Veneer type determines shear bond strength to CAD/CAM multilayer glass-fiber-reinforced polymer frameworks after thermomechanical aging

Background This study tested the null hypothesis (H0) that the type of veneering material would have no effect on the shear bond strength (SBS) to multilayer glass-fiber-reinforced polymer (MLG-FRP) frameworks. Methods Eighty-four specimens were prepared from Trinia and Zantex CAD/CAM discs (10 mm diameter, 1 mm thickness) and divided into six subgroups (n = 14). Each subgroup was veneered with composite resin (Ceramage, CE), milled hybrid ceramic (CeraSmart, CS), or 3D-printed hybrid ceramic (VarseoSmile Crown Plus, VS). All specimens underwent thermomechanical aging (240,000 chewing cycles; 5000 thermal cycles at 5–55 °C). SBS was measured using a universal testing machine, and failure modes were classified under a stereomicroscope (20×). Premature test failures (PTFs) were coded as 0 MPa in the ITT analysis, with an additional PP analysis excluding PTFs, and data were analyzed by two-way ANOVA with Tukey’s post-hoc test (α = 0.05) including Shapiro–Wilk, Levene, partial η², and Hedges g (95% CI). Results The null hypothesis that veneering material does not affect bond strength was rejected. Veneering material significantly affected shear bond strength (SBS) (ITT: F = 5.66, p = 0.005; PP: F = 4.30, p = 0.018), while MLG-FRP type and interaction were not significant (p > 0.05). In intention-to-treat analysis (PTFs = 0 MPa), composite resin (CE: 12.87 ± 2.66 MPa; 95% CI: 11.88–13.86) outperformed milled (CS: 9.19 ± 6.16 MPa) and 3D-printed hybrids (VS: 9.02 ± 4.74 MPa). Per-protocol analysis confirmed this trend (CE: 12.87 ± 2.66 MPa > CS: 12.87 ± 2.04 ≈ VS: 10.99 ± 2.25). Effect sizes were large (Hedges g up to 1.2). Kaplan–Meier analysis showed 100% survival in CE, significantly higher than CS (p = 0.002) and VS (p = 0.02), with no difference between CS and VS. Failures were mainly adhesive (> 90%). Conclusions The choice of veneering material is a critical determinant of bond strength to MLG-FRP frameworks. Composite resin provided significantly stronger and more durable adhesion than hybrid ceramics under simulated aging, supporting its use for long-term clinical reliability.