The mechanism of ablation of corneal tissue by the neodymium doped yttrium-lithium-fluoride picosecond laser.

This study examines the structural changes in cornea resulting from plasma formation and propagated acoustic shock waves produced by the neodymium doped yttrium-lithium-fluoride (Nd:YLF) picosecond laser. Human donor eyes and enucleated rabbit eyes were subjected to various ablation patterns at energies ranging from 40 to 300 microJ per pulse. Two distinctly different patterns were produced depending on the location of initial plasma formation. Plasmas initiated at the corneal surface produced smooth, straight-edged ablations of corneal tissue that consisted of collagen fibril fragmentation, fibril organizational disruption, and possible thermal effect observed along the lateral borders and wound apex. The extent of lateral damage was directly related to the energy applied. The range of acute collagen disorganization observed at the ablation edge in rabbit corneas at various pulsed energies was as follows: 50 microJ = 1.0-12 microns, 150 microJ = 3.8-12.5 microns, 250 microJ = 6.2-23.7 microns, and 300 microJ = 7.5-45.0 microns. Plasma formation initiated within the stroma at or above threshold energies (50-150 microJ per pulse) produced an inter- or intralamellar separation effect with little evidence of ablation or collagen fibril fragmentation. Intrastromal plasmas generated from higher energies (200-300 microJ per pulse) produced tissue ablation, along with ablation or disruption of tissue anterior to the intrastromal target area.