Diode laser thermokeratoplasty: Application strategy and dosimetry

Purpose: To investigate suitable application parameters for efficient hyperopic correction by laser thermokeratoplasty (LTK) using mid-infrared 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 porcine eyes to increase corneal curvature. The coagulations were equidistantly applied by a fiber-cornea contact and a noncontact focusing device that were adjusted on a ring concentric to the corneal apex. Different laser parameters and application geometries were evaluated. Refractive changes were measured by computer-assisted corneal topography before and after treatment. Polarization light microscopy and temperature calculations were used to analyze the coagulations. Results: Because of the tunability of the laser diode, the influence of the corneal absorption coefficient (between 0.9 and 1.6 mm(-1)) on the refractive change could be measured. A laser power between 125 and 200 mW was adequate to achieve refractive changes up to 10.0 diopters. In the preferable focusing device, the refractive change increased almost logarithmically with the irradiation time up to 15 seconds. The number of coagulations on a fixed application ring showed no significant influence on refractive change; however, it showed an almost linear decrease with increasing ring diameter from 5.0 to 10.0 mm. Histological analysis revealed 3 stages of thermal damage. Conclusion: Diode LTK provide defined and uniform coagulations when using a well-adapted focusing device, resulting insufficient refractive change. The results indicate that diode LTK is superior to pulsed holmium LTK.