Statement of problem

Clinicians are frequently faced with a challenge in selecting materials for adjacent restorations, particularly when one tooth requires a zirconia-based restoration and the next requires a veneer. While it may be desirable to use the same veneering ceramic on adjacent teeth, little information is available about the use of veneering ceramics over a zirconia-based material.

Purpose

The purpose of this study was threefold: (1) to study the influence of hydrofluoric acid-etched treatment on the surface topography of the zirconia veneering ceramic, (2) to test the bond strength of zirconia veneering ceramic to enamel, and (3) to evaluate the flexural strength and the elemental composition of ceramic veneers.

Material and methods

Three zirconia veneering ceramics (Cerabien CZR (CZ), Lava Ceram (L), and Zirox (Z)) and 4 conventional veneering ceramics (Creation (C), IPS d.Sign (D), Noritake EX-3 (E), and Reflex (R)) were evaluated. Twenty ceramic bars of each material were fabricated and surface treated with hydrofluoric acid according to the manufacturer's recommendations. Ten specimens from each group of materials were examined with a profilometer, and a sample of this group was selected for quantitative evaluation using a scanning electron microscope (SEM). Another 10 acid-etched specimens from each group of materials were treated with silane prior to cementing with resin cement (Variolink II) on enamel surfaces. These luted specimens were loaded to failure in a universal testing machine in the shear mode with a crosshead speed of 0.05 mm/min. The data were analyzed with a 1-way ANOVA, followed by Tukey's HSD test (α=.05). An additional 10 ceramic bars from each material group were fabricated to evaluate flexural strength and elemental composition. The flexural strength (MPa) of each specimen was determined by using a 4-point-1/4-point flexure test. A Weibull statistic tested the reliability of the strength data; pairwise differences among the 7 groups were evaluated at confidence intervals of 95%. The chemical composition of each bar was determined by energy dispersive spectroscopy (EDS).

Results

There was a significant difference in the surface roughness in all testing groups. Conventional veneering ceramics (groups C and R) had a mean surface roughness higher than the groups of zirconia veneering ceramics (P<.001). Group D showed no difference in surface roughness compared with the groups of zirconia veneering ceramics. The SEM micrographs revealed differences in the acid-etched surfaces of ceramics. Zirconia veneering ceramics were smooth, with some groove formations, while conventional veneering ceramics had an amorphous, spongy-like structure with numerous microporosites. The mean bond strength (SD) of zirconia veneering ceramics to enamel revealed a significant difference. Group R (25.16 (3.40) MPa) followed by group C (22.51 (2.82) MPa) had significantly higher mean bond strength than the groups of zirconia veneering ceramics (P<.001, P=.009 respectively). Groups D (16.54 (2.73) MPa) and E (17.92 (3.39) MPa) showed no differences. Only group L (9.45 (1.62) MPa) exhibited significantly lower mean bond strength when compared with conventional veneering ceramics (P<.001). For flexural strength, only 1 group, group CZ, had a significantly lower flexural strength than all other groups (P<.001).

Conclusions

Effective ceramic interface management, such as acid etching and enamel bonding, is essential for successful ceramic laminate veneer restorations. Not all zirconia veneering ceramics display the same quality of surface roughness after hydrofluoric acid etching and the same bond strength to enamel when used as laminate veneer materials. (J Prosthet Dent 2008;100:194-202)