Morphology of the Middle Mesial Canal in Permanent Mandibular First Molars and Its Relationship to Pulpal Floor Anatomy in a Thai Subpopulation: A Micro-Computed Tomographic Analysis.

OBJECTIVE: To evaluate the relationship between the middle mesial canal (MMC) and pulpal floor anatomy and to compare root dentine thickness among the mesial canals across MMC types in the Thai subpopulation. METHODS: A total of 724 mature permanent mandibular first molars were obtained and initially screened using cone-beam computed tomography to identify teeth with MMCs. These teeth (n=49) were rescanned with micro-computed tomography (micro-CT) to analyse canal configurations. Pulpal floor types were observed under a dental operating microscope, and minimum dentine thickness was measured using Fiji ImageJ software. Linear regression assessed relationships among dentine thickness, canal level and MMC type, while chi-square tests evaluated correlations between the presence of MMC and pulpal floor type. RESULTS: In this Thai subpopulation, the confluent MMC was most common (63.3%), followed by fin (28.6%), independent (6.1%) and unclassified (2%) types. Type 2 pulpal floors, characterised by mesiobuccal and mesiolingual orifices connected by an isthmus, were most common (61.3%). The mean vertical distance from the pulpal floor to the MMC orifice was 0.7 ± 1.00 mm, and 90% of MMC orifices were ≤1.5 mm from the floor. Dentine in MMCs was significantly thinner than in MB canals (middle and apical levels) and ML canals (apical levels). No significant differences were observed in dentine thickness among MMC types. There was no statistically significant association between pulpal floor type and the presence of MMC (P=.106), although MMCs were observed more frequently in type 2 floors (30.6%) than in type 1 floors (16.7%). CONCLUSION: In this Thai subpopulation, the confluent MMC, particularly the subtype with an isthmus, was the most common. No significant association was found between the presence of a pulpal floor isthmus and MMC occurrence. The MMC orifice was located within 1.5 mm of the pulpal floor in 90% of cases. The MMC exhibited the smallest dentine thickness, especially in the middle and apical thirds.