C-Shaped Root Canal Morphology in Mandibular Posterior Teeth—A Retrospective CBCT Study Based on Two Classification Systems

Background/Objectives: Understanding the root canal morphology is crucial for successful endodontic therapy, particularly in teeth with anatomical variations such as C-shaped canals. These canals have an unusual shape, making clinical treatment challenging due to irregularities and inter-canal connections between roots. This study aimed to determine the prevalence of C-shaped root canals in mandibular posterior teeth in a Saudi subpopulation and to characterize their morphology using Fan’s classification and the Ahmed and Dummer classification system. The distribution of C-shaped canals was further described according to tooth type, age, gender, side, and bilateral occurrence. Methods: A retrospective cross-sectional study was conducted using 228 scans acquired from December 2023 to December 2025, comprising 1961 mandibular posterior teeth. Teeth were evaluated for C-shaped canals and classified using Fan’s classification at the coronal, middle, and apical thirds, as well as the Ahmed and Dummer classification system. Data were analyzed using descriptive statistics, the chi-square test, and Fisher’s exact test. Holm–Bonferroni correction was applied for exploratory comparisons, and Cramer’s V was used as an effect size measure. Results: The overall prevalence of C-shaped canals was 2.49% (49/1961 teeth). Mandibular second molars (MSMs) showed the highest percentage (8.2%), followed by third molars (3.6%), and first premolars (0.8%). First molars and second premolars did not exhibit such morphology. Age group was not clearly associated with C-shaped prevalence (p = 0.406; Cramer’s V = 0.059). Fan’s classification showed the predominance of C1 shape coronally and C3 type in the middle/apical thirds of second molars. The most common Ahmed and Dummer morphology was (CsCIII)2 TN M/D (51%). No clear gender- or side-related pattern was observed. Tooth-level contralateral symmetry was observed in 24 out of 49 C-shaped teeth (49%). Conclusions: C-shaped root canal morphology was uncommon and occurred predominantly in MSMs. No gender-, side-, or age-related differences were observed. Since tooth-level anatomical data are presented, bilateral findings should prompt careful contralateral assessment and should not be interpreted as a conclusive indicator of bilateral occurrence.