Root canal morphology and incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography of two different scanning modes

Introduction: The mesial root of the mandibular molar is one of the most complex internal anatomies of human dentition. This study aimed to detect root canal morphology and the incidence of isthmus in the mesial root canal of the mandibular first molar using cone-beam computed tomography (CBCT) of different voxel size scanning modes compared to stereomicroscopic evaluation after teeth clearance. Materials and Methods: Sixty extracted mandibular first molars were collected, fixed in foam blocks, and scanned using CBCT with 150 and 400 μm voxel sizes. All the teeth were cleared and fixed in clear epoxy blocks for serial cross-sectioning from the apex, then examined using a stereomicroscope. Comparisons between cross-sectioned slices were achieved using Friedman's test (P ≤ 0.05). Results: There was a statistically significant difference between the cleared teeth, voxel sizes of 400 and 150 μm in the detection of the root canal and isthmus. Only voxel size 150 μm was able to detect the isthmus at 1 mm and 2.5 mm from the apex. Vertucci Types IV and II were the most common canal configurations reported, with a high incidence of isthmus at 4 mm and 5.5 mm from the apex. Conclusion: The 150 μm voxel size mode provides more accurate detection of the isthmus portion and root canal morphology. Vertucci Types IV and II were the most common canal configurations, and an isthmus with a high incidence was found at 4 mm and 5.5 mm from the apex of the mesial root of the mandibular first molar.