Laser ablation as a function of the primary absorber in dentin.

Abstract

Infrared transmission spectra of dentin reveal a broad absorption band between 6.0 and 7.0 microns composed of absorption peaks of water, collagen and carbonated hydroxyapatite. The nearly constant absorption and the existence of absorption peaks of different tissue components were used to investigate ablation as a function of the primary absorber. Laser ablation of dentin as a function of fluence was studied in the wavelength range between 6.0 and 7.5 microns using the Vanderbilt Free-Electron Laser (FEL). Depth and volume of the ablation crater were determined with a silicon replica method and subsequent confocal laser topometry. SEM investigations were performed on the irradiated surfaces. For the description of the experimental data an ablation model is developed. At all applied wavelengths we found a linear increase of ablation depth as a function of fluence above a threshold fluence. The lower absorption of dentin at 7.5 microns compared to the absorption at 6.0, 6.5 and 7.0 microns results in a greater ablation threshold. At 6.0, 6.5 and 7.0 microns wavelengths the ablation thresholds are comparable. The experimental data are in good agreement with an ablation model using a mean absorption coefficient of the target material. No thermal cracking is observed after ablation in dentin. The post ablative surface structure at 6.0 and 7.0 microns looks similar whereas at 7.5 microns the surface reveals a greater roughness. The ablation efficiency and threshold depend on the mean absorption but do not depend upon the chemical identity of the primary absorber in dentin. Calculations show that heat conduction during the laser pulse leads to a thermal equalization between the heated microstructures and surrounding tissue resulting in an ablation with little dependence on the primary absorber.