Effect of glass-ceramic thickness on the degree of conversion, polymerization shrinkage stress, and bond strength of resin cements

The aim of this study was to evaluate the effect of ceramic disc thicknesses on light attenuation, degree of conversion (DC), polymerization shrinkage stress (PSS), and microshear bond strength (μSBS) of dual-cured (RelyX Universal, and RelyX Ultimate) and light-cured (Variolink N LC, and Variolink Esthetic LC) resin cements. Forty ceramic discs were fabricated using lithium disilicate glass-ceramic in thicknesses of 0.7, 1.5, and 2.5 mm. For the μSBS test (n = 10), resin cement rods were bonded to the ceramic discs through Tygon tubes with 0.7 mm internal diameter and tested after 24 h in water at 37 °C. For the PSS test (n = 5), glass rods were aligned with a 1 mm gap filled with resin cement and light-cured, with displacement measured by software. Light attenuation analyses (n = 1) involved positioning discs at the light-curing unit tip and measuring radiant emittance. For the DC test (n = 3), an ATR-FTIR spectrometer measured light-cured specimens through each ceramic disc. Data were analyzed using two-way ANOVA and the Tukey test (α = 0.05). The results showed that the 1.5- and 2.5-mm thick ceramics significantly attenuated the light transmission and, consequently, reduced the mean values of μSBS, DC, and PSS for all resin cement used. Clinically, thicker ceramics reduce light transmission, which may compromise the polymerization of the resin cement. Dual-cure cements are preferable for thicker restorations because of their ability to compensate for the reduced light.