Micro-CT evaluation of dentin thickness in distal root canals of mandibular first molars: assessing the influence of dimension and correlation with sulcus depth.

This study aimed to evaluate the relationship between two-dimensional orifice shape measurements (major diameter, minor diameter, perimeter, area, roundness, form factor, and aspect ratio) and dentin thickness in the mesial and distal aspects of the distal root canals of mandibular first molars as well as developmental sulcus depth, using micro-CT imaging. A total of 100 distal roots of mandibular first molars showing Vertucci type I configuration was selected and scanned with micro-CT. The 2D micro-CT analysis of area, perimeter, major diameter, minor diameter, roundness, form factor and aspect ratio of the canal orifice was performed. The dentin thickness and developmental sulcus depth were measured in both mesial and distal directions at every mm for 6 mm apical from the furcal level. Whether there was a relationship between the dimensions of the orifice and the difference according to the root level, orifice shape and direction ​​were analyzed with Pearson's correlation followed by multiple linear regression and Kruskal-Wallis H and Mann-Whitney U analyses in SPSS using 5% significance threshold. Significant correlations were found between major diameter and perimeter with dentin thickness, particularly in the mesial aspect of the distal root (r = 0.38, p < .05 and r = 0.35, p < .05). Orifice area also showed a moderate correlation with dentin thickness (r = 0.29, p < .05). No significant relationship was found between the sulcus depth and any orifice shape parameters. The regression model showed that major diameter and perimeter explained 42% of the variance in dentin thickness (R = 0.42, p < .05). The most common orifice shape was long oval (58%), followed by oval (28%). Dentin thickness varied significantly between the mesial and distal directions (p < .05), and sulcus depth was significantly greater in the mesial direction at all levels (p < .05). The study identified major diameter and perimeter as important predictors of dentin thickness in distal root canals, with larger orifices associated with thicker surrounding dentin. However, sulcus depth did not show strong correlations with orifice shape, suggesting it may play a limited role in predicting root canal morphology. This study highlights the importance of major diameter and perimeter in predicting dentin thickness during post space preparation, emphasizing the need for customized approaches in cases with long oval or flattened orifices to minimize root perforation and fractures.