Effect of chemical removal or mechanical modification of smear layer on dentin adhesion with universal resin adhesives

This study aims to investigate the effect of chemical removal or mechanical modification of the smear layer on dentin adhesion with universal adhesives. Dentin surfaces (92 teeth in total) were burnished using 320-grit silicon carbide abrasive paper, and further divided into four groups: control (Con, no treatment), phosphoric acid etching (Etc, chemical removal of smear layer), polishing with 800-grit silicon carbide abrasive paper, and ultrasonic cleaning (Pol and Ult, mechanical modification of smear layer). The scanning electron microscope was used to observe the dentin surface. Two universal resin adhesives with 'mild' self-etching (Clearfil universal bond quick, pH = 2.3) and 'ultra-mild' self-etching (All bond universal, pH = 3.2) were used. Resin-dentin bonding interface was observed by the confocal laser scanning microscope. The micro-tensile bond test (n = 20) was performed to evaluate the bond strength before and after 10,000 times thermal cycles. Failure mode was evaluated by stereomicroscope. Three-way ANOVA was used to analyze the effect of different smear layer treatments, adhesives, and thermal aging on & mu;TBS. Smear layer treatment, adhesive, and thermal aging all had a significant effect on bond strength. Pol and Ult groups of two adhesives had no difference in bond durability, which were significantly higher than Etc and Con groups. Etc and Con groups of 'mild' adhesive had better bond durability than that of 'ultra-mild' adhesive. After 24 h storage, the & mu;TBS of 'mild' adhesive in the Ult group was the highest (44.39 & PLUSMN; 10.71 MPa), so as after thermal cycles (41.53 & PLUSMN; 13.29 MPa). However, the & mu;TBS of 'ultra-mild' in the Etc group was the lowest (16.70 & PLUSMN; 4.72 MPa) among all groups. After thermal cycles, it also showed the lowest & mu;TBS (12.83 & PLUSMN; 6.75 MPa) as well. The mechanical modification of the smear layer could lead to more efficient dentin adhesion with universal resin adhesives.