FACTORS DETERMINING THE REFRACTIVE EFFECTS OF INTRASTROMAL PHOTOREFRACTIVE KERATECTOMY WITH THE PICOSECOND LASER

Purpose: To determine the relationship between laser parameters and ti ssue removal with picosecond laser intrastromal photorefractive kerate ctomy (ISPRK) and to assess the effect of the parameters on the healin g process and the long-term refractive changes. Setting: Medical Laser Center Lubeck, Germany. Methods: A modified Intelligent Surgical Lase rs (ISL) 2001 system with a cone angle of 30 degrees was used for in v itro investigations of the laser effects in water and on the porcine c ornea. Photographic methods were used to determine the plasma volume a nd to investigate the thickness of the intrastromal bubble layer as a function of the laser pulse energy, the number of layers in which the pulses were applied, and the layer separation. The data were used to c alculate the amount of tissue removal. Histologic evaluation was done by polarization microscopy after Sirius-red staining. Results: The las er pulses produced a sponge-like appearance of the stroma in a layer e xtending through about one third the corneal thickness, accompanied by mechanical distortions of the stromal lamellae. Thermal changes were weak. Tissue removal was impaired by the cavitation bubbles from prece ding laser pulses: The amount of evaporated tissue depended only weakl y on laser pulse energy and number of layers in which the pulses were applied. The maximum amount of tissue that could be evaporated without damaging the outer corneal layers was a 10.0 mu m thick layer. With a 6.0 mm optical zone, the tissue removal produced an immediate refract ive effect of only 0.85 diopter. Conclusions: Intrastromal PRK does no t work as originally envisioned because the amount of evaporated tissu e is very small. The greater long-term refractive changes observed in animal experiments and clinical studies must therefore be due to the h ealing response of the cornea. Because the refractive effects are stro ngly influenced by the healing response of the cornea, they are poorly predictable.