Thermal and chemical modification of dentin by 9-11-mu m CO2 laser pulses of 5-100-mu s duration

Background and Objectives: Previous studies have shown that dentin can be thermally modified by pulsed CO2 laser irradiation to form a more highly mineralized tissue. The implications are important for the potential laser modification or removal of dentinal and root caries and the transformation of dentin to a more acid resistant mineralized tissue. Study Design/Materials and Methods: Time resolved radiometry measurements with TEA CO2 laser pulses were used to determine the magnitude of the absorption coefficients of dentin at the highly absorbed CO2 laser wavelengths and to measure the temperature excursions during lambda = 9.3, 9.6, 10.3, and 10.6 mum laser irradiation at irradiation intensities of 0.1-8 J/cm(2) per pulse. In addition, photoacoustic and transient reflectance measurements were used to monitor the loss of water and organics and to detect the thresholds for surface modification and tissue ablation. Results: The absorption coefficients were measured to be 5,000; 6,500; 1,200; and 800 cm(-1) at k = 9.3, 9.6, 10.3, and 10.6 mum, respectively. The surface temperatures of dentin were markedly higher than those measured on enamel for similar irradiation intensities due to the lower reflectance losses of dentin and the lower thermal diffusivity of dentin at the respective wavelengths. Hence, lower fluences are required for the thermal decomposition of dentin. Ablation typically occurred with the first few laser-pulses during multiple pulse irradiation and eventually ceased after modification of dentin to a more highly mineralized enamel-like tissue. The debris ejected during the initial laser pulses shielded the surface by as much as 60% at the low fluences employed in this study. Optical and electron microscopy and IR spectroscopy indicated that incident laser pulses with incident fluence as low as 0.5 J/cm(2) at 9.3 and 9.6 mum wavelengths with a duration of 5-8-mus were sufficient to induce chemical and morphological changes in dentin. Conclusions: In this study, the laser parameters for the efficient thermal modification of dentin with minimum heat deposition at CO2 laser wavelengths were firmly established.