Bonding Performance of Lithium Disilicate Veneers to Bioactive and Conventional Resin Cores using three types of luting agent: an in vitro study

Aims: The present study aimed to evaluate the shear bond strength (SBS) and internal adaptation of lithium disilicate veneers bonded to bioactive and conventional composite cores using various luting agents and bonding approaches. Materials and Methods: A total of 96 specimens were fabricated from two core materials (Predicta Bulk Fill and 3M Filteks Z250 composites) and luted with three cements: Predicta Bioactive Cement, RelyX U200, and Variolink Esthetic LC, applied either with or without GC Premio Bond. Samples were then subjected to 5,000 thermocycles (5–55°C). SBS was measured using a universal testing machine, while internal adaptation was assessed with micro-CT. Failure modes were analyzed under stereomicroscopy. Statistical analysis involved three-way ANOVA with post hoc comparisons at P ≤ 0.05. Core material and cement type significantly affected SBS outcomes. Results: Predicta Bulk Bioactive cores demonstrated the highest SBS, particularly when luted with Predicta Bioactive Cement (320–330 MPa bonded; 300–310 MPa unbonded) or RelyX U200 (310–330 MPa). Filtek Z350 XT showed greater variability, with RelyX U200 unbonded samples achieving 280–300 MPa, while Variolink Esthetic LC consistently produced the lowest values (40–60 MPa). Application of GC Premio Bond improved SBS for Predicta and Variolink groups, whereas RelyX U200 performed strongly regardless of bonding protocol. Micro-CT analysis revealed uniformly small internal gaps (2–3 µm) across groups. RelyX U200 showed the best overall adaptation, while Predicta Cement exhibited slightly larger gaps, especially without bonding. Conclusion:  RelyX U200 provided the most favourable combination of bond strength and internal adaptation for lithium disilicate cementation.