Effect of Multiple Coats of Ultra-mild All-in-One Adhesives on Bond Strength to Dentin Covered with Two Different Smear Layer Thicknesses

Purpose: To evaluate the effect on bond strength of 1, 2, or 3 coats of two one-step self-etching adhesives on dentin covered with two different smear layer thicknesses. Materials and Methods: Flat dentin surfaces from recently extracted third molars were wet ground with 60- or 600-grit SiC paper to produce smear layers of different thicknesses. Adper Easy Bond (3M ESPE) and Clearfil S3 Bond (Kuraray) were applied in 1, 2, or 3 consecutive coats after air drying each coat. ScotchBond Multi Purpose (3M) and Clearfil SE Bond (Kuraray) were used as controls. Composite resin crowns (Filtek Z250, 3M ESPE) were built up, stored for 24 h, and cut in x and y directions. Eight to twelve sticks (0.65 ± 0.05 mm2 of area) from the central area of each tooth were fractured in tension (0.5 mm/min). Sticks from each group were processed for interfacial micromorphological evaluation with SEM. Results: Three-way ANOVA revealed a significant interaction between factors (p 0.05). Tukey's post-hoc test showed that only when Adper Easy Bond was applied on dentin prepared with 600-grit SiC paper, microtensile bond strength was not significantly affected by the number of coatings. However, the effect of smear layer thickness was only statistically significant when the adhesives were applied in one coat, for both adhesives. When they were applied in two or three coats, smear layer thickness did not significantly affect bond strength (p > 0.05). Conclusions: The thickness of the smear layer affected the bond strength of both all-in-one adhesives. Additional coats can be beneficial to bond strength, especially with Clearfil S3 Bond. Keywords: smear layer, all-in-one adhesives, microtensile, bond strength, dentin