Abstract
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. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12903-025-07247-w.