Free-running versus q-switched Er : YAG-laser (scanning mode) in photorefractive keratectomy

Background: Examination of morphology in plane corneal ablation (in vitro) induced by an Er:YAG-laser (2.94 mu m) in two modes: free-running (50 mu s) and q-switched (200 ns). Methods: Sequences of different fluences in each mode were applied to fresh ly enucleated swine eyes. Parameters of free-running mode: fluences 1.21-4. 77 J/cm(2), frequency 80 Hz, spot size 500 mu m FWHM, hexagonal spot shape. Parameters of q-switched mode: fluences 0.79-2.33 J/cm(2), frequency 20 Hz , spot size 500 mu m FWHM, round spot shape. Results: Histology showed thermal damage of 10-25 mu m in depth caused by t he free running mode compared with 4.5-7.5 mu m by the q-switched mode. In both gross photography and scanning electron microscopic examination, the s urface was found to be more homogeneous and smoother in the q-switched made . Conclusions: Depending on the different application modes, both laser syste ms could be used fora defined corneal ablation in photorefractive keratecto my. However, at the moment, results using the Er:YAG laser are not as favor able as with the excimer laser.