Correlation between ceramics translucency and polymerization efficiency through ceramics

ObjectivesThe aim of this study was to analyse the effects of curing with a high intensity curing unit for different exposure times, for different ceramic types, thicknesses and corresponding different ceramic translucencies. The relationship between ceramic translucency and hardness, as well as the critical translucency value for sufficient curing were also determined.MethodsAll these effects were expressed in terms of Vickers hardness measured with an automatic micro hardness indenter on thin luting composite films (Variolink II), stored for 24 h in distilled water at 37 °C. Two preliminary studies determined the time necessary to achieve maximum hardness in the luting composite, with and without an additional chemical catalyst. The main study aimed to estimate the effect on luting composite hardness, without an additional chemical catalyst, of the following parameters: curing time (5, 10 and 15 s), ceramic thickness (0.5, 1, 2 and 3 mm), ceramic type (two leucite-reinforced and two lithium disilicate glass-ceramics) and ceramic translucency (TP), measured using a reflection spectrophotometer as a function of wavelength.ResultsThe minimum curing time necessary to achieve maximum hardness in the luting composite was 15 s for both groups, with and without an additional chemical catalyst. However, dual curing caused a hardness enhancement of ca. 50%. The two leucite-reinforced glass-ceramics did not reduced the hardness of the luting composite up to a ceramic thickness of 2 mm, whereas the more dense lithium disilicate glass-ceramics had already caused this effect at a thickness of 1 mm. ANOVA analyses revealed that the greatest effect on the luting composite hardness resulted from the curing time (eta square = 0.62) followed by translucency (eta square = 0.32 TP650 nm and 0.28 for TP470 nm), ceramic type (eta square = 0.17) and ceramic thickness (eta square = 0.03).SignificanceHigh-power curing units are not able to consistently reduce the exposure time. In both systems, at least 15 s were necessary to properly cure the luting composite. Furthermore, the effect of light activation on the dual curing luting composite cannot be neglected.