Shear bond strength of metal and plastic orthodontic brackets bonded to 3D‑printed UDMA provisional resin restorations after surface treatments and aging: an in vitro study.

This in vitro study aimed to compare the shear bond strength (SBS) of metal (stainless steel) and plastic (polycarbonate) orthodontic brackets (OBs) bonded to 3D-printed urethane dimethacrylate (UDMA) provisional resin restorations (PRRs) across experimental conditions defined by surface treatment (sandblasting vs. diamond bur) and hydrothermal aging (1100 vs. 2200 thermocycles). One hundred 3D-printed UDMA specimens were allocated to 10 subgroups (n=10 each) based on bracket type, surface treatment, and aging protocol. SBS was evaluated using a universal testing machine at a crosshead speed of 1 mm/min, and bonding failure modes were categorized using the Adhesive Remnant Index (ARI). Overall factorial effects across the eight intervention subgroups (excluding the untreated/unaged controls) were evaluated using an Aligned Rank Transform factorial ANOVA, whereas subgroup differences across all predefined groups were analyzed using Kruskal-Wallis tests with Dunn's post hoc pairwise comparisons after Bonferroni correction (overall significance level α = 0.05; Bonferroni-adjusted alpha level applied according to the number of pairwise comparisons). Metal brackets consistently exhibited significantly higher SBS values (7.85 to 10.48 MPa) than plastic brackets (4.16 to 6.17 MPa) across all intervention subgroups. Sandblasting significantly improved the SBS for metal brackets, with the highest micromechanical retention observed at the 1100-cycle aging interval median 10.48 MPa (IQR 0.93). Conversely, diamond bur treatment resulted in the lowest SBS values for both bracket types. Plastic and metal brackets differed in their debonding patterns during ARI-based failure category analysis; surface-treated plastic brackets were primarily associated with mixed failures and lower ARI scores, whereas similarly treated metal brackets showed higher ARI scores and a more distributed pattern of adhesive, mixed, and cohesive failures. Within the limitations of this in vitro study, sandblasting of 3D-printed UDMA PRRs produced more favorable SBS than diamond bur treatment, with metal brackets showing consistently higher SBS than plastic brackets.