[Structure and dynamics of photo-acoustic shock-waves in 193 nm excimer laser photo-ablation of the cornea].

Abstract

The structure and dynamics of acoustic shock waves generated with a 193-nm ArF-excimer laser pulse (20 ns) in corneal photoablation were investigated using piezoelectric transducers (PVDF foily). The shock waves passed through the cornea at the speed of sound (1630 +/- 120 m/s). The duration of the acoustic half-value thickness was approximately 60 ns. At clinically relevant laser energy densities (200 mJ/cm2) the amplitude of the shock-waves was 80 bar. At higher energy densities (500 mJ/cm2) the amplitude reaches 150 bar. When laser pulses were applied with a spot size of 100 microns the amplitude of the acoustic transient decreased inversely with the distance to the interaction zone. At large beam diameters (4 mm), however, even at a distance of 3 mm from the interaction zone no significant decrease of the shock-wave amplitude could be found. It is assumed that the mechanical stress involved in laser-induced acoustic shock-waves may be the cause of cellular alterations and the structural damage to adjacent collagen layers leading to the onset of postoperative scar formation.