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 the millijoule range is an established tool for intraocular surgery. This study investigates tissue effects in cornea, lens, and retina to assess whether picosecond (ps) pulses with energies in the microjoule range can increase the surgical precision, reduce collateral damage, and allow applications requiring more localized tissue effects than can be achieved with ns pulses.
METHODS: Both ps and ns Nd:YAG laser effects on Descemet's membrane, in the corneal stroma, in the lens, and at the retina were investigated in vitro in bovine and sheep eyes and in cataractous human lens nuclei. For each tissue, the optical breakdown threshold was determined. The morphology of the tissue effects and the damage range of the laser pulses were examined by light and scanning electron microscopy. The cavitation bubble dynamics during the formation of corneal intrastromal laser effects were documented by time-resolved photography.
RESULTS: The optical breakdown threshold for ps pulses in clear cornea, lens, and vitreous is, on average, 12 times lower than that for ns pulses. In cataractous lens nuclei, it is lower by a factor of 7. Using ps pulses, Descemet's membrane could be dissected with fewer disruptive side effects than with ns pulses, whereby the damage range decreased by a factor of 3. The range for retinal damage was only 0.5 mm when 200 microJ ps pulses were focused into the vitreous. Picosecond pulses could be used for corneal intrastromal tissue evaporation without damaging the corneal epithelium or endothelium, when the pulses were applied in the anterior part of the stroma. The range for endothelial damage was 150 microns at 80 microJ pulse energy. Intrastromal corneal refractive surgery is compromised by the laser-induced cavitation effects. Tissue displacement during bubble expansion is more pronounced than tissue evaporation, and irregular bubble formation creates difficulties in producing predictable refractive changes.
CONCLUSIONS: The use of ps pulses improves the precision of intraocular Nd:YAG laser surgery and diminishes unwanted disruptive side effects, thereby widening the field of potential applications. Promising fields for further studies are intrastromal corneal refractive surgery, cataract fragmentation, membrane cutting, and vitreolysis close to the retina.