Optimal laser parameters for intrastromal corneal surgery

We characterized the effects of pulse duration, pulse energy, and spot separation on intrastromal corneal photodisruption to determine parameters that achieve optimal surface quality and tissue plane separation. Experiments utilized two laser systems, a 60 picosecond Nd:YLF laser and a 450 femtosecond Nd:Glass laser, both operating at 1.06 mu m wavelength. Photodisruption was performed by tightly focusing the laser beam 150 microns below the tissue surface and scanning it in a spiral pattern to create a plane. A cut to the surface was made with the laser and the two surfaces separated to form a flap. Tissue plane separation was graded according to the additional mechanical dissection required. Internal surfaces were analyzed with standard histologic methods and scanning electron microscopy. We found that the Nd:YLF laser required approximately three times the pulse energy to achieve intrastromal cuts. Picosecond parameters also required more mechanical dissection and produced lower surface quality than optimal femtosecond parameters. We conclude that femtosecond laser pulses offer significant advantages that make them ideal candidate tools for high precision intrastromal corneal surgery, The flexibility in laser pulse delivery opens up a number of potential surgical applications not possible with current mechanical or laser devices.