Comparison of surface microhardnes of resin cements polymerized under different thicknesses of ceramic hybrid and nano hybrid composites

The aim of this in vitro study was to evaluate and compare the surface microhardness of resin cements polymerized using different curing modes beneath indirect restorations fabricated from a hybrid ceramic CAD/CAM block and two nano-hybrid composite resins with different thicknesses. Specimens with thicknesses of 1 mm, 2 mm, and 3 mm were prepared from three restorative materials: a hybrid ceramic CAD/CAM block (Lava Ultimate; LU), a nano-hybrid composite resin (IPS Empress Direct; IPS), and a nano-hybrid composite resin (Neo Spectra ST HV; NST). Four resin cements with light-cure and dual-cure polymerization modes (Nova Resin, Imicryl and NX3 Nexus, Kerr) were placed beneath the restorative materials using teflon molds measuring 6 mm in diameter and 0,5 mm in thickness. The resin cements were polymerized for 20 seconds using an LED lightcuring unit through the restorative materials (n=15). In the control groups, resin cements were polymerized directly for 20 seconds through a transparent glass slide. To prevent adhesion, transparent tape was applied to the specimen surfaces. After polymerization all samples were stored in a dark environment for 24 hours. Microhardness measurements were performed on the top and bottom surfaces using a hardway microvickers hardness tester. Three indentations were made on each surface with a load of 50 g applied for 15 seconds. Statistical analyses were conducted using SPSS 24.0 software. One-way analysis of variance (ANOVA), followed by Tukey and Scheffé post-hoc tests, was applied at a significance level of α= 0,05. The highest microhardness values on both surfaces were observed in the Imicryl control groups. After the control groups, the highest values were recorded in the 1 mm Imicryl dual-cure groups (p< 0,05). NX3 control and dual-cure groups showed significantly lower microhardness values compared with Imicryl groups (p< 0,05). In both cement systems, control groups demonstrated significantly higher microhardness values than the experimental groups (p < 0,05). Overall, increasing restorative material thickness resulted in decreased microhardness values. The microhardness of resin cements was influenced by the type and thickness of the restorative material. Therefore, resin cement selection in clinical practice should consider the thickness and light-transmittance properties of the restorative material. Keywords: Indirect restorations; Microhardness; Resin cement