Microtensile bond strength of different components of core veneered all-ceramic restorations. Part 3: double veneer technique.

PURPOSE: The bond strength of zirconia veneer has been considered a weak link in the layered all-ceramic restoration. In a previous study, this bond was improved using a new category of veneering ceramics adopting the pressing technique; however, the resulting esthetic quality lacked the individual characterization built using the layering technique. The aim of this work was to evaluate the effect of combining both press-on and layering veneer ceramics in one restoration on the bond strength with zirconia frameworks.
MATERIALS AND METHODS: Zirconia discs (19.4 mm in diameter, 3-mm thick) were veneered with 3-mm thick press-on veneer ceramic or veneered with 1-mm thick press-on veneer ceramic and an additional 2-mm thick of layering veneer ceramic. Two commercial layering veneer ceramics were tested. The specimens were sectioned into microbars, and the zirconia veneer microtensile bond strength (MTBS) was measured in a universal testing machine. Scanning electron microscopy (SEM) was used to examine core veneer interface quality and to assess failure type. Energy dispersive X-ray microanalysis (EDAX) was used to analyze the chemical structure of the tested veneer ceramics, which may affect the structural integrity of the double veneered restoration. One-way analysis of variance and Tukey HSD post hoc tests were selected to analyze the data (p < 0.05 was significant).
RESULTS: The MTBS of zirconia and press-on ceramic (34.4 +/- 2.9 MPa) was not affected by the addition of a second layer of either veneer ceramic. SEM analysis revealed defect-free zirconia press-on veneer interface, while the interface between the press-on and the layering veneer ceramics demonstrated a different crystal structure and glass matrix, which did not affect wetting and contact between the two materials. EDAX analysis revealed differences, which account for the observed structural differences, in the chemical composition between the tested veneers.
CONCLUSION: The double veneer technique combines the high bond strength and superior interface quality achieved using press-on ceramics with the superior esthetics and individual characterization obtained using layering ceramics. The technique promises superior function and performance of the double veneered restoration.