Repair Bond Strength of Self‐Cure Bulk‐Fill Composites: Influence of Substrate Composition, Surface Conditioning, and Repair Material Type

OBJECTIVE: This in vitro study aimed to evaluate the effects of substrate composition, surface conditioning protocol, and repair material type on the repair shear bond strength (SBS) of an aged self-cure bulk-fill resin composite. MATERIALS AND METHODS: A total of 80 specimens were fabricated using two substrate compositions of a self-cure bulk-fill composite (Stela; Automix and Capsule). After initial aging by thermocycling, specimens were repaired according to a 2 × 2 × 2 factorial design based on surface conditioning (mechanical roughening with or without 37% phosphoric acid etching) and repair material type (self-cure composite with proprietary primer or light-cure nanohybrid composite with a universal adhesive). SBS was measured using a universal testing machine. Failure modes were analyzed using a stereomicroscope. Data were analyzed using three-way ANOVA, Tukey's HSD, and Fisher's exact tests (p < 0.05). RESULTS: Substrate composition, surface conditioning protocol, and repair material type each significantly affected repair bond strength (p < 0.001). Capsule-based substrates showed higher SBS values than Automix substrates. Phosphoric acid etching significantly increased SBS compared with mechanical roughening alone. Repairs performed with the same self-cure composite exhibited significantly higher SBS than those repaired with a light-cure composite. Significant two-way interactions were observed between substrate composition × repair material type (p < 0.001) and surface conditioning × repair material type (p = 0.015). The highest SBS was observed in the Capsule + acid etching + self-cure repair group. Failure mode analysis revealed predominantly cohesive or mixed failures in self-cure repair groups, whereas light-cure repair groups showed a higher frequency of adhesive failures. CONCLUSION: Repair bond strength of a self-cure bulk-fill composite is highly material- and protocol-dependent. Utilizing a chemically compatible self-cure material system combined with adjunctive phosphoric acid etching significantly optimizes repair longevity, especially when applied to mechanically triturated capsule-based substrates. CLINICAL SIGNIFICANCE: The selection of repair material and surface conditioning protocol significantly influences the repair bond strength of aged self-cure bulk-fill composites. When the original restorative material is known, repair with the same composite system following phosphoric acid conditioning may provide more durable bonding.