Bond failure at dentin–composite interfaces with ‘single-bottle’ adhesives

Objectives. Significant differences in interfacial fracture toughness (intK1c) among six dentinal adhesives have been reported. Resulting fractured test specimens were examined under SEM to determine differences in micromorphology, which might account for the respective intK1c results. Methods. Interfacial fracture toughness specimens were assembled from bovine dentin and P50 resin composite under moist conditions, using one of five ‘single-bottle’ adhesives; either Single Bond, One Step, Optibond Solo, Prime & Bond 2.1, Bond 1 or a resin-modified glass ionomer, Fuji Bond LC. After fracture toughness testing, four fractured specimens from each group were sectioned transversely, critical-point dried and examined under SEM. Results. Most bond failures occurred at the interface between adhesive resin and the top of the hybrid layer. Single Bond and One Step gave the highest intK1c results and showed good resin infiltration at this interface. Average film thickness of unfilled adhesives was 30 μm. When this intermediate adhesive layer was too thin, resin infiltration was poor and associated with low fracture toughness. Filled adhesives, Optibond Solo and Fuji Bond LC, formed thick films varying from 60 to 250 μm which failed cohesively, effectively sealing the dentin surface despite moderate fracture toughness results. Conclusions. Most bond failures occurred between the adhesive and hybrid layers. Good resin infiltration at the top of the hybrid layer combined with an intermediate adhesive layer of adequate width produced a fracture-resistant interface. Filled adhesives failed cohesively, providing a good dentin seal, despite material fracture.