High intensity focused ultrasound enhances bonding of universal adhesive to dentin in self-etch mode

Objective: To investigate the effect of high intensity focused ultrasound (HIFU) exposure on enhancing the bonding of universal adhesives, in self-etch mode, to dentin. To achieve this aim, the effects of HIFU exposure on dentin micro-topography, structure, and nanoscale mechanical and chemical properties were investigated. In addition, resin-dentin interfacial morphology and micro-tensile bond strength (mu TBS) were evaluated. Methods: The effect of HIFU exposure at 5, 15, or 30W at varying time intervals (30-120 s) on the dentin surface properties were characterised by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and nano-indentation testing. The interfacial junctions between Scotchbond universal adhesive, in self-etch mode, and HIFU-treated dentin were characterised using confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). Furthermore, mu TBS tests were conducted before and after thermocycling. Results: HIFU exposure at 30W for 120 s resulted in almost complete removal of the smear layer. HIFU treatment at 15W for 60 and 90 s, and at 30W for 60 s significantly increased dentin surface roughness. Raman analysis indicated progressive mineral removal at higher HIFU power levels. HIFU treatment at 5, 15, and 30W did not significantly affect dentin reduced elastic-modulus and nano-hardness. Following HIFU application at 30W for 120 s, CLSM examination indicated marked increase in adhesive penetration. HIFU exposure for 120 s at 30 W was effective in removing the overlying smear layer facilitating partial demineralisation of the underlying superficial dentin substrate. Whilst HIFU treatment did not improve the immediate mu TBS of Scotchbond universal adhesive in self-etch mode, 120 s HIFU treatment at 30W significantly improved the aged mu TBS values compared to the control. Significance: HIFU treatment of dentin surface at 30W for 120 s enhanced the structural integrity of resin-dentin interfacial junctions and the mu TBS, after thermocycling, of a mild universal adhesive applied in self-etch mode.