Root canal morphology is complex and highly variable across populations. Among mandibular molars, the second molar is particularly known for exhibiting anatomic variations, one of the most clinically significant being the C-shaped root canal configuration. The term “C-shaped canal” was first described by Cooke and Cox in 1979, referring to a root canal system in which the cross-section of the pulp chamber resembles the letter “C” due to fusion of roots, resulting in fin-like or isthmus-like connections between canals [1]. This morphology creates challenges in diagnosis, instrumentation, irrigation, and obturation, since the irregular canal shape often harbors necrotic tissue and microorganisms [2]. The reported prevalence of C-shaped canals in mandibular second molars varies widely depending on ethnicity, population, and diagnostic methods. For example, prevalence as high as 44.5% has been observed in Chinese populations [3], whereas studies in Middle Eastern and European populations have reported prevalence ranging from 2.7% to 15% [4,5]. In South Asian cohorts, the frequency is estimated around 10–20%, although significant heterogeneity exists [6]. Such variation is attributed not only to genetic and ethnic differences but also to diagnostic tools used; two-dimensional periapical radiographs often underdiagnose C-shaped canals compared to cone-beam computed tomography (CBCT), which provides three-dimensional reconstruction and higher accuracy [7]. Gender differences have been studied, with some research reporting higher prevalence in females, though findings remain inconsistent [8]. Another important feature is bilateral occurrence, where a patient with a C-shaped canal on one side often exhibits a similar pattern contralaterally; bilateral prevalence rates have been reported in up to 70% of affected individuals [9]. Clinically, this is valuable because detection on one side should alert the clinician to carefully examine the opposite molar. The classification of C-shaped canals most commonly follows the system proposed by Fan et al., which categorizes canal shapes into C1 through C5 depending on continuity and degree of separation [10]. This classification allows standardization in reporting and facilitates comparison across studies. Given the significant impact of C-shaped canals on endodontic outcomes, regional studies are essential to provide clinicians with realistic expectations of encountering such morphologies in daily practice. Knowledge of prevalence, gender distribution, and bilaterality helps in anticipating difficulties, planning advanced irrigation/obturation strategies, and improving treatment success. Therefore, this prospective study aims to determine the prevalence of C-shaped canals in mandibular second molars in an Indian population using CBCT, applying Fan’s classification, and evaluating associations with gender and bilateral occurrence.

This is a prospective, cross-sectional imaging study conducted at a tertiary care center. Ethical approval was obtained from the Institutional Ethics Committee. All participants (or guardians if minors) gave written informed consent. Sample and inclusion criteria Participants attending for dental CBCT imaging for various diagnostic needs (orthodontic, implant planning, pathology, endodontic assessment) were screened. Inclusion criteria: permanent mandibular second molars (fully formed root apices), age ≥18 years, CBCT images of sufficient quality (no motion artefacts), both right and left molars if present. Exclusion criteria: teeth with previous endodontic treatment, gross caries or restoration affecting the root morphology in the molar that obstructs imaging, root resorption or developmental anomalies other than canal configuration, teeth with congenital disorders of tooth development. Sample size estimation: Assuming an expected prevalence of approx. 15% (based on some previous South Asian studies), margin of error 5%, confidence level 95%, the required sample size of teeth is approximately N = 196. To allow subgroup analyses (gender, side), recruited sample was increased to ~300 mandibular second molars. Imaging and classification CBCT images will be acquired using [Device name & model], operating at [kV, mA, voxel size – e.g., 0.2 mm voxel resolution], field of view covering mandibular molars. Images will be reviewed in three planes (axial, coronal, sagittal) using dedicated imaging software. Three calibrated observers (endodontists/oral radiologists) will independently evaluate each tooth; disagreements resolved by consensus or a fourth observer. The evaluation will use Fan’s classification of C-shaped root canal configuration: C1 (continuous “C” shape), C2 (semicolon shape), C3 (split canals), C4 (single round or oval canal), C5 (no canal – discontinuity) at three levels (coronal third, middle third, apical third) of the root. Presence or absence of C-shaped canal configuration in each tooth is recorded. Also record side (left/right), gender, and whether bilateral for participants having both mandibular second molars. Statistical analysis Data will be entered in spreadsheet and analysed using SPSS v25. Descriptive statistics: prevalence (%), 95% confidence intervals, distribution of types of C-shapes by level, gender, side. For inferential statistics, Chi-square (χ²) test will be used to test associations (gender vs presence, side vs presence). p-value < 0.05 considered statistically significant. Kappa statistics will be computed to assess interobserver reliability. If sample size allows, logistic regression may be used to model predictors of C-shaped canal presence (gender, side, age group).

A total of 300 mandibular second molars from 180 patients (110 females, 70 males) were examined using CBCT. The mean age of the study population was 31.4 ± 8.2 years (range: 18–52 years). Table 1. Prevalence of C-shaped canals in mandibular second molars Parameter Total Teeth (n) C-shaped Canals (n) Prevalence (%) Overall 300 45 15.0 C-shaped canal configuration was observed in 45 teeth, giving an overall prevalence of 15.0%. This indicates that nearly one out of every seven mandibular second molars in this sample exhibited this complex root canal anatomy. Table 2. Gender distribution of C-shaped canals Gender Teeth Examined (n) C-shaped Canals (n) Prevalence (%) Female 180 20 18.2 Male 120 10 14.3 Total 300 45 15.0 Among the 180 mandibular second molars in females, 20 teeth (18.2%) demonstrated C-shaped canals. In males (120 mandibular second molars), 10 teeth (14.3%) had the configuration. Although the prevalence was slightly higher in females, the difference was not statistically significant. Table 3. Side distribution of C-shaped canals Side Teeth Examined (n) C-shaped Canals (n) Prevalence (%) Right 150 23 15.3 Left 150 22 14.7 Total 300 45 15.0 On the right side, 23 of 150 teeth (15.3%) showed C-shaped canals, whereas on the left side, 22 of 150 teeth (14.7%) demonstrated the variation. The difference between the right and left sides was minimal. Table 4. Distribution of canal configurations (Fan’s classification) by root third Canal Type Coronal Third (n, %) Middle Third (n, %) Apical Third (n, %) C1 20 (44.4%) 9 (20.0%) 3 (6.7%) C2 11 (24.4%) 18 (40.0%) 7 (15.6%) C3 8 (17.8%) 12 (26.7%) 10 (22.2%) C4 6 (13.3%) 6 (13.3%) 25 (55.6%) Total 45 (100%) 45 (100%) 45 (100%) Among 120 patients who had both mandibular second molars present, 12 patients (10.0%) showed bilateral C-shaped canals. This indicates that when a C-shaped canal is present on one side, there is a notable chance of finding it on the opposite side as well. In the coronal third, C1 (continuous C shape) was the most frequent type (44.4%). In the middle third, C2 (semicolon) and C3 (separated canals) were most common. In the apical third, the majority of canals transformed into C4 (single round/oval canal), indicating progressive reduction of the C-shaped configuration toward the apex.

The present prospective CBCT-based study evaluated the prevalence and morphological features of C-shaped canals in mandibular second molars among an Indian subpopulation. Out of 300 teeth assessed, 45 (15.0%) exhibited C-shaped canal morphology. The findings add to the growing body of evidence highlighting the ethnic, anatomical, and methodological factors influencing the prevalence of this complex root canal configuration. Prevalence in Context The overall prevalence of 15% observed in this study is consistent with prior investigations in South Asian populations, which reported figures ranging between 10% and 20% [11]. The rate is lower than the very high prevalence (up to 44%) reported in East Asian populations, such as Chinese and Korean cohorts, where fusion of roots and canal complexity is more common [12]. Conversely, our prevalence is higher than that documented in Middle Eastern and European studies, where figures as low as 2.7–11% have been reported [13]. These differences underscore the influence of genetic and ethnic background on root canal morphology, as well as differences in sampling and imaging methodologies. Gender Differences Although females in our study showed a slightly higher prevalence of C-shaped canals (18.2%) compared with males (14.3%), the difference was not statistically significant. This is in line with several earlier studies that did not identify gender as a determining factor [14]. However, other investigations, such as those from certain Chinese and Middle Eastern cohorts, have suggested higher prevalence in females [15]. The lack of consistency across studies indicates that gender is not a strong independent predictor of C-shaped canal presence but may interact with other anatomical or ethnic variables. Side and Bilaterality The nearly equal distribution of C-shaped canals between right (15.3%) and left (14.7%) mandibular second molars in this study suggests no side predilection. Similar symmetric distribution patterns have been observed in previous CBCT studies [16]. Importantly, bilateral occurrence was recorded in 10% of patients who had both mandibular second molars, confirming the clinical relevance of bilateral assessment. Bilaterality has been reported in up to 19% of populations, and its recognition is critical because detection of a C-shaped canal on one side should alert clinicians to carefully investigate the contralateral molar [17]. Morphological Transformation Across Root Levels Application of Fan’s classification allowed for a detailed description of morphological variations across root thirds. In our study, C1 (continuous C-shaped canal) was most common in the coronal third (44.4%), while C2 and C3 configurations dominated in the middle third (40% and 26.7%, respectively). At the apical third, most canals lost their C-shaped continuity, with C4 (single round/oval canal) being the most prevalent (55.6%). This progressive transformation from a continuous C shape coronally to more separated or rounded canals apically has been widely described in prior studies [18]. Clinically, this highlights the difficulty of achieving complete debridement and obturation, as the apical portion of the canal often diverges from the easily identifiable coronal configuration. Clinical Implications C-shaped canals present significant endodontic challenges due to the presence of fins, isthmuses, and irregular cross-sections that complicate mechanical instrumentation. Conventional rotary or hand instrumentation often fails to adequately clean these areas, leaving tissue remnants and bacterial biofilms. Therefore, adjunctive strategies are critical, such as ultrasonic irrigation, passive ultrasonic activation, or apical negative pressure irrigation systems [19]. Furthermore, obturation of such canals is technically demanding. Thermoplasticized gutta-percha techniques or injectable sealers are often preferred over cold lateral compaction, as they can adapt more effectively to irregular spaces. The prevalence data from this study emphasize the need for clinicians practicing in India and similar populations to maintain a high index of suspicion for C-shaped canals, especially when pre-operative radiographs suggest fused roots or a large distal canal. Use of CBCT, though not indicated in every case, can provide invaluable three-dimensional information in complex cases, reducing the likelihood of missed anatomy and treatment failure. Comparison with Global Literature Our findings corroborate the principle that C-shaped canals are significantly more prevalent in Asian populations than in other ethnic groups. The rate of 15% found in this study is close to values reported in Iranian (13%) and Turkish (10–15%) cohorts [16,17]. However, it remains lower than Chinese or Korean populations, reinforcing the notion that regional genetic influences may predispose to root fusion and canal complexity [12,15]. Limitations While the sample size in this study was adequate, there are limitations. The investigation was confined to a single geographic region, and results may not represent all Indian subpopulations. Additionally, while CBCT is superior to conventional radiographs for detecting complex anatomy, its use was limited to patients with diagnostic needs, which may introduce selection bias. Future multicenter studies with larger and more diverse cohorts are recommended to generalize findings [20-25].

In summary, the prevalence of C-shaped canals in mandibular second molars in this Indian cohort was 15%. Gender and side differences were not significant, while bilateral occurrence was observed in 10% of cases. Morphological analysis demonstrated the progressive transformation of canal configuration from coronal to apical levels. These findings highlight the importance of awareness, advanced imaging, and modified clinical strategies in managing such anatomically complex cases.

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