Micro-CT and Finite Element Analysis of Dentin Preservation and Stress Distribution in Mesial Roots of Mandibular Molars after Shaping with Different Tapers

Introduction: The aim of this study was to evaluate the dentin preservation and fracture resistance of mandibular molar mesial roots after shaping with different tapers using micro–computed tomography (micro-CT) and finite element analysis (FEA). Methods: Forty-eight mandibular first molars with independent mesial canals were selected. The mesio-lingual and mesio-buccal canals were randomly assigned (n = 12) to ProTaper Gold (PG), ProTaper Next (PTN), ProTaper Ultimate (PTU), and B4U instrumentation systems. Pre- and post-shaping micro-CT scans were performed to compare root canal volume and cervical dentin volume. The centroid shift and the percentage and distribution of the dentin removal in correspondence of the mesial canals were analyzed in coronal, middle, and apical points of analysis and below the furcation at the “danger zone.” The residual dentin thickness in the distal and mesial aspects of the mesial canals was measured. A linear elastic model of a mesial root of a mandibular molar was created through the finite element method (FEM) and a 200N perpendicular load was applied on the root canal coronal third. One-way factorial analysis of variance with Bonferroni correction and post hoc Tukey-Kramer tests were used (P < .05). Results: The mean cervical dentin volume removal was statistically lower for the B4U compared with the PG system (P < .05). The canal centering ability was statistically more accentuated for the B4U group in the coronal and middle third and higher centroid shift variations occurred for PG system (P < .05). The residual dentin thickness appeared statistically higher after shaping with B4U through the mesial and distal root aspect at the coronal point of analysis and at the danger zone (P < .05). PG removed more dentin in the coronal root canal third (P < .05). The FEM analysis showed no statistically significant differences between groups (P > .05). Conclusions: The reduction of the root canal taper may improve the instruments’ centering ability and the preservation of the residual dentin tissue, especially in the coronal and middle third. The root canal taper seems not to influence the stress distribution pattern through the mesial root of the lower molars.