Prevalence of non-single canals in mandibular premolars and correlations with other anatomical variants: an in vivo cone beam computed tomography study

Background A knowledge regarding anatomical variants is important to achieve success in endodontic treatment. Root canal treatment of mandibular rst premolars (PM1s) is challenging due to the existence of numerous variations in canal congurations, including a C-shaped variant. We aim to determine the prevalence and morphologic characteristics of non-single canals of mandibular rst (PM1s) and second (PM2s) premolars in a Korean population using cone beam computed tomography (CBCT) and to evaluate correlations between non-single canals of PM1s and other anatomical variants, such as distolingual roots (DLRs) in mandibular rst molars (M1s) and C-shaped canals in mandibular second molars (M2s). Methods A total of 971 PM1s and 997 PM2s from 500 patients were examined in vivo by CBCT. Root canal congurations and C-shaped canals were determined in accordance with the Vertucci classication and Fan classication, respectively. The correlation between non-single canals in PM1s and DLRs in M1s was evaluated using logistic regression analysis. Results PM2s typically had one root (99.89%) with one canal (98.4%). Among PM1s with non-single canals (21.2%), Vertucci type V (10.9%) and C-shaped (3.7%) canals were prevalent. Among C-shaped PM1 canals, the majority were Vertucci type V (77.8%); a C-shaped conguration (C2) was predominant mostly at the middle and/or apical third of the root. After adjusting for other variables (i.e., sex, age, and side), C-shaped canals in PM1s was signicantly correlated with the presence of DLRs in M1s (odds ratio = 2.616; 95% condence interval, 1.257–5.443; p = 0.010). Conclusions The presence of C-shaped PM1 canals was positively related to the presence of DLRs in M1s. Although C-shaped canals in PM1s are dicult to distinguish, this nding could aid clinicians in predicting C-shaped canal congurations in PM1s of


Background
A lack of knowledge regarding anatomical variants may result in untreated canal space, potentially leading to endodontic treatment failure [1]. Mandibular premolars often exhibit complex anatomy that cannot be clearly detected in two-dimensional periapical radiographs [2,3]. Previous studies have shown high prevalences of non-single canal systems in mandibular premolars (12.9-34.8% and 2-9.9% in mandibular rst and second premolars, respectively). [4][5][6][7][8][9] These variations are caused by differences in methodology, ethnicity, and characteristics of participants, such as age and sex [4,5].
In vivo cone beam computed tomography (CBCT) has comparable accuracy to that of micro-computed tomography (micro-CT) for detecting root canal morphology [6]. Previous studies have suggested that CBCT analysis could be useful for determining root canal anatomy [7][8][9][10][11][12]. Notably, its noninvasive application allows collection of a greater number of samples than in previous studies that were limited to the use of extracted teeth; these samples are thus su ciently large to be representative of the general population. In addition, differences in sex, side, and relationships with other variations in root canal morphology can be easily compared in CBCT scans.
Endodontic treatment of mandibular rst premolars (PM1s) is challenging due to the existence of numerous variations in canal con gurations, including a C-shaped variant; moreover, the relatively small diameter of PM1s limits direct access to additional canals [13,14]. To the best of our knowledge, little information is available regarding the prevalence of C-shaped canals in PM1s (as shown in CBCT images) and their correlations with other anatomical variants, such as distolingual roots (DLRs) in mandibular rst molars (M1s) and C-shaped canals in mandibular second molars (M2s). Therefore, this study aimed to retrospectively investigate the roots and root canal con gurations in PM1s and PM2s in a Korean population using a large number of CBCT images and to assess correlations between non-single canals in PM1s and other anatomical variations, including DLRs in M1s and C-shaped canals in M2s.

Subjects
We retrospectively included 500 subjects from among 1,393 patients who met the following inclusion criteria: 1. Age between 18 and 69 years; 2. Imaging data available, including scans of fully erupted mandibular premolars; 3. Presence of mandibular premolars with fully matured apices and without apical periodontitis; and 4. Presence of mandibular premolars without root canal llings, posts, or crown restorations.
The protocol of this retrospective study was approved by the Ethics Committee of Ewha Womans University Hospital, Seoul, Korea (no. EUMC 2018-01-064). Images of mandibular premolars were obtained from patients who had undergone CBCT scanning at the hospital between January 2011 and November 2012. CBCT images were acquired using a Dinnova system (Willmed, Gwangmyeong, Korea) with the following parameters: 80 kVp, 9.0 mA, 10 × 10-cm eld of view, and 0.167-mm 3 voxel size. Cross-sectional images in the axial, coronal, and sagittal planes were reconstructed using OnDemand3D software (Cybermed, Seoul, Korea). CBCT scans were generally acquired for implant surgery or surgical removal of impacted molars. Therefore, no subjects in this study were exposed to unnecessary radiation to obtain information regarding root canal anatomy; moreover, the "as low as reasonably achievable" principle was followed with respect to radiation dose. Before examining the images, data were anonymized by numbering the subjects from 1 to 500 to prevent any possible bias.

Image assessment
Axial-, coronal-, and sagittal-plane CBCT images were closely examined based on the reconstruction parameters. The numbers of roots and root canals were recorded, as were the con gurations of the root canals and the prevalences of unilateral and bilateral root canal con gurations.
The numbers of roots in PM1s were determined by examining axial-plane images. Single-rooted teeth had conical-shaped roots; these included teeth with two canals with a fused root. Double-rooted teeth exhibited bifurcation at a certain root level; these included teeth with two canals in a single fused root and a third canal in a separate root. Triple-rooted teeth exhibited three independent roots. The presence of DLRs in M1s and C-shaped M2s was also examined in axial-plane images. Root canal con gurations were determined in accordance with the Vertucci classi cation [15]. Thus, root canals other than Vertucci type I were regarded as "non-single canals"; root canals with more than one canal, except C-shaped canals, were regarded as "complicated canals." C-shaped canal con gurations were determined in accordance with the Fan classi cation [16], as follows: C1 (continuous C-shaped canal: an uninterrupted "C" without separation or division); C2 (semicolon-shaped canal: caused by discontinuation of the "C" outline); C3 (separated canals: two or three separate canals); C4 (a single canal subdivided into round (C4a), oval (C4b), or at canals (C4c)); C5 (≥3 separate canals); or C6 (no visible canal lumen). The distribution of radicular grooves was noted. The presence of a C1 or C2 con guration at any position of the root canal was taken to indicate a C-shaped root canal system.
All images were independently assessed by two endodontists who were experienced in CBCT imaging. Both experts viewed the images on a 27-inch monitor (SE2717H; Dell, Round Rock, TX, USA) with a screen resolution of 1920 × 1080 and 32-bit color depth. Kappa values for intra-and interobserver reliability were calculated by evaluating 60 randomly selected images twice, with an interval of 1 week between evaluations; intra-and interobserver values were 0.85 and 0.79, respectively, which were considered adequate. After calibration, all study subjects were examined independently. The examiners magni ed the images as necessary for proper assessment of both root and root canal morphology. In instances of disagreement, images were re-evaluated and discussed until a consensus was reached.

Statistical analysis
Statistical analyses were performed using SPSS software (ver. 21; SPSS, Inc., Chicago, IL, USA). The chi-squared test was performed for analyses of differences based on sex, tooth location (left or right side), and bilateral PM1 canal con gurations. The chi-squared test was also used to compare the prevalences of DLRs in M1s and C-shaped canals in M2s according to PM1 root canal con guration and to compare unilateral and bilateral M1 DLRs and M2 C-shaped canal con gurations.
To evaluate correlations between root canal con gurations and other anatomical variants, multivariate logistic regression and multinomial logistic regression analyses were used, with adjustments for sex, age, and side. Multivariate logistic regression analysis was used to assess correlations of root canal con gurations (single and non-single canals) in PM1s with other anatomical variants. Multinomial logistic regression analysis was used to evaluate correlations of root canal con gurations (single, complicated, and C-shaped canals) in PM1s with other anatomical variants.

Canal con guration by sex and bilateral distribution of root canal con guration in PM1s
Compared with women, men more frequently showed complicated canal con gurations and C-shaped canals in PM1s (p < 0.001; Table 3). No left-or right-sided predominance was detected among Vertucci root canal con gurations or C-shaped canals (p = 0.95). In PM1s, the respective prevalences of bilateral root canal con guration for a single canal, complicated canals, and C-shaped canals were 93.2% (700 of 751), 69% (116 of 168), and 62.9% (22 of 35). The overall bilateral prevalence rate was 87.8% (838 of 954 canals, p < 0.001). Of the complicated canals on both sides (69%), 60.7% of root canals had the same Vertucci type, and 8.3% had different Vertucci types. With respect to unilateral C-shaped canals, more than half of the contralateral canals were complicated canals (Table 3).

C-shaped canals in PM1s
All C-shaped PM1 canals showed a radicular groove or concavity of the external root surface; most grooves (94.4%) were on the mesiolingual surface of the root. Root canal con guration varied with respect to vertical position in the root. In the coronal third of the root, most canals were single canals (C4) ( Table 4); a C2 con guration was present mostly in the middle and/or apical thirds. An axial view of a C-shaped root PM1 canal is shown in Fig. 2.
Chi-squared analysis revealed that the prevalence of M1 DLRs varied signi cantly according to root canal con guration (p < 0.05), whereas the prevalence of C-shaped M2s did not (p = 0.181, Fig. 4). Thus, logistic regression analysis was used to assess the correlation between M1 DLRs and PM1 root canal con guration.
Multivariate logistic regression analysis showed that, after adjusting for other variables (i.e., sex, age, and side), the presence of non-single PM1 canals was signi cantly correlated with the presence of M1 DLRs (odds ratio [OR] = 1.539; 95% con dence interval [CI], 1.077-2.200; p = 0.018, Table 5). Multinomial logistic regression analysis showed that, after adjusting for other variables (i.e., sex, age, and side), the presence of C-shaped canals in PM1s was signi cantly correlated with the presence of DLRs in M1s (OR = 2.616; 95% CI, 1.257-5.443; p = 0.010, Table 6); conversely, the presence of complicated canals in PM1s was not associated with the presence of DLRs in M1s (p = 0.107, Table 6). The prevalence of C-shaped canals in PM1s was positively related to the bilateral presence of DLRs in M1s (p < 0.05, Table 7).

Discussion
This in vivo retrospective study used CBCT scanning to investigate the root and root canal morphology of premolars, as well as correlations between nonsingle canals of premolars and other anatomical variants (i.e., DLRs in M1s and C-shaped canals in M2s). The prevalence 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][20][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 [18]. This nding agreed with our results, which showed that only 1.6% of PM2s contained two canals. In the present study, men had signi cantly 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 ndings regarding anatomical conditions and sex showed were con icting [12,24]. Bilateral root canal con gurations were noted in a signi cantly higher proportion of PM1s (85.9%, p < 0.05; Table 3), in agreement with the ndings 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 con guration was more prevalent than the other types. This is consistent with the ndings of previous in vivo studies that analyzed CBCT images of Chinese [7,8,21], German [9], and Turkish populations [11,25], as well as with the ndings of an in vitro study that analyzed the micro-CT data of a Chinese cohort [20]. However, our results are not consistent with the ndings 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 re ect 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 con guration 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%) [4]. 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 prevalence 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 di culty for clinicians; moreover, these show the highest rates of failure after root canal treatment [28]. This might be due to the complexity of the root canal morphology and the appearance of the ori ce in C-shaped mandibular premolars. In the present study, the canal con guration of mandibular C-shaped premolars was typically oval at the coronal third of the root (C4 con guration), whereas the C con guration was observed mainly at the middle third (Table 5). Our ndings 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 ori ces had a continuous C-shape or an incomplete C con guration, 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 de ne canal con guration based on coronal canal morphology. For straight-line access, the use of an operating microscope is recommended to detect bifurcation and establish whether the ori ce extends in the buccolingual direction [13,14]. [17,18] Our results regarding the prevalence of DLRs in M1s (25.3%) are consistent with those of previous studies in Asian populations (22)(23)(24)(25).9%) [4,12,32]. A recent study showed a positive correlation between in M1 DLRs and complicated PM1 canal con gurations [12]. In that study, C-shaped canals were categorized as complicated canals, a major difference relative to our study. In the present study, we con rmed a positive correlation between non-single PM1 canals and DLRs in M1s. We then subdivided non-single canals into complicated and C-shaped canal con gurations. We found that the presence of C-shaped canals was signi cantly correlated with the presence M1 DLRs (p = 0.010; Table 6), whereas complicated canals did not show a signi cant 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 con gurations was signi cantly more frequent when bilateral DLRs were present (p < 0.05; Table 7). As noted above, C-shaped PM1 canal con gurations may be di cult to recognize due to the coronal appearance, with bifurcation in the middle and/or apical third of the root. This nding 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 prevalence of C-shaped canals in M2s is very high (43.8%) in the Korean population. Clinicians should note that C-shaped canal con gurations are commonly found in M2s in the Korean population regardless of the presence of C-shaped canals in PM1s. CBCT is recommended for future in vivo studies in different regions to investigate the true prevalences and morphologic characteristics of various root canal con gurations and their correlations with other anatomical variants.

Conclusions
This retrospective in vivo study showed that more than one-fth (21.2%) of PM1s in a Korean population had ≥2 root canals. The most common canal con gurations were Vertucci type V (10.9%), followed by C-shaped canals (3.7%); the prevalence of complicated canals in PM2s was lower (1.6%). We found that the presence of C-shaped PM1 canals was signi cantly correlated with the presence of DLRs in M1s. This nding could help clinicians to predict the presence of C-shaped canal con gurations in the PM1s of patients who exhibit DLRs in M1s.    *The signi cance level is P = .05.     Frequencies of distolingual roots in mandibular rst molars and C-shaped canal con gurations in mandibular second molars according to unilateral or bilateral status.