Evaluation of bond strength and internal adaptation between the dental cavity and adhesives applied in one and two layers.

This research evaluated the influence of the number of adhesive layers of three adhesive systems on microtensile bond strength (microTBS) to dentin and the internal adaptation between the dental structure and the resin composite restoration. Two cavities (C-factor = 3) were made on the buccal surfaces of 30 bovine incisors. Scotchbond Multi-Purpose (MP), Adper Single Bond 2 (SB) and Clearfil SE Bond (CSB) were applied in one layer (MP-I, SB-I and CSB-I) and in two layers (MP-II, SB-II and CSB-II). The cavities were restored with Z250 resin composite. After 48 hours, beams were obtained (n = 15/group) for microTBS testing, and slices containing the two restorations were obtained for internal adaptation analysis. The beams were submitted to the microTBS test on a universal testing machine (EMIC DL-2000) and the failures were analyzed by SEM. The slices were analyzed under an optical microscope and the types and amounts of internal failures (gap, crack, rupture) were classified. The mean values of microTBS (MPa) were: MP-II: 56.92; MP-I: 52:23; CSB-II: 47:71; CSB-I: 42.25; SB-I: 35.12 and SB-II: 34.69. According to two-way ANOVA (alpha = 0.05), the independent variables adhesive system (p = 0.001) and the number of layers (p = 0.025) presented significant difference. The mixed failure was predominant in all groups. For the internal adaptation analysis, the Kruskal-Wallis test (alpha = 0.05) showed statistical differences for cracks and ruptures between the adhesive systems. SB-I and II were shown to have the highest values for cracks, and MP-I demonstrated the lowest. SB-II showed significantly higher values for rupture. The application of two layers promoted a slight increase in microTBS for Scotchbond MP and Clearfil SE Bond, but it did not improve the internal adaptation for the tested adhesive systems.