Optoacoustic measurements were carried out in order to obtain better understanding of the ablation mechanisms during the illumination of hard dental tissue by Er:YAG laser radiation. A broadbard microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of tht laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the har i dental tissue independently of other spikes. This is in agreement with the model of ablation by means of micro explosions. The optoacoustic signal is observed to be approximately linearly related to the ablation efficiency, and is thus demonstrated to be a good measure of the ablation efficiency. The experiments als) show a significant difference in optoacoustic signals obtained during ablation in caries, enamel and dentin.
Enamel and dentin samples were exposed extraorally to pulsed TEA CO2 lasers with pulse durations of 1 tsec and 0.1 JLsec. The ablation rate is for energy densities above 5 J/cm2 independent of the CO2 laser energy. For 1js long CO2 pulses the ablation rate is 3 pm/pulse for drilling in enamel, and 8 pm/pulse for drilling in dentin. Drilling with 0.1 jis CO2 laser results in lower ablation rates of approximately 1 pm/pulse in enamel, and 4 pm/pulse in dentin. At all experimental energy densities plasma formation is observed, effectively reducing the amount of energy deposition. Compared with these results, experiments with the Er:YAG laser show that 200 ps long Er:YAG laser pulses achieve better ablation in the high energy density range because ablation is not diminished as rapidly by the plasma formation. The Er:YAG laser saturated ablation rates are approximately 60 pm/pulse for drilling in dentin and 40 pm/pulse for drilling in enamel.
The dependence of the output-energy efficiency and thermal load of an Er:YAG laser on the spectrum of exciting flash-lamp light has been measured. In particular, the influence of the direct and crossrelaxation pumping process has been studied. For pump pulses of 400 µs cross-relaxation pumping processes provide four times the efficiency and 1.5 times less thermal load normalized to fixed output energy.
In vitro study on extracted human teeth of the influence of Er:YAG laser power on the ablation efficiency and thermal damage has been carried out. Dependence of the diameter and depth of ablation holes in dentin and enamel was measured as a function of laser pulse energy and repetition rate. The drilling speed per pulse was found to be independent of the repetition rate in the range of 1-20 Hz. In the studied laser power range, and under water spray cooling, no visible damage was observed in enamel. In dentin, carbonization was observed only at very high laser powers. Experimental results show that at high laser powers thermal damage occurs more readily at higher energies per pulse than at higher repetition rates. The experiment therefore seems to indicate that dental lasers which allow higher repetition rates are better suited for laser drilling of hard dental tissue.
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