Root canal anatomy of human permanent mandibular incisors and mandibular canines: A systematic review.

Mandibular incisors and canines show variations in their root canal anatomy. It is imperative to be aware of these variations to lower the frequency of missed canals. The objective was to systematically review the existing studies to recognize the root and root canal anatomy of the mandibular incisors and canines among the population of various geographical locations, as studied by different methods. This systematic review was registered in the International Prospective Register of Systematic Reviews database under the number CRD42020185146. An exhaustive search was undertaken in three electronic databases - MEDLINE PubMed, EBSCOhost, and Google Scholar - along with hand-searching for the identification of studies. Studies were selected following strict inclusion and exclusion criteria. Selected studies were scored using the Joanna Briggs Institute Critical Appraisal tool for prevalence studies to determine the risk of bias. This review included data from 26 countries including the analysis of 71,404 mandibular anterior teeth. The risk of bias of all included studies was either low or moderate. Overall, it was seen that the percentage of second canals was higher in lateral than in central incisors. Mandibular canines occasionally showed the presence of two roots. Deviations in anatomy were high in the Middle Eastern European countries and the Indian subcontinent. Limitations are as follows: lack of homogeneity across data reported in the studies concerning the methods used to study the root canal anatomy. Diversities seen in the root canal configurations of mandibular incisors and canines vary according to ethnicity, and thus, one must have a thorough knowledge before commencing endodontic treatment. Copyright:

INTRODUCTION

Human teeth have an extremely complicated pulp space anatomy. Each tooth in the dental arch has unique anatomy which shows high degree of variability among populations. Mandibular incisors and canines are known to have a single root and one or two canals. However, often, they show variations in their root canal anatomy such as the presence of an extra lingual canal. There is a high prevalence of a second canal in the mandibular incisors and canines.[1]

It is critical to know these variations in the anatomy to lower the frequency of missed canals during endodontic treatment, as this can cause the failure of root canal treatment. A periapical lesion is 4.38 times more likely to occur in a tooth with a missed canal.[2] In a vital tooth, a missed canal contains pulp tissue which may cause pain at first and then may get infected. In case of a nonvital tooth, the canal that is missed may be infected and thus would lead to persistence of infection and failure of treatment. Therefore, complete debridement and disinfection of the root canal system is critical to a successful endodontic treatment outcome. The knowledge of canal systems along with their frequent variations is of utmost importance for the success of root canal treatment.

In addition, root canal anatomy configurations may vary according to ethnicity,[34] sex,[54567] and age.[8] Several methods to study the root canal anatomy have been published in the literature which includes radiographic methods such as intraoral periapical (IOPA) radiograph - film[9] digital IOPA,[10] cone-beam computed tomography (CT),[11] and micro-CT,[12] as well as nonradiographic methods such as tooth clearing and canal staining technique,[13] modeling,[14] and the sectioning technique.[1015]

Root canal anatomy was studied[16] and then classified by various authors to enable standardization and reproducibility.[17181920] At present, several studies on the different types of root canal anatomy seen in the mandibular incisors and canines are available in the literature. These studies have origins in different countries and use different methods to study root canal anatomy. The systematic review aims to combine these existing studies to understand the variations in the root and root canal configurations of human permanent mandibular incisors and canines among the population of various geographical locations, as studied by different methods.

MATERIALS AND METHODS

This systematic review was registered in the International Prospective Register of Systematic Reviews database under CRD42020185146. It is formulated and written in accordance with the Preferred Reporting Items for Systematic Review and Meta-analysis guidelines, 2020 [Supplemental Table 1].[21]

Supplemental Table S1

Filled PRISMA checklist, 2020

Section and topic	Item #	Checklist item	Location where the item is reported (Page No.)
Title
Title	1	Identify the report as a systematic review.	1
Abstract
Abstract	2	See the PRISMA 2020 for Abstracts checklist. 21	1
Introduction
Rationale	3	Describe the rationale for the review in the context of existing knowledge.	1
Objectives	4	Provide an explicit statement of the objective (s) or question (s) the review addresses.	2
Methods
Eligibility criteria	5	Specify the inclusion and exclusion criteria for the review and how studies were grouped for the syntheses.	3
Information sources	6	Specify all databases, registers, websites, organisations, reference lists and other sources searched or consulted to identify studies. Specify the date when each source was last searched or consulted.	2
Search strategy	7	Present the full search strategies for all databases, registers and websites, including any filters and limits used.	2
Selection process	8	Specify the methods used to decide whether a study met the inclusion criteria of the review, including how many reviewers screened each record and each report retrieved, whether they worked independently, and if applicable, details of automation tools used in the process.	3
Data collection process	9	Specify the methods used to collect data from reports, including how many reviewers collected data from each report, whether they worked independently, any processes for obtaining or confirming data from study investigators, and if applicable, details of automation tools used in the process.	3
Data items	10a	List and define all outcomes for which data were sought. Specify whether all results that were compatible with each outcome domain in each study were sought (e.g. for all measures, time points, analyses), and if not, the methods used to decide which results to collect.	3
	10b	List and define all other variables for which data were sought (e.g. participant and intervention characteristics, funding sources). Describe any assumptions made about any missing or unclear information	3, 7
Study risk of bias assessment	11	Specify the methods used to assess risk of bias in the included studies, including details of the tool (s) used, how many reviewers assessed each study and whether they worked independently, and if applicable, details of automation tools used in the process.	3
Effect measures	12	Specify for each outcome the effect measure (s) (e.g. risk ratio, mean difference) used in the synthesis or presentation of results.	3
Synthesis methods	13a	Describe the processes used to decide which studies were eligible for each synthesis (e.g. tabulating the study intervention characteristics and comparing against the planned groups for each synthesis.	3
	13b	Describe any methods required to prepare the data for presentation or synthesis, such as handling of missing summary statistics, or data conversions.	3, 7
	13c	Describe any methods used to tabulate or visually display results of individual studies and syntheses.	3
	13d	Describe any methods used to synthesise results and provide a rationale for the choice (s). If meta-analysis was performed, describe the model (s), method (s) to identify the presence and extent of statistical heterogeneity, and software package (s) used.	3
Reporting bias assessment	14	Describe any methods used to assess risk of bias due to missing results in a synthesis (arising from reporting biases).	3
Certainty assessment	15	Describe any methods used to assess certainty (or confidence) in the body of evidence for an outcome.	3
Results
Study selection	16a	Describe the results of the search and selection process, from the number of records identified in the search to the number of studies included in the review, ideally using a flow diagram	4
	16b	Cite studies that might appear to meet the inclusion criteria, but which were excluded, and explain why they were excluded.	3
Study characteristics	17	Cite each included study and present its characteristics.	5-7, 11-13
Risk of bias in studies	18	Present assessments of risk of bias for each included study.	4
Results of individual studies	19	For all outcomes, present, for each study: (a) summary statistics for each group (where appropriate) and (b) an effect estimate and its precision (e.g. confidence/credible interval), ideally using structured tables or plots.	5-7, 11-13
Results of syntheses	20a	For each synthesis, briefly summarise the characteristics and risk of bias among contributing studies.	3, 4, 7
	20b	Present results of all statistical syntheses conducted. If meta-analysis was done, present for each the summary estimate and its precision (e.g. confidence/credible interval) and measures of statistical heterogeneity. If comparing groups, describe the direction of the effect.	3
Reporting biases	21	Present assessments of risk of bias due to missing results (arising from reporting biases) for each synthesis assessed.	3, 4
	22	Present assessments of certainty (or confidence) in the body of evidence for each outcome assessed.	3, 4, 7
Discussion
Discussion	23a	Provide a general interpretation of the results in the context of other evidence.	4, 7, 9
	23b	Discuss any limitations of the evidence included in the review.	9
	23c	Discuss any limitations of the review processes used.	9, 14
	23d	Discuss implications of the results for practice, policy, and future research.	14
Other information
Registration and protocol	24	Provide registration information for the review, including register name and registration number, or state that the review was not registered.	2
Support	25	Describe sources of financial or non-financial support for the review, and the role of the funders or sponsors in the review.	14
Competing interests	26	Declare any competing interests of review authors.	14
Availability of data, code, and other materials	27	Report which of the following are publicly available and where they can be found: template data collection forms; data extracted from included studies; data used for all analyses; analytic code; any other materials used in the review.	3

The question in focus was, “What is the prevalence of Vertucci canal configuration with additional types in human mandibular incisors and canines as studied by various techniques and in different geographical locations?” It followed the PCC[22] – Population (P), Condition (C), Context (C) criteria for prevalence studies, where the Population (P) was human mandibular incisors and canines; Condition (C) was Vertucci types of canal configuration with its additional modifications, and Context (C) was the various geographical locations and different study designs used to study it.

Search strategy

A concept table was made based on the PCC[22] criteria. It included the key concepts, controlled vocabulary terms (Medical Subject Headings Terms), as well as free-text terms. These terms were used to formulate a search strategy. The terms were combined using suitable Boolean operators (AND, OR, and NOT). A similar search strategy was implemented in all the three electronic database searches – The National Library of Medicine (MEDLINE PubMed), EBSCOhost, Google Scholar – for the identification of studies for this review. Table 1a summarizes the terms and filters used in each database. The full electronic search strategy of the MEDLINE PubMed database is presented in Supplemental Figure 1. Hand-searching of the above databases was also done. References were checked of all eligible studies for other relevant studies. The databases were searched between 1965 and September 30, 2020, using the aforementioned search strategy. In addition, hand-searching of two scientific journals (Journal of Endodontics and International Endodontic Journal) was carried out.

Table 1a

Terms and filters used in each electronic database

Database	Terms used	Filters
PubMed	((((root canal anatomy) OR (root canal morphology)) OR (root canal configuration)) OR (canal morphology)) OR (root canal system) AND (((((((((((((((((((((((human permanent mandibular incisors and mandibular canines) OR (mandibular anterior teeth)) OR (permanent dentition)) OR (human permanent teeth)) OR (human permanent mandibular incisors)) OR (permanent mandibular incisors)) OR (human permanent mandibular canines)) OR (permanent mandibular canines)) OR (mandibular incisor)) OR (mandibular canines)) OR (anterior teeth)) OR (mandibular permanent anterior teeth)) OR (mandibular incisors)) OR (human anterior teeth)) OR (human mandibular canines)) OR (lower incisors)) OR (human mandibular anterior teeth)) OR (lower anterior teeth)) OR (mandibular central and lateral incisors)) OR (permanent teeth)) OR (mandibular permanent teeth)) OR (central and lateral mandibular incisors)) OR (mandibular canine)) AND ((((((Vertucci’s canal configuration) OR (Vertucci’s classification)) OR (root canal pattern)) OR (Vertucci)) OR (Weine classification)) OR (Gulabivala’s modification for Vertucci’s classification))	N/F Search - “All fields”
EBSCOhost	((((root canal anatomy) OR (root canal morphology)) OR (root canal configuration)) OR (canal morphology)) OR (root canal system) AND (((((((((((((((((((((((human permanent mandibular incisors and mandibular canines) OR (mandibular anterior teeth)) OR (permanent dentition)) OR (human permanent teeth)) OR (human permanent mandibular incisors)) OR (permanent mandibular incisors)) OR (human permanent mandibular canines)) OR (permanent mandibular canines)) OR (mandibular incisor)) OR (mandibular canines)) OR (anterior teeth)) OR (mandibular permanent anterior teeth)) OR (mandibular incisors)) OR (human anterior teeth)) OR (human mandibular canines)) OR (lower incisors)) OR (human mandibular anterior teeth)) OR (lower anterior teeth)) OR (mandibular central and lateral incisors)) OR (permanent teeth)) OR (mandibular permanent teeth)) OR (central and lateral mandibular incisors)) OR (mandibular canine))) AND ((((((Vertucci’s canal configuration) OR (Vertucci’s classification)) OR (root canal pattern)) OR (Vertucci)) OR (Weine classification)) OR (Gulabivala’s modification for Vertucci’s classification))	Limiters - Full-text expanders - Apply equivalent subjects Search modes - Boolean/Phrase
Google Scholar	Root canal anatomy OR root canal morphology OR root canal configuration OR canal morphology OR root canal system AND human permanent mandibular incisors and mandibular canines AND Vertucci’s canal configuration	N/F

N/F: No filter

Study selection

Initially, the titles and abstracts of the studies were screened and labeled as “appropriate” or “inappropriate.” This was done according to the previously decided inclusion and exclusion criteria [Table 1b]. Following this, the full text of all the appropriate studies was assessed for eligibility according to the same criteria.

Table 1b

Inclusion and exclusion criteria

Inclusion criteria	Exclusion criteria
Studies using all nonradiographic methods such as tooth clearing and canal staining technique, modeling, and sectioning technique to investigate the root canal anatomy	Reviews and expert opinions
Studies using all radiographic methods such as IOPN radiograph - manual and digital, CBCT, and micro-CT system to investigate the root canal anatomy	Only abstracts
Studies done on the root canal anatomy of human permanent mandibular incisors and canines in various world populations	Animal studies
Publications in English with full text available in hard or soft copy	Articles where the outcome of the study was not clearly specified
Articles published between 1965 and September 2020	Studies done on deciduous mandibular incisors and mandibular canines

CT: Computed tomography, CBCT: Cone-beam CT, IOPA: Intraoral periapical

Data extraction

Two review authors independently collected data using a specially designed data extraction form. The data extraction form had undergone a pilot test using a few articles. Using those results, it was modified before its usage. Data where the root canal anatomy was studied but was not presented as per any classification system were extracted, but it was included only if both the review authors could convert the data presented into a classification system (provided there was sufficient data given to enable conversion) and if both the review authors independently had the same result. Disagreements were resolved by discussion until a consensus was reached. For each of the included studies and each tooth category, data were extracted under the following headings: author, year of publication, country, method, number of subjects (with their age and gender, if specified), number of teeth, number of roots, Vertucci classification, additional modifications of Vertucci classification (given by Gulabivala et al.,[20] Sert and Bayirli,[6] and Kartal and Yanikoğlu[19]), and the risk of bias.

Scientific merit assessment

The quality of the selected studies was checked using the checklist from the Joanna Briggs Institute (JBI) Critical Appraisal tool for systematic reviews of prevalence studies [Supplemental Table 2].[23] The included studies were evaluated by two endodontists, who scored each JBI question as yes (score 2), no (score 0), unclear (score 1), or not applicable. An interrater reliability test was performed. Any differences in opinion between the evaluators were discussed till a consensus was reached.

Supplemental Table 2

Joanna Briggs Institute critical appraisal form

JBI Critical Appraisal tool for systematic reviews of prevalence studies questions
Number	JBI Question	Yes	No	Unclear	Not applicable
1	Was the sample frame appropriate to address the target population?
2	Were study participants sampled in an appropriate way?
3	Was the sample size adequate?
4	Were the study subjects and the setting described in detail?
5	Was the data analysis conducted with sufficient coverage of the identified sample?
6	Were valid methods used for the identification of the condition?
7	Was the condition measured in a standard, reliable way for all participants?
8	Was there appropriate statistical analysis?
9	Was the response rate adequate, and if not, was the low response rate managed appropriately?

JBI: Joanna Briggs Institute

When there was no sample size calculation done in the study, we conducted our sample size analysis using the given formula.[24] The prevalence values of Type I root canal anatomy[13] of previous studies[46132526] using tooth clearing and canal staining technique as a method of study (gold standard) were used for calculating the adequate sample size. The minimum sample size required was found to be 207 teeth for mandibular central incisors, 246 teeth for mandibular lateral incisors, and 113 teeth for mandibular canines. A sample size lesser than these values for any one of the mandibular anterior teeth was considered to be inadequate.

The final score of each study applied to the JBI questions was calculated. Then, the risk of bias of each study was categorized according to the final score as “high” (score below 8), “moderate” (score 8–12), or “low” (score 13–16).

RESULTS

Included studies

An exhaustive electronic database search resulted in 225 articles on PubMed, 646 articles on EBSCOhost, 357 articles from Google Scholar search, and 24 articles through a hand search. Mendeley software (Version 1.19.4, Elsevier, London, United Kingdom) was used for duplicate removal of the articles. After removing duplicates, 1130 articles were evaluated. From these, 1012 articles were excluded because they were labeled as “inappropriate” based on titles and abstracts. After a full-text analysis for eligibility, 33 articles were excluded [Supplemental Table 3] because they did not meet the inclusion criteria. Therefore, the study selection process resulted in 85 full-text articles [Figure 1].

Supplemental Table 3

A list of all the excluded studies from this systematic review along with the reason for their exclusion

Study	Reason
Aggarwal, 2016	Root canal anatomy is not studied
Agholor et al., 2020	No details of the canal configuration mentioned-like the number of foramina. cannot be classified
Ahmad, 2015	Review article
Aldawla et al., 2019	Review article
Amreen et al., 2020	Data is mismatched
Andrei et al., 2011	Outcome of the study not clearly specified with respect to any type of classification system
Barker et al., 1973	Outcome of the study not clearly specified with respect to any type of classification system
Bellizzi et al., 1983	Root with two canals are not categorized as to whether the canals exit by a common apical foramen or separate apical foramina. Cannot be classified
Beshkenadze et al., 2015	Each type of Vertucci classification is not specified for a particular tooth type
Haghanifar et al.,2017	Each type of Vertucci classification is not given as a separate percentage
Hession, 1977	Details of the apical foramina not given in the roots with multiple canals; therefore, not possible to deduce Vertucci classification
Jaiswal et al., 2013	Root canal anatomy is studied
Kaffe et al., 1985	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Kayaoglu et al.,2015	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Kerekes et al., 1977	Number of root canals neither studied nor classified
Kulkarni et al.,2019	Percentage of prevalence of each type of Vertucci classification in a particular tooth not mentioned
Mauger et al., 1998	Number of root canals neither studied nor classified
Mazzi-Chaves et al.,2020	Number of root canals neither studied nor classified
Monsarrat et al.,2016	No details of the number of apical foramina mentioned, therefore not possible to deduce Vertucci classification
Nattress et al.,1991	Outcome of the study not clearly specified with respect to any type of classification system
Neelakantan et al.,2010	Percentage of prevalence of each type of Vertucci classification in a particular tooth not mentioned
Neo et al., 1990	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Oliveira et al., 2009	Number of root canals neither studied nor classified
Prado et al., 2016	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Razumova et al.,2018	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Shaikh et al., 2014	Details of the apical foramina not given in the roots with two canals; therefore, not possible to deduce Vertucci classification
Tiku et al., 2005	Root canal anatomy not studied
Tsujimoto, 2009	Review article
Versiani et al., 2013	Number of root canals neither studied nor classified
Vertucci, 2005	Review article
Wang et al., 2018	Number of root canals neither studied nor classified
Xu et al., 2019	Number of root canals neither studied nor classified
Zhu et al., 2020	Teeth with only double canals are studied for the presence of bilateral symmetry, not studied the root canal anatomy

Figure 1

A flow diagram showing the process from identification to the inclusion of studies

Results of Cohen Kappa interrater reliability performed for the studies submitted to the JBI questionnaire were above 0.7 [Supplemental Table 4]. The risk of bias of each question as answered for all the studies is depicted graphically [Figure 2]. Figure 3 shows the percentage of studies with a moderate and low risk of bias. The average JBI score for the 85 included studies was 84.85%.

Supplemental Table 4

Joanna Briggs Institute Critical Appraisal tool for systematic reviews of prevalence studies questions

JBI question	Cohen Kappa interrater reliability between evaluators
Was the sample frame appropriate to address the target population?	0.96
Were study participants sampled in an appropriate way?	0.92
Was the sample size adequate?	0.81
Were the study subjects and the setting described in detail?	0.87
Was the data analysis conducted with sufficient coverage of the identified sample?	0.83
Were valid methods used for the identification of the condition?	1.00
Was the condition measured in a standard, reliable way for all participants?	0.78
Was there appropriate statistical analysis?	0.94
Was the response rate adequate, and if not, was the low response rate managed appropriately?	Not applicable

JBI: Joanna Briggs Institute

Figure 2

Risk of bias of each question as answered for all the studies represented graphically

Figure 3

Percentage of studies showing a moderate and low risk of bias

The present systematic review includes data of at least 71,404 mandibular anterior teeth (27,852 mandibular central incisors, 27,808 mandibular lateral incisors, and 15,744 mandibular canines) acquired from 26 countries, namely Australia, Brazil, China, Egypt, Germany, Greece, India, Indonesia, Iran, Iraq, Italy, Israel, Japan, Jordan, Malaysia, Myanmar, Pakistan, Poland, Portugal, Saudi Arabia, Serbia, Sri Lanka, Syria, Taiwan, Turkey, and the United States of America, studied by various radiographic and nonradiographic methods.

Root and root canal configuration types

Table 2 summarizes the data extracted from the included studies for mandibular central incisors. Table 3 summarizes the studies that have shown modifications in their root canal configuration types other than those given by Vertucci. Tables 4 and 5 summarize all the data extracted from the included studies for mandibular lateral incisors and canines, respectively. Mandibular incisors show a greater variation in the root canal configuration rather than in the number of roots, with a predominance of Vertucci Type I configuration followed by Type III and in some cases Type II. The overall range of these anatomical deviations seen in mandibular incisors at different geographical locations is – Type II, 0.45%–40.1%; Type III, 0.4%–55.9%; Type IV, 0.1%–15.4%; and Type V, 0.05%–17.9%. Mandibular canines show a greater prevalence of two roots (0.2%–12.08%) and only sometimes show the presence of two canals in a single root (Type II, 0.5%–15.5%; Type III, 0.4%–17.5%; Type IV, 0.15%–12.8%; and Type V, 0.24%–6.94%).

Table 2

Root and root canal anatomy of mandibular central incisors

Author, year	Methodology	Demographics				Root and root canal anatomy												Risk of bias
Mandibular central incisor		Country	Number of subjects	Men/women	Age (years)	Number of teeth	Number of roots (%)		Vertucci classification (%)
							1	2	I	II	III	IV	V	VI	VII	VIII	Other‡
Ajinkya MP, 2017	CBCT	India	NA	NA	NA	100	NA	NA	57	12	24	4	Nil	Nil	Nil	Nil	3	Moderate
Al-Fouzan, 2012	Tooth clearing and canal staining	Saudi Arabia	NA	NA	NA	40	100	Nil	70	Nil	30	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Al-Qudah,* 2006	Tooth clearing and canal staining	Jordan	NA	NA	NA	450	100	Nil	73.8	10.9	6.7	5.1	3.6	Nil	Nil	Nil	Nil	Low
Almeida-Gomes,* 2017	CBCT	Brazil	NA	NA	NA	148	NA	NA	72.3	25.67	Nil	2.03	Nil	Nil	Nil	Nil	Nil	Moderate
Altunsoy, 2014	CBCT	Turkey	827	410/417	14-70	1582	NA	NA	84.45	0.45	0.8	4.25	10.05	Nil	Nil	Nil	Nil	Low
Aminsobhani, 2013	CBCT	Iran	400	NA	NA	632	100	Nil	72.7	11.3	4.7	7.7	3.6	Nil	Nil	Nil	Nil	Low
Arslan, 2015	CBCT	Turkey	101	47/54	10-70	184	100	Nil	51.9	4.3	41.6	Nil	0.5	Nil	Nil	Nil	1.6	Low
Assadian,* 2016	Digital radiography	Iran	NA	NA	NA	76	NA	NA	52.6	1.3	31.6	Nil	14.5	Nil	Nil	Nil	Nil	Low
	Sectioning					76	NA	NA	69.7	5.3	25	Nil	Nil	Nil	Nil	Nil	Nil
	CBCT					76	NA	NA	43.4	5.3	50	Nil	1.3	Nil	Nil	Nil	Nil
Aung, 2020	Tooth clearing and canal staining	Myanmar	NA	NA	NA	58	NA	NA	72.41	1.72	17.24	Nil	Nil	Nil	Nil	Nil	8.62	Low
Basha, 2018	CBCT	Egypt	100	50/50	15-60	200	NA	NA	85.5	Nil	11	Nil	3.5	Nil	Nil	Nil	Nil	Low
Baxter, 2020	CBCT	Germany	302	116/186	18-78	604	NA	NA	76.15	22	Nil	0.65	1.15	Nil	Nil	Nil	Nil	Low
Benjamin,* 1974	Radiograph	USA	NA	NA	NA	364	NA	NA	58.6	40.1	Nil	1.3	Nil	Nil	Nil	Nil	Nil	Moderate
Boruah,* 2010	Tooth clearing and canal staining	India	NA	NA	NA	480	NA	NA	63.75	7.08	22.92	Nil	6.25	Nil	Nil	Nil	Nil	Low
Boruah,* 2011	Tooth clearing and canal staining	India	NA	NA	NA	480	NA	NA	63.75	7.08	22.9	Nil	6.25	Nil	Nil	Nil	Nil	Low
Caliskan, 1995	Tooth clearing and canal staining	Turkey	NA	NA	NA	100	NA	NA	68.63	13.73	13.73	Nil	1.96	Nil	Nil	1.96	Nil	Low
Chaturvedi, 2019	Tooth clearing and canal staining	NA	NA	NA	NA	27	NA	NA	66.66	3.7	18.51	11.11	Nil	Nil	Nil	Nil	Nil	Moderate
Dizayee,* 2019	CBCT	Iraq	429	229/200	14-75	1716	NA	NA	79.25	0.47	20.28	Nil	Nil	Nil	Nil	Nil	Nil	Low
Estrela, 2015	CBCT	Brazil	618	224/394	M=43.4	100	100	Nil	65	35	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Low
Ghabbani, 2020	CBCT	Saudi Arabia	406	300/106	20–80	812	NA	NA	49.38	Nil	43.22	0.25	4.93	Nil	2.22	Nil	Nil	Low
Gomes, 1996	Modeling	NA	NA	NA	40-60	58	NA	NA	63.8	22.4	1.7	Nil	5.2	5.2	1.7	Nil	Nil	Moderate
Goran, 2020	CBCT	Iraq	194	72/122	16-40	388	NA	NA	67	1	22.6	Nil	7.7	Nil	1.8	Nil	Nil	Low
Green,* 1973	Sectioning	NA	NA	NA	NA	500	100	Nil	79	17	Nil	4	Nil	Nil	Nil	Nil	Nil	Moderate
Han, 2014	CBCT	China	648	NA	NA	1286	100	Nil	84.29	3.42	6.53	1.17	3.89	Nil	0.31	Nil	0.39	Low
Hassani, 2016	CBCT	NA	81	NA	NA	160	100	Nil	63.1	13.1	21.2	Nil	2.5	Nil	Nil	Nil	Nil	Moderate
Jaju, 2013	CBCT	India	300	NA	NA	130	NA	NA	54.6	6.9	38.45	Nil	Nil	Nil	Nil	Nil	Nil	Low
Kalaitzoglou, 2018	CBCT	Greece	NA	NA	NA	143	NA	NA	71.3	4.2	21	Nil	1.4	Nil	Nil	Nil	2.1	Moderate
Kamtane,* 2016	CBCT	India	NA	NA	NA	102	100	Nil	64.71	23.53	8.82	2.94	Nil	Nil	Nil	Nil	Nil	Moderate
Kartal,* 1992	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	55	16	20	4	3	Nil	Nil	Nil	2	Moderate
Kartika, 2018	Radiograph	Indonesia	55	NA	NA	220	NA	NA	87.72	1.82	7.27	Nil	Nil	Nil	Nil	Nil	3.18	Low
Kelsen, 1999	Tooth clearing and canal staining	NA	66	NA	8-40	54	NA	NA	96.3	Nil	3.8	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Kurumboor, 2018	CBCT	India	NA	NA	NA	100	100	Nil	73	1	13	10	3	Nil	Nil	Nil	Nil	Moderate
Leoni, 2014	Micro-CT	Brazil	NA	NA	NA	50	NA	NA	50	Nil	28	Nil	Nil	Nil	4	Nil	18	Moderate
Lin, 2014	CBCT	China	353	163/190	15-75	706	100	Nil	89.1	2.4	6.2	1.7	0.6	Nil	Nil	Nil	Nil	Low
Liu, 2014	CBCT	China	398	190/208	16-50	768	NA	NA	91.1	2	5.3	1.3	0.3	Nil	Nil	Nil	Nil	Low
Madeira, 1973	Tooth clearing and canal staining	NA	NA	NA	NA	683	NA	NA	88.7	11	Nil	0.3	Nil	Nil	Nil	Nil	Nil	Moderate
Martins, 2017	CBCT	Portugal	646	228/418	M=51	1160	100	Nil	72.3	2.5	24.2	0.1	0.3	Nil	5	Nil	0.1	Low
Martins, 2018	CBCT	China	120	54/66	M=28	240	100	Nil	99.6	Nil	0.4	Nil	Nil	Nil	Nil	Nil	Nil	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1203	100	Nil	72.6	2.4	24	0.1	0.3	Nil	0.5	Nil	0.1	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1203	NA	NA	72.57	2.41	24.02	0.08	0.33	Nil	0.5	Nil	0.08	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=50.8	1203	100	Nil	72.57	2.41	24.02	0.08	0.33	Nil	0.5	Nil	0.08	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17-59	410	100	Nil	73.66	Nil	26.34	Nil	Nil	Nil	Nil	Nil	Nil	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17-59	410	100	Nil	73.7	Nil	26.3	Nil	Nil	Nil	Nil	Nil	Nil	Low
Milanezi de Almeida,* 2013	Micro- CT	Brazil	NA	NA	NA	324	NA	NA	75	0.62	16	Nil	2.47	Nil	0.31	Nil	4.94	Low
Mirhosseini, 2019	CBCT	Iran	180	NA	NA	330	NA	NA	76.1	Nil	15.8	0.6	7.6	Nil	Nil	Nil	Nil	Low
Mirzaie, 2012	CBCT	Iran	66	NA	NA	66	100	Nil	84.8	10.6	1.5	3.1	Nil	Nil	Nil	Nil	Nil	Moderate
Miyashita,* 1997	Tooth clearing and canal staining	NA	NA	NA	NA	1085	100	Nil	87.6	9.3	1.4	1.7	Nil	Nil	Nil	Nil	Nil	Moderate
Naz,* 2015	Radiograph Sectioning	Pakistan	100	23/77	≥10	100	NA	NA	91	Nil	6	Nil	3	Nil	Nil	Nil	Nil	Low
Nogueira,* 2017	Tooth clearing and canal staining	Brazil	NA	NA	NA	100	100	NIL	72	Nil	17	Nil	1	Nil	Nil	Nil	Nil	Moderate
Paes da Silva Ramos Fernandes,* 2014	Micro-CT	NA	NA	NA	NA	40	NA	NA	60	Nil	40	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Pan, 2019	CBCT	Malaysia	208	90/118	15-66	408	100	Nil	94.9	Nil	1	Nil	4.2	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	17-79	54	NA	NA	57.4	1.9	37	Nil	3.7	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	15-65	54	NA	NA	57.4	1.9	37	Nil	3.7	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Japan	NA	NA	20-88	94	NA	NA	86.2	Nil	10.6	Nil	2.1	Nil	1.1	Nil	Nil	Low
Perlea,* 2013	Radiograph	Rome	NA	NA	NA	575	NA	NA	81	17	Nil	1	1	Nil	Nil	Nil	Nil	Low
Pineda, 1972	Radiograph	NA	NA	NA	≤25-55≥	179	NA	NA	72.4	2	23.5	1	Nil	1.1	Nil	Nil	Nil	Moderate
Popovic, 2018	CBCT	Serbia	NA	NA	NA	296	100	Nil	73	4.7	21.6	Nil	0.7	Nil	Nil	Nil	Nil	Low
Rahimi, 2013	Tooth clearing and canal staining	Iran	NA	NA	NA	186	100	Nil	64.52	18.28	16.67	0.54	Nil	Nil	Nil	Nil	Nil	Moderate
Raman, 2017	CBCT	India	50	32/18	NA	100	NA	NA	65	Nil	33	Nil	2	Nil	Nil	Nil	Nil	Moderate
Rankine-Wilson,* 1965	Radiograph	Australia	NA	NA	NA	111	NA	NA	59.5	35.14	Nil	5.41	Nil	Nil	Nil	Nil	Nil	Moderate
Saati, 2018	CBCT	Iran	207	86/121	NA	207	100	Nil	54.5	Nil	34.2	Nil	11.3	Nil	Nil	Nil	Nil	Low
Sert, 2004	Tooth clearing and canal staining	Turkey	200	100/100	NA	200	NA	NA	32.5	27	26	9	0.5	Nil	Nil	1	4	Low
Sert, 2004	Tooth clearing and canal staining	Turkey	NA	NA	NA	200	NA	NA	32.5	27.5	27	10	0.5	Nil	Nil	2	0.5	Moderate
Shemesh, 2017	CBCT	Israel	1016	446/570	13–89	1472	NA	NA	59.5	4	33.7	0.8	0.5	Nil	Nil	1.2	Nil	Low
Silva, 2016	CBCT	NA	432	211/221	NA	200	NA	NA	64.5	Nil	18	Nil	14.5	0.5	2.5	Nil	Nil	Moderate
Singh, 2016	Tooth clearing and canal staining	India	NA	NA	NA	100	100	Nil	84	8	4	4	Nil	Nil	Nil	Nil	Nil	Moderate
Sroczyk-Jaszczyńska, 2019	CBCT	Poland	111	47/64	9-72	212	100	Nil	65.39	0.96	26.44	Nil	5.3	Nil	0.96	Nil	0.96	Low
Sunil, 2019	CBCT	India	40	18/22	18-49	67	100	Nil	55.2	3	28.4	4.5	9	Nil	Nil	Nil	Nil	Low
Uma,* 2004	Radiograph	India	NA	NA	NA	50	NA	NA	44	2	52	Nil	Nil	2	Nil	Nil	Nil	Moderate
Valenti-Obino, 2019	CBCT	Italy	250	130/120	18-79	487	100	Nil	55	34.3	9.3	0.6	Nil	Nil	0.8	Nil	Nil	Low
Verma, 2017	CBCT	India	200	103/97	15-60	400	100	Nil	68.25	11	15.25	1.75	3.75	Nil	Nil	Nil	Nil	Low
Vertucci, 1974	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	70	5	22	3	Nil	Nil	Nil	Nil	Nil	Moderate
Vertucci, 1984	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	70	5	22	3	Nil	Nil	Nil	Nil	Nil	Moderate
Walker, 1988	Radiograph	China	151	NA	NA	100	NA	NA	78	21	Nil	1	Nil	Nil	Nil	Nil	Nil	Moderate
Wu, 2018	CBCT	Taiwan	400	NA	NA	800	NA	NA	84.4	Nil	13.5	Nil	2.1	Nil	Nil	Nil	Nil	Low
Zhengyan, 2016	CBCT	China	1725	923/802	≤20-60≥	3375	100	Nil	96.25	0.15	2.7	0.1	0.75	Nil	Nil	Nil	0.05	Low

*The studies which have not differentiated between mandibular central and lateral incisors, † Root canal anatomy studied in patients with Down syndrome, ‡ Other=Additional modifications of Vertucci classification [Table 3]. Micro-CT: Micro-computed tomography, CBCT: Cone-beam CT; NA: Not available, M: Mean age of the population (as stated in the study), ≤25-55≥: Indicates an age range in a study where the population groups were up to 25 years, between 25 and 55 years, and over 55 years of age

Table 3

Additional modifications of Vertucci classification

Additional modifications of Vertucci classification
Author, year	Mandibular tooth	Other‡ types (%)	Root canal anatomy	Classification
Ajinkya MP, 2017	Central	3	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
	Lateral	2	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Amardeep, 2014	Canine	1.60	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Arslan, 2015	Central	1.60	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
	Lateral	0.50	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Aung, 2020	Central	1.72	1-3-2	NA
		6.90	1-2-1-2-1
	Lateral	2.63	2-3-1	Type XII - Sert and Bayirli
Han, 2014	Central	0.39	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
	Lateral	0.08	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Kalaitzoglou, 2018	Central	2.10	1-2-1-2-1	NA
	Lateral	1.37	1-2-1-2-1	NA
Karataslioglu, 2019	Canine	0.70	NA	NA
Kartal,* 1992	Central and lateral	1	2-3-1	Type VI - Kartal and Yanikoğlu
		1	1-2-1-3	Type VII - Kartal and Yanikoğlu
Kartika, 2018	Central	2.73	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.45	1-2-1-2-1	NA
	Lateral	1.82	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Leoni, 2014	Central	4	1-2-1-2-1	NA
		2	1-2-3-2-1
		2	1-2-3-2-3
		2	1-3-2-1-2-1-2-1
		2	1-2-1-2-3-2-3-2-1
		2	1-2-1-2-3-2-1-2-2-1
		2	1-2-1-2-3-2-1-2-1-2-1
		2	1-2-3-2-3-2-3-2-1-2-1
	Lateral	2	1-2-3-2	Type X - Sert and Bayirli
		2	1-2-1-2-1	NA
		2	1-2-3-2-1
		2	1-2-3-2-3
Martins, 2017	Central	0.10	1-2-1-2-1	NA
	Lateral	0.20	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.10	1-3-1	Type XVII - Sert and Bayirli
	Canine	0.10	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Martins, 2018	Central	0.10	Other 2 root canal types	NA
	Lateral	0.20	Other 2 root canal types	NA
		0.10	Other 3 root canal types
	Canine	0.10	Other 2 root canal types	NA
Martins, 2018	Central	0.08	1-2-1-2-1	NA
	Lateral	0.16	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.08	1-3-1	Type XVII - Sert and Bayirli
	Canine	0.08	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
Martins, 2018	Central	0.08	Other 2 root canal types	NA
	Lateral	0.16	Other 2 root canal types	NA
		0.08	Other 3 root canal types
	Canine	0.08	Other 2 root canal types	NA
Milanezi de Almeida,* 2013	Central and lateral	2.16	1-2-1-2-1	NA
		0.93	1-3-1	Type XVII - Sert and Bayirli
		0.62	1-2-3-1	NA
		0.31	1-3	Type IX - Sert and Bayirli
		0.31	1-2-1-2-1-2-1	NA
		0.31	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.31	1-3-1-3	NA
Sert, 2004	Central	0.50	1-2-3-2	Type X - Sert and Bayirli
		0.50	1-2-3-2-1-3	NA
		0.50	1-2-4-2
		0.50	1-3-1-2
		0.50	1-2-4-3-1
		0.50	1-2-3-1
		0.50	1-2-3-2-1
		0.50	1-2-3-2	Type X - Sert and Bayirli
	Lateral	0.50	1-2-3-2	Type X - Sert and Bayirli
		0.50	2-3-2	NA
		0.50	2-1/2-1
	Canine	0.50	1-3-4-1	NA
Sert, 2004	Central	0.50	1-2-3-2	Type X - Sert and Bayirli
	Lateral	0.50	1-2-3-2	Type X - Sert and Bayirli
Sroczyk-Jaszczyńska, 2019	Central	0.48	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.48	1-3	Type IX - Sert and Bayirli
	Lateral	0.97	2-1-2-1	Type IV - Gulabivala et al./Type XIX - Sert and Bayirli
		0.50	1-2-1-2-1	NA
		0.50	1-3	Type IX - Sert and Bayirli
	Canine	0.49	1-3	Type IX - Sert and Bayirli
Zhengyan, 2016	Central	0.05	NA	NA
	Lateral	0.30	NA	NA
	Canine	0.85	NA	NA

*The studies which have not differentiated between mandibular central and lateral incisors. NA: Not available

Table 4

Root and root canal anatomy of mandibular lateral incisors

Author, year	Methodology	Demographics				Root and root canal anatomy												Risk of bias
Mandibular lateral incisor		Country	Number of subjects	Men/women	Age (years)	Number of teeth	Number of roots		Vertucci classification (%)
							1	2	I	II	III	IV	V	VI	VII	VIII	Other‡
Ajinkya MP, 2017	CBCT	India	NA	NA	NA	100	NA	NA	54	15	27	2	Nil	Nil	Nil	Nil	2	Moderate
Al-Fouzan, 2012	Tooth clearing and canal staining	Saudi Arabia	NA	NA	NA	40	100%	Nil	70	Nil	30	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Al-Qudah,* 2006	Tooth clearing and canal staining	Jordan	NA	NA	NA	450	100%	Nil	73.8	10.9	6.7	5.1	3.6	Nil	Nil	Nil	Nil	Low
Almeida-Gomes,* 2017	CBCT	Brazil	NA	NA	NA	148	NA	NA	72.3	25.67	Nil	2.03	Nil	Nil	Nil	Nil	Nil	Moderate
Altunsoy, 2014	CBCT	Turkey	827	410/417	14-70	1603	NA	NA	80.2	1.3	1	5.4	12.1	Nil	Nil	Nil	Nil	Low
Aminsobhani, 2013	CBCT	Iran	400	NA	NA	614	100%	Nil	70.6	7.1	3.7	15.4	3.2	Nil	Nil	Nil	Nil	Low
Arslan, 2015	CBCT	Turkey	101	47/54	10-70	190	100%	Nil	52.9	2.6	42.3	Nil	1.6	Nil	Nil	Nil	0.5	Low
Assadian,* 2016	Digital radiography	Iran	NA	NA	NA	76	NA	NA	52.6	1.3	31.6	Nil	14.5	Nil	Nil	Nil	Nil	Low
	Sectioning					76	NA	NA	69.7	5.3	25	Nil	Nil	Nil	Nil	Nil	Nil
	CBCT					76	NA	NA	43.4	5.3	50	Nil	1.3	Nil	Nil	Nil	Nil
Aung, 2020	Tooth clearing and Canal staining	Myanmar	NA	NA	NA	38	NA	NA	68.42	Nil	28.95	Nil	Nil	Nil	Nil	Nil	2.63	Low
Basha, 2018	CBCT	Egypt	100	50/50	15-60	200	NA	NA	89.5	Nil	8	Nil	2.5	Nil	Nil	Nil	Nil	Low
Baxter, 2020	CBCT	Germany	302	116/186	18-78	604	NA	NA	76.65	21.35	Nil	1	1	Nil	Nil	Nil	Nil	Low
Benjamin,* 1974	Radiograph	USA	NA	NA	NA	364	NA	NA	58.6	40.1	Nil	1.3	Nil	Nil	Nil	Nil	Nil	Moderate
Boruah,* 2010	Tooth clearing and canal staining	India	NA	NA	NA	480	NA	NA	63.75	7.08	22.92	Nil	6.25	Nil	Nil	Nil	Nil	Low
Boruah,* 2011	Tooth clearing and canal staining	India	NA	NA	NA	480	NA	NA	63.75	7.08	22.9	Nil	6.25	Nil	Nil	Nil	Nil	Low
Caliskan, 1995	Tooth clearing and canal staining	Turkey	NA	NA	NA	100	NA	NA	68.63	13.73	15.69	Nil	1.96	Nil	Nil	Nil	Nil	Low
Chaturvedi, 2019	Tooth clearing and canal staining	NA	NA	NA	NA	27	NA	NA	65.21	8.69	21.73	4.34	Nil	Nil	Nil	Nil	Nil	Moderate
Dizayee,* 2019	CBCT	Iraq	429	229/200	14-75	1716	NA	NA	79.25	0.47	20.28	Nil	Nil	Nil	Nil	Nil	Nil	Low
Estrela, 2015	CBCT	Brazil	618	224/394	M=43.4	100	100%	Nil	58	42	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Low
Ghabbani, 2020	CBCT	Saudi Arabia	406	300/106	20-80	812	NA	NA	51.35	Nil	41.5	0.25	5.79	Nil	1.1	Nil	Nil	Low
Gomes, 1996	Modeling	NA	NA	NA	40-60	53	NA	NA	64.1	20.7	3.8	Nil	5.7	5.7	Nil	Nil	Nil	Moderate
Goran, 2020	CBCT	Iraq	194	72/122	16-40	388	NA	NA	67	0.5	18	Nil	13.9	Nil	0.7	Nil	Nil	Low
Green,* 1973	Sectioning	NA	NA	NA	NA	500	100%	Nil	79	17	Nil	4	Nil	Nil	Nil	Nil	Nil	Moderate
Han, 2014	CBCT	China	648	NA	NA	1294	100%	Nil	72.64	4.02	15.53	2.32	5.1	0.15	0.15	Nil	0.08	Low
Hassani, 2016	CBCT	NA	81	NA	NA	160	100%	Nil	56.9	12.5	28.1	Nil	2.5	Nil	Nil	Nil	Nil	Moderate
Jaju, 2013	CBCT	India	300	NA	NA	130	NA	NA	52.3	5.8	41.95	Nil	Nil	Nil	Nil	Nil	Nil	Low
Kalaitzoglou, 2018	CBCT	Greece	NA	NA	NA	143	NA	NA	69.86	5.48	23.28	Nil	Nil	Nil	Nil	Nil	1.37	Moderate
Kamtane,* 2016	CBCT	India	NA	NA	NA	102	100%	Nil	64.71	23.53	8.82	2.94	Nil	Nil	Nil	Nil	Nil	Moderate
Kartal,* 1992	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	55	16	20	4	3	Nil	Nil	Nil	2	Moderate
Kartika, 2018	Radiograph	Indonesia	55	NA	NA	220	NA	NA	84.55	0.9	12.73	Nil	Nil	Nil	Nil	Nil	1.82	Low
Kelsen,†1999	Tooth clearing and canal staining	NA	66	NA	8–40	44	NA	NA	88.6	Nil	4.5	Nil	6.8	Nil	Nil	Nil	Nil	Moderate
Kurumboor, 2018	CBCT	India	NA	NA	NA	100	100%	Nil	67	3	17	8	5	Nil	Nil	Nil	Nil	Moderate
Leoni, 2014	Micro-CT	Brazil	NA	NA	NA	50	NA	NA	62	Nil	28	Nil	Nil	Nil	2	Nil	8	Moderate
Lin, 2014	CBCT	China	353	163/190	15–75	706	100%	Nil	74.5	3.7	19.3	2.1	0.4	Nil	Nil	Nil	Nil	Low
Liu, 2014	CBCT	China	398	190/208	16–50	785	NA	NA	82.5	3.9	10.4	2.8	0.3	Nil	Nil	Nil	Nil	Low
Madeira, 1973	Tooth clearing and canal staining	NA	NA	NA	NA	650	NA	NA	88.2	11.1	Nil	0.8	Nil	Nil	Nil	Nil	Nil	Moderate
Martins, 2017	CBCT	Portugal	646	228/418	M=51	1191	100%	Nil	69.8	6.3	23.1	Nil	0.3	Nil	0.2	Nil	0.3	Low
Martins, 2018	CBCT	China	120	54/66	M=28	240	100%	Nil	95	2.9	0.8	Nil	1.3	Nil	Nil	Nil	Nil	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1234	100%	Nil	70.1	6.1	23.1	Nil	0.2	Nil	0.2	Nil	0.3	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1234	NA	NA	70.1	6.08	23.1	Nil	0.24	Nil	0.24	Nil	0.24	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=50.8	1234	100%	Nil	70.1	6.08	23.1	Nil	0.24	Nil	0.24	Nil	0.24	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17-59	412	100%	0	69.17	Nil	29.85	Nil	0.97	Nil	Nil	Nil	Nil	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17-59	412	100%	0.01	69.2	Nil	29.8	Nil	1	Nil	Nil	Nil	Nil	Low
Milanezi de Almeida,* 2013	Micro-CT	Brazil	NA	NA	NA	324	NA	NA	75	0.62	16	Nil	2.47	Nil	0.31	Nil	4.94	Low
Mirhosseini, 2019	CBCT	Iran	180	NA	NA	351	NA	NA	65	0.6	15.7	0.9	17.9	Nil	Nil	Nil	Nil	Low
Mirzaie, 2012	CBCT	Iran	66	NA	NA	66	100%	Nil	78.8	12.1	1.5	7.6	Nil	Nil	Nil	Nil	Nil	Moderate
Miyashita,* 1997	Tooth clearing and canal staining	NA	NA	NA	NA	1085	100%	Nil	87.6	9.3	1.4	1.7	Nil	Nil	Nil	Nil	Nil	Moderate
Naz,* 2015	Radiograph Sectioning	Pakistan	100	23/77	≥10	100	NA	NA	91	Nil	6	Nil	3	Nil	Nil	Nil	Nil	Low
Nogueira,* 2017	Tooth clearing and canal staining	Brazil	NA	NA	NA	100	100%	Nil	72	Nil	17	Nil	1	Nil	Nil	Nil	Nil	Moderate
Paes da Silva Ramos Fernandes,* 2014	Micro-CT	NA	NA	NA	NA	40	NA	NA	60	Nil	40	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Pan, 2019	CBCT	Malaysia	208	90/118	15-66	400	100%	Nil	87.8	Nil	3.8	0.3	8.3	Nil	Nil	Nil	Nil	Low
Papic, 2019	CBCT	Serbia	63	32/31	M=28.84	126	NA	NA	54	4.8	41.3	Nil	Nil	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	17-79	59	NA	NA	35.6	8.5	55.9	Nil	Nil	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	15-65	60	NA	NA	36.7	8.3	55	Nil	Nil	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Japan	NA	NA	20-88	100	NA	NA	66	Nil	30	Nil	3	Nil	1	Nil	Nil	Low
Perlea,* 2013	Radiograph	Rome	NA	NA	NA	575	NA	NA	81	17	Nil	1	1	Nil	Nil	Nil	Nil	Low
Pineda, 1972	Radiograph	NA	NA	NA	≤25-55≥	184	NA	NA	76.2	3.2	19.3	1.3	Nil	Nil	Nil	Nil	Nil	Moderate
Popovic, 2018	CBCT	Serbia	NA	NA	NA	294	100%	Nil	73.5	5.4	18.4	0.7	2	Nil	Nil	Nil	Nil	Low
Rahimi, 2013	Tooth clearing and canal staining	Iran	NA	NA	NA	128	1	Nil	61.71	16.41	21.09	0.78	Nil	Nil	Nil	Nil	Nil	Moderate
Raman, 2017	CBCT	India	50	32/18	NA	100	NA	NA	51	Nil	48	Nil	1	Nil	Nil	Nil	Nil	Moderate
Rankine-Wilson,* 1965	Radiograph	Australia	NA	NA	NA	111	NA	NA	59.5	35.14	Nil	5.41	Nil	Nil	Nil	Nil	Nil	Moderate
Saati, 2018	CBCT	Iran	207	86/121	NA	207	1	Nil	56.5	Nil	26.1	Nil	17.4	Nil	Nil	Nil	Nil	Low
Sert, 2004	Tooth clearing and canal staining	Turkey	200	100/100	NA	200	NA	NA	37	26.5	26	9	Nil	Nil	Nil	Nil	1.5	Low
Sert, 2004	Tooth clearing and canal staining	Turkey	NA	NA	NA	200	NA	NA	37	27	26.5	9.5	Nil	Nil	Nil	Nil	0.5	Moderate
Shemesh, 2017	CBCT	Israel	1016	446/570	13-89	1508	Na	NA	62.1	4.3	31.9	0.4	0.05	Nil	Nil	0.8	Nil	Low
Silva, 2016	CBCT	NA	432	211/221	NA	200	Na	NA	60.5	0.5	25.5	Nil	12	Nil	1.5	Nil	Nil	Moderate
Singh, 2016	Tooth clearing and canal staining	India	NA	NA	NA	100	100%	Nil	80	8	4	8	Nil	Nil	Nil	Nil	Nil	Moderate
Sroczyk-Jaszczyńska, 2019	CBCT	Poland	111	47/64	9-72	208	100%	Nil	67.25	0.99	24.7	0.5	3.89	Nil	Nil	Nil	1.97	Low
Sunil, 2019	CBCT	India	40	18/22	18-49	80	1	Nil	57.5	5	22.5	8.8	6.3	Nil	Nil	Nil	Nil	Low
Uma,* 2004	Radiograph	India	NA	NA	NA	50	NA	NA	44	2	52	Nil	Nil	Nil	2	Nil	Nil	Moderate
Valenti-Obino, 2019	CBCT	Italy	250	130/120	18-79	491	100%	Nil	57	35.7	6.9	Nil	Nil	Nil	0.4	Nil	Nil	Low
Verma, 2017	CBCT	India	200	103/97	15-60	400	1	Nil	65	13.25	15.25	3	3.5	Nil	Nil	Nil	Nil	Low
Vertucci, 1974	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	75	5	18	2	Nil	Nil	Nil	Nil	Nil	Moderate
Vertucci, 1984	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	75	5	18	2	Nil	Nil	Nil	Nil	Nil	Moderate
Walker, 1988	Radiograph	China	151	NA	NA	100	NA	NA	68	31	Nil	1	Nil	Nil	Nil	Nil	Nil	Moderate
Wu, 2017	CBCT	Taiwan	300	179/121	M=38.55	600	NA	NA	75	Nil	23	Nil	2	Nil	Nil	Nil	Nil	Low
Zhengyan, 2016	CBCT	China	1725	923/802	≤20-60≥	3257	1	0	89.4	1.05	7.7	0.3	1.15	Nil	Nil	Nil	0.3	Low

*The studies which have not differentiated between mandibular central and lateral incisors, † Root canal anatomy studied in patients with Down syndrome, ‡Other=Additional modifications of Vertucci classification [Table 3]. Micro-CT: Micro computed tomography, CBCT: Cone-beam computed tomography, NA: Not available, M: Mean age of the population (as stated in the study), ≤25-55≥: Indicates an age range in a study where the population groups were up to 25 years, between 25 and 55 years, and over 55 years of age

Table 5

Root and root canal anatomy of mandibular canines

Author, year	Methodology	Demographics				Root and root canal anatomy												Risk of bias
Mandibular canine		Country	Number of subjects	Men/women	Age (years)	Number of teeth	Number of roots (%)		Vertucci classification (%)
							1	2	I	II	III	IV	V	VI	VII	VIII	Other‡
Al-Dahman, 2019	CBCT	Saudi Arabia	707	396/311	16–79	454	99.80	0.20	95.4	2.6	1.8	0.2	Nil	Nil	Nil	Nil	Nil	Low
Altunsoy, 2014	CBCT	Turkey	827	410/417	14–70	1604	NA	NA	92.8	2.1	1.2	1.35	2.65	Nil	Nil	Nil	Nil	Low
Amardeep, 2014	CBCT	India	NA	NA	NA	250	100	Nil	79.6	3.2	13.6	Nil	2	Nil	Nil	Nil	1.6	Low
Aminsobhani, 2013	CBCT	Iran	400	NA	NA	608	96	4.70	71.8	10.3	2.8	12.8	2.3	Nil	Nil	Nil	Nil	Low
Basha, 2018	CBCT	Egypt	100	50/50	15–60	200	NA	NA	100	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Low
Caliskan, 1995	Tooth clearing and canal staining	Turkey	NA	NA	NA	100	NA	NA	80.39	3.92	13.73	Nil	1.96	Nil	Nil	Nil	Nil	Low
Doumani, 2020	CBCT	Syria	418	172/246	16–60	418	97.85	2.15	95.87	0.73	3.18	Nil	0.24	Nil	Nil	Nil	Nil	Low
Estrela, 2015	CBCT	Brazil	618	224/394	M=43.4	100	97	3	78	19	Nil	3	Nil	Nil	Nil	Nil	Nil	Low
Goran, 2020	CBCT	Iraq	194	72/122	16–40	388	NA	NA	90.7	0.5	1.5	Nil	6.7	Nil	0.5	Nil	Nil	Low
Green, 1973	Sectioning	NA	NA	NA	NA	100	100	Nil	87	10	Nil	3	Nil	Nil	Nil	Nil	Nil	M
Han, 2014	CBCT	China	648	NA	NA	1291	98.68	1.32	93.73	0.62	3.25	Nil	0.54	Nil	Nil	Nil	Nil	Low
Karataslioglu, 2019	CBCT	Turkey	220	99/121	15–60	419	96.60	3.40	87.8	Nil	9	Nil	2	Nil	0.5	Nil	0.7	Low
Kelsen, † 1999	Tooth clearing and canal staining	NA	66	NA	8–40	12	NA	NA	100	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Kurumboor, 2018	CBCT	India	NA	NA	NA	100	100	Nil	79	3	6	9	1	2	Nil	Nil	Nil	Moderate
Marceliano-Alves, 2018	Micro-CT	Brazil	NA	NA	NA	80	100	Nil	100	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Martins, 2017	CBCT	Portugal	646	228/418	M=51	1200	97.20	2.80	90.2	3.3	2.7	1.4	2.3	Nil	Nil	Nil	0.1	Low
Martins, 2018	CBCT	China	120	54/66	M=28	240	99.20	0.80	97.1	1.7	0.4	0.4	0.4	Nil	Nil	Nil	Nil	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1244	97	3	90.2	3.3	2.6	1.4	2.5	Nil	Nil	Nil	0.1	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=51	1244	NA	NA	90.19	3.3	2.57	1.37	2.49	Nil	Nil	Nil	0.08	Low
Martins, 2018	CBCT	Portugal	670	243/427	M=50.8	1244	97.03	2.97	90.19	3.3	2.57	1.37	2.49	Nil	Nil	Nil	0.08	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17–59	410	97.32	2.68	90.73	Nil	6.1	Nil	3.17	Nil	Nil	Nil	Nil	Low
Mashyakhy, 2019	CBCT	Saudi Arabia	208	100/108	17–62	410	97.30	2.70	90.7	Nil	6.1	Nil	3.2	Nil	Nil	Nil	Nil	Low
Mirzaie, 2012	CBCT	Iran	66	NA	NA	66	100	Nil	95.4	3.1	Nil	1.5	Nil	Nil	Nil	Nil	Nil	Moderate
Naseri, 2019	CBCT	Iran	NA	NA	NA	33	NA	NA	93.9	Nil	6.1	Nil	Nil	Nil	Nil	Nil	Nil	Low
	Tooth clearing and canal staining					33	NA	NA	90.9	Nil	9.1	Nil	Nil	Nil	Nil	Nil	Nil
Pan, 2019	CBCT	Malaysia	208	90/118	15–66	411	98.78	1.22	95.13	4.87	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Low
Pecora, 1993	Tooth clearing and canal staining	NA	NA	NA	NA	830	98.30	1.70	92.2	4.9	Nil	2.9	Nil	Nil	Nil	Nil	Nil	Moderate
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	17–79	57	NA	NA	70.2	8.8	17.5	Nil	3.5	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Sri Lanka	NA	NA	15–65	58	NA	NA	70.7	8.6	17.2	Nil	3.5	Nil	Nil	Nil	Nil	Low
Peiris, 2008	Tooth clearing and canal staining	Japan	NA	NA	20–88	107	NA	NA	95.3	Nil	4.7	Nil	Nil	Nil	Nil	Nil	Nil	Low
Pineda, 1972	Radiograph	NA	NA	NA	≤25–55≥	187	NA	NA	81.5	13.5	Nil	5	Nil	Nil	Nil	Nil	Nil	Moderate
Popovic, 2018	CBCT	Serbia	NA	NA	NA	312	94.20	5.80	92.9	0.6	0.6	Nil	5.8	Nil	Nil	Nil	Nil	Low
Rahimi, 2013	Tooth clearing and canal staining	Iran	NA	NA	NA	149	87.92	12.08	91.6	6.11	2.29	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Raman, 2017	CBCT	India	50	32/18	NA	98	NA	NA	82.65	Nil	17.35	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Sert, 2004	Tooth clearing and canal staining	Turkey	200	100/100	NA	200	NA	NA	76	15.5	6.5	1.5	Nil	Nil	Nil	Nil	0.5	Low
Sert, 2004	Tooth clearing and canal staining	Turkey	NA	NA	NA	200	NA	NA	76	16	6.5	1.5	Nil	Nil	Nil	Nil	Nil	Moderate
Silva, 2016	CBCT	NA	432	211/221	NA	200	NA	NA	90.5	1	4	2.5	2	Nil	Nil	Nil	Nil	Moderate
Singh, 2016	Tooth clearing and canal staining	India	NA	NA	NA	100	100	NIL	92	8	Nil	Nil	Nil	Nil	Nil	Nil	Nil	Moderate
Soleymani, 2017	CBCT	Iran	150	64/86	M=42.5	300	98.67	1.33	89.7	3.7	5.7	Nil	1	Nil	Nil	Nil	Nil	Low
Sroczyk-Jaszczyńska, 2019	CBCT	Poland	111	47/64	9–72	204	94.08	5.92	85.12	2	3.93	0.5	6.94	0.49	Nil	Nil	0.49	Low
Vaziri, 2008	Sectioning	Iran	NA	NA	NA	100	NA	NA	88	5	7	Nil	Nil	Nil	Nil	Nil	Nil	Low
Vertucci, 1974	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	78	14	2	6	Nil	Nil	Nil	Nil	Nil	Moderate
Vertucci, 1984	Tooth clearing and canal staining	NA	NA	NA	NA	100	NA	NA	78	14	2	6	Nil	Nil	Nil	Nil	Nil	Moderate
Zhengyan, 2016	CBCT	China	1725	923/802	≤20–60≥	3014	99.20	0.80	95.8	0.7	2.1	0.15	0.4	Nil	Nil	Nil	0.85	Low

† Root canal anatomy studied in patients with Down syndrome, ‡ Other=Additional modifications of Vertucci classification [Table 3]. Micro-CT: Micro-computed tomography, CBCT: Cone-beam CT, NA: Not available, M: Mean age of the population (as stated in the study), ≤25-55≥: Indicates an age range in a study where the population groups were up to 25 years, between 25 and 55 years, and over 55 years of age

DISCUSSION

The knowledge of the root and root canal configurations of mandibular incisors and canines is crucial to avoid the possibility of a missed canal during root canal treatment. A missed root canal is one of the possible causes of failure of endodontic therapy. The signs and symptoms associated with this could range from no clinical symptoms to severe acute apical abscess.[27] A study reported the frequency of posttreatment apical periodontitis in the teeth with at least one untreated canal to be 98%.[28] Another study stated that 82.6% of the teeth with missed canals were associated with periapical lesions.[27] Therefore, clinicians should be completely informed and aware of the root anatomy and root canal configurations, with their possible variations, before commencing endodontic treatment to minimize the possibility of missing canals during treatment.

The numerous methods that have been used to study root canal anatomy include radiographic methods as well as nonradiographic methods.[29] Tooth clearing and canal staining method is generally considered the gold standard in studying root canal anatomy.[203031] Micro-CT assessment is a relatively recent method to study root canal anatomy.[12] The differences in the methodology and the various origins of the investigated teeth account for the highly variable results of the present systematic review.

Demographic factors such as sex and age also play a role in governing the variability of root canal anatomy. Females show a significantly higher number of roots in the mandibular canines and simple root canal configurations in the mandibular incisors.[732] A progressive decrease of Vertucci Type I configuration was seen in the mandibular lateral incisors and mandibular canines, when compared to the younger groups. The opposite situation was noted with mandibular central incisors.[8] In the current systematic review, these variations in configurations between different ages and genders have not been mentioned separately but rather calculated as a single value for each type of canal configuration by taking an average of the values given for each age or sex. This has been done to provide a consolidated datasheet to understand the variations in the root and root canal configurations between different populations, as per the aim of this study.

Root canal anatomy was initially studied by Hess and Zurcher in 1925.[33] Since then, it has been studied and classified by various authors for ease of understanding. In this review, the root canal system configurations have been presented according to various classification systems.[6131920] There have also been a few studies where the root canal anatomy was studied but not presented as per any classification system. An attempt was then made to convert the data given into the Vertucci classification system (done only if the concerned study provided sufficient data to enable conversion).[15163435363739]

In the present systematic review, all the mandibular central incisors showed the presence of a single root. The most predominant root canal configuration was Vertucci Type I followed by Type III and in some cases Type II. The population of the Middle Eastern European countries showed the predominance of Vertucci Type I (32.5%–85.5%) and Vertucci type III (13.7%–50%). In previous studies, almost an equal proportion of Vertucci Types I and III canal configurations were seen in the mandibular central incisors of a Turkish and Saudi Arabian population.[3240] Another study showed almost equal proportions of Type I (32.5%), Type II (27.5%), and Type III (27%) canal configurations in the mandibular central incisors of the Turkish population.[6] In this review, the mandibular central of the population of the Western European countries showed Vertucci Type I (55%–81%) followed by Vertucci type III (9.3%–26.4%) and Vertucci Type II (2.5%–34.3%). Similarly, the population in the Indian subcontinent showed Type I (44%–73%) followed by Type III (4%–38.4%) and Type II (1.7%–23.5%). The Brazilian population also showed Type I (50%–75%) followed by Type III (16%–28%) and Type II (0.6%–35%). On the other hand, Far East Asian countries showed Vertucci Type I (84.4%–99.6%) followed by Vertucci Type II (2%–21%) and Vertucci Type III (0.4%–13.5%). One study found a relatively high percentage of Vertucci Type II canal configuration in the mandibular incisors of the population of the United States, whereas Type III was not found.[35] A similar finding was reported in the mandibular incisors of the Australian population.[16] Overall, the percentages of Vertucci Types IV–VIII were relatively low.

All the mandibular lateral incisors showed the presence of a single root, except in two instances. 0.5% of permanent mandibular lateral incisors in a Saudi Arabian population and 0.3% of the lateral incisors in a Chinese population presented with two roots. The root canal configurations of the mandibular lateral incisors showed a considerable amount of variation. The most predominant configuration was Vertucci Type I followed by Type III. The population of the countries in Middle Eastern Europe showed the presence of Vertucci Type I (37%–89.5%) followed by Vertucci Type III (1.5%–50%), Vertucci Type II (0.5%–26.5%), Vertucci Type IV (0.25%–15.4%), and Vertucci Type V (0.05%–17.9%). Similarly, the Indian subcontinent showed the presence of Type I (35.6%–68.42%) followed by Type III (4%–55.9%), Type II (2%–23.53%), Type IV (2.94%–8.8%), and Type V (3%–6.3%). The Western European countries having a lesser amount of variability showed Vertucci Type I (67.25%–81%) followed by Vertucci Type III (23.1%–24.7%) and Vertucci Type II (0.99%–21.35%). Similarly, the South American population had a Type I (58%–75%), followed by Type III (16%–28%) and Type II (0.62%–42%) canal configuration. Far East Asian countries showed the presence of Vertucci Type I (66%–95%), Vertucci Type II (2.9%–31%), and Vertucci Type III (0.8%–30%). The South East Asian population showed the predominance of Vertucci Type I (84.55%–87.8%), with a lesser number of Type III (3.8%–12.73%). The North American and Australian populations showed canal configurations similar to those seen in the mandibular central incisors. A very interesting finding was reported by Peiris et al., in 2008, who found a higher percentage of Vertucci Type III canal configuration (55.9%) than Type I (35.6%) in the mandibular lateral incisors of the Sri Lankan population.[441] However, when a similar study was conducted by the same author in a Japanese population, a higher percentage of Vertucci Type I (66%) was found as compared to Type III (30%).[4] This finding reinforces that root canal morphology is variable among different population groups. Vertucci Types IV–VIII percentages were low but still greater than those seen in the mandibular central incisors.

A small proportion of mandibular canines in almost all the studies showed the presence of two roots. The frequency of two-rooted canines ranged from 0.2% to 12.08%, the greatest being in the Iranian population. Regarding the root canal configurations, a vast majority of mandibular canines showed a Vertucci Type I configuration, whereas the other types were seen to a minimum. Some variations were seen in the root canal configurations of the Middle Eastern European region (Vertucci Type II - 0.5%–16%, Vertucci Type III - 0.6%–13.73%) and the Indian subcontinent (Vertucci Type III - 6%–17.5%, Vertucci Type II - 3.2%–8.8%). However, these were still less than those seen in the mandibular incisors.

The root canal anatomy of the mandibular anterior teeth in patients suffering from Down syndrome has also been studied.[42] The root canal configurations were found to be relatively simple and mostly in Type I canal configuration. This finding can help the conventional endodontic treatment of Down syndrome patients.[42]

The lack of homogeneity across the data reported in the studies concerning the patient age, patient gender, and the methods used to study the root canal anatomy could have led to a bias in the interpretation of the results. The difficulty of standardizing clinicians’ interpretation of Vertucci canal configuration also poses limitations for comparing studies. All these factors must be taken into consideration when interpreting results from this systematic review. The variations in the root and root canal anatomy of the mandibular anterior teeth in genetic disorders other than Down syndrome should be investigated. Further research to study the variations in root canal anatomy of the mandibular incisors and canines due to functional changes occurring over time should also be carried out.

CONCLUSION

There are several prevalence studies on the root canal anatomy of the mandibular anterior teeth studied in different populations with various radiographic and nonradiographic methods. Our systematic review included data from 26 countries including the analysis of 71,404 mandibular anterior teeth. The risk of bias of all the included studies was either low or moderate. Deviations in anatomy were high in the Middle Eastern European countries and the Indian subcontinent. The population of the Far East Asian countries showed the least amount of variations. Taken together, these studies show that the mandibular incisors have a single root, and the most frequent canal configuration is Vertucci Type I followed by Type III and in some cases Type II. The percentage of second canals is higher in lateral incisors than in central incisors. Mandibular canines are mostly single rooted with a Vertucci Type I canal configuration but occasionally show the presence of two roots, though in a relatively low frequency.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.