NONCONTACT LASER PHOTOTHERMAL KERATOPLASTY .3. HISTOLOGICAL STUDY IN ANIMAL EYES

BACKGROUND: Laser photothermal keratoplasty has been studied as a pote ntial refractive procedure. The purpose of this study is to investigat e the histological response to various laser treatments including geom etrical patterns, radiant exposure levels, and pulse numbers. MATERIAL S AND METHODS: A noncontact laser photothermal keratoplasty system was used in this study. Epithelial and endothelial response to the laser photothermal keratoplasty annulus treatment pattern were studied on an owl monkey model with a 5-millimeter annulus ring pattern, 8 J/cm2, 2 5 consecutive pulses at 1 Hz. Epithelial and endothelial response to t he laser photothermal keratoplasty spot pattern were then studied and compared on cat and rabbit models for safety monitoring. One pulse and five consecutive pulses of eight different radiant exposures (5.00 J/ cm2 to 18.01 J/cm2) were applied on each cornea. A cadaver eye model w as used to study the collagen shrinkage induced by the laser spot trea tment following the same protocol as the cat and rabbit model. Finally , the biological healing response to the laser photothermal keratoplas ty treatment with the optimal laser parameters obtained in our experim ent was studied on the cat model. Five cats were treated by the laser photothermal keratoplasty procedure with eight spots on a 3-millimeter ring, 15.6 J/cm2, and 1 pulse. RESULTS: Epithelial and endothelial da mage were observed after annulus treatment on an owl monkey's cornea a t 8 J/cm2, 25 pulses, and after spot treatment on cat and rabbit corne as at 18.01 J/cm2, five pulses. No endothelial damage was observed on cat corneas for the single pulse treatment at 18.01 J/cm2. For the tis sue shrinkage study, no laser photothermal keratoplasty lesion could b e detected for a radiant exposure setting below 10.26 J/cm2. Histologi cal cross-sections showed that the five-pulse treatment reached the en dothelium for the radiant exposure range (5 J/cm2 to 18 J/cm2) studied . The cat model showed that the laser-induced mechanical octagonal str ess-lines by collagen shrinkage were maintained after 3 months. The hi stological sections across the lesion showed a denser keratocyte popul ation indicating scar formation. CONCLUSION: The volume of collagen sh rinkage, its location, and its geometrical shape can be accurately and precisely controlled by a 2.10-micrometer Ho:YAG laser coupled to an optical delivery system.