Computerized Tomography Technique for the Investigation of the Maxillary First Molar Mesiobuccal Root

The aim of this paper was to review the literature about the use of computerized tomography to evaluate the presence and characteristics of the second mesiobuccal canal in the maxillary first molar. An electronic search was performed. Frequencies of the presence of second mesiobuccal canal and root anatomy characteristics were extracted from the selected studies. Pooled frequencies were calculated as weighted means. Seven articles were included. A second mesiobuccal canal was present in 59.32% of the teeth, and it was noncommunicating in 58.45% of teeth presenting the canal itself. The most common root canal morphology was single canal or two separated canals. The present paper showed that cone beam CT is a viable radiologic device for the evaluation of the mesiobuccal root of maxillary first molars. In fact, it was observed that the frequency of second mesiobuccal canal detection is similar to those presented by clinical studies or micro-CT evaluations.


Introduction
A sound knowledge of root canal anatomy is mandatory in order to perform an adequate root canal treatment. Studies reported that failure to detect all the canals present in a root canal system was one of the causes of failure of endodontic therapy [1][2][3][4][5][6].
A number of studies that evaluated the anatomy of mesiobuccal roots of maxillary permanent first molars reported a wide range of anatomical variations [7][8][9][10].
It was hypothesized that failure to detect, debride, and fill a second mesiobuccal canal (MB2) of first permanent maxillary molars was one of the main causes of poor long-term prognosis after root canal treatment in these teeth [11,12].
While many ex vivo studies investigated the presence of a mesiobuccal canal using canal staining, cross-sectioning, and dentine examination through magnification devices [13][14][15], the most used technique to investigate the anatomy of these teeth prior to an endodontic treatment is periapical radiography, which does not allow a complete detailed evaluation of the root canal anatomy [16].
Cone-beam computed tomography (CBCT) was developed in the 1990s with the aim of producing maxillofacial three-dimensional images using a lower radiation dose than conventional computed tomography (CT) [17,18]. The characteristics of the CBCT scanning were described as well suited to the endodontic field because of the higher accuracy of the device in comparison to that of the standard CT [17,19].
Despite the known limitations of the CBCT (scattering [20], lower resolution than conventional radiography [17,21]), it proved to be valuable in endodontic therapy for the diagnosis and evaluation of root canal anatomy [17].
The aim of the present paper was to systematically review the current scientific literature about the use of CT for the investigation of the mesiobuccal roots and canals of first permanent maxillary molars.

Study Selection Criteria.
The following inclusion criteria had to be met in order to be included in the review: (1) any study design (prospective or retrospective); (2) ex vivo or in vivo studies; (3) root canal anatomy evaluated using CT; (4) at least 10 teeth analyzed; (5) description of presence/absence of MB2 in maxillary first molars; (6) clear description of tooth type and location.
Studies not meeting the above criteria were excluded from the review.

Data Extraction and Analysis.
The following parameters were recorded in an electronic form: (i) presence/absence of the MB2 in maxillary first molars; (ii) any anatomical characteristics of the MB2, as described by Vertucci [29]; (iii) methods for detection and CBCT machine characteristics.
Risk of bias was assessed for each of the included studies, considering the following parameters: (i) number of examined teeth, posing that when less than 50 teeth were evaluated, the study had a moderate risk of bias; otherwise, the risk of bias for this parameter was judged as low; (ii) quality of data reporting, considering that if the authors did not report the characteristics of the root canal anatomy of MB2, but only the presence/absence of the canal, the study was judged at moderate risk of bias; (iii) fulfillment of the aims of the study, reporting of individual data instead of frequency percentages, and clear description of population characteristics were evaluated, and when missing, a moderate risk of bias was assigned.
Pooled data were analyzed through an evaluation of the weighted mean prevalence of MB2. Weighted means were calculated for the frequencies of different mesiobuccal canals morphologies following the Vertucci classification [29]. Moreover, a critical evaluation of differences among the various detection methods in terms of presence/absence of MB2 was performed.

Results
Article selection process is summarized in the flowchart shown in Figure 1. The electronic search yielded 367 articles. After title and abstract screening, 20 articles were selected for full-text evaluation. The article selection was performed independently by two authors (SC and MDF). In case of disagreement, a joint decision was taken by discussion. A total of seven articles were finally included in the review: one article described the use of dental-CT in vivo [28], and six described the use of CBCT both in in vivo and in ex vivo studies [22][23][24][25][26][27]. Data about study design, population, and device used are summarized in Table 1.
Each of the included studies was evaluated by two authors (SC and ST) for risk of bias analysis, whose results are presented in Table 2.
In pooled analysis, the second mesiobuccal canal was present in 59.32% of the examined teeth (out of a total of 1964 teeth), and it was evaluated as completely independent (noncommunicating) in 58.45% of the teeth presenting the canal ( = 1165). Data about frequency of the presence of the second mesiobuccal canal with different detection methods are summarized in Table 3.
International Journal of Dentistry 3  Four studies presented data that could be classified according to the Vertucci classification of root canal morphology [24][25][26][27]. The data are summarized in Table 4. More than 37% of the classified 1741 teeth included in this analysis belonged to type IV, while 36.18% belonged to type I. These two types were the most frequently detected morphologies.
Comparison of the different detection systems showed that in CBCT the MB2 was detected in 61.84% of teeth, while dental-CT showed MB2 presence in 39% of teeth.

Discussion
The present study was aimed to systematically review the root canal anatomy of the mesiobuccal roots of permanent maxillary first molars as shown in CT images.
The retrieved data showed that the detection of a separate mesiobuccal canal occurred in nearly two-thirds of the cases. The common occurrence of two canals in the mesiobuccal roots has a major implication in endodontics when performing a root canal therapy in maxillary first molars.
A systematic review of the literature performed by Cleghorn and coworkers in 2006 [8] estimated the anatomy of the permanent maxillary first molar through a meta-analysis of data of about 8399 teeth from 34 laboratory studies and of 2576 teeth from 14 clinical studies. They reported that the incidence of two canals in the mesiobuccal root was 56.8%, which is similar to the results of the present study.
An issue to be considered is the age and gender of the subjects. Several authors found that a second mesiobuccal canal is less frequent in older subjects due to progressive calcification and obturation [8,[30][31][32][33]. As reported by other authors, there is conflicting evidence regarding differences related to gender of the subjects [11,32].
The present study also showed that, if an MB2 was present it was mostly not merged with the primary one there were two separate apexes and two entrances without any merging (Vertucci Class IV [29]). This finding, which was also confirmed by previous systematic reviews [8,9,34], may have clinical relevance. The detection of a second mesiobuccal canal is mandatory for complete sterilization and filling of the root canal system, in order not to leave a pathway for bacterial migration towards the apex or a reservoir for microorganisms.
Some studies investigated the use of magnification devices (microscope or magnifying loupes) as an adjuvant for the detection of a second mesiobuccal canal [34][35][36][37]. One of these studies showed that the MB2 was detected in 71.1% of maxillary first molars when using a microscope, in 62.5% of these teeth when using dental loupes, and only in 17.2% of these teeth without any magnification [36]. These results are similar to those reported in other studies and are comparable to the frequencies reported in the present review for CT.
However, the use of CBCT is considered very important for an adequate planning of endodontic surgery because of the capability to detect anatomical variations [17,38]. The use of CBCT enables reproduction of three-dimensional anatomy, allowing the evaluation of bone thickness and the relationships between root apices, lesions, and anatomical structures such as the maxillary sinus [38][39][40][41].  In the present review, the included articles seemed to be very heterogeneous in population characteristics and study methodology, limiting the external validity of the results. Also, different CBCT devices were used, which should be considered as an important confounding factor, thus limiting the ability to perform a full meta-analysis.
Despite these limitations, the present study showed that CBCT may be useful in detecting the presence of a second mesiobuccal canal in permanent first molars. This can be considered as an important adjunct in pretreatment assessment for endodontic procedures, especially in periapical surgery.