Laser thermokeratoplasty: analysis of in-vitro results and refractive changes achieved in a first clinical study

Laser thermokeratoplasty (LTK) is a minimally invasive method to correct hyperopia and astigmatism. A cw mid-IR laser diode emitting at wavelengths around 1.86 μm was used to perform LTK on a first clinical trial. The coagulations were applied to the cornea by means of a specially designed focusing handpiece which was introduced into a corneal application mask fixed by a suction ring. Coagulation patterns consisting of 8 spots per ring were performed with a laser power between 100-150 mW and an irradiation time of 10 seconds both on single and on double rings. Significant refractive changes up to 19 D could initially be achieved followed by a strong regression within the first month. Three months post LTK, refractive changes achieved with the single and double ring have stabilized, yielding 1.2 and 1.8 D on the average, respectively. The method reveals only little adverse effects limited to the first days post-op. Force measurements were performed on corneal stripes, which were submerged for 10 s into an oil bath of constant temperature in order to investigate the absolute temperatures required for corneal collagen contraction. Only temperatures exceeding 90°C induced a significant force. Analyzing the clinically used LTK parameters by temperature calculations revealed that only a small part of the heated stromal volume experienced sufficient high temperatures to induce significant collagen shrinkage.