The effect of interface stiffness on dentin–composite interfacial fracture resistance

Objectives: The dentin–composite interface should withstand stresses that develop initially during composite polymerization and later during clinical function. The elastic behavior of the dentin–composite interface, which could be represented by an interfacial stiffness parameter, is not completely understood. The purpose of this study was to measure the relative interfacial stiffness of dentin–composite interfaces formed using commercially available dentin bonding agents and to relate these relative interfacial stiffness results to previously reported interfacial fracture toughness (KICi) values. Methods: The miniature short-rod fracture toughness specimen geometry containing a dentin–composite interface was used for testing. Specimens were aged in distilled water at 37°C for 180 days prior to tensile loading at an extension rate of 0.5 mm/min. The relative interfacial stiffness was determined from the initial slope of the force–displacement curve that was obtained from each fracture toughness test. ANOVA, Fishers LSD, and linear regression tests were used for statistical analyses (p<0.05). Results: The relative interfacial stiffnesses ranged from 16 to 92 N/mm. The groups containing dentin–composite interfaces with greater depths of surface dentin demineralization generally exhibited higher interfacial stiffness than those with less dentin demineralization. Bonding to “wet” dentin produced higher relative interfacial stiffness results than bonding to “dry” dentin. A significant positive linear relationship was found among the individual (p=0.0001, r2=0.58) and the mean (p=0.0004, r2=0.97) relative interfacial stiffness and KICi results. Conclusions: The significant positive relationship between the relative interfacial stiffness and KICi results suggest a benefit (increased interfacial fracture resistance) of a relatively stiff dentin–composite interface.