Mechanism of dye-enhanced pulsed laser ablation of hard tissues: implications for dentistry

Alexandrite laser ablation of enamel enhanced by an indocyanine green dye was studied. A microjet system was employed to deliver precisely measured small amounts of absorbing dye solution to the site of irradiation. A sequence of physical phenomena involved in dye-enhanced laser ablation of dental enamel was revealed when laser pulse profiles were compared with the profiles of laser-induced pressure, laser-induced plasma, and ablation plume kinetics. To understand photomechanical effects on the enamel ablation, the absolute values of pressure waves were measured by a calibrated wide-band acoustic transducer. Absolute amplitude and temporal profile of pressure waves, plasma emission, ablation plume kinetics, ablation efficiency, and crater quality under free-running and and Q-switched ablation of enamel were studied. It was found that there is an optimal dye solution volume (100-200 nL) when the maximum ablation efficiency (30 μm/pulse) can be obtained. It was shown that the ablation efficiency under Q-switched laser irradiation is approximately one order of magnitude lower than that under free-running ablation. It was shown that Q-switched enamel ablation with dye solution is caused by the powerful recoil pressure wave with an amplitude 3-6.5 kbar. In contrast, dye-enhanced free-running enamel ablation is caused by plasma-mediated evaporation of enamel and accompanied by recoil pressure waves of lower amplitude (0.5-1 kbar) that is below mechanical damage threshold in enamel. Uneven crater walls after Q-switched ablation were observed by scanning electron microscopy (SEM). Free-running ablation makes precise craters with smooth and even crater walls.