Characterisation of universal adhesive bonded resin-dentin interface after focused ultrasound smear layer conditioning

Objective: This laboratory study aimed to characterize the effect of thermally selected high intensity focused (HIFU) ultrasound parameters on the resulting self-etch mode universal adhesive bonded resin-dentin interface after smear layer conditioning using ultrasonic gel mediated acoustic energy transfer. Methods: Dentin slabs/beams were subjected to HIFU conditioning at four previously reported parameters (HIFU power levels: 20 W and 25 W, treatment time: 30 s and 60 s) and restored with Scotchbond universal adhesive in self-etch mode. The resulting resin-dentin interfaces were morphologically assessed for adhesive penetration via confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). Subsequently the interface was evaluated for matrix metalloproteinase (MMP) activity and nano-leakage via gelatin zymography (via CLSM) and scanning electron microscopy (SEM) respectively. Additional resin-dentin beams were fabricated and mechanical strength determined via immediate and aged (10000 thermocycles) micro-tensile bond strength (μTBS) tests. Fractographic analysis of the aged resin-dentin beams were also assessed using SEM. Results: Both 20 W and 25 W HIFU treatment for 60 s significantly (p < 0.05) increased rhodamine-B doped adhesive penetration, and the immediate and aged μTBS compared to the self-etch control. Minimal nano-leakage was present in the experimental group exposed to 25 W HIFU treatment for 60 s and analysis of the fractured μTBS samples show predominantly adhesive failure between the adhesive and composite interface, and cohesive failure within the adhesive layer. Gelatinolytic activity was detected in the dentin tubules for both the control and HIFU treated groups. However, only the self-etch control exhibited a continuous line of fluorescence at the resin-dentin interface, suggesting hydrolyzation of the fluorescein conjugated gelatin by MMPs at the hybrid layer. Significance: HIFU treatment at 25 W for 60 s optimises the dentin surface for bonding with Scotchbond universal adhesive in self-etch mode via smear layer removal. The removal of the obstructive smear debris promotes the formation of a durable resin-dentin interface via increased adhesive-dentin interaction.