Corneal photocoagulation with continuous wave and pulsed holmium: YAG radiation

In this study, the effectiveness of pulsed and continuous wave (CW) holmium: YAG lasers in coagulating in vitro pig corneas was analyzed. With the CW laser, irradiance and exposure time were varied; irradiance, from 162 to 324 W/cm2 and exposure time, from 200 to 800 ms. With the pulsed laser, number of pulses and radiant exposure were varied; number of pulses per lesion, from 4 to 30 and radiant exposure, from 10 to 25 J/cm2. Laser-induced corneal damage was determined by analyzing histological cross sections of each lesion. Depth and diameter of the lesions were plotted against the varying laser parameters. Light and birefringent photomicrographs of typical lesion histology show that the pulsed laser significantly damaged superficial layers of the cornea and could not achieve the coagulation depths produced by the CW laser. Additional histology demonstrates that minimal surface damage (intrastromal coagulation) occurred when the CW laser beam was delivered with a sapphire-tipped contact probe. The results provide empirical data on the sensitivity of each parameter in producing a range of coagulation end points. In addition, the experimental results describe trends between the parameters of either laser and the extent of coagulation.