Microcomputed tomography evaluation of cement shrinkage under zirconia versus lithium disilicate veneers

Computer-aided design and computer-aided manufacturing (CAD-CAM) technology and the improved translucency of recently developed high-strength monolithic zirconia could make them clinically acceptable for veneers if bonding to zirconia was as predictable as to glass-ceramics. Few studies have compared how resin cements behave between glass-ceramic and zirconia veneers before and after polymerization.

The purpose of this in vitro study was to evaluate the volumetric polymerization shrinkage of resin cement, marginal discrepancy, and cement thickness before and after polymerization for glass-ceramic and zirconia veneers with light-polymerizing resin cement.

Ten lithium disilicate veneers and 10 zirconia veneers were fabricated with a CAD-CAM workflow on extracted human maxillary anterior teeth with intact enamel surfaces. Zirconia veneers were treated with airborne-particle abrasion, and lithium disilicate veneers were etched with 5% hydrofluoric acid. All specimens were treated with ceramic primer and cemented with a light-polymerized resin cement. All specimens were scanned before and after resin cement polymerization by microcomputed tomography. The data were processed by the Amira software program to compare polymerization volumetric shrinkage, cement thickness, and marginal discrepancy. The data were compared by using a t test and analysis of variance (α=.05). Two bonded veneers were loaded in a mastication simulator for 400 000 cycles to investigate the effect of cyclic fatigue loading.

Mean volumetric polymerization shrinkage was 4.2 ±0.8% for the lithium disilicate group and 6.4 ±3.5% for the zirconia group. No significant difference was found for volumetric shrinkage between materials (P=.132). The mean ±standard deviations of the marginal discrepancies before and after polymerization were 178 ±41 μm and 158 ±37 μm for lithium disilicate and 115 ±33 μm and 107 ±32 μm for zirconia. A smaller marginal discrepancy was found for both materials after polymerization (P=.011) and for zirconia compared with lithium disilicate (P=.004). The mean ±standard deviation cement thickness values before and after polymerization were 157 ±27 μm and 147 ±27 μm for lithium disilicate and 162 ±53 μm and 147 ±52 μm for zirconia. Smaller cement thickness was found after polymerization (P<.001), whereas no significant difference was found in cement thickness between materials (P=.144). No changes were noted in marginal discrepancy and cement thickness as a result of the fatigue loading.

The difference in the volumetric polymerization shrinkage of cement between lithium disilicate and zirconia veneers was not statistically significant. Polymerization shrinkage resulted in smaller marginal discrepancy and cement thickness for both veneer materials.