Influence of Surface Conditioning Protocols and Lining Materials on the Fracture Resistance of Aged <scp>3D</scp> ‐Printed Onlay Restorations

ABSTRACT Objective This in vitro study evaluated the effects of surface conditioning protocols and lining material type on fracture resistance and failure behavior of 3D‐printed methacrylate resin onlay restorations. Materials and Methods One hundred twenty 3D‐printed onlays were fabricated and allocated to four surface conditioning protocols: no treatment, airborne particle abrasion with aluminum oxide (Al 2 O 3 ), universal adhesive application, and combined Al 2 O 3 abrasion followed by adhesive application. Each group was further subdivided according to the lining material used: bis‐acryl composite, polymethylmethacrylate, or flowable composite ( n = 10). Following surface conditioning and lining procedures, all specimens were subjected to thermomechanical aging. Fracture resistance was subsequently measured using a universal testing machine. Results Two‐way ANOVA revealed that the surface conditioning protocol ( p &lt; 0.001) and lining material ( p &lt; 0.001) had a significant influence on fracture resistance, with a significant interaction between the two factors ( p = 0.003). Surface‐treated specimens showed significantly higher fracture resistance than untreated controls ( p &lt; 0.05). The combined Al 2 O 3 and adhesive protocol produced the highest fracture resistance values, although differences among surface‐treated groups were not consistently significant ( p &gt; 0.05). Flowable composite liners demonstrated significantly higher fracture resistance than bis‐acryl composite and polymethylmethacrylate across all protocols ( p &lt; 0.05), whereas polymethylmethacrylate showed the lowest values. Failure analysis revealed a shift from predominantly adhesive failures in control groups to mixed and cohesive failures in surface‐treated specimens. Conclusions Optimized surface conditioning combined with flowable composite lining enhances fracture resistance and interfacial integrity of 3D‐printed resin onlay restorations. Clinical Significance From the clinical perspective, enhanced surface conditioning accompanied by flowable composite lining seems to be an effective approach for augmenting the fracture resistance and longevity of repaired or relined 3D‐printed resin onlay restorations postaging.