[Photoablation of the cornea with pulsed 2790 nm ErCr:YSGG laser irradiation. Basic studies]

The potential of 3 microns solid-state lasers as an alternative to excimer lasers for photoablative corneal surgery was investigated. A Q-switched ErCr:YSGG laser (2790 nm, 200 ns) was used for irradiation of porcine corneas and agar-agar samples. Mechanical tissue effects (stroma, endothelium) were documented by micromorphology. Laser-induced shock-waves were analyzed by piezo-electric transducers. No sharp ablation threshold, as in excimer laser photoablation, could be determined. Energy fluences < 2 J/cm2 led to dehydration of the irradiated samples. Higher fluences are necessary for the evaporation of tissue water to be so vigorous that the tissue matrix is expelled along with the organic matrix. At high fluences, the ablation rate exceeds the absorption depth of the laser radiation (up to 25 microns/pulse). At fluences between 2.5 and 28 J/cm2 the thermal necrosis zone adjacent to the crater was 7 +/- 3 microns. The intensity of the laser-induced acoustic shock waves can peak to some hundred bar. Small gas bubbles up to 1 mm were found in the surrounding area of the ablation crater. Apparently, they were pressed between the collagen lamellas by the explosive force of the ablative process. In deep excisions (> 75%) endothelial defects underneath the beam axis could be documented. Large-area tissue ablation, with a resolution in the range of 1 micron, as necessary in myopia correction, will not be possible with the present generation of ErCr:YSGG lasers. Its high ablation rate makes this laser suitable as a cutting (astigmatism, keratoplasty, vitreous surgery) and drilling (glaucoma) device.