Effect of core stiffness on the in vitro fracture of crowned, endodontically treated teeth

Statement of problem. Dentin and core materials that substitute for missing dentin are dissimilar materials. A core material with a lower elastic modulus may deform more under applied stress and therefore result in reduced stress concentration at the core/dentin junction. Purpose. This in vitro study examined the effect of core stiffness on the fracture resistance and failure characteristics of a crowned, endodontically treated tooth under simulated occlusal load. Material and methods. Forty extracted human mandibular premolars were divided equally into 4 groups and prepared for posts and cast crowns as follows: group 1 = cast post and core, cast crown; group 2 = preformed metal post, composite core, and cast crown; group 3 = preformed metal post, amalgam core, and cast crown; and group 4 (control) = preformed metal post, no core, and cast crown. All prepared teeth had 2 mm of sound dentin on which the cemented crown rested. A continuous load (kg) was applied to the buccal cusp at a 30-degree angle to the long axis of each tooth at a crosshead speed of 2 mm/min until failure. Collected data were subjected to 1-way analysis of variance with the Welch modification to compare groups (P<.05). Results. Failure loads for the 4 test groups were as follows: 98.1 +/- 34.6 kg (group 1), 94.4 +/- 41.8 kg (group 2), 105.5 +/- 18.6 kg (group 3), and 101.1 +/- 55.3 kg (group 4). No significant difference in failure load values was found among the 4 groups. The primary mode of failure (80%) in all groups was an oblique radicular fracture, either apical to the post or at the post level. Horizontal fracture of the root and post was found in groups 1, 2, and 3 (20%). Loosening of the crown, post, and core was found only in group 2(20%). Conclusion. Within the limitations of this study, core stiffness did not affect the failure resistance of teeth restored with posts and cores and complete-cover age cast metal crowns. The dominant pattern of failure was unrepairable root fracture. Only the composite core exhibited repairable fractures.