INTRAOCULAR PHOTODISRUPTION WITH PICOSECOND AND NANOSECOND LASER-PULSES - TISSUE EFFECTS IN CORNEA, LENS, AND RETINA

Purpose. Nd:YAG laser photodisruption with nanosecond (ns) pulses in t he millijoule range is an established tool for intraocular surgery. Th is study investigates tissue effects in cornea, lens, and retina to as sess whether picosecond (ps) pulses with energies in the microjoule ra nge can increase the surgical precision, reduce collateral damage, and allow applications requiring more localized tissue effects than can b e achieved with ns pulses. Methods. Both ps and ns Nd:YAG laser effect s on Descemet's membrane, in the corneal stroma, in the lens, and at t he retina were investigated in vitro in bovine and sheep eyes and in c ataractous human lens nuclei. For each tissue, the optical breakdown t hreshold was determined. The morphology of the tissue effects and the damage range of the laser pulses were examined by light and scanning e lectron microscopy. The cavitation bubble dynamics during the formatio n of corneal intrastromal laser effects were documented by time-resolv ed photography. Results. The optical breakdown threshold for ps pulses in clear cornea, lens, and vitreous is, on average, 12 times lower th an that for ns pulses. In cataractous lens nuclei, it is lower by a fa ctor of 7. Using ps pulses, Descemet's membrane could be dissected wit h fewer disruptive side effects than with ns pulses, whereby the damag e range decreased by a factor of 3. The range for retinal damage was o nly 0.5 mm when 200 mu J ps pulses were focused into the vitreous. Pic osecond pulses could be used for corneal intrastromal tissue evaporati on without damaging the corneal epithelium or endothelium, when the pu lses were applied in the anterior part of the stroma. The range for en dothelial damage was 150 mu m at 80 mu J pulse energy. Intrastromal co rneal refractive surgery is compromised by the laser-induced cavitatio n effects. Tissue displacement during bubble expansion is more pronoun ced than tissue evaporation, and irregular bubble formation creates di fficulties in producing predictable refractive changes. Conclusions. T he use of ps pulses improves the precision of intraocular Nd:YAG laser surgery and diminishes unwanted disruptive side effects, thereby wide ning the field of potential applications. Promising fields for further studies are intrastromal corneal refractive surgery, cataract fragmen tation, membrane cutting, and vitreolysis close to the retina.