Background and Aim: All ceramic, inlay-retained resin bonded fixed partial denture is a conservative method for replacement of missing teeth, because of minimal tooth reduction. The connector between the retainer and the pontic is the weak point of these bridges. Reinforcement of ceramic core will increase the clinical longevity. The aim of this study was to determine the effect of zirconia on flexural strength of IPS Empress 2 core ceramic.

Materials and Methods: In this experimental in vitro study, twenty eight bar shape specimens (17´3.1´3.1 mm) were made of four different materials: (1) Slip casting in-ceram alumina core (control group) (2) Hot-pressed lithium disilicate core ceramic (IPS Empress 2) (3) IPS Empress 2 with cosmopost (zirconia post) inserted longitudinally in the center of the bar (4) IPS Empress 2 with cosmopost (zirconia post) inserted longitudinally in bottom of the bar. Specimens were subjected to three-point flexure loading with the span of 15mm, at a cross-head speed of 0.5 mm/min. Failure loads were recorded and analyzed using one-way ANOVA and Tomhane Post-hoc tests and p<0.05 was set as the level of significance. Fractured surfaces were then observed by scanning electron microscope (SEM). Four additional samples were made as the third group, and zirconia-IPS interface was observed by SEM before fracture.

Results: Mean values and standard deviations of three point flexural strengths of groups 1 to 4, were: 378.4±44.6, 258.6±27.5, 144.3±51.7, 230±22.3 MPa respectively. All the groups were statistically different from each other (P<0.05), except groups 2 and 4. The flexural strengths of groups 2, 3, 4 were significantly lower than group 1. Group 3 had the lowest flexural strength. SEM analysis showed that the initiated cracks propagated in the interface of zirconia post and IPS Empress 2 ceramic.