Expansion characteristics of laser-induced stress waves in eye

Background: The characteristics of shock waves during photoablation were investigated for an IR and a UV laser. These stress waves may be harmful to ocular structures. Material and methods: The amplitude of shock waves was measured by a needle-shaped hydrophone in enucleated porcine eyes during excimer laser (193 nm, 23 ns, diameter of ablation 1.5-7.5 mm) and Er:YAG laser photoablation (2.94 mu m, 200 mu s, 1.2 mJ/cm(2), diameter of ablation 4 mm). Results: With the excimer laser at ablation zones larger than 4.5 mm, a pressure focus occurs at a distance of 4-6 mm behind the cornea. The pressure amplitudes are smaller than 80 bar for a fluence of 180 mJ/cm(2) and decrease steadily to values below 10 bar towards the retinal level. Higher fluences produce higher pressure values; in the range of 60 to 220 mJ/cm(2) the relation is linear. For the Er:YAG laser, pressure amplitudes are smaller than 0.5 bar. Conclusions: Mechanical damage of the retina is unlikely during excimer-or Er:YAG-laser ablation. The existence of a pressure focus may result in mechanical damages of the posterior lens or anterior vitreous at large ablation diameters. During Er:YAG laser ablation, shock waves could not be detected with our measurements. Theoretical estimations yield values of less than 700 mbar at a fluence of 1.2 J/cm(2). The pressure load of the endothelium is independent of diameter but dependent on fluence.