Thermal shock resistance of core reinforced all-ceramic crown systems

Abstract

The coefficient of thermal expansion of core and veneer porcelains for all ceramic crowns must be compatible to prevent formation of stresses during the firing cycles. The purpose of this study was to use a thermal shock test to evaluate the crazing resistance of six all-ceramic crown systems. The systems tested were: Vita In-Ceream®, Vita Hi-Ceram®, Dicor™ veneered with Vitadur® N, magnesia core veneered with Ceramco® II, magnesia core veneered with Silhouette™, and magnesia core veneered with Vita VMK® 68. A maxillary central incisor was prepared with 1.0 mm axial and 1.5 mm incisal reduction. Refractory dies were prepared and crowns were fabricated using the manufactures recommendations. Six specimens of each system were placed inside a beaker in a furnace at 90°C. After 15 min heat soak, ice water was poured into the beaker through a funnel. The samples were dried in air, reheated, and inspected for crazing using light microscopy. If crazing was observed, this would constitute a failure at a temperature difference (ΔT) of 90°C. If no failure occurred, the test was repeated at increasing temperature increments of 10°C until failure. The crazing resistance of each system, expressed as the mean ΔT, was: > 200 (Hi-Ceram®), 152 ± 19 (Dicor™ / Vitadur® N), 143 ± 5 (magnesia core/Vita VMK® 68), 122 ± 21 (magnesia core/Silhouette™), and 118 ± 10 (magnesia core/Ceramco® II) °C. All systems tested resisted crazing due to quenching in ice cold water from 100°C. Systems with lower coefficients of expansion showed significantly greater resistance to thermal shock using ANOVA. © 1994 John Wiley & Sons, Inc.