DIODE-LASER THERMOKERATOPLASTY - APPLICATION STRATEGY AND DOSIMETRY

Purpose: To investigate suitable application parameters for efficient hyperopic correction by laser thermokeratoplasty (LTK) using mid-infra red laser diodes. Setting: Medical Laser Center Lubeck, Lubeck,Germany . Method: A tunable continuous-wave laser diode in the spectral range between 1.845 and 1.871 mu m was used. Transmitted by waveguides, the laser energy was used to induce coagulations on freshly enucleated por cine eyes to increase corneal curvature. The coagulations were equidis tantly applied by a fiber-cornea contact and a noncontact focusing dev ice that were adjusted on a ring concentric to the corneal apex. Diffe rent laser parameters and application geometries were evaluated. Refra ctive changes were measured by computer-assisted corneal topography be fore and after treatment. Polarization light microscopy and temperatur e calculations were used to analyze the coagulations. Results: Because of the tunability of the laser diode, the influence of the corneal ab sorption coefficient (between 0.9 and 1.6 mm(-1)) on the refractive ch ange could be measured. A laser power between 125 and 200 mW was adequ ate to achieve refractive changes up to 10.0 diopters. In the preferab le focusing device, the refractive change increased almost logarithmic ally with the irradiation time up to 15 seconds. The number of coagula tions on a fixed application ring showed no significant influence on r efractive change; however, it showed an almost linear decrease with in creasing ring diameter from 5.0 to 10.0 mm. Histological analysis reve aled 3 stages of thermal damage. Conclusion: Diode LTK provide defined and uniform coagulations when using a well-adapted focusing device, r esulting insufficient refractive change. The results indicate that dio de LTK is superior to pulsed holmium LTK.