Bond Strengths of Veneering Ceramics to Reinforced Ceramic Core Materials

Reinforced core ceramics are commonly used to deliver more esthetic ceramic restorations with weaker, but more translucent, veneering ceramics. The aim of this study was to evaluate the shear bond strengths of four individual veneering ceramics-three feldspathic and one fluorapatite-to their corresponding core ceramics: leucite-reinforced ceramic (Evopress, Wegold); low-leucite-reinforced ceramic (Finesse, Ceramco); glass-infiltrated alumina (In-Ceram Alumina, Vita); and lithium disilicate (Empress 2, Ivoclar Vivadent), respectively. Materials and Methods: Ceramic cores (n = 10/group, total = 40) were fabricated according to manufacturers' instructions (thickness 3 mm, diameter 5 mm) and ultrasonically cleaned for 15 minutes in ethanol and deionized water. The veneering ceramics were condensed in a stainless-steel mold (diameter 5 mm, height 5 mm, core 3 mm, veneer 2 mm) and fired on the core materials. The samples were tried in the mold for minor adjustments, ultrasonically cleaned, and embedded in polymethyl methacrylate. All groups of core-veneering ceramic combinations were randomly divided into two groups (n = 5/group) for dry and thermocycled storing conditions. Dry samples were kept in a dessicator at room temperature for 24 hours prior to testing, and the other groups were subjected to thermocycling (5 cycles; 5 and 55°C; 30-second dwell time). The shear bond strength tests were performed in a universal testing machine (cross-head speed 0.5 mm/min) (Fig 1). The bond strengths (mean, in MPa, ± standard deviation) and modes of failures were recorded. The means of each group were analyzed by one-way analysis of variance, and multiple comparisons were made by repeated measures test (a = .05) (SAS 8.02, SAS Institute). Results: In dry conditions, the shear bond strength of veneering ceramic to core material in the Empress 2 system was significantly higher (41 ± 8 MPa; P , .05) than those of the Finesse (28 ± 4 MPa), In-Ceram Alumina (26 ± 4 MPa), and Evopress (23 ± 3 MPa) systems (Fig 2). Thermocycling significantly decreased the bond strengths in the Empress 2 system (31 ± 4 MPa) when compared with dry conditions, but the decrease was not significant in the Finesse, Evopress, and In-Ceram systems ( P > .05). Although the failure mode was mainly adhesive at the core-veneer interface for In-Ceram Alumina, predominantly cohesive fractures in the core materials were observed in the Empress 2, Finesse, and Evopress systems. Scanning electron microscopic images exhibited cohesive failures, with partially delaminated surfaces revealing no clear crack sites, and the adhesive failures, particularly in glass-infiltrated alumina/feldspathic ceramic, exhibited visible delamination sites at the core-veneer interface. Conclusion: Bilayered ceramic specimens exhibited complex failure modes that could be attributed to differences in the flexural strengths between the two ceramics, as well as to the differences in their thermal expansion coefficients. Although the thickness of the core ceramic was standard for all groups, it was reported that small variations could affect the strength of the restoration. Fluorapatite veneering ceramic demonstrated higher bond strength to lithium disilicate ceramic than the leucite-glass ceramic/feldspathic ceramic or glass-infiltrated alumina/feldspathic core-veneer ceramic combinations did. After thermocycling, core-veneer bond strength was affected the most in lithium disilicate/fluorapatite combinations.