The most common form of a mandibular first molar is the presence of two roots with three canals: two canals located in the mesial root and one canal in the distal root. There is little difference between countries and ethnic groups [5, 27-31]. In our study, the most common root canal configuration of the mandibular first molar was type IV (65.84%) in the mesial roots and type I (78.19%) in the distal roots. The result is in accordance with the findings of most earlier studies [5, 16, 27, 29-35].
As mentioned before, the detection rates of the MMC in mandibular first molars varied among studies, ranging from 0.26%-46.15% [16, 17]. This varied detection rates can be due to the varying ethnic groups and ages as well as the study design or methods of detection. In China, this anatomical variation has been found to range from 0.8% to 22% [36-39]. In the present study, the detection rate of the MMC was only 3.41%. The mean distances between the MMC and MBC orifice and the MMC and MLC orifice were 1.84 mm and 1.95 mm, respectively. These distances were obviously longer than those detected by Versiani et al. , who found that the average distances were 1.35 mm and 1.34 mm, respectively. In addition, Akbarzadeh et al.  showed that in molars with MMC, the mean distance between the MBC and MLC orifice was 3.1 mm, while the distance was 3.7 mm in those without MMC; But in a most recent study, that the mean distance between the mesial canals was 3.643 mm in teeth with MMC, and 3.818 mm in teeth without MMC, and Weinberg et al. suggested that there is no significant difference between above-mentioned distance in molars with or without MMC. But these distances are also apparently shorter than those detected in our study. Therefore, clinicians can carefully explore the MMC within a wide range between the MBC and MLC to avoid missing root canals in western Chinese population as well as in Chinese populations. In addition, the analysis of data by Nosrat et al.  showed that the incidence of MMC among different age groups has significant difference. They found that the incidence of MMC was 32.1% , 23.8%, 3.8% in patients ≤20 years old, 21-40 years old, and >40 years old, respectively. This finding suggests that clinicians should be more careful and spend more time searching for an MMC when handling younger patients.
In the clinic, MMCs are difficult to find and treat. Versiani et al.  reported that the mean minor diameter of the MMC (0.16 mm) orifice was significantly smaller than that of the MBC (0.46 mm) and MLC (0.50 mm) orifices; it was always too small to detect and was also prone to root strip perforation during instrumentation. In addition, according to Akbarzadeh et al. , there could be an isthmus in 87% of mandibular first molars with an recognizable MMC. Due to the existence of an isthmus in most cases with an MMC, it is difficult to completely clean up the microbial biofilm attached to the isthmus, which can lead to failure in endodontic treatment. Interestingly, Tahmasbi et al.  also found that an MMC originated from a separate apical foramen in only 2.4% of the total cases. They proposed that the omission of an MMC in mandibular molars may not be severe and would not definitely contribute to the failure of endodontic treatment in contrast to the omission of a second MB canal in maxillary molars .
In terms of the RE, a previous study by Zhang et al.  in western Chinese population, in which CBCT images of 232 mandibular first molars were detected, showed a detection rate of 30% for distal extra roots, and all of the extra roots had a type I configuration. Another study in western Chinese population  found that of the 558 CBCT images of mandibular first permanent molars, 24.7% exhibited a distolingual root. The results of the present study showed a detection rate of 22.15% (260/1174) for the total teeth and a detection rate of 25.04% (147/587) for individuals, which is consistent with the results described above. These results suggest that the prevalence of a RE in western Chinese populations is relatively high, and one out of every four people may have a distolingual root. Therefore, clinicians should carefully and consciously explore the presence of distolingual roots during endodontic treatment.
According to Schneider’s method, Chen et al.  found that most REs had a more severe curvature in the BL orientation (36.35°± 9.38°) than in the MD orientation (9.24°±6.10°). The present study evaluated 487 mandibular first molars with an RE using Schneider’s method and showed that the angles of the RE root curvature in the BL and MD orientations were 40.69±14.37° and 17.58±7.84°, respectively; it was also found that most RE canals exhibited severe curvature in the BL orientation and exhibited moderate curvature or a straight appearance in the MD orientation, which is in agreement with previous studies. It is usually known that there is an increased risk of instrument fracture with an increasing angle of curvature [45, 46]. Attention should be taken to avoid instrument fracture or perforation for the treatment of this additional unexpected canal since they are usually short and severely curved. The present study also found that the mean inter-orifice distances from the DLC to the DBC and from the DLC to the MLC were 4.22 and 4.01 mm, respectively. These distances are higher than those reported by Zhang et al. , in which the distances found between the two distal canal orifices in most (65.2%) mandibular first molars with an RE were 2.5-3.5 mm. This suggests that clinicians could try to explore the presence of an orifice of the RE canal within a larger range in western Chinese populations, and a highly trapezoidal cavity may be helpful for locating the RE canal orifice .
It is also necessary to learn the similarity and symmetry of the number and morphology of roots and root canals between the left and right sides if bilateral mandibular first molars are to be treated. Of the 587 patients we studied, 91.14% had bilateral symmetry in terms of the number of roots, 84.50% had bilateral symmetry in terms of the number of root canals, and 71.72% had bilateral symmetry in terms of the root canal morphology (both sides had the same canal configuration according to Vertucci’s classification in each root). In addition, our study showed that the right molars had a higher incidence of having two roots (80.4%), while the left molars had a higher incidence of having three roots (28.1%), which is consistent with the results of Wang et al. . In addition, the frequency of bilateral occurrence of the MMC was only 0.05%, with no significant differences between the right and left sides. The frequency of bilateral occurrence of the RE was 76.87%, and there was a statistically significant difference between the right and left sides. The incidence of a RE according to the side of occurrence still has some dispute. Some studies have found a right-side predominance [24, 48, 49], whereas other investigators have reported REs seen more frequently on the left side [32, 50-52]. The results of the current study support the latter evidence. These contradictory findings might be attributed to the ethnic backgrounds, sample sizes and methods used.