Root Canal Configuration of Burmese (Myanmar) Maxillary First Molar: A Micro-Computed Tomography Study

Aim To investigate the root canal anatomy of Burmese (Myanmar) permanent maxillary first molar (BMFM) with micro-computed tomography. Methodology. One hundred and one extracted BMFMs were scanned by a SkyScan 1272 scanner (Bruker microCT, Belgium) and reconstructed with NRecon software (Bruker microCT). CTAn software (Bruker microCT) was used to create 3D models of root and internal canal anatomy, while CTVol software (Bruker microCT) was used to visualize 3D models. In each root, Vertucci's canal types, incidence and location of the lateral canal, incidence, location, and type of isthmus, and number and position of foramina were examined. Results In 101 specimens, 83 (82.18%) mesiobuccal roots had multiple canals. The most common canal type is type IV (45.5%), followed by type II (17.8%) and I (17.8%) canals. Type III, V, VI, VII, and VIII canals are less than 10% in total. Seven additional canal types were seen for 10% in total. Fourteen (13.86%) distobuccal roots had multiple canals, and the predominant canal type is type I (86.1%), followed by type II (5.9%) and V (4%) canals. Three additional canal types were observed for 4% in total. All palatal roots possessed the simplest type I canal. Apical ramification occurred in 69 mesiobuccal roots (68.3%), 36 distobuccal roots (35.6%), and 37 palatal roots (36.6%). A total of 240 lateral canals were observed in 101 specimens. Each specimen had 2.38 ± 2.22 lateral canals on average. The highest incidence, 136 (56.67%) lateral canals, occurred in the mesiobuccal root, followed by 57 (23.75%) and 47 (19.58%) lateral canals from the distobuccal root and the palatal root, respectively. Each specimen had 6.17 ± 2.42 foramina. Mesiobuccal root had the highest incidence of apical foramina compared to other roots. Seventy-two mesiobuccal roots (71.29%) had isthmus, while only 7 distobuccal roots (6.93%) had isthmus somewhere along the root. Conclusions The root canal anatomy of BMFM was quite complex, especially in the mesiobuccal root. The predominant canal type was Vertucci type IV in the mesiobuccal root and type I in the distobuccal and palatal roots. In addition, this micro-computed tomography study disclosed complemented canal types and a higher prevalence of lateral canal than the previous studies.


Introduction
A thorough understanding of root canal anatomy (RCA) and its variation is extremely important for endodontic treatment [1]. Maxillary first molar (MFM) is one of the most frequently endodontically treated teeth [2,3] and the posterior teeth with the highest endodontic failure rate [4]. e majority of MFMs have three roots and four canals [5]. Because of its internal anatomic intricacy, it has been extensively studied by various methods [5]. Historically, the clearing method was considered as the gold standard for three-dimensional (3D) study of human RCA; however, this method is not suitable to reveal the complex and fine anatomic features of RCA in the mesiobuccal root of MFM, and micro-computed tomography (µCT) is currently becoming the reference method for this purpose [6]. Several studies used µCT to investigate the RCA of MFM in different population [7], especially in mesiobuccal root, and showed more complex internal RCA than previously expected and higher incidence of the additional canal [6,[8][9][10][11][12] than previous studies that used traditional methods such as clearing, sectioning, or grinding [5]. Ethnicity, age, and gender differences might influence the diversity of RCA [12,13]. One previous study also applied µCT to study the RCA of MFM in the Egyptian population [7]. In 2001, Ng et al. studied the RCA of Burmese MFM by clearing method [14]. Burmese MFM possesses the complex RCA and shows additional Vertucci's canal types [14]. To date, there was no µCT-based study for the RCA of Burmese MFM. erefore, this study aimed to investigate the RCA of Burmese MFM by using the current gold standard µCT analysis.

Materials and Methods
e ethics committee of the University of Dental Medicine, Mandalay, Myanmar, approved this study (Ethical/UDMM/ 2018/17). One hundred and one extracted human permanent MFMs with three distinct roots were selected from the Burmese population. Specimens with immature apex, apical root fracture, and previous endodontic treatment were not included in this study. Debris and stain on tooth surface were cleansed by hand and ultrasonic scalar. Specimens were stored in saline solution during the study period.
en, specimens were scanned by a SkyScan 1272 scanner (Bruker microCT, Kontich, Belgium) with 10 µm voxel size, 125 µA, 80 mV, 1 mm aluminum filter, 0.4°rotation steps, and 180°r otation. After reconstruction with NRecon software v1.6.1 (Bruker microCT), CTAn software v1.14.4 (Bruker microCT) was used to create 3D models of root and internal canal anatomy, while CTVol software v2.2.3 (Bruker microCT) was used to visualize these 3D models in colorcoded and transparent models. e segmentation or thresholding of the scan dataset was performed by CTAn Software. e proper threshold value for enamel, dentine, and pulp in the crown and root was determined in the density histogram. Original grayscale images and segmented images were compared and checked to obtain accurate result. e global thresholding method was used. resholding, morphological operation, despeckle, bitwise operation, and 3D model tasks from the custom processing function tab in CTAn software were applied as necessary to produce the 3D models of the root and root canal systems. en, these 3D models were visualized in color-coded and transparent modes by CTVol software. In each root, Vertucci's canal type, incidence and location of the lateral canal, incidence, location, and type of isthmus, and number and position of foramina were investigated. e main canal is defined as any canal extending from pulp chamber floor to apex and possessing its own or shared orifice and main apical foramen. e lateral canal is defined as any branch emerging from the main canal to communicate with the periodontium. Apical delta or ramification is defined as the presence of any branch, apart from the main canal, communicating the periodontium in the apical third of the root. Isthmus is defined as any communication, sheet-or tubelike structure, between main canals whether completely or incompletely connecting to each other. e isthmuses observed were three-dimensionally categorized into six types, such as sheet complete type: a narrow sheet-like structure connecting the main canals anywhere, sheet incomplete type: a narrow sheet-like structure extending from either of main canals but not completely connecting each other at any point, tubular complete type: a tube-like structure extending from either of main canals connecting each other at any point, tubular incomplete type: a tube-like structure extending from either of main canals but not completely connecting each other at any point, mixed complete type: a narrow sheet-and tube-like structure together connecting the main canals anywhere, and mixed incomplete type: a narrow sheet-and tube-like structure extending together from either of main canals but not completely connecting each other at any point [15].

Root Canal Types and Incidence of Apical Ramification.
Vertucci's canal types with additional canal types observed in mesiobuccal, distobuccal, and palatal roots are summarized in Table 1.
In the palatal root, 100% of the specimen possessed the simplest type I canal.

Incidence, Location, and Type of 3D Isthmus.
e distribution of 3D isthmus types in the mesiobuccal and distobuccal roots at different root thirds is expressed in Table 7. Mesiobuccal root revealed a higher number of isthmuses compared to distobuccal root in all root thirds. Seventy-two mesiobuccal roots (71.29%) had isthmus, while only 7 distobuccal roots (6.93%) had isthmus somewhere along the root.

Discussion
is study investigated the root canal configurations of Burmese MFM by µCT analysis. In the endodontic literature, Weine's and Vertucci's classifications are commonly and classically used in RCA studies [12]. Previous studies [10,12] showed that Vertucci's classification had wider coverage to classify the complex RCA of human teeth than Weine's classification. Recently, a 4-digit system and another new system were introduced to classify the RCA of human teeth [7,16]. In the 4-digit system, the first three digits represent the canal number at the coronal limit of the respective thirds only and the last digit behind the slash is for the number of   main apical foramina. We tried to use these systems; however, these systems did not fit our study and may not reflect the actual complexities of human RCA. In our opinion, Vertucci's classification with modification for complementary canal types is suitable for our study and applied in this study. It was reported that the prevalence of mesiobuccal root with multiple canals in laboratory studies by traditional methods (60.5%) is higher than that of clinical studies (54.7%) [5]. Six previous µCT studies showed a high prevalence of mesiobuccal root with multiple canals (71.3-100%) [6,8,10,12,17]. Only two µCT studies showed a relatively lower prevalence of mesiobuccal root with multiple canals (46.6% and 55.6%) [7,11]. e prevalence of mesiobuccal root with multiple canals in this study (82.18%) is within the range of that (46.6-100%) from previous µCT studies and is quite similar to that of Korean (71.3%), American (100%), and Italian (100%) but different from Egyptian (46.6%) and Japanese (55.6%) [6,12,17]. However, one recent cone-beam computed tomography (CBCT) clinical study in Egyptian at 133 µm voxel size stated that 74.55% of the mesiobuccal root of MFM had additional      International Journal of Dentistry canals [18]. It might be due to the sample size, the study method, the different tribes, or different canal classification systems used, for example, the 4-digit system in Egyptian study and modified Weine's canal classification in the Japanese study. Additionally, 82.18% of mesiobuccal root in this study had multiple canals, which is higher than that of the previous studies by clearing method (70%) for Burmese MFM [14] and (67.2%) for ai MFM [19]. Success in endodontic treatment depends on the adequate cleaning, shaping, and obturation of the entire root canal system. e frequent failure of endodontic treatment was likely due to the failure to treat the additional canals [20]. A previous CBCT clinical study showed that teeth with a missed canal were 4.38 times more likely to be associated with apical lesions and that the incidence of missed canals was highest in the MFM [21]. e ability of the CBCT method to detect the MB2 canal mainly depends on its voxel size. e results may be varied from study to study due to the different spatial and contrast resolutions used. e currently available voxel size for the best resolution of CBCT analysis is 76 µm. In one previous clinical study, even with the 76 µm voxel size, three MB2 canals were not seen on the CBCT images but were identified after troughing under magnification and illumination [22]. CBCT is a very useful state-of-the-art 3D imaging technique for clinical cases; however, it still has the limitation to detect the fine anatomic structure of complex root canal systems, like MB2 canal.
Although the clearing method was previously considered as the best for the root canal anatomy study, it has some weaknesses, like the distortion of specimen during decalcification and subsequent difficulty in dye penetration, resulting in the inadequate presentation of fine anatomic structures including additional canals [6]. Nowadays, µCT analysis can disclose more real 3D complex anatomic structures and the actual prevalence of additional canals in the RCA of MFM, especially in mesiobuccal root, compared to traditional methods, grinding, sectioning, or clearing techniques [6]. Because of radiation hazards, µCT analysis can only be applied in in vitro studies. From these results, the prevalence of additional canals in the mesiobuccal root of MFM in different populations by traditional laboratory methods should be revised by µCT analysis. e majority (70%) of the inhabitants of Myanmar possess Mongoloid traits [23]. In this study, the prevalence of type IV canal (45.5%) is highest, followed by type I canal (17.8%) and type II canal (17.8%) in the mesiobuccal root, showing the typical Mongoloid trait [12], whereas Caucasian population shows more type II canal [24]. Also, the previous studies stated that type IV is predominant over type II canal in the mesiobuccal root of MFM of Burmese [14], Indian [24], Japanese [11,25], Korean [26], and ai [19], which conforms to our result. e higher prevalence of type IV (2 orifices with 2 foramina) compared to type II (2 orifices with single foramen) is the endodontic significance for the presence of multiple portals of exits and more microleakages readily [27]. Considering these facts, the clinician should take attention to find the additional canal in the mesiobuccal root of Burmese MFMs during the treatment until it is excluded.
is might be attributed to the better ability of µCT, nondestructive and higher resolution 10 µm, to reproduce the fine and complex anatomic structures accurately by compensating the drawbacks of the clearing methods, distortion of specimen, and limited dye penetration into minute structures.
Variation in additional canal occurrence in the distolingual root was less frequently seen in this study. However, the prevalence of distobuccal root with multiple canals in this µCT study is 13.9%, which is higher than that of previous clearing studies (5.5% and 1.9%) [14,19]. Only one   [14], while the present µCT study identified three additional canal types, (1)(2)(3), , and (2-1-2-1-2), for 1%, 2%, and 1%, respectively. So, the relatively more complex RCA of distobuccal roots was disclosed by the benefit of µCT compared to the previous clearing study that showed a more simple type I canal [14]. All palatal roots (100%) in this study possessed the simplest type I canal configuration and lack of complexity and isthmus, which is similar to the previous findings [14,19], showing less variation of this root. In contrast, one previous µCT study demonstrated that 1.2% of palatal roots of Egyptian had more than one canal and it might be due to the different canal classification systems used and different tribes [7].
In this study, the highest incidence of apical ramification was observed in the mesiobuccal root (68.3%) possessing 56.7% of lateral canals and 49.6% of main and lateral canal foramina compared to the distobuccal root (35.6%) with apical ramification possessing 23.75% of lateral canals and 26.64% of main and lateral canal foramina and the palatal root (36.6%) with apical ramification possessing 19.58% of lateral canals and 23.76% of main and lateral canal foramina. ese complex anatomic structures were commonly located at the apical region and similar to previous findings [10,14,28]. In contrast, the present µCT study identified 240 lateral canals in 101 MFMs, mainly in the apical portion of the mesiobuccal root, which is much higher than only 22 lateral canals in 90 MFMs by the clearing method [14]. is discrepancy could be explained by the better performance of µCT technology. ese minute structures harboring microbes and/or necrotic tissues could not be shaped, cleaned, and properly filled and may cause postoperative pain and subsequent failure [20]. By these facts, clinicians should pay attention to clean these fine and complex anatomic structures by using the copious amount of proper chemical disinfectant with agitation in the very apical portion of mesiobuccal root during cleaning and shaping.
In this study, isthmuses were more common in the mesiobuccal root compared to the distobuccal root, especially at the coronal and middle thirds of roots, and this is agreed with the previous results [6,14]. In 101 specimens, the mesiobuccal root had 149 isthmuses whereas the distobuccal root had only 10 isthmuses, which is in contrast to the previous study [7]. Regarding the occurrence of isthmus, 71.29% of mesiobuccal roots and 6.93% of distobuccal roots had isthmus somewhere along the root, which is much higher than that of the previous study by clearing technique for the same population [14]. e previous µCT study [7] mentioned about the connecting canals for only 2.9% of specimens, which might be similar to the tube complete isthmus type in our study.
e isthmus observed in this study were three-dimensionally categorized into six types based on the previous classification [15]: sheet complete type, sheet incomplete type, tubular complete type, tubular incomplete type, mixed complete type, and mixed incomplete type. Two 3D isthmus classifications were introduced by two previous studies [29,30]. Here, we also devised the new 3D isthmus types according to the nature of anatomic structure and connectivity, which is based on the previous one [15] and it might be more convenient and wide-ranging than the previous two classifications.
One hundred and one extracted Burmese MFMs, not specified by age and gender, were used for this µCT analysis, and this might be the limitation of this study.

Conclusions
e results of this study reiterate that the root canal configuration of Burmese MFMs is quite complex, especially the mesiobuccal root possessing the highest incidence of additional canals, lateral canals and apical delta, and isthmuses among three roots. e predominant canal type was Vertucci type IV in the mesiobuccal root, and type I in the distobuccal and palatal roots. is µCT study showed a more detailed and much complicated root canal feature of Burmese MFMs than the previous study with Indian ink and clearing technique. e previous RCA studies by clearing technique for other tribes need to be revisited by the current gold standard µCT analysis.

Data Availability
e data used to support this study are provided within the article.

Conflicts of Interest
e authors deny any conflicts of interest related to this study.