Effect of resin-matrix and leucite-reinforced glass ceramic CAD/CAM restorative materials with different thicknesses on the time-dependent surface microhardness of resin-based luting materials.

This study evaluated the time-dependent surface microhardness of different resin-based luting materials beneath CAD/CAM restorative materials of varying types (resin-matrix ceramic-RMC and leucite-reinforced glass ceramic-LRGC) and thicknesses (1 mm and 2 mm) as an indirect indicator of their polymerization behavior. A RMC(Cerasmart 270, GC) and a LRGC (Initial LRF, GC) CAD/CAM ceramic were used. Four luting materials (light-cured/LC, dual-cured/DC, self-adhesive resin cements/SC, and a flowable composite resin/FCR) were evaluated. Specimens (n = 10 per group) were polymerized under no restoration (control), 1 mm and 2 mm thick CAD/CAM materials using an LED curing unit (0 mm distance). Top and bottom surface microhardness values were measured at 1 h, 24 h, 7 days, and 30 days. Data were analyzed using non-parametric statistical tests. Restorative material type, thickness, luting material, and time significantly influenced microhardness values (p < 0.05). Increasing restorative thickness resulted in reduced microhardness values, particularly in LC. LC consistently exhibited bottom/top ratios below the 0.80 threshold under all conditions, whereas DC and FCR maintained values above this threshold. RMC materials yielded higher microhardness values compared to LRGC. Time-dependent increases in microhardness were observed, particularly in DC and SC systems. The microhardness of resin-based luting materials beneath CAD/CAM restorations is influenced by the combined effects of restorative material type, thickness, luting material, and time. Increasing restorative thickness reduced microhardness values, particularly in LC materials, whereas time-dependent increases were most evident in DC and SC materials. Not applicable. This study is a laboratory-based investigation and does not involve a clinical trial. Therefore, no clinical trial number has been assigned.