Abstract: Objective To explore the effect of ultrasound on dentin smear layer's surface and bonding strength of the universal resin adhesive under self-etching mode. Methods Forty mandibular third molars without caries were randomly divided into two groups; one was polished with silicon carbide sandpaper; the other was polished with silicon carbide sandpaper followed by ultrasonic treatment. Scanning electron microscope (SEM) was used to observe surface of the dentin. Treated teeth were bonded with two universal resin adhesives, Clearfil Universal Bond (pH=2.3) and All-Bond Universal (pH=3.1), and the penetration of the bonding interface was observed with a confocal laser scanning microscope (CLSM) after Rhodamine B staining. Finally, the micro tensile bond strength test was conducted to test the adhesion. Results The SEM showed that after polishing with silicon carbide sandpaper, the smear layer of the dentin surface was scratched, and dentin tubules were almost completely blocked, with no obvious dentin tubules exposed. After ultrasonic treatment, the scratches were reduced, and a large number of dentin tubules were exposed. CLSM showed that both adhesives could penetrate the dentin along the dentin tubules more deeply after ultrasound treatment. Micro tensile bond strength tests showed that ultrasonic treatment could enhance the bonding strength of two universal resin adhesives. However, there was no statistical difference in bonding strength between these two universal resin adhesives under the same treatment. Conclusion Ultrasound can partially remove the smear layer on dentin's surface, expose dentin tubules, and increase universal resin adhesives' penetration depth and bonding strength under self-etching mode.

Key words: ultrasound, dentin, smear layer, universal resin adhesive, micro tensile bond strength