Root anatomy and canal configuration of human permanent mandibular second molar: A systematic review.

The objective of the study was to review the literature on root anatomy and canal configuration in permanent mandibular second molar based on Vertucci classification. Online electronic databases such as PubMed-Medline, Embase, Scopus, and Cochrane Library were searched using appropriate keywords from the earliest available date till 10th February 2021 restriction on language. Additional sources such as Google Scholar, major journals, unpublished studies, conference proceedings, and cross-references were explored. Information curated for data extraction included methodology, population, sample size (number of teeth), number of root canals, and type of root canal configuration (RCC). Thirty-seven articles were selected with a total of 12,393 permanent mandibular second molar teeth. In the mesial root, Type IV canal configuration with 37.3% was more frequent, having two canals with 70.4%. In the distal roots, one canal was more prevalent with 77% occurrence, and Type I canal configuration was more frequent with 85.2%. Comprehensive knowledge of tooth anatomy and root canal morphology is crucial to root canal treatment. Therefore, an understanding of the canal configuration and anatomy is necessary for the dentist to enhance successful endodontic treatment. Our systematic review findings reveal a wide variety of possible canal configurations among permanent mandibular second molar, while the number of canals ranged from 1 to 5 in number. The evidence is intended to help dental practitioners to avoid mishaps such as perforation and missing canals and anticipation of potential complications during treatment. Copyright:

INTRODUCTION

Endodontic treatment's primary goal is to provide curative measures for pulpal inflammatory responses, causing pain and discomfort.[1] Root canal therapy aims to thoroughly clean and shape all pulp spaces and their complete obturation with a biocompatible filling material.[2] However, the root canal anatomy complexity presents clinical challenges and difficulties that often jeopardize such therapy's primary goal.[34] Root canal treatment (RCT) involves a series of steps for its successful elimination and prevention of microbial contamination.[5]

From the reported reasons for root canal therapy failure in the literature, one explanation for failure could be the existence of an untreated canal. A canal can be left unnoticed because of the dentist's ignorance and lack of experience. It can be due to the complexity of the root canal morphology.[6] Thus, before undertaking endodontic therapy, it is essential to visualize and know the internal anatomy relationships. It becomes a necessity to evaluate the case through periapical radiographs at different angles.[7] Peters et al.[8] reported that the type of canal geometry before cleaning and shaping procedures holds more influence in treatment success than the instrumentation technique alone.

Whether it is an old dated work by Hess and Zurcher[9] or the latest studies reported, all have presented data on the complexity of root canals' anatomical structure. It is well known to the practitioners that a root with a single tapering canal terminating in one foramen, according to the law, remains an exception. Instead, studies have shown numerous additional canals, deltas, loops, intercanal connections, fins, multiple foramina, and C-shaped canals increasing the difficulty level of the treatment and compromising the treatment success.[10]

The pulp canal system is complex, and it can branch, split, and rejoin canals. To date, many classifications have been listed based on the canal morphology. Weine[10] classified the network of root canals into four specific forms in any root. Using the clearing technique, Vertucci[11] in 1984 divided the root canal structures into eight forms. Another study reported on Burmese teeth added seven forms to preceding classifications.[12] Sert and Bayirli[13] introduced 15 different types of root canal systems to the Vertucci list and ended up with 23 types of root canal systems in total.

While understanding the canal morphology, it becomes essential to look into the technique used for assessing this information. Many studies have employed various techniques to determine root canal morphology, including clearing technique,[14] radiography,[14] micro-computed tomography (micro-CT),[15] and cone-beam computed tomography.(CBCT)[16] To date, voluminous studies have been published reporting data on the canal morphology according to Vertucci classification. According to Vertucci classification, a systematic review was conducted considering both, in vitro and in vivo studies on permanent mandibular second molar to have a pooled prevalence on the canals and classification type.

PICO

P – Population: Human permanent mandibular second molar

O – Outcomes: Root canal morphology according to Vertucci classification

S – Study Design: In vitro and in vivo studies.

METHODS

Focused question

What is the prevalence of root anatomy and canal configuration in permanent mandibular second molar based on Vertucci classification?

Search strategy

An exhaustive literature search was conducted to identify root anatomy and canal configuration in permanent mandibular second molar based on Vertucci classification. Online electronic databases such as PubMed-Medline, Embase, Scopus, and Cochrane Library were searched from the earliest available date till February 10, 2021 without restriction on language. Additional sources such as Google Scholar, major journals, unpublished studies, conference proceedings, and cross-references were explored. Contact with authors was done for any unpublished studies. A detailed search strategy is given in Figure 1 and tailored to each database when necessary.

Figure 1

Search strategy

Domains	Keywords
Canal Morphology	“root canal morphology” OR “root canal structure” OR “root canal anatomy” OR “tooth root canal” OR “Root canal system configuration” OR “canal configuration” OR “Root canal number” OR “number of canals” OR “root canal systems” OR “study of root” or “mesial canal” OR “Distal canal”
Teeth	“Permanent Dentition” OR “Secondary Dentition” OR “Mandibular molars” OR “permanent molar” OR “Permanent mandibular molar” OR “mandibular second molar” OR “Permanent mandibular second molar” OR “Permanent tooth” OR “second permanent molar” OR “mandibular second molars”
Vertucci Classification	“Vertucci classification” OR Vertucci OR “Vertucci type I” OR “Vertucci type II” OR “Vertucci type III” OR “Vertucci type IV” OR “Vertucci type V” OR “Vertucci type VI” OR “Vertucci type VII” OR “Vertucci type VIII”

Eligibility criteria

All in vitro and in vivo studies including root anatomy and canal configuration of human permanent mandibular second molar based on Vertucci classification were included

Case reports, letters, and reviews were not included in the search

Studies assessing root anatomy and canal configuration according to classifications other than Vertucci were excluded.

Screening and selection

The papers were independently scanned by two reviewers (VM and PJ), first by the title and abstract. Reviews, commentary, or clinical trials were not included in the search. If the search keywords were present in the title and/or the abstract, the papers were selected for full-text reading. Papers without abstracts but with titles suggesting that they were related to the objectives of this review were also selected to screen the full text for eligibility. After selection, full-text papers were read in detail by two reviewers (VM and PJ). Those papers that fulfilled all of the selection criteria were processed for data extraction. Two reviewers (VM and PJ) hand searched the reference lists of all selected studies for additional relevant articles. Disagreements between the two reviewers were resolved by discussion. If a disagreement persisted, the judgment of a third reviewer (RJ) was considered decisive.

Risk of bias

Two reviewers (VM and PJ) assessed the quality of the included studies. Checklist for Reporting In vitro Studies Guidelines[17] were used to assess risk of bias. Any disagreement was resolved by consensus.

Data extraction

Information curated for data extraction included methodology, population, sample size (number of teeth), number of root canals, and type of root canal configuration (RCC).

The systematic review was registered with the International Prospective Register of Systematic Reviews on January 23, 2021, which was in accordance with the guidelines (Registration Number CRD42021227881).

RESULTS

The reporting is based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses algorithm.

Search selection and results

The PubMed-Medline, Embase, Scopus, Cochrane Library, and additional sources identified 461 search results, of which 345 were duplicates. The remaining 116 unique studies were screened for the titles and abstracts, and fifty articles were selected for full-text screening [Figure 2]. A total of 37 articles,[11121318192021222324252627282930313233343536373839404142434445464748495051] that matched the eligibility criteria, were processed for data extraction.

Figure 2

Flowchart summarizing the article selection process. n: Number of studies

Study characteristics

Thirty seven studies[11121318192021222324252627282930313233343536373839404142434445464748495051] identified a total of 12,393 permanent mandibular second molar teeth among varied populations. Eighteen studies were in vivo,[293031343637383940424344454647484950] while the rest 19 were in vitro/ex vivo studies.[11121318192021222324252627283233354151] Nineteen studies used CBCT technique,[29303133343637383940424344454647484950] two studies used micro-CT,[2332] and other studies used various clearing techniques in their methodology.[11121318192021222425262728354151] Gender comparison was done in eight studies.[1330313439434950] In contrast, one study[44] used the age group as a comparing factor. Two studies used different population subgroups to compare the anatomy of the molars between different ethnicity.[2542] Seven studies[12283646474851] were divided based on the number of roots present in the permanent mandibular second molar and three studies reported the presence of additional root.[303134] In comparison, six studies presented data according to the anatomical variation in root shape.[121927284046] One study[20] presented with single-rooted mandibular molars [Table 1].

Table 1

Overview of included studies

Author/years	Population	Type of study	Methods	Type	Total number of teeth
Vertucci et al., 1984[11]	Not reported	In vitro	5% hydrochloric acid+5% potassium hydroxide+haematoxylin	-	100
Caliskan et al., 1995[18]	Turkish population	In vitro	5.25% sodium hypochloride+Indian ink+5% nitric acid	-	100
Gulabivala et al., (i) 2001[12]	Burmese population	In vitro	2.5% sodium hypochlorite+10% nitric acid+ethanol+methyl salicylate	2 rooted	78
				Fused	20
				1 rooted C-shaped	30
				1 rooted (conical)	6
Gulabivala et al., (ii) 2002[19]	Thai population	In vitro	3% sodium hypochlorite, Indian ink, 10% nitric acid	2 rooted	60
				1 rooted
Sert and Bayirli 2004[13]	Turkish population	In vitro	5.25% sodium hypochlorite+5% nitric acid+ethyl alcohol+India ink	Male	100
				Female	100
Cimilli et al., 2005[20]	Caucasian Turkish population	In vitro	5% solution of sodium hypochlorite+ultrasonic cleaner+spiral CT scanner	1 rooted	112
Peiris et al., (i) 2007[21]	Sri Lankan population	In vitro	China ink+5% nitric acid+ethanol+benzoic acid+benzene and methyl salicylate	-	100
Ahmed et al., 2007[22]	Sudanese population	In vitro	5% sodium hypochlorite+10% nitric acid+ethanol+methyl salicylate+Indian ink	-	100
Cheung et al., 2007[23]	Chinese population	In vitro	Y micro computerized tomography and stereomicroscope	-	44
Peiris et al., (ii) 2008[24]	Sri Lankan population	In vitro	5% sodium hypochlorite+10% nitric acid+ethanol+China ink	-	165
Peiris (iii) 2008[25]	Sri Lankan and Japanese population	In vitro	Boiled in 5% NaOH+cleaned with 10% NaOCl in a supersonic cleaner+examined visually beneath a quartz-halogen light with a hand lens aid	Sri Lankans	275
				Japanese	19
Rwenyonyi et al., 2009[26]	Ugandan population	In vitro	5% sodium hypochlorite+10% nitric acid+99% methyl salicylate+India ink	-	223
Al-Qudah and Awawdeh 2009[27]	Jordanian population	In vitro	3.25% sodium hypochlorite+10% nitric acid+ethyl alcohol+methyl salicylate	2 rooted	310
				C-shaped	37
				Conical	8
Neelakantan et al., 2010[28]	Indian population	In vitro	2.5% sodium hypochlorite+Indian ink+10% nitric acid+dehydrated with ethanol+methyl salicylate (star chem) + digital images	2 rooted	288
				3 rooted	31
				C-shaped root	26
Zhang et al., 2011[29]	Chinese population	In vivo	CBCT	2 rooted	119
Demirbuga et al., 2013[30]	Turkish population	In vivo	CBCT	Female	516
				Male	409
				Extra root	40
Nur et al., 2014[31]	Turkish population	In vivo	CBCT	Female	1165
				Male
				Extra root	6
Barsness et al., 2015[32]	Not reported	In vitro	Micro-CT	-	18
Ceperuelo et al., 2014[33]	Spanish population	In vitro	CBCT	-	16
Celikten et al., 2016[34]	Turkish cypriot population	In vivo	CBCT	Extra root	421
				Male
				Female
Akhlaghi et al., 2016[35]	Iranian population	Ex-vivo	5.25% sodium hypochlorite+5% nitric acid+100% ethanol+Indian ink	-	130
Kim et al., 2016[36]	Korean subpopulation	In vivo	CBCT	1 rooted	1148
				2 rooted
				3 rooted
Madani et al., 2016[37]	Northern Iranian population	In vivo	CBCT	2 rooted	147
Martins et al., (i) 2017[38]	Caucasian population	In vivo	CBCT	-	567
Pawar et al., 2017[39]	Indian population	In vivo	CBCT	Male	489
				Female	494
Pérez-Heredia et al., 2017[40]	Spanish population	In vivo	CBCT	2 rooted	121
				Fused
Madjapa and Minja 2018[41]	Tanzanian population	In vitro	Pacegrove UK+methylene blue+5% Nitirc acid	-	85
Martin et al., (ii) 2018[42]	Asian and white ethnic groups	In vivo	CBCT	Asian	927
				Caucasian
Martin et al., (iii) 2018[43]	Portuguese subpopulation	In vivo	CBCT	Male	226
				Female	421
Martin et al., (iv) 2018[44]	Caucasian population	In vivo	CBCT	≤20 years	687
				21-40 years
				41-60 years
				≥61 years
Tassoker and Sener 2018[45]	Not reported	In vivo	CBCT	-	444
Pan et al., 2019[46]	Malaysian population	In vivo	CBCT	2 rooted	192
				3 rooted	1
				C-shaped roots
Donyavi et al., 2019[47]	Iran population	In vivo	CBCT	2 rooted	406
				Additional root	9
				1 rooted	41
Kantilieraki et al., 2019[48]	Greek population	In vivo	CBCT	2 rooted	434
				1 rooted	11
				3 rooted	26
Popovic et al., 2020[49]	Serbian population	In vivo	CBCT	Male	162
				Female
Rasidi et al., 2020[50]	Chennai population	In vivo	CBCT	Male	50
				Female	33
Singh et al., 2020[51]	Indian population	In vitro	2% India ink dye+10% nitric acid+methanol+methyl salicylate+special halogen lighting	2 rooted	91
				3 rooted	4
				C-shaped root	5
Total sample size					12,393

CBCT: Cone-beam computed tomography systems, CT: Computed tomography, NaOCl: Sodium hypochlorite, UK: United Kingdom

Vertucci classification

All the 37 studies presented data on canal configuration of mandibular second molar based on Vertucci classification.[11121318192021222324252627282930313233343536373839404142434445464748495051] Out of which, 34 studies[11121318192122242526272829303132333435363738394041424344454647485051] reported frequency of occurrence of RCC separately for mesial root and distal roots. The concurred data for most of these studies included both the percentage of occurrence as well as the number of cases for such canal types seen in the sample. For few studies, only the percentage of occurrence was given, and the exact value of the number of teeth for each specific canal type was calculated from the given percentage and sample size (number of teeth) taken for the study. Our results demonstrate Vertucci Type IV as the most common RCC for mesial root with 37.3% occurrence, followed by Type II with 34.6%. For the distal root, Type I was most common, with 85.2%. All other types of RCC in distal root were infrequent [Table 2].

Table 2

Root canal configuration based on vertucci classification in mesial and distal root separately

Author’s name	Sample size	Type	Canal morphology - vertucci types (%)
			Type I (1-1)		Type II (2-1)		Type III (1-2-1)		Type IV (2-2)		Type V (1-2)		Type VI (2-1-2)		Type VII (1-2-1-2)		Type VIII (3-3)
			Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal
Vertucci 1984[11]	100	-	27	92	38	3	0	0	26	4	9	1	0	0	0	0	0	0
Caliskan et al., 1995[18]	100	-	10	70	19	14	0	12	53	0	0	4	4	0	0	0	2	0
Gulabivala et al., (i) 2001[12]	78	2 rooted	24	70	28	4	3	1	21	1	1	1	0	0	0	1	0	0
Gulabivala et al., (ii) 2002[19]	60	2 rooted	8	38	11	5	0	2	31	6	1	1	2	0	0	0	0	0
Sert and Bayirli 2004[13]	100	Male	14	78	36	4	20	14	22	2	3	1	2	0	0	1	0	0
	100	Female	11	74	27	7	23	12	33	2	1	4	0	0	0	1	0	0
Peiris et al., (i) 2007[21]	100	-	20	93	11	0	42	2	5	1	15	3	2	0	3	0	0	0
Ahmed et al., 2007[22]	100	-	5	65	18	11	0	0	63	10	0	1	3	2	1	1	0	0
Peiris et al., (ii)/2008[24]	165	-	51	159	24	1	42	2	25	0	20	3	1	0	0	0	0	0
Peiris R (iii) 2008[25]	275	Sri Lankans	86	265	38	1	72	3	43	0	32	6	1	0	0	0	0	0
	19	Japanese	8	19	4	0	4	0	2	0	0	0	0	0	0	0	0	0
Rwenyonyi et al., 2009[26]	223	-	73	210	26	0	7	3	111	6	4	2	2	1	0	0	0	0
Al-Qudah and Awawdeh 2009[27]	310	2 rooted	50	245	101	24	11	8	125	14	11	17	3	0	0	0	1	0
Neelakantan et al., 2010[28]	288	2 rooted	29	224	7	16	5	2	218	38	18	6	0	0	0	0	0	2
	31	3 rooted	58	26	1	1	0	0	3	2	0	2	0	0	0	0	0	0
Zhang et al., 2011[29]	119	2 rooted	50	115	0	0	0	0	29	0	16	0	0	0	0	0	0	0
Demirbuga et al., 2013[30]	516	Female	101	490	188	3	22	3	180	3	11	6	3	1	1	0	0	0
	409	Male	79	392	145	2	10	3	163	6	6	3	2	1	2	0	0	0
	40	Extra root	40	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Nur et al., 2014[31]	1165	Female	86	562	10	19	1	0	440	10	45	2	0	0	0	0	0	0
		Male	59	537	15	20	7	0	487	17	15	1	0	0	0	0	0	0
	6	Extra root	2	0	0	0	0	0	0	1	0	0	0	0	0	0	0	0
Barsness et al., 2014[32]	18	-	0	11	0	0	0	0	0	0	0	0	0	0	6	0	0	0
Ceperuelo et al., 2014[33]	16	-	2	13	9	1	3	0	0	1	1	0	0	0	0	0	0	0
Celikten et al.,2016[34]	421	Extra root	4	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
		Male	9	174	69	6	0	0	103	0	0	0	0	1	0	0	0	0
		Female	20	217	62	4	1	0	142	4	0	0	0	0	0	0	0	0
Akhlaghi et al., 2016[35]	130	-	112	81	87	0	0	49	43	0	0	0	0	0	0	0	0	0
Kim et al., 2016[36]	1102	2 rooted	153	1064	416	23	13	0	490	10	29	5	0	0	0	0	0	0
Madani et al., 2017[37]	147	2 rooted	22	111	34	4	7	1	52	2	4	2	1	0	0	0	0	0
Martins et al., (i) 2017[38]	567	-	45	530	362	3	29	24	103	2	3	8	9	0	0	0	0	0
Pawar et al., 2017[39]	983	-	71	601	320	179	9	0	444	74	10	0	0	0	0	0	0	0
FL Pérez et al., 2017[40]	121	2 rooted	3	93	79	2	0	3	15	3	1	0	0	0	0	0	0	0
Madjapa and Minja 2018[41]	85	-	0	39	44	27	1	0	40	19	0	0	0	0	0	0	0	0
Martins et al., (ii) 2018[42]	927	Asian	39	129	39	1	15	0	29	0	9	1	0	0	0	0	0	0
		Caucasian	47	547	370	5	30	23	114	3	3	11	10	0	0	0	0	0
Martins et al., (iii) 2018[43]	226	Male	19	232	142	0	13	7	60	1	1	6	4	0	0	0	0	0
	421	Female	28	315	228	5	17	16	54	2	2	5	6	0	0	0	0	0
Martins et al., (iv) 2018[44]	687	≤20 years	1	9	5	0	0	0	3	0	0	0	0	0	0	0	0	0
		21-40 years	16	195	126	2	8	5	49	1	1	5	4	0	0	0	0	0
		41-60 years	21	248	173	2	14	11	46	2	1	5	6	0	0	0	0	0
		≥61 years	9	95	66	1	8	7	16	0	1	1	0	0	0	0	0	0
Tassoker et al., 2018[45]	444	-		412	122	11		16	179	0	0	0	0	0	0	0	0	0
Pan et al., 2019[46]	192	2 rooted	34	186	8	0	0	0	150	6	0	0	0	0	0	0	0	0
Donyavi et al., 2019[47]	406	2 rooted	65	396	222	5	0	0	116	5	3	0	0	0	0	0	0	0
	9	Additional root	9	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Kantilieraki et al., 2019[48]	434	2 rooted	77	424	278	6	12		61	2	6	2	0	0	0	0	0	0
Rasidi and Ramakrishnan 2020[50]	50	Male	21	25	8	5	5	5	10	10	4	4	2	1	0	0	0	0
	33	Female	13	22	5	1	3	2	7	6	2	2	3	0	0	0	0	0
Singh et al., 2020[51]	91	2 rooted	16	80	67	8	0	1	8	2	0	0	0	0	0	0	0	0
	4	3 rooted	4	4	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Total	11,818		1751	10,077	4088	436	457	237	4414	278	289	121	70	7	13	4	3	2
Percentages			14.8	85.2	34.6	3.7	3.9	2	37.3	2.35	2.45	1.03	0.6	0.06	0.1	0.03	0.03	0.02

Studies[121920232728364046474849] reported findings where the percentage of various types of RCC for mandibular molar are given in totality and not segregated for each root. One rooted molar with Type I canal configuration is seen most commonly in all of them [Supplementary Table 1].

Supplementary Table 1

Root canal configuration based on vertucci classification

Author/years	Type	Canal morphology - vertucci types (%)
		Type I	Type II	Type III	Type IV	Type V	Type VI	Type VII	Type VIII
Gulabivala et al., (i) 2001[12]	Fused	5 (1)	20 (4)		50 (10)			15 (3)
	1 rooted (C shaped root)	30 (9)			20 (6)	3.3 (1)	3.3 (1)		13.4 (4)
	1 rooted (conical)	50 (3)			50 (3)
Gulabivala et al., (ii) 2002[19]	1 rooted (C-shaped root)	33.3 (2)			33.3 (2)
Cimilli et al., 2005[20]	1 rooted	40 (16)	0	22.5 (9)	0	20 (8)	0	0	0
Cheung et al., 2007[23]	-	9 (4)	7 (3)	0	29 (13)	5 (2)	2 (1)	0	28 (12)
Al-Qudah and Awawdeh 2009[27]	C shaped	21.6 (8)			48.6 (18)				5.4 (2)
	Conical	100 (8)
Neelakantan et al., 2010[28]	C shaped root	2.8 (10)	0	0.57 (2)	2.02 (7)	0.28 (1)	0	0.86 (3)	0
Kim et al., 2016[36]	1 rooted	90.3 (29)	0	6.25 (2)	0	3.13 (1)	0	0	0
	3 rooted	100 (14)	0	0	0	0	0	0	0
Pérez-Heredia et al., 2017[40]	Fused	50 (10)	40 (8)	0	0	0	0	0	0
Pan et al., 2019[46]	3 rooted	100 (2)			100 (1)
	C shaped roots	48.7 (183)
Donyavi et al., 2019[47]	1 rooted	24.4 (10)	34.4 (14)	7.3 (3)	17.1 (7)	4.9 (2)	0	0	12.2 (5)
Kantilieraki et al., 2019[48]	1 rooted	27.3 (3)	63.6 (7)
	3 rooted	100 (10)	55 (11)		25 (5)	1 (5)
Popovic et al., 2020[49]	Male	58.3 (14)	63.3 (42)	40 (4)	58.1 (36)	0	0	0	0
	Female	41.7 (10)	36.4 (24)	60 (6)	41.9 (26)	0	0	0	0
	Total	14.8 (24)	40.7 (66)	6.2 (10)	38.3 (62)	0	0	0	0

Data of studies which were not segregated into mesial and distal roots. The sample is written in brackets and the percentages outside the bracket

Root canals

Twenty-one studies[192122252627282930313233343639414243474951] presented with the findings on the Supplementary Table 2 shows number of canals in permanent mandibular second molar. Nine studies[222628314142434749] with a total of 4480 teeth presented data on number of roots separated for mesial and distal roots. In the mesial root, two canals were most commonly seen with 70.4%, followed by one canal with 11.5%. In the distal root, 1 canal was most prevalent with 77% [Table 3].

Supplementary Table 2

Number of canals in permanent mandibular second molar

Author/years	Type	Percentage of canals
		1	2	3	4	5
Gulabivala et al., (ii) 2002[19]	-	0	23.3% (14)	58.3% (35)	16.7% (10)	1.7% (1)
Peiris et al., (i) 2007[21]	-	0	20% (20)	73% (73)	7% (7)	0
Peiris (iii) 2008[25]	Sri Lankan	2.8% (9)	32.3% (101)	61.7% (192)	3.2% (10)	0
	Japanese	10.3% (3)	41.4% (13)	44.9% (14.3)	3.4% (1)	0
Al-Quad and Awawdeh 2009[27]	-	4.5 (16)	18.9% (67)	58.3% (207)	16.6% (59)	1.7% (6)
Neelakantan et al., 2010[28]	C root	2.89% (10)	3.76% (13)	0.28% (1)	0.57% (2)	0
Zhang et al., 2011[29]	-	0	38% (60)	46% (72)	0	0
Demirbuga et al., 2013[30]	-	2.05 (19)	22.8% (211)	72.8% (674)	2.27% (21)	0
	Extra root	66.66 (2)	33.33% (1)
Barsness et al.,2015[32]	2 rooted	0	11.1 (2)	44.4% (8)	33.3% (6)	11.1% (2)
Ceperuelo et al., 2014[33]	-	0	81.25% (13)	18.75% (3)	0.00%	0
Celikten et al.,2016[34]	Males	2.10%	4.30%	89.70%	3.70%	0
	Females	4.60%	8.05%	83.80%	3.30%	0
	Total	3.5% (15)	6.3% (27)	86.4% (364)	3.5% (15)	0.00%
Kim et al., 2016[36]	1 rooted	41.15% (790)	0	0	0	0
		(Conical shape 1.67%, C shape 39.38%, O shape 0.10%)
	2 rooted	0	8.02% (154)	47.34% (909)	2.03% (39)	0
	3 rooted	0	0	0.72% (14)	0	0
				0.26% had MB, ML, distal0.36% had mesial, DB, DL0.10% had mesial, distal, ML
Pawar et al., 2017[39]Singh et al., 2020[51]	-		16.09% (158)	53.5% (527)	17.29% (169)	0
	1 rooted	24.1% (10)	63.4% (26)	12.2% (5)	0	0
	Additional root	100% (9)	0%	0	0	0
	-	5% (5)	91% (91)	4% (4)	0	0

MB: Mesiobuccal, ML: Mesiolingual, DB: Distobuccal, DL: Distolingual

Table 3

Number of canals in mesial and distal roots

Author/years	Sample size	Types	Percentage of canals
			1		2		3		4		5
			Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal	Mesial	Distal
Ahmed et al., 2007[22]	100	-	7	69	83	21	10	10	0	0	0	0
Rwenyonyi et al., 2009[26]	223	-	87	215	139	8	0	0	0	0	0	0
Neelakantan et al., 2010[28]	288	2 rooted	29	224	248	62	110	2	0	0	0	0
	31	3 rooted	58	26	4	5	0	0	0	0	0	0
Nur et al., 2014[31]	1165	Female	70	553	512	13	0	0	0	0	0	0
		Male	44	553	539	22	0	0	0	0	0	0
Madjapa and Minja et al., 2018[41]	85	-	0	39	0	46	0	0	0	0	0	0
Martins et al., (ii) 2018[42]	927	Asian	39	129	92	2	0	0	0	0	0	0
		Caucasian	47	547	529	42	13	0	0	0	0	0
Martins et al., (iii) 2018[43]	266	Males	19	232	221	14	6	0	0	0	0	0
	421	Females	28	315	308	28	7	0	0	0	0
Donyavi et al., 2019[47]	812	2 rooted	65	396	341	10	0	0	0	0	0	0
Popovic et al., 2020[49]	162	-	24	160	138	2	0	0	0	0	0	0
Total	4480		517	3458	3154	275	146	12	0	0	0	0
Percentage			11.5	77	70.40	6	3.2	0.27	0	0	0	0

Additional types

Fifteen studies had canal configuration other than the eight types of Vertucci classification.[121319202123242527283642434448] The findings in these studies were random, and no distinct pattern was noted. The frequency of occurrence of these RCC outside of Vertucci classification was low. These variations were seen more in certain groups of the population. It was noted that the Chinese population had a maximum percentage of occurrence of RCC, which could not be classified by Vertucci classification [Supplementary Table 3].

Supplementary Table 3

Different population

Population	Total	Sample size (%)
Burmese	12	134 (9)
Thai	5	60 (8.30)
Turkish	15	100 (15)
Sri Lankan	8	540 (1.50)
Chinese	9	44 (20.45)
Japanese	1	19 (5.30)
Jordan	9	310 (3)
Indian	14	345 (4)
Korean	1	1920 (0.05)
Caucasian	29	1374 (2.10)
Portuguese	14	647 (2.16)
Greek	1	434 (0.23)

DISCUSSION

For successful RCT, prior knowledge of anatomy is essential. The morphology of root anatomy can vary depending on the age, population, ethnicity, gender, number of teeth, and methodology. The selected articles' reporting presented diverse population groups with different characteristics and sample sizes. The study designs showed a near equal trend for both in vivo as well as in vitro studies. While comparing the methodologies, it was noted that clearing technique and staining were used as a methodology by in vitro studies. In contrast, contemporary studies were mostly in vivo and used three-dimensional (3D) imaging techniques commonly. No distinct difference was noted in the findings due to variation in methodology.

The most common feature reported by available literature on mandibular molar was its canal configuration. In 34 studies,[11121318192122242526272829303132333435363738394041424344454647485051] the data for each root was presented in the form of percentages according to the sample size (number of teeth) of the study. Thus, a study with a smaller sample size (number of teeth) showed a higher percentage for a particular type even though the actual number of the said type was low. Vice-versa occurred in studies with a large sample size (number of teeth). Hence, it would have been difficult to draw a definite conclusion among the myriad of data extracted from the studies. Thus, to interpret the data properly, each type's total percentage was calculated for mesial and distal roots separately. The calculated percentage showed Type IV as the most frequent RCC, closely followed by Type II for mesial roots. The distal root presented a higher prevalence of Type I. Gaêta-Araujo et al. and Plotino et al. in their study reported a similar finding of higher prevalence of type IV followed by type II for mesial and type I for distal roots.[5253] However, the study by Zare Jahromi et al. revealed type III as the most prevalent RCC for mesial root followed by type II; however, the prevalent RCC for distal root was type I, same as in our study.[54]

The similar theme of prevalence in the findings of the studies gives us an overall insight into the canal configuration of mandibular second molars. However, our review also reports other canal configuration types even though they are infrequent. Thus, to get a comprehensive idea about mandibular second molars' canal configuration, these findings should also be considered. The occurrence of type VIII canal configuration was rare and noted in only a few studies on the Chinese, Burmese, Turkish, Jordan, Indian, and Iranian populations.

RCC undefined by Vertucci classification was also found in several studies. The different RCCs were more in occurrence for single-rooted and third rooted molars. Even though such RCCs were in small percentages and were very low compared to the other canal configuration, it explains how varied the canal configuration of permanent mandibular second molar can be and warrants further investigation [Supplementary Table 3].

Supplementary Table 3 displays the occurrence of these RCCs according to the population. The studies on Chinese, Turkish, and Thai people reported maximum cases indicating a wide variation in RCC. The Chinese population showed the highest percentage in RCC variation, with more than 20% of the population lying outside the Vertucci classification. An overview of the global map [Figure 3] demonstrates the correlation between varied RCC presence outside of Vertucci classification with the geographical location of specific populations. This provides us with an insight into variation in RCC based on demographics. However, it should be acknowledged that these values can also be influenced by the gender, age, and sample size (number of teeth) collected.

Figure 3

Geographic incidence of root canal configuration undefined by Vertucci classification in human permanent mandibular second molar

Among anatomical variations in shape, canals with C, O, and conical shapes were being reported.[36] The occurrence of C-shaped canals was most frequent. This finding commensurate with the results of a study by Roy et al. that C-shaped canals are one of the most common reported variations in mandibular second molar.[55] Shemesh et al. reported that C-shaped canals were reported rarely in teeth other than the second mandibular molar, and the prevalence of C-shaped canals in their study was 4.6%.[56] According to the study conducted by Wadhwani et al., prevalence of C-shaped canals in mandibular second molars was found to be 9.7%; slightly greater than the mandibular third molars.[57] Roy et al. in their research noted a great variability in C-shaped canal configuration according to ethnicity and gender.[58]

The presence of a number of canals was the most common finding reported in the studies. A strategy similar to RCC was followed to find the number of canals most frequently occurring in the mandibular second molar. For mesial root, two canals were found to be most frequent and one canal for distal root. Thus, in a two rooted permanent mandibular second molar, three canals are noted, 2 in mesial and 1 in distal. Manning reported similar results in their study stating that, in two rooted mandibular molars, more than 90% of distal roots have one canal and more than 70% of mesial roots have two canals.[59]

Most studies reported two rooted mandibular second molars. Only a few reported a third root or an additional root. Zare Jahromi et al. and Manning report a similar finding with mandibular molars having two roots frequently.[5459] Three rooted molars showed mostly Vertucci type I configuration. Single-rooted mandibular molars were noted to have only one canal but with anatomic variations in the root shape and RCC of which C-shaped roots and type 1 canal configuration were most frequently occurring.

Lack of knowledge of internal structure can lead to failure in endodontic treatment. Thus, every tooth should be judiciously analyzed based on ethnicity, race, and population, keeping in mind the peculiarities in the population around us. Gaêta-Araujo et al. in their study on RCT errors revealed that technical errors were most likely, for type IV RCC. This was due to two-dimensional imaging adopted for RCT that can cause overlap of the two canals causing clinicians to confuse it with type I. Moreover, as RCC's complexity increased chances of RCT errors also increased.[52] Mandibular second molar presents with the high frequency of C-shaped canals, and a wide range of unclassified complex RCC variation. Furthermore, its mesial root most frequently occurs in type IV canal configuration. Thus, every tooth should be judiciously analyzed and approached with prior knowledge carefully.

No exclusion criteria were kept for the type of method used in the study, and all study's findings of both in vivo and in vitro/ex vivo methodology have been discussed in this review. Thus, a discrepancy in reporting of the anatomy could be present between clearing techniques and 3D techniques. Furthermore, the included were not about a specific population, ethnicity, or geographic distribution, but across the globe, so the changes in the findings may be present due to certain genetic predispositions. The sample size (number of teeth) of the studies was also varied. Hence, the percentages of studies with a small sample size (number of teeth) were higher even though we tried to get a conclusive finding by calculating the percentages. Some studies did not segregate the data into mesial and distal roots.

CONCLUSION

Comprehensive knowledge of tooth anatomy and root canal morphology is crucial in the success of RCT. Therefore, an understanding of the canal configuration and anatomy is necessary for the dentist to enhance the chance of successful endodontic treatment. Our systematic review findings reveal a wide variety of possible canal configurations among permanent mandibular second molar, while the number of canals ranged from 1 to 5 in number. No noted difference was seen in the comparative studies. Among the various population, type IV was commonly seen in mesial roots, and type I in distal roots. Most mandibular molars were two rooted and had three canals present. Mesial roots had two canals, while distal roots contained only 1 canal. Rarely, canal configuration other than Vertucci type was noted, and the majority of different types were seen among Chinese population groups. The evidence is intended to help dental practitioners to avoid mishaps such as perforation and missing canals and anticipation of potential complications during treatment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.