High-intensity focused ultrasound: an innovative approach for micro-manipulation of demineralized dentine

BACKGROUND: Treatment of deep carious lesions poses significant challenges in dentistry, as complete lesion removal risks compromising pulp vitality, while selective removal often reduces the longevity of restorations. Herein, we propose a minimally invasive approach using High-Intensity Focused Ultrasound (HIFU) for microscale removal of carious dentine. Concurrently, HIFU's antimicrobial effects against associated cariogenic biofilms and the corresponding thermal and biological impacts on surrounding tissues were investigated. METHODS: A total of 238 sound human molars were utilized, with 203 samples of artificial carious-simulated dentine (ACSD) prepared for HIFU exposure. HIFU (250 kHz) was applied at 20 W for varying durations (60, 120, and 180 s). The acoustic waves were administered via a collimated cone coupled to the dentine surface using ultrasonic gel. RESULTS: Advanced characterization techniques including scanning electron microscopy (n = 5/group), Raman spectroscopy, atomic force microscopy, and nano-indentation (n = 5/group), demonstrated HIFU's effectiveness in removing demineralized collagen-fibrils. This was reflected in the increased mineral content, nano-hardness, and reduced elastic-modulus of ACSD lesions. Micro-CT (n = 6/group) confirmed the increase in mineral density post-HIFU exposure. Confocal microscopy of Rhodamine-B stained ACSD (n = 5/group) quantified the depth of dentine microscale removal post-HIFU exposure in a time-dependent manner. HIFU's potent anti-biofilm effect (n = 9/group) against Streptococcus mutans biofilms was evidenced by microscopic characterizations and significant reductions in metabolic-activity and colony-forming units. Furthermore, HIFU promoted the proliferation of dental pulp stem cells (n = 3/group) while maintaining the associated temperature-rise within the physiological tolerance. CONCLUSION: HIFU's potential as an innovative, minimally invasive, non-ionizing tool for dentine carious lesion micromanipulation was demonstrated through the interaction between focused acoustic waves and dentine, warranting further studies for future clinical translation in restorative and/or preventive dentistry.