The middle mesial canal (MMC) is a variation found in mandibular molars, and its prevalence varies among different populations, exhibiting a wide range in the literature. For instance, Barros-Costa M et al. reported a prevalence of 11.1% in a Brazilian population [18], Tahmasbi et al. found a rate of 26% in an American subpopulation [19], and Liu J et al. documented a rate of 15.2% in a northern Chinese population [20]. In a transnational cross-sectional meta-analysis conducted in 2023, the worldwide prevalence of MMC in mandibular first molars was approximately 7% [17]. Another global meta-analysis yielded a prevalence of 4.15% [21]. All of these conclusions were drawn from observations using CBCT. Recently, Nosrat A. et al. conducted a large-scale clinical study on mandibular molars and reported a prevalence of 8.8% [22]. The present study included 449 patients and 898 M1Ms from northwest China, with a prevalence of 9.6% based on the number of patients and 7.2% based on the number of teeth. These findings were also analyzed through CBCT, and the results fall within the aforementioned range. Pertek Hatipoğlu F noted that variations in MMC prevalence are not only related to ethnicity but also influenced by factors such as sample size, CBCT parameters, inter-examiner differences, and interference of isthmus [17].

In this study, there was no significant difference in the prevalence of MMC between genders, consistent with the results reported in previous extensive studies [17–18, 23−24]. This underscores the importance of conducting a thorough examination for the presence of MMCs in the M1Ms of both males and females during treatment.

Similarly, in this study, there was no significant difference in the prevalence of MMC across age groups, consistent with some previous research findings [17–18, 25−26]. However, some studies have indicated that the prevalence of MMC is higher in younger patients and decreases with age [22, 27], which may be related to secondary dentin deposition. Conversely, other studies have reported that the prevalence of MMC peaks in the 41–60 age group [19], possibly due to dentin formation at the isthmus between the MB and ML root canals, allowing previously connected root canals to be separated by dentin, thereby forming another root canal. The relationship between the prevalence of MMC and age remains controversial and requires further investigations.

Professor Pomeranz initially proposed three classifications of MMC morphology based on apical radiographs. Compared with CBCT, two-dimensional images have certain limitations in some cases [28]. Barros-Costa M et al. used CBCT to analyze the morphology of MMC and divided it into four types: independent, confluent, mesiolingual confluent, and mesiobuccal confluent. They pointed out that the prevalence of the independent type was the highest, accounting for 60.7% [18]. However, this is inconsistent with the results of a recent systematic review, which indicated that the most common form of MMC is the confluent type, which is more frequently merged with MB canals [29]. Versiani MA et al. employed microCT to observe 48 M1Ms with MMC and also noted that the most common morphology is the confluent type. Furthermore, 3D images in this literature demonstrated that the morphology of MMC is significantly diverse [30]. Among the 65 teeth in this study, 19 different root canal types were identified, with the most common type being 1-3-2, and only one independent type was found. The varying results of studies on MMC morphology may be related to racial differences and require further investigation. In clinical practice, it is essential to conduct a thorough interpretation of radiological images. to ensure that the diameter and taper of root canal preparation can be rationally decided based on the morphology of MMC [31].

In addition, this study measured that the first appearance of MMC was 1.99 ± 1.60 mm below the canal orifice, and MMC mainly appeared within 4 mm beneath the orifice. This is basically consistent with the research findings reported by Alroomy R et al. [16]. However, there is little literature reporting this measurement, and more data are expected to be reported in the future. During clinical procedures aimed at identifying the MMCs, practitioners are advised to thoroughly inspect the area situated approximately 3 to 4 millimeters beneath the canal orifice.

Regarding the relationship between the presence of MMC and the MB-ML distance, there remains controversy in the literature. Akbarzadeh et al. evaluated 210 CBCT data and found that the MB-ML distance was negatively correlated with the presence of MMC [15]. Conversely, studies by Alroomy R, Barros-Costa M, et al. have shown that the average distance between MB and ML is greater when MMC is present [16, 18]. Our study also concurs with this finding, demonstrating that the MB-ML distance is larger at 3.84 mm when MMC is present, compared to 3.62 mm when it is absent. Although we cannot determine a specific MB-ML distance that definitively indicates the presence of MMC, we can infer that a larger space between MB and ML is necessary to accommodate MMC. In clinical practice, a distance close to 4 mm can serve as a reference point to raise awareness of the potential presence of MMC.

With regard to the relationship between the presence of MMC and the buccolingual width, mesiodistal width, as well as the degree of flattening of the mesial root, there are only a handful of related studies. One study has pointed out that teeth with MMC exhibit a larger buccolingual distance (8.79 mm compared to 8.44 mm), whereas no significant difference is observed in the mesiodistal distance [18]. However, our study failed to uncover a significant correlation between these factors. We argue that when detecting MMC, greater emphasis should be placed on the distance between MB and ML, rather than focusing on the anatomical morphology of the mesial root. Nevertheless, this issue necessitates further research to provide definitive proof.

This study has several limitations. First, the 250 μm voxel size used in the CBCT device may reduce sensitivity in detecting fine anatomical structures like the MMC; literature suggests that voxel sizes of 200 μm or smaller yield more accurate results [32–33]. The study sample comprises a relatively young population with an average age of 34.89 years, which limits the assessment of potential differences in MMC prevalence across age groups. Secondary dentin deposition is a well-documented phenomenon, potentially leading to the occlusion or disappearance of MMCs. In this study, there is a large variability in age, ranging from a minimum of 18 to a maximum of 94, which may affect the results. Including only patients requiring orthodontic or restorative treatment may reduce the generalizability of the results to the general population. Additionally, using only CBCT means that the advantages of higher-resolution techniques, such as micro-CT, could not be leveraged. The assessments being conducted by a single observer may lead to a lack of inter- and intra-observer reliability, which could impact reproducibility. Finally, as the study is based on a Northwest Chinese population, its generalizability regarding MMC prevalence to other ethnic groups or geographic regions is limited.