Isthmus Incidence in Human Permanent Mandibular First Molars of a South Indian Population: A Cone-Beam Computerized Tomographic Study.

AIM: This study aimed to analyze incidence of isthmus in human permanent mandibular first molar teeth using cone-beam computed tomographic imaging techniques in a South Indian population.
MATERIALS AND METHODS: Three hundred permanent mandibular first molar teeth were collected, cleaned, and stored in normal saline. They were divided into groups (GPs) I and II based on number of roots, and were further subdivided (right and left [RL] subgroups A and B for GP I; and RL subgroups C and D for GP-II). Samples were processed and isthmus incidence was evaluated by cone-beam tomography, compared, and statistically analyzed.
RESULTS: Overall in mandibular first molars, the isthmus incidence in mesial root was 97.2%, distal root was 39%, and distolingual root was 0%. There was no statistically significant difference between the right and left mandibular first molar teeth with regard to incidence of isthmus (P > 0.05). There was an incidence of type I (38.67%), type II (56.33%), type III (3%), and type IV (2%) isthmuses in mesial root and type I (12.33%), type II (16%), and type III (10.67%) in distal root.
CONCLUSION: Incidence of isthmus was very high in the mesial root of the mandibular first molar and should be factored during nonsurgical and surgical endodontic treatment procedures to achieve successful treatment outcomes.

INTRODUCTION

The human permanent mandibular first molar teeth due to variations in anatomy presents the clinician challenges, with the need for more diagnostic investigations and clinical skill levels. Successfully treating complex root canal systems often requires specialized training. A number of differences in the anatomy of permanent mandibular first molar have been reported as specific to ethnic groups and populations. Failure to successfully identify and understand the ramifications of root canal space can lead to inadequate canal obturation, which will eventually lead to failure of therapy.[1] Of the total failures of root canal therapy, 58.66% could be attributed to the incomplete obturation and 9.68% to perforations of the root.[2] Cone-beam computed tomography is a useful diagnostic technique in endodontic cases where intraoral radiography and clinical examination alone are unable to provide adequate insight regarding the tooth and the surrounding structures. The presence of anatomical entities such as isthmus and middle mesial canal is common in mesial root of the permanent mandibular first molar. In the distal root, presence of two canals, isthmus and distolingual canal, has been reported.[3] An isthmus is a passage connecting two larger structures or cavities, which can be considered as a lateral connection between canals of the same root.[4] The presence of an isthmus can act as a reservoir for organic debris and microorganisms.[5] The presence of an isthmus is often missed, and when noticed, it is difficult to cleanse. The reported incidence of isthmus ranged from 49.5% to 87.9% in mesial root.[6] The detection, cleansing, and debridement of these areas during nonsurgical or surgical endodontic therapy are vital for the successful outcome of therapy.[78] The isthmus is classified into different types and might vary in different population groups. This study aimed to analyze incidence of isthmus in mandibular first molars using cone-beam computed tomographic techniques in a South Indian population.

MATERIALS AND METHODS

Permanent mandibular first molar teeth were collected after extraction, rinsed, cleansed, and stored. The samples were visualized under a magnifying loupe with illumination for intact occlusal and root morphology and selected as per accepted criteria.[9] The root surfaces were cleaned and analyzed using digital radiography, and those with canal calcifications were discarded. A total of 300 teeth (n = 300) were selected and divided into 2 groups namely group I (2 rooted mandibular first molars [n = 285]) and group II (3 rooted mandibular first molars [n = 15]). Group I was further subdivided into two: subgroup A (n = 133) and subgroup B (n = 152). Group II was further subdivided into two: subgroup C (n = 3) and subgroup D (n = 12). The teeth belonging to respective groups were mounted on an occlusal rim, coded, and numbered separately with markings so that they were visible during the image analysis procedure and stored for further analysis. In a single imaging scan, two arches were kept in such a way that its occlusal surfaces face each other with a layer of wax in between them for separation and stabilization. They were then mounted on the cone-beam tomography machine, imaging was carried out, and presence of isthmus was assessed using the classification of isthmuses by Hsu and Kim.[10] The group-wise data observed on the incidence of isthmus, depth of isthmus, were recorded and compared statistically between the groups, and the results were tabulated.

RESULTS

Overall in mandibular first molars the isthmus incidence in mesial root was 97.2%, in distal root 39% and 0% in the distolingual root. There was no statistically significant difference between the right and left mandibular first molar teeth with regard to incidence of isthmus (p > 0.05). There was an incidence of TypeI (38.67 %), TypeII (56.33%), TypeIII (3%) and TypeIV (2%) pattern of isthmuses in mesial root. TypeI (12.33%), TypeII (16%), and TypeIII (10.67%) pattern of isthmus incidence was seen in the distal root.

DISCUSSION

The mesial root in mandibular first molar is buccolingually wide and provides an ideal environment for intercanal communications and isthmuses to be formed during development of the root.[10] A higher incidence of isthmus was found in mandibular first molars.[6] Detection and efficient management of canal isthmus is considered as an important factor for successful endodontic therapy. A high prevalence of mandibular molars with isthmuses necessitates timely detection and biomechanical cleaning of these areas during nonsurgical or surgical root canal therapy. The presence of isthmus in mandibular first molar mesial root was 54.8% and 20.2% in distal root as reported in a systematic review on canal configuration.[11] Different anatomical features of isthmuses were found between first and second mandibular molars in the apical part of the mesial root. Sheet type of connections between the canals was more common in the second mandibular molars.[12] A high prevalence of isthmus (78.4%) and middle mesial canals (18.2%) was observed and was significantly higher in the age group of 31–50 years.[13] The frequency of incidence of isthmus in the apical 5 mm of the mesial root has been reported to be as high as 85%. Less isthmuses were found in 1 mm, and more isthmuses in 3 mm from the apical tip. Lateral canals start from the main part and calcifications were found in most isthmuses. Clinical and surgical procedures performed in the mesial root in mandibular first molars may be affected by this type of canal anatomy.[14] Two- and three-dimensional analyses of the mesial roots of mandibular molars using microcomputed tomography revealed that band-shaped isthmuses had complex shapes.[15] Isthmus incidence is very common in flat roots and in lower molars; these structures were most frequently located at about 7 mm from the apex.[16] The mesial canals were found to be much more variable than distal, and the furcal aspect of the entire mesial root should be considered a danger zone.[17]

Incidence of isthmus has been reported from different populations. Isthmuses were observed in 44.6% of mesial and 27.3% of distal roots in an Iranian population.[18] Isthmus incidence was found to be common in the mesial root of the mandibular permanent first molar in an Iranian population, with the highest prevalence at 6-mm distance from the root apex.[19] In a mandibular first molar mesial root analysis using micro-computed tomography in a Brazilian population, isthmus types I and II were the most prevalent; mesial root canals showed high morphological variability[20] [Figures 1-3]. In this study of isthmuses of mandibular first molar on a South Indian population, the overall isthmus incidence in mesial root was 97.2%, and in distal root it was 39% [Figure 4].

Figure 1

Comparison with other studies—mesial root

Figure 3

Mesial and distal root studies comparison

Figure 4

Incidence of isthmus

Comparison with other studies—distal root

During rotary instrumentation of mesial canals of lower molars, a lot of dentinal debris is packed into the isthmus despite continuous irrigation during and after instrumentation. The debris blocks penetration of filling material and sealer into the isthmus area.[21] The location and pattern of isthmus when visualized correctly by the clinician result in more efficient microsurgical procedures. By successfully locating and deciphering the anatomy of isthmus, a more efficient endodontic microsurgery can be guaranteed. Untreated isthmuses can be a cause of endodontic treatment failure.[22] From a mechanical perspective, the isthmus connecting root canals can be regarded structurally as a natural weak zone. Two canal mesial roots are much more prone to vertical root fractures than single canal distal roots. Fractures may occur during clinical condensation of gutta-percha in mesial roots of mandibular molars as well as other roots with canals connected by isthmus.[23] The success rates for endodontic microsurgery on teeth with isthmus were found to be lesser. Considering the potential risk of structural weakening of the root during surgical procedures, the techniques of isthmus preparation need to be improvised.[24]

In this study, in mandibular first molars the types of isthmus in mesial root were type II (56.33%), type I (38.67%), type III (3.0%), and type IV (2.0%), and in distal root were type II (16.0%), type I (12.33%), and type III (10.67%). In the distolingual root, isthmus was absent [Chart 1]. The incidence in the subgroups was also analyzed. A statistical comparison between incidence of various types of isthmus on left and right side of the mandible for group I and II showed no difference (P > 0.05) [Figure 5, Chart 2 A and B].

Chart 1

Isthmus type and incidence

Figure 5

Isthmus types

Chart 2

(A) Isthmus incidence and type—subgroups A and B. (B) Isthmus incidence and type—subgroups C and D

Depth of isthmus

In this study, in mandibular first molars the average isthmus depth was 1.14 mm. The average isthmus depth in mesial root was 1.69 mm and in distal root was 0.58 mm [Chart 3].

Chart 3

Depth of isthmus

The map reading dynamics was found to be precise to detect the isthmus while using cone-beam tomographic investigation.[25] Every attempt at identifying the presence and the pattern of the isthmus present should be carried out during preoperative analysis. Successful preparation and cleansing of isthmus is a major challenge during root canal therapy.[2627] Isthmus cleansing is important for conventional and surgical endodontic treatment because it is difficult to clean and manipulate endodontically. It serves as a potential space for accumulation of debris as a result of instrumentation procedures and proliferation of microorganisms. Therefore, clinicians need to correctly identify the presence of the isthmus and use effective methods of disinfection.[27] Irrigating solution activation provided better cleaning of the canal and isthmus.[2829]

The mandibular first molar presents the clinician with a complex anatomy requiring more diagnostic modalities, suitable modification of access, and developing sufficient clinical skills to successfully localize, negotiate, debride, disinfect, and seal the root canal system. The presence of isthmus as communication channels between canals should be considered during biomechanical preparation as well as during periapical surgical procedures. Preoperative assessment and analysis of the tooth to be treated and variations in a given population is important as knowledge of these canal variations helps the operator to effectively manage these cases successfully. Therefore, detection, cleaning, and filling of these isthmuses should be carried out meticulously and would greatly contribute to long-term success.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.