Importance of the hybrid layer on the bond strength of restorations subjected to cyclic loading

The present study evaluated the long-term microtensile bond strength of collagen-depleted dentin followed by cyclic loading. Bovine flat, midcoronal dentin received one of the following surface treatments: acid-etch or acid-etched + 5% NaOCl for 2 min. The teeth were restored with Single Bond, Scotchbond Multi-purpose, One-Step Plus, or All-Bond 2 adhesive systems. Half of the specimens were randomly assigned to receive 200,000 cycles (50 N force). Teeth were sectioned into 1 x 1 mm thick slices and stored for 24 h, 1 month, 3 months, or 6 months. Specimens were subjected to tensile testing after elapsed storage time. Samples were analyzed by three-way ANOVA and Fisher's PLSD (p < 0.05). The results showed that the deproteinized groups without cyclic loading presented lower bond strength when compared with the control group, but the difference was only statistically significant for Single Bond and All-Bond 2 (p < 0.05). All adhesives presented a decrease in bond strength over time, regardless of the dentin treatment. When cyclic loaded, the decrease in bond strength for the deproteinized group was even greater when compared with the control group (p < 0.05). The results suggest that the hybrid layer is important as a stress-buffering layer when loading is applied, and thus the presence of collagen is essential.