Effect of different dentin etching protocols on the immediate microtensile bond strength of two bulk-fill restorative systems in class I cavities

Reliable bonding to dentin in high C-factor cavities remains a clinical challenge due to polymerization stress. Bulk-fill composites, including self- and dual-cured systems, aim to simplify placement and improve adaptation in deep cavities. However, limited data exist on how different dentin surface treatments affect their bond strength. Sixty extracted human molars with standardized Class I cavities were randomly assigned to six groups based on restorative system (self-curing bulk-fill, and a dual-cured bioactive composite system) and dentin treatment (self-etch, short 3-second etch, conventional etch-and-rinse). µTBS was evaluated after one month of water storage. Two-way ANOVA was used to assess the effects of material and dentin treatment, and failure modes were analyzed descriptively using Chi-square test. Dentin treatment significantly affected µTBS (p < 0.001), whereas the restorative system did not (p = 0.56). The highest µTBS was observed in the etch-and-rinse groups for both the self-cure (26.4 MPa) and the dual-cure systems (26.0 MPa), while the lowest values occurred in self-etch groups (self-cure: 18.0 MPa; dual-cure: 19.5 MPa). Failure mode analysis showed predominantly adhesive failures in self-etch groups and mixed failures in etch-and-rinse groups. Within the limitations of this study, etch-and-rinse dentin treatment appears to provide higher µTBS than self-etch. While short dentin etching could potentially offer a practical compromise, particularly for the self-curing bulk-fill composite. A short 3-second etch may improve bond strength over self-etch protocols, especially for the self-cure restorative system (Stela, SDI), suggesting a simple strategy to optimize adhesion while minimizing potential over-etching.