The MMC is an anatomical variation relevant to mandibular molars, and its identification is important to successful endodontic treatment. The non-obturation of all root canals is a factor that could persist the symptoms of apical periodontitis in patients submitted to endodontic treatment [3, 4]. The known anatomical of the root system is important to plan the endodontic treatment and some variations of the common anatomy can be found in teeth and make their treatment difficult. So it is important to know the prevalence of these variations to understand the probability of the clinicians finds challenging endodontic treatment. This study assessed the prevalence of MMC in a Brazilian subpopulation in mandibular first and second molars and the anatomical aspects of the teeth and found 11.1% and 1.75%, respectively.
The presence of the MMC was first introduced by Pomeranz in 1981; the prevalence of this third canal in mesial root of mandibular first molars is being referred in the literature differently, which makes its study important. Qiao et al. (2020) reported the lowest prevalence of MMC in mandibular first molars, being 3.41% in a Chinese subpopulation [14], followed by Kuzekanani et al. (2020), who found 8.1% in the Kerman subpopulation [13]. The closest prevalence to that of the present study was found by Weinberg et al. (2020) and Akbarzadeh et al. (2017), who found 13.73% and 14.7% in American subpopulations, respectively [11, 12]. Higher prevalence values were also reported in the literature, ranging from 22.0–37.5% [8–10]. Tahmasbi et al. (2017) reported a 26% prevalence of MMC in an American subpopulation as well [10]. As the present study, these five previous studies have also used CBCT scans for analysis; however, the spatial resolution varied among the studies.
In contrast, other studies have assessed the prevalence of MMC based on their findings during root canal treatment. Nosrat et al. (2015) found a 22% prevalence of MMC using a dental operating microscope, while Azim et al. (2015) found MMC in 37.5% of their sample with a similar method, both in American subpopulations [8, 9]. The main source of discrepancies among the prevalence of different studies may be the studied population. Other important hypotheses are the method used to evaluate the presence of MMC and the sample size, which varied among the studies. Although we have raised the method as a possible cause for variation because it appears that the prevalence with dental operating microscope tends to be higher, a study showed that dental operating microscope and CBCT imaging are equally effective in detecting the presence of MMC [9]. Considering the studies that used CBCT scans, the voxel size could also influence the results, since a recent study found that smaller voxel size increased the detection of second mesiobuccal canals in maxillary molars [23]. It is important to note that the present study used high spatial resolution CBCT scans, which may favor the visualization of the root system. Further studies in different countries should be carried out to compare variations in the prevalence of MMC among the global population.
We found that the prevalence of MMC in second molars was significantly lower, at only 1.75%. This significance may alert the clinicians to the possibility of an anatomical variation to mandibular first molars, which are the teeth most subjected to endodontic procedures [1, 2]. Only a few studies have reported the prevalence of MMC separately for mandibular second molars, and the values also varied considerably, being 8%, 16% and 60% [11, 23, 14], probably due to the factors raised for the first molars. Of those, only one tested whether the distribution of MMC cases was different between molar types; although the raw data pointed out that the prevalence was higher in first molars than in second molars (22% versus 16%), the difference was not statistically significant, which is opposite to our findings.
The CBCT is a suitable imaging method to evaluate teeth with complex anatomy. In endodontics, the use of CBCT is commonly requested when patients had persisted symptoms and the intraoral radiography is limited in diagnosing. The justification principle needs to be clear for the clinicians before this request, that is, the benefits succeed the potential risks [16–18]. In cases of mandibular first molars with persisted symptoms after endodontic treatment, the dentist can suspect of the presence of a non-obturated canal. The intraoral radiography can be the first imaging method to assess the origin of the persisted apical periodontitis, but in some cases, this method could be limited and the CBCT should be considered. Regardless, other technical errors could also be the origin of persisted apical periodontitis, as underfilling canal and even vertical root fractures [3, 4]. Therefore, the clinicians need to know the indication of the exams, clinical signs, previous medical history to be more accurate about the diagnosis.
The MMC classification by Pomeranz et al. (1981) was the first and is currently used [15]. The authors classified this canal using intraoral radiographs, which could be limited to an appropriate visualization of the root canal system. The CBCT allows a multisectional and dynamic evaluation of the root canal system and, when compared to bidimensional radiography, CBCT has a more accurate visualization for endodontic examination [20, 21]. Still, CBCT allows enhancement of brightness, contrast, and application of filters during the evaluation. This dynamic evaluation allows clinicians to explore more accurately the morphology of the root system [19–21]. In this study, the evaluation was performed in all multiplanar reconstructions and the application of filters, enhancement of brightness and contrast was used for the examiner to achieve an appropriated visualization. Thus, this evaluation was carefully performed to visualize the root canal path, the pulp chamber orifice and apical orifice to define the proposed classification. The use of three-dimensional examinations in the present study allowed the Pomeranz’s classification to be further developed since it was possible to evaluate the type of confluence that the MMC showed.
In the present study, the MMC presented four different types with a variable frequency. The most frequent was Independent, with 60.7%, and the less frequent was Mesiobuccal confluent. The Independent MMC represents a canal with a separate orifice and independent apical foramen. This configuration represents less complexity when compared to canals with ramifications and confluences [1]. Few studies have evaluated the MMC configuration and found that the confluent type was the most prevalent [13, 14]. However, it appears that the configuration was evaluated using periapical radiographs rather than using 3D imaging as the present study. Assessment of the canal system using 2D imaging may mask independent canals by the overlapping nature of 2D and may lead to misinterpretations. Some studies, based on CBCT imaging, reported that the more complex the root system configuration, the greater the occurrence of endodontic technical errors [4, 24, 25]. Further research is encouraged to verify if the complexity of MMC is related to the occurrence of endodontic technical errors. Furthermore, applying this classification in different imaging methods, such as micro-CT and different populations could be considered.
This study evaluated the anatomical aspects aiming to correlate the presence of MMC with some of them, because this topic is still controversy in the literature. While one study concluded that there does not seem to be a correlation between the presence of MMC and mesial intracanal distance [9], other investigation found that distance is shorter in teeth with MMC [10]. The latter finding does not seem reasonable, because the distance between mesiobuccal and mesiolingual canals should be larger to fit the third canal, or at least it should be similar to that of teeth without MMC. Our data reinforced our hypothesis about the need for more space in the tooth to fit the third canal, since both mesial root width and the distance between mesiobuccal and mesiolingual canals were statistically significantly greater in teeth with the MMC, and the intracanal distance was also indicated as a predictor of the presence of MMC in the regression analysis. It is not possible to establish a measurement at which MMC will be present, but these findings may alert the clinicians when the presence of MMC should be suspected during the three-dimensional evaluation of the CBCT scan. In the clinical routine, the use of the operating microscope can also evaluate the distance between the regular canals and alert the clinicians.
Although the present study used high-resolution scans, CBCT examination is not the gold standard for the presence of MMC; other imaging methods, such as micro-CT and nano-CT could be more specific in defining the anatomy. Nonetheless, these methods are exclusive to laboratory analysis and could not be used in patients such as those in the present study. However, studies with extracted mandibular molars could be performed to analyze MMCs and describe their anatomy in these imaging modalities. Despite the limitations, the prevalence of MMC is relevant for clinicians’ knowledge because it could predict the possibility of encountering this variation during the endodontic procedure.