Corneal morphology in vitro after superficial keratectomy with Q-switched Er:YSGG and free-tunning Er:YAG lasers

PURPOSE: Examination of morphology in corneal ablation induced by a q-switched Er:YSGG (2.79 mu m) laser and a free-running Er:YAG laser (2.94 mu m). METHODS: Defined ablation of 6-mm diameter and 15, 30, 55, 90, and 120-mu m depth was performed on freshly enucleated swine eyes. Er:YSGG laser parameters: fluence 1.6 J/cm(2), frequency 6 Hz, spotsize 465 mu m FWHM, scanning-mode, pulse number 1520 to 6210. Er:YAG laser parameters: fluence 3 J/cm(2), frequency 1.5 Hz, spotsize 6 mm, wide area ablation, pulse number 2 to 13. Corneal morphology was analyzed by gross photography, histology, scanning electron microscopy, and scanning nearfield acoustic microscopy. RESULTS: Histology showed thermal damage of 5 to 15 mu m in depth caused by the Er:YSGG laser in comparison with 10 to 20 mu m by the Er:YAG laser. Average roughness of the ablated surface measured with scanning nearfield acoustic microscopy was 20 to 40 mu m for the Er:YSGG laser and 5 to 15 mu m for the Er:YAG laser. These data confirm the subjective impression of images created by scanning electron microscopy and gross photography. CONCLUSIONS: Although the Er:YAG laser system appeared to demonstrate a smoother corneal surface than the q-switched Er:YSGG laser, the thermal damage in either case poses a potential limitation for clinical use in lamellar refractive surgery.