Histological comparison of corneal ablation with Er : YAG laser, Nd : YAG optical parametric oscillator, and excimer laser

PURPOSE: To use histological techniques to assess and compare the ablation depth, local damage, and surface quality of corneal ablations by a Q-switch ed Er:YAG laser, an optical parametric oscillator laser at 2.94 mu m, a lon g pulse Er:YAG laser, and a 193-nm excimer laser. METHODS: Human cadaver eyes and in vivo eat eyes were treated with a 6.0-mm diameter, 30-mu m-deep phototherapeutic keratectomy ablation and a 6.0-mm diameter, -5.00-D photorefractive keratectomy ablation. Human cadaver eyes mere also treated with a 5.0-mm diameter, -5.00-D laser in situ keratomileu sis (LASIK) ablation, Fluences and pulse widths used were 200 mJ/cm(2) and 70 ns for the Q-switched Er:YAG, 150 mJ/cm(2) and 70 ns for the optical par ametric oscillator laser (OPO), 500 mJ/cm(2) and 50 microseconds for the lo ng pulse Er:YAG, and 160 mj/cm(2) and 20 ns for the excimer laser In the ab lation rate study, 12 porcine eyes were ablated by the OPO laser with a ran ge of layers and at different fluences ranging from 60 to 150 mJ/cm(2), all using a 1.5-mm spot on the eye. The ablation depth of these acute ablation s was evaluated by Light microscopy examination. RESULTS: In the acute damage study, light microscopy showed a thin surface layer in all samples with minimal thermal damage except on the long pulse E r:YAG corneas. Transmission electron microscopy revealed less than 0.3-mu m surface damage for an specimens of both the optical parametric oscillator and the excimer laser samples with no evidence of collagen shrinkage. Trans mission electron microscopy showed damage layers of 0.5 to 3 mu m for Q-swi tched Er:YAG and 3 to 10 mu m for long pulse Er:YAG. Scanning electron micr oscopy showed smooth surfaces in all eyes, although the excimer was the rou ghest. In the porcine eye study ablations were produced in both PTK and PRK modes with the ablation rate per layer increasing with the fluence. At 120 mJ/cm(2), the average ablation rate was 1.9 mu m per layer. CONCLUSIONS: The histology from the short pulse mid-infrared optical parame tric oscillator laser at 2.94 mu m was comparable to the 193-nm excimer wit h a smooth, damage-free, ablation zone when performing PRK and LASIK.