Conservative endodontic cavity (CEC) may improve fracture resistance of teeth but compromise instrumentation of canals. Extracted human intact maxillary incisors, mandibular premolars and molars were imaged with micro-CT and assigned to CEC or traditional endodontic cavity (TEC) groups (n=10/group/type). Canals were prepared and post-treatment micro-CT images obtained. These teeth along with the negative controls were then loaded to fracture. Mean proportion of untouched canal-wall was significantly higher only in distal canals of molars with CEC compared to TEC. Mean dentin volume removed was significantly smaller for CEC in all tooth types. Mean load-at-fracture for CEC was significantly higher in premolars and molars without differing significantly from the negative controls. While CEC was associated with compromised canal instrumentation only in the distal canals of molars, it conserved coronal dentin in all three tooth types and increased resistance to fracture in the mandibular molars and premolars.iii ACKNOWLEDGEMENTS

Introduction:The aim of this study was to understand the mechanism by which iatrogenic root dentin removal influences radicular stress distribution and subsequently affects the resistance to vertical root fractures (VRF) in endodontically treated teeth.Materials and Methods:The experiments were conducted in two phases. Phase 1: freshly extracted premolar teeth maintained in phosphate-buffered saline were instrumented to simulate three different degrees of dentin removal, designated as low, medium, and extreme groups. Micro-Ct analyzes were performed to quantitatively determine: (a) the amount of dentin removed, (b) the remaining dentin volume, and (c) the moment of inertia of root dentin. The specimens were then subjected to thermomechanical cycling and continuous loading to determine (a) the mechanical load to fracture and (b) dentin microcracking (fractography) using scanning electron microscopy. Phase 2: Finite element analysis was used to evaluate the influence of dentin removal on the stress distribution pattern in root dentin. The data obtained were analyzed using one-way ANOVA and Tukey's post hoc test (P < 0.05).Results:Phase 1: A significantly greater volume of dentin was removed from teeth in extreme group when compared to low group (P < 0.01). The mechanical analysis showed that the load to fracture was significantly lower in teeth from extreme group (P < 0.05). A linear relationship was observed between the moment of inertia and load to fracture in all experimental groups (R2 = 0.52). Fractography showed that most microcracks were initiated from the root canal walls in extreme group. Phase 2: The numerical analysis showed that the radicular stress distribution increased apically and buccolingually with greater degree of root canal dentin removal.Conclusions:The combined experimental/numerical analyses highlighted the influence of remaining root dentin volume on the radicular bending resistance, stress distribution pattern, and subsequent propensity to VRF.