Laser thermokeratoplasty - Experimental study in minipigs with a CW-IR laser diode

Purpose: Laser thermokeratoplasty (LTK) by means of a pulsed Holmium-laser to perform hyperopic corrections is currently under clinical examination. Laser energy in the spectral region of high water absorption in the 2 μm range is used to create heat induced collagen shrinkage. To achieve a considerably lasting temperature rise - which is needed to coagulate the corneal stroma - high repetition rates of μs pulses inducing temperature peaks of up to 200°C were used. The procedure was found to be save but of rather poor predictability and stability. We have shown in-vitro that a continuously emitting laser source is able to create refractive effects of the same dimension as with pulsed LTK. The purpose of our study was now to evaluate the reliability and safety of a continously emitting laser diode for laser thermokeratoplasty in minipigs. Methods: We used a continuously emitting laser diode (SDL 6472-P1) at 1,81 μm wavelength with a power of 150 mW and an irradiation time of 10 seconds per treatment spot. A pattern of 8 LTK lesions on a ring of 6 mm diameter, concentric to the apex corneae was performed on eyes of adult minipigs. Prior to the operation and at the time of routine control examinations at 1 week, 1 month and 3 month postop the corneal refractive power was meassured by means of a commercially available hand held keratometer A complete ophthalmological examination was performed. Results: Hyperopic corrections of initially up to 4 D were achieved postoperatively. No serious adverse effects could be noted. We will report on the further follow-up. Conclusion: To our opinion LTK with a continuously emitting laserdiode is able to correct a considerable amount of hyperopia. Further studies on human eyes will follow to compare safety and longterm stability of this method to LTK with a pulsed lasersystem. (This study was conducted following the ARVO guidelines for animal subjects in biomedical research.).