[Intrastromal refractive corneal surgery with pico-second Nd:YAG laser pulses]

Intrastromal laser surgery with picosecond pulses aims to achieve refractive changes of the cornea without damaging the epithelium, Bowman's membrane, or the endothelium. For that, a tissue layer with well-defined thickness has to be evaporated by creating laser plasmas within the corneal stroma. We investigated the plasma formation and the plasma-induced shock wave emission and bubble generation (cavitation) in the cornea, as well as the tissue effects and the range for endothelial damage. The laser light source used was an Nd:YAG laser emitting pulses with a duration of 30 ps at a repetition rate of 10 Hz. Intrastromal plasma formation and cavitation were investigated in sheep eyes in vitro by means of time-resolved macro-photography with 20 ns exposure time. Photographs were taken at various delay times (3 microseconds-2 min) after the release of the Nd:YAG laser pulse. The morphology of the laser effects and the incidence of endothelial damage was investigated by light-microscopic inspection of histological cross sections of the irradiated corneas. The minimal plasma size at energies close to the breakdown threshold was about 40 microns. Using a laser effects could be created without causing microscopically detectable damage to the epithelium, endothelium, or Bowman's membrane. To avoid damage, the distance between endothelium and laser focus had to be larger than 150 microns. Shock wave-induced tissue damage was not observed, although the maximum shock wave pressure was up to 13 kbar. The laser-generated intrastromal cavities are at least 10 times larger than the plasma volume.(ABSTRACT TRUNCATED AT 250 WORDS)