Effect of firing protocols on cohesive failure of all-ceramic crowns

ObjectivesThe mechanical properties of veneering material in all-ceramic restorations are inferior to those of high-strength ceramics. The objective of this study was to assess the effect of three different firing protocols (different cooling times) on cohesive failure of veneered all-ceramic crowns using a newly developed in vitro test design.MethodsA prepared anterior maxillary incisor was used to produce geometrically identical crowns (n = 68) with a zirconia framework 0.4 mm thick veneered with two different ceramics and prepared by use of three different firing protocols. Twenty crowns were loaded until fracture without artificial ageing; the other crowns underwent thermocycling and chewing simulation before ultimate-load testing. Statistical analysis was performed using non-parametric tests.ResultsThe results showed that a reduced temperature decrease after firing led to better performance of all-ceramic anterior crowns in the in vitro tests. A significant difference with regard to firing protocol (p = 0.006) was observed for one veneering ceramic material only, however.ConclusionIt can therefore be concluded that the test design presented enabled sensitive measurement of the effect of firing protocol on cohesive failure of all-ceramic crowns. The modified firing protocol with 6 min additional cooling time resulted in greater resistance.