Corneal cell responses after PRK performed with a 213-nm solid-state and a 193-nm excimer laser were compared in a rabbit model. The results showed that both lasers have similar cell death-inducing properties, but less keratocyte proliferation followed treatment with the 213-nm laser.
New Orleans-Results of an animal study show that less keratocyte proliferation occurs after surface ablation performed with a 213-nm solid-state laser compared with a 193-nm excimer laser, reported Paul P. van Saarloos, MSc, PhD, at the annual meeting of the American Academy of Ophthalmology.
"Since proliferating keratocytes may transform into activated fibroblasts that give rise to haze, the findings in this study suggest the 213-nm laser is potentially superior to the 193-nm excimer laser as a tool for refractive surgery," said Dr. van Saarloos, chief executive officer, CustomVis, Perth, Australia.
"In fact, in animal studies with longer follow-up after PRK treatment with the two lasers, both groups healed well, but there was evidence of less haze following treatment with the 213-nm laser. In addition, in clinical experience using the 213-nm laser for surface treatments, we are definitely seeing better stability and some suggestion of less haze compared with 193-nm excimer laser procedures."
"The 213-nm wavelength is absorbed about 20-fold less in balanced salt solution and about 10,000 times less in normal saline. However, 193-nm and 213-nm have a similar absorption spectrum in corneal tissue," Dr. van Saarloos said. "Together, this information suggests 213-nm energy is absorbed more by corneal collagen than 193 nm. This suggests the ablation process is more efficient with 213-nm energy, with less energy wasted heating fluids and less heat generated that can damage surrounding tissue."
The animal study Dr. van Saarloos reported was performed independently by researchers at the University of Western Australia using New Zealand white rabbits. Groups of animals underwent identical PRK treatments for –5.00 D of sphere with a 6.5-mm optical zone and 7-mm translational zone using one of the two lasers. Animals within each group then were selected to be killed on post-treatment days 1 and 3 for evaluation of the corneas using immunohistochemical staining techniques to identify apoptotic cells and proliferating keratocytes. Frozen sections were cut, and six photographs of each section were taken, including two inside the laser-created crater, two at the edge of the crater within the transition zones, and two from non-lasered portions of the cornea.
At 1 day after both treatments, the epithelium had not yet recovered. Eyes treated with the 193-nm excimer laser had higher levels of both apoptotic cells and keratocytes beneath the ablative zone compared with eyes in the 213-nm solid-state laser group, although the differences between groups were not statistically significant.
At 3 days after surgery, the number of apoptotic cells had decreased to nearly zero in both groups, but the number of keratocytes was significantly higher in eyes treated with the 193-nm laser.
Dr. van Saarloos also noted that previous studies suggest the rabbit eyes provide a good representative model for outcomes in human eyes, with the major difference being that the healing responses to laser treatment occur much faster in the animals.