The influence of water storage and C-factor on the dentin–resin composite microtensile bond strength and debond pathway utilizing a filled and unfilled adhesive resin

Objective: To test the elastic wall concept utilizing adhesive resins of varying stiffness in a low- and high-C-factor cavity design after short- and long-term water storage. Methods: A flat and box-shaped cavity was restored on occlusal dentin with a resin composite using a filled and unfilled adhesive resin from which microtensile specimens with a 0.5 mm2 cross-sectional area were formed. After storage for 30- and 150-days the microtensile bond strength (μTBS) was determined in a Zwick materials testing machine and the subsequent debond pathway was examined under scanning electron microscopy. Fisherʹs exact test was used to determine differences in joint and substrate failure modes and a Weibull regression model with gamma frailties was used to test for differences between failure distributions. Tests for three-way and two-way interactions were also completed for storage time, C-factor and adhesive. All tests were at 95% confidence levels. Results: The characteristic strength (TBS°) for the Optibond FL adhesive applied on a flat cavity was 47.57 and 20.90 MPa and a box-shaped cavity was 49.26 and 17.49 MPa for short- and long-term storage, respectively, while the corresponding TBS° for the unfilled Optibond adhesive on the flat cavity design was 36.93 and 32.68 MPa and in a box-shaped cavity was 32.84 and 15.46 MPa. Combining all groups according to storage time revealed a three-fold increase in the debond pathway including the bottom of the hybrid layer. Significance: Evidence suggests that the durability of the bonded joint is threatened by hydrolysis and the most susceptible region is the bottom half of the hybrid layer and in low C-factor cavity designs a more flexible adhesive resin liner was more durable.