[Thermal damage to the corneal endothelium in diode laser thermokeratoplasty]

Laser thermokeratoplasty (LTK) can be applied for correction of hyperopia and astigmatism by means of concentrically placed coagulations. Because of the temperature rise during coagulation, damage of the endothelial cells directly below the irradiated areas is possible. In this study, we examined the extent of the thermally denatured zones for different laser parameters and the threshold of thermal endothelial damage as a function of of temperature and duration of elevated temperatures. The threshold for thermal damage of endothelial cells was determined in isotonic NaCl solution for temperature exposures of 10 s and 1 min in a water bath. To determine the damage zones, corneas were irradiated under standardized conditions with a continuously emitting infrared (cw-IR) laser diode at various wave-lengths and different power values and were stained after preparation with trypan blue and alizarine red. The extensions of the damage zones were compared with calculated isotherms. Fifty percent cell damage was found at temperatures of 65 degrees C for heating times of 10 s and at 59 degrees C for 1 min. With thicker corneas, less laser power and higher absorption coefficients, the damage zone was reduced. The damage range determined corresponded to the calculated isotherms of 60 degrees C and 70 degrees C. Regarding clinical LTK, a loss of endothelial cells can be predicted and minimized or totally avoided by choosing the appropriate irradiation parameters.