Effects of Surface Pretreatment Protocols on Interfacial Gap Volume and Shear Bond Strength in the Composite Repair of a Definitive 3D-Printed Dental Resin: An In Vitro Study

Surface pretreatment may influence adaptation and mechanical performance in repaired 3D-printed restorations. This in vitro study evaluated six protocols for interfacial gap volume and shear bond strength (SBS) and examined their association. Seventy-two Ceramic Crown specimens were randomly allocated to an adhesive-assisted control or protocols involving air abrasion, a silane-containing primer, and hydrofluoric acid etching, alone or combined (n = 12). After standardized adhesive application and composite repair, interfacial gap volume was quantified by micro-computed tomography, followed by SBS testing of the same specimens. Both outcomes were analyzed using log-transformed one-way analysis of variance, and Spearman correlation was used to assess their association. No significant group effect was detected for interfacial gap volume (p = 0.222) or SBS (p = 0.074). Interfacial gap volume was not significantly correlated with SBS (ρ = 0.125; 95% CI, −0.085 to 0.327; p = 0.294). Failure-mode distributions differed significantly among the surface pretreatment groups (p = 0.0027). Following short-term water storage without additional artificial aging, no pretreatment protocol demonstrated a statistically significant advantage for either outcome in the primary six-group comparisons; these findings cannot be directly extrapolated to repair after prolonged clinical service.