This in vivo retrospective study used CBCT scanning to investigate the root and root canal morphology of premolars, as well as correlations between non-single canals of premolars and other anatomical variants (i.e., DLRs in M1s and C-shaped canals in M2s). The frequency of PM1s with a single canal (760 of 971, 78.27%) was comparable to those reported in two systematic reviews (75.8% and 73.55%) [17, 18], as well those reported in East Asian populations (e.g., Chinese and Taiwanese [65.2–87.1%]) [7, 8, 19-21]. Regarding the root canal morphology of PM2s, a recent review reported a markedly lower incidence of a second canal (2%) in East Asian populations compared with other populations . This finding agreed with our results, which showed that only 1.6% of PM2s contained two canals. In the present study, men had significantly more root canals and C-shaped canals in PM1s than did women (Table 3); this is also consistent with previous results [9, 11, 19, 22, 23]. The findings regarding anatomical conditions and sex showed were conflicting [12, 24]. Bilateral root canal configurations were noted in a significantly higher proportion of PM1s (85.9%, p < 0.05; Table 3), in agreement with the findings of previous studies [12, 19]. In the present study, when complicated or C-shaped canals were observed in PM1s, the canals were bilateral in 68% of subjects. Thus, when non-single canals are present in PM1s, clinicians should consider the possibility of complicated canals in contralateral premolars.
In PM1s with two canals, our results indicated that the Vertucci type V configuration was more prevalent than the other types. This is consistent with the findings of previous in vivo studies that analyzed CBCT images of Chinese [7, 8, 21], German , and Turkish populations [11, 25], as well as with the findings of an in vitro study that analyzed the micro-CT data of a Chinese cohort . However, our results are not consistent with the findings of other in vitro studies [26, 27], which showed that other Vertucci types (i.e., II or IV) were more prevalent. These variations in root canal morphology may reflect differences in ethnicity, age, sex, and/or research methodology [5, 17]. Although no consensus has been reached regarding ethnic differences in the most common internal canal configuration of complicated canals in PM1s, a recent study indicated that among complicated canals, Vertucci type V was most frequent in both Asian (12.6%) and White ethnic groups (12.2%) . Information regarding the most common internal root canal morphology of PM1s could help clinicians to anticipate bifurcation at the middle third of PM1 roots with a single coronal canal.
The frequency of C-shaped root canal systems was 3.6% in our study, which was consistent with that of previous studies in which CBCT analysis was used [7, 8]. PM1s typically cause the greatest difficulty for clinicians; moreover, these show the highest rates of failure after root canal treatment . This might be due to the complexity of the root canal morphology and the appearance of the orifice in C-shaped mandibular premolars. In the present study, the canal configuration of mandibular C-shaped premolars was typically oval at the coronal third of the root (C4 configuration), whereas the C configuration was observed mainly at the middle third (Table 5). Our findings are in agreement with those of previous studies [13, 29, 30], which[17, 30, 31] reported that C-shaped canals were primarily located in the apical half of the root. However, previous investigations of C-shaped canals in M2s showed that the majority of the canal orifices had a continuous C-shape or an incomplete C configuration, whereas 0–9% were round or oval in shape [16, 31]. The coronal oval canal is a distinguishing characteristic of C-shaped canals in mandibular premolars. Clinicians should be aware of this feature and should not define canal configuration based on coronal canal morphology. For straight-line access, the use of an operating microscope is recommended to detect bifurcation and establish whether the orifice extends in the buccolingual direction [13, 14].[17, 18]
Our results regarding the frequency of DLRs in M1s (25.3%) are consistent with those of previous studies in Asian populations (22–25.9%) [4, 12, 32]. A recent study showed a positive correlation between in M1 DLRs and complicated PM1 canal configurations . In that study, C-shaped canals were categorized as complicated canals, a major difference relative to our study. In the present study, we confirmed a positive correlation between non-single PM1 canals and DLRs in M1s. We then subdivided non-single canals into complicated and C-shaped canal configurations. We found that the presence of C-shaped canals was significantly correlated with the presence M1 DLRs (p = 0.010; Table 6), whereas complicated canals did not show a significant relationship with in M1 DLRs (p = 0.107). After adjusting for sex, age, and side, we found that the frequency of C-shaped canals was 2.616-fold greater than that of single canals in subjects with DLRs. In addition, the presence of C-shaped PM1 canal configurations was significantly more frequent when bilateral DLRs were present (p < 0.05; Table 7). As noted above, C-shaped PM1 canal configurations may be difficult to recognize due to the coronal appearance, with bifurcation in the middle and/or apical third of the root. This finding of a correlation between C-shaped PM1 canals and M1s DLRs could provide an advantage to clinicians, as they might anticipate the possibility of C-shaped canals in PM1s when DLRs are observed.
There was no correlation between non-single PM1 canals and C-shaped M2 canals (p > 0.05). Notably, the frequency of C-shaped canals in M2s is very high (43.8%) in the Korean population. Clinicians should note that C-shaped canal configurations are commonly found in M2s in the Korean population regardless of the presence of C-shaped canals in PM1s. Although CBCT imaging is somewhat less accurate to detect sophisticated root canal anatomy (e.g., multiple apical foramen or accessory canal) compared to micro-CT, CBCT is recommended for future in vivo studies in different regions/races to investigate the true frequencies and morphologic characteristics of various root canal configurations and their correlations with other anatomical variants.