Mechanisms of tissue damage during ArF excimer endolaser microsurgery

摘要

The novel fiberoptic delivery system for the 193 nm excimer laser has been developed for vitreoretinal microsurgery. During the application of this laser in a liquid environment both the short-living cavitation bubbles and hydrogen gas-containing insoluble bubbles are produced. In present work we study the influence of these bubbles generated in free liquid on membranous tissue. Damage zones resulting from application of pulse trains at various repetition rates were investigated using vital stains which indicate the increase of cell membrane permeability. Cavitation bubbles were created by laser above the tissue in a highly absorbing liquid -Hartmann's solution with an addition of 7% albumin. These conditions simulate a situation in which a thin membrane separated from the underlying retina by layer of liquid is cut. After application of 50 pulses at 20 Hz at energy levels varying from 14 to 68 $mu@J per pulse we have detected cell damage at corresponding distances varying from 100 to 1200 microns. In Hartmann's solution (physiological medium), where the cavitation bubbles could not be formed at the same applied energies, the laser damage has been detected only at the distances varying from 150 to 200 microns. Penetration depth of the laser radiation in this solution is about 50 microns. The cells damage in this case probably has a photochemical nature. The difference in damage distance obtained at 1 and 20 Hz repetition rates can be explained by the influence of insoluble gas bubbles that grow at the tip exit and play a role of a transparent medium for the laser radiation. This effect probably determines the minimal distance at which the surgeon can apply the laser in standard physiological medium without being concerned with underlying cells damage. On the other hand, this phenomenon enable to destroy the upper level of cells in tissue without the deep penetrating mechanical influence associated with cavitation bubble-based tissue removal.