Root Canal Morphology and Configuration of the Mandibular Canine: A Systematic Review

The aim of this study was to systematically review the root canal morphology and configuration (RCC) of mandibular canines (MaCa). The review was registered in the PROSPERO database (ID-272297) and it was carried out following the PRISMA guidelines. Three electronic databases (MEDLINE via PubMed, Embase, Scopus) were searched. Randomized controlled trials, cross-sectional, cohort, comparative, evaluation and validation studies have been included. The anatomical quality assessment (AQUA) tool was used for a quality assessment of the anatomical studies. Of 910 studies retrieved from the systematic search, 28 studies investigating RCCs were included. Most MaCa were single-rooted (87.9–100%), while two-rooted MaCa were present up to 12.1%. The 1-1-1/1 (35.8–96.4%) was the most commonly reported RCC, followed by 2-2-1/1 (0.2–22.0%) and 1-2-1/1 (0.9–20.0%). A high frequency of 1-1-1/1 RCC in MaCa has been described. Most systematic review reports confirm that two-rooted MaCa are found considerably less frequently than single-rooted ones.


Introduction
Knowledge and understanding of the internal morphology of root canals is crucial for successful non-surgical as well as surgical endodontic therapy [1][2][3]. Although examination methods have improved significantly in recent decades, interest in the morphology of the three-dimensional root canal system and its importance has not diminished. In order to minimize or to avoid iatrogenic errors and failures in endodontic treatment, a precise knowledge of the anatomical relationships in the root canal system and the immediate recognition of possible deviations is of integral importance for the treating dentist [1][2][3].
Moreover, the realization that the morphological complexity of the root canal system can be obscured by the uniform and relatively simple radiological anatomy of the outer root surface is of great clinical benefit [4]. Various methods, such as staining and clearing [5][6][7][8][9][10], grinding [11], cross-sectional [12], microscopy [9,13,14], and radiographic analysis [15] have been used to study the morphology of the root canal system, with both ex vivo/in vitro and in vivo studies described in the literature.
Micro-CT has emerged as a non-destructive, noninvasive, and reproducible examination method when in combination with 3D image rendering software and can be considered as the gold standard for dental research purposes [35,39]. Half a century ago, Vertucci [1] and Weine et al. [2] proposed two of what nowadays are the most commonly used methods to describe root canal configuration; they used decalcification, injection with dye, and clearing [1] or sectioning [2]. However, these methods cannot describe various configurations, as is possible with the method developed by Briseño-Marroquín et al. [3]. The use of micro-CT by Briseño-Marroquín et al. has the advantage that the classification system is descriptive and can be applied individually to the internal morphology of a particular root, rather than forcing a classification based on the system of internal morphology.
Therefore, the aim of the present paper is to provide a systematic review of the root canal configuration of mandibular canines, contributing to the morphological knowledge that is a prerequisite for successful endodontic treatment.

Materials and Methods
The protocol was registered in the international prospective register of systematic reviews (PROSPERO) system of the National Institute of Health Research of the Centre for Reviews and Dissemination of the University of York (United Kingdom) (ID-272297, 7 August 2021). The systematic review followed the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines [40].

Eligibility Criteria
Cross-sectional studies, comparative studies, evaluation and validation studies, and randomized controlled trials (RCTs) were included in the review procedure. Case reports and reviews were excluded. Furthermore, only papers containing data on root canal configuration were included in the systematic review. Exclusion criteria, therefore, included studies investigating other morphological issues than root canal configuration. All duplicates were removed; the remaining articles were examined by title and abstract, and papers were discarded after consulting the title and abstract and finding that they did not refer to the topic. The papers were then reviewed in full text; several papers were excluded after consulting the full text.

Information Sources and Search Strategy
Several literature searches through three electronic databases (MEDLINE via PubMed, Embase, and Scopus) were performed up to August 2021, using an ad hoc prepared string with Medical Subject Heading (MeSH) terms and keywords: (oot canal configuration OR root canal system OR root canal morphology) AND (morphology OR anatomy) AND (mandibular canine) without any restrictions. A cross-reference search in the reference list of full-text articles was performed. Grey literature has also been retrieved (http://www. opengrey.eu). (accessed on 26 August 2021).

Study Selection
Only publications in English were considered; duplicates and those articles deemed ineligible were excluded. Three authors (T.G.W., A.L.A. and G.C.) independently examined all abstracts of the screened papers. All articles that met the inclusion criteria were reviewed by two independent observers (T.G.W. and A.L.A.) in full text.

Data Collection, Summary Measures and Synthesis of Results
Information of the reports on publication date, authors, population investigated, number of specimens/patients, methodology, data on root canal configurations and number of roots were summarized.

Assessment of Bias across Studies
The risk of bias of the included studies was assessed with the anatomical quality assessment (AQUA) tool for the quality assessment of anatomical studies [41]. Two authors (T.G.W., A.L.A.) independently screened the articles and assessed the risk of bias using the five AQUA tool domains. In case of disagreement in the assessment, a third author (G.C.) was consulted to reach to a consensus. Each report has been judged as "low", "high" or "unclear" in the categories: target and subject attributed, design of the study, methodology description, descriptive anatomy and reporting of outcomes. The tool contains five domains, each with a set of signaling dichotomous questions (Yes or No) to help assess and judge the risk of bias pertaining to it. If all questions of a category are "Yes", then the risk of bias can be judged as "low".
The list of excluded papers (Table S1), the quality assessment of the studies (Table S2), the AQUA tool evaluation (Table S3), the list of included papers after full text evaluation (Table S4) and the PRISMA checklist (S5) can be found under Supplemental Materials.

Results
The literature search through the three databases (MEDLINE via PubMed, Embase, Scopus) resulted in a total of 910 articles. After all duplicates were removed, the remaining articles (n = 833) were examined according to title and abstract, and 768 papers were discarded after consulting the title and abstract. A total of 65 articles were reviewed in full text, and a further 42 papers were excluded after consulting the full text. Through cross-referencing and a hand search of the bibliographies of the full-text articles, another five articles were added to this review. Finally, 28 articles containing randomized controlled trials, cross-sectional studies, comparative studies and evaluation studies from different study populations were included (      Table 1 shows detailed information on the articles: authors, year of publication, sample size, research methods used, number of roots and root canal configurations (RCCs) observed based on the classification systems by Vertucci [1], Weine et al. [2] and Briseño-Marroquín et al. [3].

Discussion
The present study was designed and conducted as a systematic review of the root canal configurations of mandibular canines, in order to provide the dentist with knowledge/understanding of the root canal morphology to be expected during clinical treatment.
Several reviewed studies that considered the morphology of the mandibular canines (MaCa) were performed by means of CBCT imaging, examining a relatively large sample size [6,13,14,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. Although CBCT images do not provide images that are as highresolution as those of micro-CT, it appears to be a good method to examine root canal configurations [3,43]. Few studies have investigated the MaCa root canal morphology by means of micro-CT [33,34,37,38]. However, those investigating morphological parameters different from the ones in the systematic review investigated other topics than root canal configuration; thus, they did not meet the inclusion criteria and could not be considered in the present study.
The root canal configuration systems proposed by Vertucci [1] and Weine et al. [2] have been extensively used to describe root canal configuration. With computer-assisted imaging techniques, such as micro-CT, it has been possible to depict further root canal configurations; however, these cannot be correctly classified with the stated classification systems by Vertucci [1] and Weine et al. [2].
Despite the possible differences and the superiority of the gold standard micro-CT, studies using this method cannot currently be found in the literature for the root canal configuration of the mandibular canine. Further research is needed; the investigation of accessory canals across all root thirds, observed and evaluated mainly with the micro-CT method, could provide additional information and enhance the knowledge of the dentist to increase the success of an endodontic treatment based on additional understanding, improved therapy decisions, and the appropriate selection of instruments and techniques.

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CBCT is widely and, in recent years, most frequently used for in vivo research on the root canal morphology of mandibular canines.