A curvature leads to asymmetrical removal of root dentine, which results in an increase in the incidence of canal transportation. The aim of this study was to evaluate the effect of master apical file (MAF) size on the apical transportation (AT) of teeth with severe canal curvatures. Thirty-six mesial roots of human mandibular first molars were included. Inclusion criteria consisted of mature roots with closed apices, absence of carious lesions and resorptions, and root canal curvatures (CC) of 45 degrees < CC < 60 degrees. The root canals were prepared using a crown-down pressureless technique. The samples were equally divided into groups A, B, and C based on MAF size: group A: MAF size equivalent to #20; group B: MAF size equivalent to #25, and group C: MAF size equivalent to #30. Cone beam computed tomography was used to evaluate the AT. There were no statistically significant differences in the AT between the canals with different curvature angles up to MAF size #30 (P = 0.55). Using flexible hand files for canal preparation, an increase in MAF size up to #30 does not significantly influence AT in severely curved canals.
