SURFACE MELTING AND THERMAL ABLATION PATTERNS INDUCED IN ENAMEL AND CEMENTUM BY 10.6-m TEA-CO2 LASER RADIATION. III. THEORETICAL MODELS FOR PLASMA AND SURFACE WAVES EFFECTS

Further theoretical analysis of the physical interactions between the TEA-CO2 (10.6μm) laser radiation and dental enamel is reported. The ~2 s long pulses correspond to the melt expulsion domain where plasma is produced with a high yield by colisional ionization in vaporized substance, even at 1.25 10 7 W/cm 2 only (5 J/pulse and spots of 5 mm diameter). More than half of the incident energy is probably lost in formation of plasma and its screening effects, but returned partly as shock waves or infrared radiation. The shock wave pressure produced by plasma was shown to be inferior to the tensile strength of calcified tissues, and it does not disrupt directly the enamel. Thus the cracks and calcinations beyond the spot boundaries were assigned to radiative heating by plasma. The contribution of the later to these clinically undesired effects appears to be more important than downward and radial heat transfer during and after the laser pulse. Assuming a simple description of the hydrodynamic effects in the liquid layer in the melt expulsion domain, a new model was postulated. The non-resonant as well as the resonant periodic structures (NRPS and RPS) observed in both types of enamel were interpreted in terms of frozen surface capillary and thermal capillary waves of the melted enamel layer. Surface waves were generated by a sudden pressure build-up in the liquid phase at the center of the spot by direct heat generation from the Gaussian profile laser pulse, without plasma involvement. In the ‘shallow water’ regime, the relative energy fluxes per unit of wavecrest length and the relative total power stored in the wave systems were evaluated. The total stored energy was larger in the enamel with perpendicular prisms (42 m NRPS + 11 m RPS) as compared to the one with parallel prisms (68 m NRPS), in agreement with the larger surface density of defects in the former. If the 42 m NRPS and the 11 m RPS are assumed to be thermal capillary waves due to fluctuations only, both modes undergo equipartition and store a mean energy of kT/2 each, in agreement to the melt expulsion regime.