Effect of Resin Cement Viscosity and Thickness on the Shear Bond Strength of a Lithium Disilicate Glass-Ceramic

Objective: To assess how resin cement viscosity and thickness influence the static shear bond strength (s-SBS) of adhesively bonded lithium disilicate ceramic. Methods: Cylindrical lithium disilicate samples were prepared and randomly assigned to four experimental groups (n = 10), based on two factors: resin cement thickness (thin ≈50 µm or thick ≈150 µm) and viscosity (high or low). A tri-layer configuration (ceramic–resin cement–ceramic) was used, with standardized ceramic discs obtained from CAD/CAM blocks. The bonding ceramic surfaces were etched with hydrofluoric acid, silanized, and a defined bonding area (≈6 mm2) was delimited using adhesive tape. The resin cement thickness was controlled by applying either one (thin) or three (thick) overlapping layers of adhesive tape. Resin cement was applied, and assemblies were seated under a 100 g static load. After light-curing, specimens were subjected to static shear bond strength testing (s-SBS) using a universal testing machine (crosshead speed: 1 mm/min; load cell: 1 kN). Failure mode was classified under stereomicroscopy, and representative samples were analysed under SEM. Additional specimens were used to confirm cement layer thickness using a profilometer. Results: For the thick resin cement layer (≈150 µm), viscosity significantly influenced bond strength, with high-viscosity cement showing higher mean values (27.36 MPa) compared to the low-viscosity cement (19.65 MPa). In contrast, resin cement viscosity did not affect the bond strength in the thin layer condition (≈50 µm). Most specimens exhibited adhesive failure at the ceramic–cement interface. Significance: Resin cement selection based on viscosity is particularly relevant when a thick cement layer is anticipated, whereas for ideal, thin cement films, viscosity is a less consequential factor for interfacial bond strength.