Evaluation of the degree of conversion of luting materials beneath hybrid ceramic CAD/CAM substrates of different thicknesses

This study primarily aimed to evaluate the degree of conversion (DC) of dual-cure resin cements, flowable composites, and bulk-fill composites used for cementing hybrid ceramic CAD/CAM substrates of different thicknesses. Additionally, the optical properties of the substrates were assessed to support the interpretation of DC-related outcomes. Hybrid ceramic CAD/CAM substrates (Cerasmart, GC) with three different thicknesses (1.5 mm, 2.0 mm, and 2.5 mm) were used to evaluate contrast ratio (CR), relative translucency (RTPab, RTP00), and opalescence (OP) using a spectrophotometer (VITA Easyshade Advance 4.0, Zahnfabrik) (N = 18, n = 6). Three different luting materials—dual-cure resin cement (Estecem II Plus, Tokuyama), flowable composite (Estelite Universal Flow, Tokuyama), and bulk-fill composite (Estelite Bulk-Fill Flow, Tokuyama)—were polymerized using a standardized setup to prepare a total of 63 specimens (n = 7 per group). All specimens were stored at 37 °C in the dark for 24 h. The degree of conversion (DC) was analyzed using Fourier Transform Infrared Spectroscopy (FTIR). Optical data were analyzed using one-way ANOVA, while DC data were tested for normality using the Shapiro–Wilk test and subsequently analyzed by two-way ANOVA followed by Bonferroni post hoc tests (p < 0.05). Substrate thickness significantly affected the DC values of the luting materials (p < 0.001). At 1.5 mm and 2 mm substrate thicknesses, flowable composite cements exhibited significantly higher DC values than both dual-cure resin cement and bulk-fill composites (p < 0.001), while bulk-fill and dual-cure resin cements showed comparable DC values. In 2.5 mm substrates, the highest DC was obtained with the dual-cure resin cement, whereas flowable composites demonstrated the lowest DC. Regarding optical properties, RTPab, RTP00, and OP values decreased in all groups with increasing substrate thickness, while CR values increased accordingly. The DC of the luting material varied according to substrate thickness and luting material type. For specimens ≤ 2 mm thick, flowable and bulk-fill resin composites can be alternatives to dual-cure cements. Bulk-fill composites showed potential for use in 2.5-mm restorations due to their polymerization ability in deeper regions. However, further studies are required to investigate material-specific differences.