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 Table of Contents  
REVIEW ARTICLE
Year : 2021  |  Volume : 11  |  Issue : 2  |  Page : 142-153

Prevalence and morphology of root canal isthmus in human permanent teeth using micro-computed tomography: A systematic review


1 Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research (Maher), Chennai, Tamil Nadu, India
2 Department of Periodontology and Oral Implantology, Faculty of Dentistry, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research (Maher), Chennai, Tamil Nadu, India
3 Department of Conservative Dentistry and Endodontics, Faculty of Dental Sciences, Sri Ramachandra Institute of Higher Education and Research, Chennai, Tamil Nadu, India
4 Department of Conservative Dentistry and Endodontics, Sri Venkateswara Dental College and Hospital, Chennai, Tamil Nadu, India

Date of Submission19-Nov-2020
Date of Decision17-Dec-2020
Date of Acceptance21-Jan-2021
Date of Web Publication8-May-2021

Correspondence Address:
Dr. Velmurugan Natanasabapathy
Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Meenakshi Academy of Higher Education and Research (Maher) University, No. 1, Alapakkam Main Road, Maduravoyal, Chennai - 600 095, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_290_20

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  Abstract 

Introduction: The isthmus is a communication between two or more root canals that contains pulp tissue. Its unpredictable location, variable prevalence, and anatomic complexity often result in challenging debridement. The knowledge of these factors may prove beneficial for successful endodontic treatment. The primary aim of this systematic review was to assess the prevalence and morphology of root canal isthmus (RCI) in human permanent teeth using micro-computed tomography (micro-CT) studies. The secondary aim was to assess the influence of demographic factors such as age, gender, and geographic region on the prevalence and morphology of the isthmus.
Materials and Methods: An electronic search was conducted until January 2020. Studies published in the English language using micro-CT for the evaluation of isthmus prevalence and morphology were included. Sixteen articles met the inclusion criteria and were subjected to qualitative synthesis.
Results: The prevalence of isthmus in the mesiobuccal root of maxillary first molars ranged from 8.9% to 70.8%, with maximum prevalence in the middle third of the root (49.50%–73%). The mesial root of mandibular molars had a prevalence ranging from 80.6% to 100% on full root analysis and 85%–88.89% in the apical third.
Conclusion: The prevalence of RCI was higher in the mesiobuccal roots of maxillary molars and the mesial roots of mandibular molars, extending in the corono-apical direction with a complex and variable morphology. The influence of demographic factors on the prevalence of RCI could not be established clearly.

Keywords: Isthmus, micro-computed tomography, root canal anatomy, systematic review


How to cite this article:
Natanasabapathy V, Arul B, Santosh SS, Vasudevan A, Mahendran SS, Namasivayam A, Deivanayagam K, Srinivasan M R. Prevalence and morphology of root canal isthmus in human permanent teeth using micro-computed tomography: A systematic review. Saudi Endod J 2021;11:142-53

How to cite this URL:
Natanasabapathy V, Arul B, Santosh SS, Vasudevan A, Mahendran SS, Namasivayam A, Deivanayagam K, Srinivasan M R. Prevalence and morphology of root canal isthmus in human permanent teeth using micro-computed tomography: A systematic review. Saudi Endod J [serial online] 2021 [cited 2021 Jun 17];11:142-53. Available from: https://www.saudiendodj.com/text.asp?2021/11/2/142/315648


  Introduction Top


The root canal isthmus (RCI) is defined as a narrow, ribbon-shaped communication between two or more root canals that contains pulp tissue or pulpally derived tissue.[1] It is commonly seen in mandibular molars, maxillary molars, C-shaped canals, and fused teeth often presenting with a highly complex and variable configuration.[2],[3],[4] Due to its poor accessibility, it might serve as a reservoir for residual infected pulp tissue and bacteria, leading to the failure of the root canal treatment or nonhealing of the periapical lesion.[5]

Several methodologies have been used to study the RCI. They include clearing and staining,[6] sectioning, inspection of the resected root surface, and cone-beam computed tomography (CBCT).[7] A recent study reported the overall prevalence of isthmi to be 64.6% in mandibular posteriors,[8] while other studies have reported a frequency ranging from 57.8% to 83%.[2],[9] These aforementioned studies had described the RCI to merely extend in a horizontal fashion between two root canals. However, recent nondestructive (micro-computed tomography [micro-CT]) studies which allow for an in-depth visualization of the root canal anatomy have unraveled the isthmus to be highly complex extending longitudinally in the corono-apical direction in a web-like manner.[10],[11]

Earlier systematic reviews on root canal anatomy have largely dealt with the number and configuration of the root canals with very limited literature on the RCI.[12],[13],[14] It is a well-recognized fact that precise knowledge of root canal anatomy is indispensable for successful endodontic treatment.[15] A systematic review on RCI would allow for the synthesis of evidence-based data, highlight gaps in knowledge, and enable clinicians to devise strategies that effectively debride this part of the root. Hence, the primary aim of the present study was to perform a systematic review of the literature to evaluate the prevalence and morphology of RCI in human permanent teeth assessed using micro-CT studies. The secondary aim was to assess the influence of demographic factors such as age, gender, and geographic region on the prevalence and morphology of the isthmus.


  Materials and Methods Top


Research question

What is the prevalence and morphology of RCI in human permanent teeth assessed using micro-CT studies? Do demographic factors such as age, gender, and geographic location influence the prevalence and morphology of RCI?

Protocol and registration

This systematic review was reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement[16] and the protocol of the review was registered with Open Science Framework (OSF) (Platform number: 10.17605/OSF.IO/YRZTH). Ethical clearance was sought from the Institutional Review Board but was deemed unnecessary, on account of it being a systematic review of in vitro studies.

Literature search process

Two reviewers performed a comprehensive literature search for studies evaluating the prevalence and morphology of RCI in human permanent teeth using micro-CT imaging, according to specific search terms until January 2020 using the following four electronic databases: PubMed, Scopus, Embase, and Web of Science. The search terms and filters used in each electronic database to identify relevant studies are shown in [Supplementary Table S1]. In addition, a gray literature search was conducted in Open-Grey (opengrey.org). Additional hand search was performed in three peer-reviewed endodontic journals (International Endodontic Journal, Journal of Endodontics, and Australian Endodontic Journal) to identify relevant literature.



Selection criteria

Laboratory studies using micro-CT for the evaluation of RCI prevalence and morphology in extracted human permanent teeth with fully formed apices except third molar teeth, in the English language were included. Review articles, case reports, surveys, comment letters, book chapters, conference abstracts, animal studies, studies evaluating only instrumentation, or removal of intracanal medicaments, root filling materials, and quality of obturation were all excluded. Furthermore, studies evaluating the morphology of the isthmus in fused roots and C-shaped canals were also excluded.

Study selection

Titles and abstracts of the articles were screened according to the predefined inclusion/exclusion criteria by two reviewers and “relevant” articles were subjected to a full-text reading. Any disagreements between the two reviewers were resolved by discussion with two senior endodontists. A manual search was also conducted from the reference lists of the included studies.

Quality assessment

Risk of bias assessment of the included articles was done using the modified version of the Joanna Briggs Institute (JBI) critical appraisal tool for quasi-experimental studies.[17] Four questions from the original JBI tool, pertaining to the similarity of participants in the comparison group, presence of a control group, comparisons receiving similar treatment other than the exposure of interest, and outcome measurements prior and postintervention were eliminated as they were irrelevant to the current research question. Two new questions with regard to the micro-CT parameters, namely the voxel size used and the axial slice thickness were added to the JBI risk assessment tool for a more detailed analysis. The modified JBI tool comprising of seven questions is given in [Supplementary Table S2].



Two evaluators independently assessed eligible studies and scored each JBI question as yes, no, or unclear. Discrepancies in the assessment were discussed with senior team members until a consensus was reached. Interobserver reliability tests between the evaluators were also undertaken. The final score of each article subjected to the JBI appraisal was calculated based on the percentage of positive answers (”yes”) and was classified as having a “high” risk of bias when the score was ≤49%, “moderate” risk of bias if the score ranged from 50% to 69% and “low” risk of bias if the score was >70%.[18]

Data extraction

Data extraction form was created using Microsoft Excel and the data were retrieved independently by two reviewers. It was then verified by two senior team members and disagreements between the reviewers were resolved by discussion. The data extraction form contained the following details: study title, author/year, sample size, tooth type, age, gender, geographic location, isthmus prevalence (%), classification and configuration of the isthmus, level-wise prevalence (%) of the isthmus as well as individual level-wise prevalence (%) in the apical third (1, 2, 3, 4, 5 and 6 mm). In addition, micro-CT parameters such as voxel size and axial slice followed in that particular study were also tabulated.


  Results Top


Study selection

The search flow diagram is presented in [Figure 1]. A total of 616 articles were obtained, with 385 from electronic database search, 184 from gray literature and 47 from hand searching. After application of the eligibility criteria and elimination of duplicates, 30 articles were selected for full-text assessment. Following full-text reading, 14 studies were excluded [reasons for the exclusion provided in Supplementary Table S3].[43],[44],[45],[46],[47],[48],[49],[50],[51],[52],[53],[54] Finally, 16 studies that fulfilled the eligibility criteria were included for qualitative synthesis. Due to the presence of heterogeneity among the included studies, meta-analysis was not performed for the current review.
Figure 1: Preferred Reporting Items for Systematic Reviews and Meta-Analyses literature search flow diagram

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Study quality assessment

All sixteen included studies were classified as having a low risk of bias, with an average score of 91.95% [Figure 2]. The interreviewer reliability for the risk of bias evaluation was calculated to be above 0.87 using the Cohen–Kappa statistics [Supplementary Table S2].
Figure 2: Risk of bias summary and graph

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Study characteristics

The publication year of the included studies ranged from 2005 to 2019. Four studies mentioned the mean age group of the extraction pool.[11],[19],[20],[21] The studies included covered varying geographic locations, with 3 from a Brazilian population,[22],[23],[24] 1 from a Saudi Arabian Eastern Province,[25] 1 from a Burmese population,[26] 2 from a Chinese population,[10],[19] and one each, in Japanese[27] and Italian[20] populations [Table 1]. The included studies (n = 16) reported data on a total of 1043 teeth, including 469 maxillary first molars, 100 maxillary second premolars, and 474 mandibular molars.
Table 1: Characteristics of included studies

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Prevalence of isthmus

Mesiobuccal root of maxillary first molars

Seven studies considered the mesiobuccal root for analysis of isthmus in 469 maxillary molars. The prevalence of isthmus from three studies that considered the full root for evaluation ranged from 8.9%[28] to 70.88%.[20] The prevalence in the apical third was considered in six studies, out of which only four studies had reported the overall apical third prevalence. The overall prevalence of the RCI in the apical third was found to range from 23.6%[20] to 45%.[29] Middle third prevalence was reported in three studies to range from 49.5%[28] to 73%.[29] Four studies had reported the prevalence of RCI in the coronal third with a range from 36%[29] to 41.2%,[20] while one study had evaluated the prevalence of RCI 2 mm and 3 mm apical to the furcation[24] and found it to be 41% and 53%, respectively [Figure 3].
Figure 3: Pie chart representation of studies evaluating the mesiobuccal root of maxillary first molars and mesial root of mandibular molars

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Two studies reported apical isthmus prevalence among 200 maxillary first molars in an individual level-wise manner from 1 to 4[23] and 3–6 mm,[27] respectively [Table 1].

Distobuccal and palatal root of maxillary first molars

Only one study evaluated the distobuccal and palatal roots of 100 maxillary first molars[27] and reported the prevalence of isthmus to range from 6% to 10% in the distobuccal root and from 3% to 6% in the palatal root, respectively.

Maxillary premolars

The prevalence of isthmus in 100 maxillary second premolars was evaluated in one study[25] which reported the overall prevalence of isthmus to be 2%.

Mesial root of mandibular molars

Eight studies evaluated the isthmus in 474 mandibular molars. The prevalence of RCI in the mesial root of mandibular molars based on two studies that used full root for evaluation ranged from 80.6%[11] to 100%.[30] The prevalence in the apical third was considered in five studies, out of which three studies reported the overall apical prevalence to be 85%[10],[31] to 88.89%.[19] There were no data available for the separate evaluation of the middle and coronal third levels [Figure 3].

Three studies reported apical isthmus prevalence among 155 mandibular molars in an individual level-wise manner from 1 to 5,[31] 1–4,[32] and 1–6 mm,[26] respectively [Table 1].

Distal root of mandibular molars

Only one study analyzed the distal root in addition to the mesial root and found the isthmus prevalence among the 22 distal roots to be 9.1%, upon full root analysis.[30]

Types (morphology) of root canal isthmus

There was a lack of homogeneity among the included studies in classifying the different types of RCI. Two studies merely calculated the presence or absence of isthmus in the axial cross sections of the teeth; they did not use any classification for evaluating isthmus morphology.[23],[24]

Partial versus complete

Three studies simply classified the isthmus into complete or partial[19],[26],[27] wherein, a complete isthmus (CI) had a continuous opening between the two main root canals, while the term partial isthmus (PI) was defined as a narrow projection of one root canal opening toward the second in the same section but not merging with the other canal [Figure 4].
Figure 4: Studies classifying isthmus as partial and complete. MFM: Mandibular first molar, MB: Mesiobuccal

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The ratio of PI to CI was 5.9:1 for age group 20–39 that increased to 17.1:1 for age group 60 and above.[19] The results of this study inferred that the prevalence of the isthmus decreased significantly with age. But there was an increased prevalence of incomplete isthmus amongst the older age group.

Longitudinal classification

Two studies classified isthmus in a longitudinal manner[19],[31] as “continuous,” when the isthmus was visible throughout all the analyzed cross-sections from its coronal part until the apical end, and as “sections with and without isthmuses” [Figure 5]a. Based on this longitudinal classification, Gu et al. 2009 further described the isthmus to be either a fin, web, or ribbon-shaped connection between the root canals.[19]
Figure 5: (a) Studies classifying isthmus in a longitudinal manner; (b) Studies using Hsu and Kim's classification. MB: Mesiobuccal

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Hsu and Kim classification

Two studies[27],[32] used this classification for their analysis [Figure 5]b.

Villas-Boas et al. used the criteria of Hsu and Kim to identify the type of RCI present at the apical 1–4 mm level, but further classified isthmus into complete (Hsu and Kim Type II-V) and incomplete (Ia), and reported prevalence data only based on this division.[32] The prevalence of isthmus at the 3 and 4 mm level was significantly higher when compared to the 1 mm level. The second study by Matsunaga et al. reported that Type I was the most commonly noted RCI pattern in 90% of the distobuccal and 94% of the palatal roots, respectively. In the case of mesiobuccal root, 40% were Type I, 23% were Type II, 20% were Type III, 2% were Type IV, and 15% were Type V at the apical 3 mm level.[27]

Other classifications

Fan et al. classification

Fan et al. classified the isthmus into four types. They reported the prevalence in 70 mesial roots of mandibular first molars to be 29% Type II, 27% Type IV, 23% Type I, and 21% Type III, respectively. In the 56 mandibular second molar mesial roots, the prevalence was 46% Type I, 30% Type IV, 17% Type III, and 7% Type II, respectively.[10]

Moe et al. classification

Moe et al. 2017 classified the isthmus of 75 mesial roots of mandibular first molars into five types, with the prevalence of each type being as follows: 30.1% Type I, 6.7% Type II, 4% Type III, 38.6% Type IV, and 20% Type V.[26]

Both Fan et al. and Hsu and Kim classification

Marceliano-Alves et al. used both Hsu and Kim and Fan et al. classification for evaluating 104 mesial roots of mandibular molars. Based on Fan et al. classification, the prevalence was 52.9% Type II, 46.2% Type IV, 15.4% Type I, and 5.8% Type III. When using Hsu and Kim classification, the values were 52.9% Type I, 19.2% Type IV, and 17.3% Type V.[22]


  Discussion Top


This systematic review assessed studies that used micro-computed tomography (micro-CT) for the evaluation of RCI in human permanent teeth. It was classified as JBI level 4a evidence, which was considered to be low, due to the observational nature of the 16 included studies. However, the results of this review are highly relevant as only micro-CT studies assessing the RCI were considered for analysis.

Micro-CT allows for three-dimensional reconstruction of the root canal system in a precise manner and is considered as a reference standard against which other methods such as clearing, staining, and CBCT that are used for the assessment of isthmus are compared with.[6],[7] In addition, in order to increase the strength of the review, a detailed quality assessment of the studies was done using a modified JBI risk assessment tool, the objective being to appraise the methodological quality of each study and understand the likelihood of bias in the study design, conduct, and analysis. All 16 included studies were classified as having a low risk of bias. The average score of the included studies was 91.95% indicating a low risk of bias thereby increasing the robustness of this systematic review.

Two recent articles on guidelines for reporting for root canal anatomy studies stressed the importance of precalibration of the examiners/evaluators and performing reliability tests (intra/interoperator scores) as highly essential for eliminating bias.[33],[34] Conversely, the vast majority of the articles in this review failed to mention/perform these two aspects.[10],[20],[23],[24],[25],[26],[27],[28],[29],[30],[32]

Data analysis revealed that micro-CT parameters, mainly the slice thickness and voxel size used were different among the studies ranging from 11 microns to 33 microns. Moreover, two studies had not mentioned the voxel size used,[25],[26] while seven studies made no mention of the axial slice thickness.[19],[22],[23],[24],[25],[26],[32]

In addition, there was a variation in terminologies used. Verma and Love used the term “intercanal communication” instead of the isthmus and described it as the branching of the pulp space between the main canals that did not communicate with the external surface.[29] Totally four of the studies in this review used the term “intercanal communication” instead of the isthmus.[11],[21],[28],[29] Somma et al. used both the terms “isthmus” and “intercanal connection” in their study and considered it as separate entities.[20] They described the isthmus as a ribbon-shaped canal on the same cross section for several consecutive cross sections and intercanal connection as an accessory canal between two main root canals.

The results of this systematic review revealed the prevalence of RCI to be higher in the mesiobuccal root of maxillary molars and the mesial root of mandibular molars when compared to other teeth/roots which was in accordance with other studies reported in the literature.[35],[36] In the current review, the prevalence of RCI in the mesiobuccal root of maxillary first molars on full root analysis ranged from 8.9% to 70.8%. With respect to the mesial root of mandibular molars, the prevalence of RCI on full root evaluation ranged from 80.6% to 100%, which was in accordance with a systematic review by de Pablo et al. who reported the incidence to range from 23% to 77.4%.[37]

The prevalence of isthmus in maxillary second premolars was evaluated only in one study in the current review and found to be 2%. This is in contrast to a systematic review that reported the prevalence of connecting canals to be more than 90% of maxillary second premolars.[38] One factor attributing to the striking contrast may be the type of isthmus studied, but details regarding the classification used to analyze these connecting canals were not described.

In the current systematic review, there was lack of homogeneity among the included studies in classifying the different types of RCI. Two studies[23],[24] merely calculated the presence or absence of isthmus in the axial cross sections of the teeth. Two studies[27],[32] used Hsu and Kim classification for the analysis of the isthmus. Marceliano-Alves et al. interestingly used two methods to classify the RCI, one of which was Hsu and Kim and the other being Fan et al. three-dimensional classification.[22] The remaining studies had proposed a classification of their own[10],[19],[26] based on the three-dimensional extent of the isthmus.

With regard to the different regions of the root canal that were analyzed, only eight studies[11],[20],[21],[22],[25],[29],[30],[31] among the 16 studies included for qualitative synthesis had evaluated the entire root length for the prevalence and anatomy of RCI. A vast majority of the studies considered only the apical third of the root and the apical length taken for evaluation was variable ranging from 1 to 3, 1–4, and 1–6 mm.

As for the influence of demographic factors on the prevalence and morphology of RCI, studies have concurred that ethnicity is a predisposing factor for anatomical variations,[12],[39] but the present articles included for qualitative synthesis failed to demonstrate any direct relationship between the two. While none of the studies had mentioned the gender of the patient, four studies[11],[19],[20],[21] mentioned the mean age group of the extraction pool, and only one study had assessed the influence of age on the prevalence of RCI.[19] Moreover, the RCI prevalence studies have been done largely in Asian,[10],[19],[25],[26],[27] Brazilian,[22],[23],[24] and Italian[20] populations only. Hence, future studies across different countries are essential to understand if the RCI pattern varies across different populations.

Ahmed and Rossi-Fedele proposed a new system for classifying root canal anatomy in the form of codes, which aimed to provide detailed information on the tooth, number of roots, and main and accessory canal morphology.[40] However, the authors considered intercanal communications to be an integral part of the canal system and made no separate mention about it while defining the root canal anatomy codes.[41] Karobari et al. suggested two modifications to the anatomy codes; the first being the addition of an asterisk to indicate the presence of complex canal communications, and the second being the use of alphabets C/M/A (coronal/middle/apical) to denote the position of these communications.[42]

Clinical implications and future perspectives

The present systematic review highlights the tooth, root, and section-wise prevalence of the RCI as well as provides details on the morphological variations of the same. The present findings advocate the need to pay adequate attention to the risk of concealed isthmi along the entire root length in maxillary as well as mandibular molars. Due to their poor accessibility, the mechanical instrumentation of these areas is unfeasible. Therefore, future endeavors should concentrate on efficient irrigant delivery and activation.

Upcoming micro-CT studies on the isthmus should consider using a three-dimensional classification and provide complete demographic as well as micro-CT details in order to enable an in-depth analysis. Additionally, there is a need for more studies on the isthmus pattern in fused roots and C-shaped canals.

Limitations

There was considerable heterogeneity among the studies in terms of the classification used to describe the isthmi. Furthermore, only a few of the articles considered the entire root for the analysis. Additionally, the channel communications in fused roots or complex canal morphologies have not been considered in any of the included studies.


  Conclusion Top


The prevalence of RCI was higher in the mesiobuccal roots of maxillary molars and the mesial roots of mandibular molars, extending in the corono-apical direction with a complex and diverse morphology. The influence of demographic factors on the prevalence of RCI could not be established clearly.

Acknowledgments

The authors would like to thank Dr. Nagendrababu Venkateshbabu for his advice and comments which helped in doing the systematic review.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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