Microleakage of Class II composite restorations bonded with universal adhesives containing different functional monomers: an in vitro study

BACKGROUND: This in vitro study aimed to evaluate and compare the microleakage of Class II composite restorations bonded with universal adhesives containing different functional monomers, with particular emphasis on a chemically cured universal adhesive applied using different etching strategies. METHODS: Standardized Class II cavities were prepared in 120 extracted human molars and randomly allocated into 12 groups (n = 10) according to the adhesive system and etching strategy. The adhesives evaluated included a three-step etch-and-rinse system, a two-step self-etch system, and three universal adhesives applied in total-etch, selective-etch, and self-etch modes, including a chemically cured universal adhesive. All cavities were restored with a nanohybrid resin composite and subjected to 5000 thermocycles between 5 °C and 55 °C. Microleakage at occlusal and gingival margins was assessed using 0.5% basic fuchsin dye penetration under stereomicroscopy. Data were analyzed using non-parametric statistical tests (α = 0.05). RESULTS: Significant differences in microleakage were observed among the groups (p < 0.001). The three-step etch-and-rinse adhesive demonstrated the lowest microleakage values at both occlusal and gingival margins. Universal adhesives applied in self-etch mode exhibited significantly higher microleakage, particularly at the gingival margin. The chemically cured universal adhesive showed comparable marginal sealing only when combined with phosphoric acid etching. CONCLUSIONS: Within the limitations of this in vitro study, multi-step adhesive systems and universal adhesives used with total-etch or selective-etch strategies provided superior marginal sealing compared with single-step self-etch applications. These findings highlight the clinical importance of etching strategy selection in Class II composite restorations, particularly at gingival margins where light attenuation may compromise polymerization in deep proximal boxes.