Multiple analyses on mechanisms of root fracture in endodontically treated teeth

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Extracted human upper premolars were restored with a fiber post, prefabricated metallic post or cast metallic post-core. Teeth with full crown preparations without post-core restorations served as a control. All teeth were restored with full coverage crowns. A 90-degree vertical or 45-degree oblique load was applied to the restored teeth with a crosshead speed of 0.5 mm/min, and the fracture loads and mode of fracture were recorded. Fatigue fracture tests were conducted by applying sinusoidal cyclic loads to restored teeth from vertical or oblique directions. Fatigue limits for each restoration were calculated using the staircase approach.Under the condition of vertical loading, the fracture load of teeth restored with the cast metallic post-cores was greatest among the groups. All fractures in teeth restored with all types of post-core systems propagated in the middle portions of roots, including the apices of the posts. Under the condition of oblique loading, the fracture load of teeth restored with pre-fabricated metallic posts was significantly smaller than that in other groups. Two-thirds of fractures in the fiber post group propagated within the cervical area, while most fractures in other groups extended beyond the middle of the roots. Fatigue fracture resistance of teeth with different restoration methods presented identical patterns in static fracture resistance tests. Fatigue limits of teeth restored with fiber and metallic posts presented 73% of static fracture loads under vertical loading, and 48 and 46% under oblique loading.From the results of the present investigations, it was concluded that under conditions of vertical and oblique loadings, the combination of a fiber post and composite resin core with a full cast crown is most protective to the remaining tooth structure.