Corneal endothelial cell damage after experimental diode laser thermal keratoplasty

PURPOSE: To evaluate the safety of diode laser thermal keratoplasty (LTK) with respect to corneal endothelial cell damage. METHODS: In an in vitro animal model system, porcine eyes were irradiated with a continuously emitting laser diode at wavelengths (λ) of 1.85 or 1.87 μm, corresponding to an absorption coefficient (μ(a)) of 1.1 or 2.0 mm^-1. Different irradiation and application parameters were tested serially. To determine the temperature threshold for endothelial damage, corneal buttons were analyzed separately in a waterbath experiment. The endothelial damage was assessed after trypan blue and alizarin red supravital staining under light microscopy. RESULTS: The thresholds for the 50% probability of thermal damage (ED50) were determined at corneal temperatures of 65°C for a 10-second water-bath immersion, and 59°C for 60 seconds. Coagulations that reached the deeper stromal layers revealed severe endothelial cellular alterations and areas of exposed Descemet's membrane. The thermally induced changes were dependent on laser power and the absorption coefficient (wavelength). Mean diameter of total endothelial cell damage was 245 ± 154 μm (range, 0 to 594 μm) for an absorption coefficient of 1.1 mm^-1. The maximal lateral extent of endothelial cell damage induced by the laser exposure was 594 μm in diameter. Increasing the absorption coefficient decreased the penetration depth of the laser irradiation, creating a greater temperature rise within the corneal stroma and significantly less endothelial damage (P < .01), when the same laser power was applied. The calculated total area of damage for the paracentral human corneal endothelium ranged from 1.8% to 13.6%. CONCLUSION: Data obtained in this in vitro study were transferred to an endothelial cell damage nomogram, demonstrating that appropriate parameter improvements can minimize the adverse effects to the corneal endothelium. However, model adjustment to the human cornea indicates the potential for endothelial cell damage after diode laser thermal keratoplasty, and should be considered when performing this elective procedure.