The Effect of Ceramic Thickness on the Surface Microhardness of Dual-cured and Light-cured Resin Cements

Abstract

This study was conducted to evaluate the effect of ceramic thickness on the surface hardness of light-cured and dual-cured resin cements. Forty disk-like specimens of the dual-cured resin cement and twenty-four specimens of the light-cured resin cement were prepared (with 6 mm diameter and 1mm thickness). The samples were light-cure d for 40 seconds through three different ceramic disks (2 mm, 3 mm and 4 mm). Control specimens from each group were directly polymerized under a Mylar strip. In the control group of the dual-cured resin cement, the cement setting was realized by chemical reaction alone. After storing dry in darkness (24 hours, 37°C), the surface hardness of the specimens was measured using the Vickers microhardness test. Data were statistically analyzed using a two-way analysis of variance (ANOVA) followed by the LSD's test (p <0.05). BisCem resin cement which had been dual-cured under the 4 mm thickness ceramic with the 4.3 Vickers hardness, showed minimum surface microhardness, while the light-cured resin cement which had been directly activated in the control group with the 51.8 Vickers hardness value exhibited the maximum surface microhardness. So, BisCem dual polymerized control specimens had significantly higher hardness values in comparison to the chemically polymerized and indirectly activated ones (p <0.001). An increase in the thickness of ceramic could decrease the hardness of the resin cement. An overlying ceramic thickness of 3 mm and above was found to adversely affect the polymerization of the LC and DC resin cement and it was considered as the clinical threshold. In addition, using only the self-cured mode in the dual-cured resin cement was not sufficient for achieving the optimum surface microhardness. Clinical significant: Adequate polymerization of resin cement is essential for the optimal mechanical properties and clinical performance. It affects by increasing the thickness of ceramic restorations.