Wavelength and Average Power Density Dependency of the Surface Modification of Root Dentin Using an MIR-FEL

Abstract

Surface modification of root dentin by mid-infrared (MIR) pulsed-laser irradiation is one of the candidates for a novel, non-invasive treatment to prevent root surface caries. To modify root dentin effectively and non-invasively it is essential to estimate quantitatively and qualitatively the laser parameters, such as the wavelength and the power density, required for surface modification. The key aspect is to bring about effective surface modification of root dentin while minimizing the unwanted removal of the underlying dentin. Using a tunable, MIR Free Electron Laser (FEL) in the wavelength region of 8.8-10.6 μm, we have investigated macroscopically the extent of surface modification (morphological and chemical changes) of root dentin. We have obtained experimental results related to the ablation depth, the MIR absorption spectrum, and the elemental chemical composition of the irradiated dentin. The observations showed that the surface modification of root dentin was inclined toward well-recrystallized, HAp-like material, leading to an increase in the acid resistance and dentinal tubule sealing. The laser parameters, at which efficient surface modification without enhanced ablation occurred, were estimated to be at λ= - 9.0 μm or - 9.7 μm and in the average power density region of - 10-20 W/cm2 (resulting in energy density per macropulse and peak power density regions of - 1-2 J/cm2 and - 66.7-133.3 kW/cm2). Thus, it was found that the surface modification of root dentin strongly depends on the laser conditions applied. We conclude that the optimum laser wavelengths for (1) root surface caries treatment without restorative procedure and (2) hard tissue ablation without water irrigation are around (1) λ= 9.0 μm or 9.7 μm and (2)λ= 9.0 μm in the absorption bands due to P-O stretching, respectively.