Prevalence and root canal configuration of C-shaped canals in mandibular first and second molars: a retrospective cross-sectional CBCT study in a northern Iranian population

Abstract Background C-shaped canal systems show marked anatomical variation along the root and can complicate cleaning, shaping, and obturation. This study estimated their patient- and tooth-level prevalence and described their cross-sectional configurations in mandibular first and second permanent molars. Materials and methods This retrospective cross-sectional study evaluated 435 eligible mandibular molars (157 first and 278 s molars) from 209 patients whose pre-existing CBCT examinations had been obtained for clinical reasons at a private oral and maxillofacial radiology center in Gorgan, Iran, during 2021–2023. Images were acquired with a Carestream CBCT unit (90 kVp, 15 mA, 150-µm isotropic voxel size, 5 × 8 FOV and 15 s exposure time) and reviewed in multiplanar reconstruction using CS Imaging software. Five standardized axial levels were assessed. Observer training was performed by an oral and maxillofacial radiologist; interobserver agreement on the calibration/reliability assessment was κ = 0.748. Fan’s C1-C5 classification was applied. Patient-level prevalence was the primary estimate; tooth-level estimates were reported descriptively. All prevalence estimates are presented with 95% Wilson confidence intervals. Results Twenty-five of 209 patients had at least one C-shaped mandibular molar (11.96%, 95% CI 8.23–17.06). At tooth level, 33 of 435 molars were C-shaped (7.59%, 95% CI 5.45–10.46), including 2 of 157 first molars (1.27%, 95% CI 0.35–4.53) and 31 of 278 s molars (11.15%, 95% CI 7.97–15.39). Among 97 patients with bilateral mandibular second molars, 6 had bilateral and 5 had unilateral C-shaped morphology. C1 predominated at the orifice, C3a in the middle level, and C4 at the apical-middle and apical levels. Conclusion C-shaped morphology was uncommon in mandibular first molars but relatively frequent in second molars. Because canal configuration changed substantially along the root, clinicians should not infer the apical anatomy from the orifice appearance alone.