[Silicone impression procedure. Principles for determining ablation and healing parameters in vitro and in vivo]

Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. So far, ablation rates have been determined either by tissue perforation or histological micrometry. A cast of irradiated cornea was made by using a 2-component silicon that polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. The silicon surface is extremely smooth and the accuracy of the cast better than 1 micron. Reproducibility and long-term stability were proven for casts of photoablated PMMA. Thus ablation rates and profile, volumetry and topometry can be determined at any time. The method was applied for 193 nm excimer and 2.94 microns Er:YAG laser in vitro irradiation of the human cornea. Ablation rates in Bowman's layer and stroma were observed for various radiant energies and distinct pulse numbers. The average ablation rates are in agreement with published data. It could be demonstrated that there was an incubation effect for the first pulses. The method feasibility for in vivo measurement was also proven in an animal model. After excimer photoablation in rabbit eyes (beam diameter 3.5 mm, radiant energy 185 mJ/cm3, ablation rate 0.3 micron/pulse), wound healing was recorded: for the right eye silicon casts were molded at three different moments. For the left eye only photographs were taken. There was no difference in the time course of wound healing, so the silicon does not seem to interfere with repair mechanisms of the corneal epithelium. No side effects were observed.