NONCONTACT LASER PHOTOTHERMAL KERATOPLASTY .1. BIOPHYSICAL PRINCIPLESAND LASER-BEAM DELIVERY SYSTEM

BACKGROUND: Thermal shrinkage of stromal collagen is known to produce changes in the corneal curvature. We designed a novel, noncontact lase r beam delivery system to perform laser photothermal keratoplasty. MAT ERIALS AND METHODS: The instrument consisted of a pulsed holmium:YAG l aser (2.10-micrometer wavelength, 250-microsecond pulse width, 5-hertz repetition rate) coupled via a monofilament fiber to a common slit-la mp microscope equipped with a polyprism, an adjustable mask, and a pro jection lens. The system projected an 8-spot annular pattern of infrar ed laser energy on the cornea to achieve a thermal profile within the stroma and to attain controlled, predictable collagen shrinkage. The s ystem produced treatment patterns of 8 to 32 spots of 150 to 600 mum d iameter in concentric rings, continuously adjustable between 3 and 7 m m. The versatility of the system in creating different treatment patte rns was tested on thermal paper and human cadaver eyes. RESULTS: A uni form beam profile and different treatment patterns for myopia, hyperop ia, and astigmatism were obtained. Myopic correction of 6.00 diopters was demonstrated on cadaver eyes. Corneal topography documented cornea l flattening (> 6.00 D) with the following treatment parameters: each spot size on the cornea = 300 mum, radiant exposure of each spot = 18. 0 J/cm2, number of pulses = 1, diameter of the treatment ring = 3 mm. CONCLUSIONS: Noncontact slit-lamp microscope laser delivery system for laser photothermal keratoplasty provides flexible and precise selecti on of laser treatment parameters. It may improve the efficacy of the p rocedure.