Thermal response of hard dental tissues to 9- through 11-mu m CO2-laser irradiation

The morphology and the chemistry of dental enamel and dentin can be modified by irradiation with a CO2 laser to increase the acid resistance of the intrinsic mineral. The changes induced in hard dental tissues after laser irradiation are predominately determined by the photothermally induced temperature rise in the tissue. Therefore the temperature rise in the irradiated enamel and dentin must be determined under controlled laser conditions. Surface and subsurface temperatures were monitored after multiple-pulse CO2-laser irradiation at lambda=9.3, 9.6, 10.3, and 10.6 mu m with 1- to 20-J/cm(2) pulses of 50- to 500-mu s duration using radiometry and microthermocouples. Surface temperatures were significantly higher after 9.3- and 9.6-mu m irradiation than for the more commonly utilized 10.6-mu m CO2-laser wavelength. Permanent changes in the temperature response of enamel and dentin were observed at fluences greater than 2 J/cm(2) and 100-mu s duration for dentin and 5 J/cm(2) for enamel, CO2-laser irradiation changes the thermal and the optical properties of these tissues, substantially changing the energy deposition for subsequent laser pulses. These changes affect both the amount of energy absorbed and the depth of absorption. The more efficient absorption at lambda=9.3 and 9.6 mu m may be advantageous for both caries-preventive treatments and ablation of exposed hard dental tissues while minimizing heat deposition in the tooth. (C) 1996 Society of Photo-Optical Instrumentation Engineers.