CORNEAL ABLATIONS PRODUCED BY THE NEODYMIUM-DOPED YTTRIUM LITHIUM-FLUORIDE PICOSECOND LASER

This study examines corneal ablations produced by the neodymium doped yttrium-lithium-fluoride (Nd:YLF) picosecond laser. The laser delivers a 1-KHz, 40-ps pulsed, 1,053-nm wavelength beam (with energy measured in microjoules) to a 15-mum diameter spot size. The ablation mechanism is by plasma formation, which generates acoustic shock waves. Using enucleated rabbit (n = 25) and human donor eyes (n = 29), corneas were examined after tissue ablation at energies ranging from 40 to 300 muJ per pulse with various programmed ablation depths and patterns. The histologic data were collected using light microscopy and transmission electron microscopy. The tissue effects and Nd:YLF laser functions studied were ablation thresholds, cutting ability, programmed ablation depth accuracy, and acute endothelial effects. Our study showed histologic ablation thresholds for the following human corneal layers: epithelium = 7.15 +/- 0.05 x 10(11) W/cm2 (34.1 +/- 8.1 J/cm2 per pulse, 50 muJ per pulse); Bowman's layer = 1.33 +/- 0.29 x 10(12) W/cm2 (58.5 +/- 3.3 J/cm2 per pulse, 100-110 muJ per pulse); stroma and endothelium = 7.10 x 10(11) W/cm2 (28.4 J/cm2 per pulse, 50 muJ per pulse). Depth of corneal ablation was found to be directly related to energy and independent of programmed ablation depth. This study shows the endothelial loss in rabbit corneas by energy beams (50 muJ per pulse) focused 100 mum from this layer.