Abstract
Background: Lithium disilicate veneers rely on both strong resin–ceramic bonding and intimate adaptation for long term success. Universal adhesives containing 10-Methacryloyloxydecyl Dihydrogen Phosphate may enhance the performance of contemporary resin cements, yet evidence for their effect on bioactive and self-adhesive luting agents remains limited. Aim: To compare the shear bond strength (SBS) and internal adaptation of IPS e.max CAD veneers luted to enamel with three resin cements — bioactive dual cure (Predicta), light cure (Variolink Esthetic LC) and self-adhesive (RelyX U200) — used with or without a universal bonding agent. Materials and Methods: Forty-eight sound third molars were prepared to mid enamel, CAD/CAM veneers were etched with 10% hydrofluoric acid and silanated, and teeth were etched with 37 % phosphoric acid. Specimens were allocated randomly (n = 8 per subgroup) to one of six cement–bonding combinations. Bonded subgroups received a 10 MDP universal adhesive; unbonded subgroups did not. After cementation and light curing, all samples were thermocycled 5,000X between 5 °C and 55 °C. SBS was measured using a universal testing machine (1 mm min⁻¹), failure modes were classified at 20× magnification, and marginal/internal gaps were quantified by micro computed tomography (micro-CT). Results: Predicta exhibited the highest SBS in both bonded (140 ± 24 MPa) and unbonded (80 ± 60 MPa) conditions. Variolink LC showed the lowest SBS, especially when unbonded (26 ± 35 MPa). RelyX U200 displayed moderate SBS that increased significantly with bonding (p < 0.05). Micro CT revealed smaller gap volumes for RelyX U200 overall, while Predicta’s adaptation improved notably when bonded. Failure analysis showed predominantly cohesive failures for Predicta and Variolink; RelyX U200 shifted from cohesive to adhesive failures when adhesive was applied (p < 0.05). Conclusion: Applying a universal bonding agent significantly improved both bond strength and internal adaptation of resin cements, particularly Predicta and RelyX U200. Predicta cement combined with 10 MDP adhesive delivered the most favorable enamel–ceramic interface, supporting its clinical use for lithium disilicate veneers.