Lamellar refractive surgery with scanned intrastromal picosecond and femtosecond laser pulses in animal eyes

PURPOSE: To evaluate the use of scanned intrastromal picosecond and femtosecond laser pulses in lamellar refractive surgical procedures. METHODS: Intrastromal corneal photodisruption was performed in fresh porcine and primate cadaver eyes with a solid-state femtosecond laser. Laser pulses were focused 150 to 200 μm below the epithelial surface and scanned in a spiral pattern to create a plane. A flap was made by scanning an arc pattern from the plane of the spiral to the surface of the cornea. Tissue plane separation was graded using a standard scale, while internal surfaces were analyzed by scanning electron microscopy. Comparison was made to a picosecond laser system using the same delivery system device. Creation of a stromal lenticule for in situ keratomileusis was also demonstrated and compared with both laser systems. RESULTS: For femtosecond pulses, tissue separation was achieved best with pulse energies from 4 to 8 μJ and spot separations from 10-15 μm. Picosecond pulses accomplished less complete separations with pulse energies of 25 μJ and spot separations from 10 to 20 μm. Surface quality corresponded to dissection results, with high-grade dissections resulting in a smooth surface appearance, versus a more irregular surface for low-grade dissections. Although high-grade dissections could be created with picosecond pulses (with optimal parameters) in ex vivo porcine eyes, only femtosecond parameters produced similar results in ex vivo primate eyes. CONCLUSION: In contrast to previous attempts using picosecond lasers which require additional mechanical dissection, high precision lamellar refractive surgery may be practical with femtosecond laser pulses.