, Japan) 0.25 mm - 90 Kv/5–8 mA/17.5 s. Morita’s i-Dixel 3D Mirhosseini et al./2019 [21]Iran 23.9% Planmeca ProMax 3D (Planmeca, Helsinki, Finland) 0.2 mm 100 ×70 ×50 mm 90 Kv/10 mA/14 s. Planmeca Romexis Pan et al./2019 [22]Malaysia 17.4% KaVo 3D eXam (Imaging Sciences International, Hatfield, PA, USA) 0.25 mm - 120 kV/5 mA/26.9 s. eXam Vision Valenti-obino et al./2019 [23]Italy 44% GXDP-500 system (Gendex Dental, Biberach, Germany) 0.2 mm 13 ×9×13 cm 90 kV/7 mA/23 s. Horos Martins et al./2018 [15]China 2.7% Kodak 9500 (Carestream, Atlanta, GA, USA) 0.2 mm Full arch. 90 kV/10 mA/10.8 s. CS 900 3D imaging Martins et al./2018 [15]Portugal 28.6% Planmeca ProMax 3D (Planmeca, Helsinki, Finland) 0.2 mm Full arch 80 kV/15 mA/12.0 s. Planmeca Romexis Wu et al./2018 [24]Taiwan 15.6% NewTom 5G (QR, Verona, Italy) - Full arch 110 kV/11.94 mA/7 s. - Appl. Sci. 2020,10, 4914 8 of 15 Table 2. Cont. Study/Year Country % 2nd Root Canal CBCT Model Voxel Size FOV Settings CBCT Software Visualization Shemesh et al./2018 [25]Israel 39.2% Asahi Alioth (Asahi Roentgen IND, Kyoto, Japan) 0.155 mm 80 ×80 mm 85 kV/6 mA OnDemand 3D Saati et al./2018 [26]Iran 18.6% NewTom 5G (QR, Verona, Italy) 0.25 mm - 110 kV/2.5–6.7 mA/12 s. NNT Viewer Verma et al./2017 [27]India 33.5% Galaxis Galileo (Sirona, Bensheim, Germany) - - 98 kV/5–15 mA - Da Silva et al./2016 [28]Brazil 37.5% i-CAT (Imaging Sciences International, Hatfield, PA, USA) 0.2 mm - 120 kV/7 mA/40 s. i-CAT Zhengyan et al./2016 [29]China 7.2% - 0.125 mm - 120 kV/5 mA/9–18 s. i-CAT Geduk et al./2015 [30]Turkey 3.6% Galaxis Galileo (Sirona, Bensheim, Germany) - - 98 kV/15–30 mA SIDEXIS XG Altunsoy et al./2014 [31]Turkey 17.3% i-CAT (Imaging Sciences International, Hatfield, PA, USA) 0.3 mm - 120 kV/9–14 mA/6 s. - Han et al./2014 [32]China 21.7% Galaxis Galileo (Sirona, Bensheim, Germany) 0.125 mm - 85 kV/35.0 mA/2–6 s. SIDEXIS XG Lin et al./2014 [33]- - - - - - Liu et al./2014 [34]China 13.2% i-CAT (Imaging Sciences International, Hatfield, PA, USA) - - 120 kV/5 mA/9–18 s. eXam Vision Aminsobhani et al./2013 [35]Irán 28.35% Planmeca ProMax 3D (Planmeca, Helsinki, Finland) - - - Planmeca Romexis FOV (Field of View). Appl. Sci. 2020,10, 4914 9 of 15 Table 3. Specific Preferred Reporting Items for Cross-sectional Studies on Root and Root Canal Anatomy Using Cone-beam Computed Tomographic (CBCT). Section and Item n(%) 1. Title 19 100 Introduction 2. Keywords 19 100 3. Aim 19 100 Methods 4. Participants (in vivo assessment) 19 100 5. CBCT 19 100 6. Morphology concept and assessed teeth (variables) 19 100 7. Assessment 19 100 8. Observers 13 68.4 9. Potential sources of bias 15 78.9 10. Final simple size 15 78.9 11. Reliability 9 47.4 12. Statistical analysis 13 68.4 13. Ethics Committee 13 78.9 Results 14. Primary outcomes 19 100 15. Other analysis 15 78.9 16. Visual documentation support 16 84.2 Discussion 17. Outcomes interpretation 19 100 18. Strength and limitations 6 31.5 19. Generalizability 19 100 20. Future research 4 21.0 3.3. Synthesis of Studies Included There is unanimity across the studies regarding the fact that type I is the most frequent configuration. The rest of Vertucci’s configurations were considered incisors with two canals, regardless of their path in the canal system. When there were two canals, the most frequent classification was type III, with the exception of four studies [ 8 , 22 , 23 , 35 ]. There was no heterogeneity as to the frequency of the rest of configurations, with studies where the next most frequent configurations were type II [ 14 , 23 , 26 , 27 , 30 , 33 , 34 ], type V [ 8 , 19 – 21 , 24 , 25 , 28 , 29 , 32 ], or type IV [ 31 ]. The remaining types [VI, VII and others] were generally present in very low or non-existent percentages. Five studies reported symmetry between the incisors on the right and left side of the patient [ 8 , 19 , 20 , 23 , 26 , 33 ]. Lateral incisors showed a higher prevalence in the existence of a second canal compared to central incisors in fourteen studies [ 7 , 14 , 20 – 22 , 25 , 27 – 29 , 31 – 35 ]. The central incisor had a higher prevalence in only three [ 19 , 23 , 26 ]. Likewise, we calculated the weighted average of all the percentages in the existence of a second canal, obtaining 17.2% in the central and 23.7% in the lateral incisors. Only two studies reported a relationship between age and the presence of a second canal [ 24 , 29 ]. The remaining studies did not assess patient age or, if so, did not find any significant differences [ 8 , 30 ]. Regarding gender influence, there was no unanimity as to a higher prevalence of second canals in men [ 19 , 20 , 24 , 31 ] than in women [ 27 , 29 ], and there were even certain studies where no significant differences were found [ 8 , 22 , 25 , 30 , 34 , 35 ]. Geographical distribution indicated a lower incidence of a second canal in the Chinese population [ 14 , 29 , 32 – 34 ]. There was no direct relationship between voxel size and the increased prevalence of a second canal.