Theo Lasser

Mathematical simulation of retinal image contrast after photorefractive keratectomy with a diaphragm mask.

BACKGROUND Photorefractive keratectomy (PRK) using a dilating diaphragm mask engraves a delicate three-dimensional staircase pattern into a formerly smooth corneal surface. The created steps are later smoothed by tear film and wound healing processes. The present study investigates, in a mathematical simulation, the effects that such staircase patterns and their smoothing may have on retinal image contrast. METHODS All simulations are based on the Gullstrand eye model and calculate retinal image contrast from point spread function (PSF) analysis of Gullstrand eyes treated by simulated PRK under various conditions. RESULTS The simulations indicate that PRK can reduce retinal contrast markedly. The most critical factor for such a reduction is the step height of the ablation pattern. With step heights below 0.4 microns, loss of contrast due to the created staircase pattern is always moderate and should be restored during early wound healing. Complete wound healing may smooth out larger step heights. Micromovements during PRK also can lead to partial loss of retinal image contrast. CONCLUSIONS Simulation of retinal contrast after PRK shows that step heights below 0.4 microns seem to be acceptable. A minimization of the micromovements during PRK can offset some of the reduction of retinal contrast.

Photoablation on bovine cornea with a Q-switch Er:YAG laser

Photoablation of ocular tissues can be performed with free running Er:lasers, but their application leads to significantly more thermal tissue damage than photoablation with ArF excimer lasers. This means a serious drawback to clinical investigations on treatments with Er:lasers. However, an essential reduction of the thermal damage zone is achieved by photoablation with a Q-switched Er:YAG laser (TEMoo, tp equals 100 nsec). Detailed results of our work on bovine cornea are presented.