Transmitted irradiance through ceramics: effect on the mechanical properties of a luting resin cement.

Abstract

The study aims to characterise the curing behaviour of a light-curing luting composite (Variolink® Aesthetic LC, Ivoclar Vivadent) polymerised at different exposure times (10s, 20s) through different ceramics (IPS Empress CAD and IPS e.max CAD, Ivoclar Vivadent) and ceramic thicknesses (no ceramic, 0.5, 1, 1.5 and 2mm). Curing units' (Bluephase Style, Ivoclar Vivadent) variation in irradiance delivered up to 10-mm exposure distance as well as the incident and transmitted irradiance and radiant exposure up to 6-mm ceramic thickness were assessed on a laboratory-grade spectrometer. A total of 216 (18 groups, n=12) thin and flat luting composite specimens of 500-μm thickness were prepared and stored after curing in a saturated vapour atmosphere for 24h at 37°C. Micro-mechanical properties (Vickers hardness, HV and indentation modulus, YHU) were determined by means of an automatic micro-hardness indenter. Within the study design, the radiant exposure received by the luting composite varied from 2.56 to 24.75J/cm2, showing a high impact on the measured properties. Comparing the effect of the parameters exposure time, ceramic thickness and type, the highest influence on the micro-mechanical parameters was identified for exposure time, while this influence was stronger on HV (p<0.001, η P2=0.452) than on YHU (p<0.001, η P2=0.178). The influence of ceramic type was significant but low (η P2=0.161 on HV and 0.113 on YHU), while the influence of ceramic thickness was the lowest (η P2=0.04 and 0.05, respectively). Slightly higher irradiance values were transmitted through Empress CAD up to a ceramic thickness of 3mm (p<0.001), while being comparable with e.max for thicker slices. Differences in translucency between ceramics were reflected in the micro-mechanical properties of the luting composite. The radiant exposure reaching the luting composite is determined by the incident irradiance, exposure time, ceramic type and ceramic thickness. At the analysed incident irradiance, exposure time was the most consistent parameter affecting the micro-mechanical properties of the luting composite, and this effect was strongly reflected in the more translucent ceramic Empress CAD. Within the curing conditions, an exposure time of 20s is recommended.