The aim of this study was to determine the fatigue strength of two contemporary crown ceramics, li-thium disilicate e.maxsCAD (LD) and polymer-in filtrated ceramic Enamics (PIC) and examine their failure mode. Twenty extracted human maxillary premolars were divided into two groups. The teethwere prepared and restored with either LD or PIC crowns. The restored teeth were subjected to cyclic chewing simulation (9 Hz). The sinusoidal loading was started with 500 N followed by a stepwise in- crease of 100 N for every 100,000 cycles until fracture occurred or, alternatively, after one million cycles. The Kaplan-Meier survival analysis was applied on fatigue loadings. Differences in the failure modes were analyzed by Fisher Exact tests. The average failure load of LD group was 1400 N (904,728 cycles) with 70% survival rate, while the average failure load of PIC group was 870 N (378,167 cycles) with 0% survival. Regarding the failure mode in both test groups, the crowns tended to fail above the cemento- enamel junction (CEJ) and both combined adhesive and cohesive failure mode were commonly observed rather than cohesive failure. No signi ficant difference was found between the test groups. The failures were initiated from a contact point or central fossa. The fatigue resistance of LD crowns on premolars was signi ficantly higher than the PIC ceramic crowns.

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