EyeWorld Asia-Pacific March 2019 Issue

50 March 2019 EWAP REFRACTIVE New noninvasive refractive treatment on the horizon by Liz Hillman EyeWorld Senior Staff Writer Animal model tests of femtosecond laser crosslinking showed positive induction of steepening and flattening effects S urgery in all specialties is continuing to move toward the least invasive modes possible, and refractive surgery is no exception. Two methods are making headway based on the concept of collagen crosslinking to stiffen and change corneal shape, inducing a refractive change. Avedro (Waltham, Massachusetts) has been pioneering photorefractive intrastromal crosslinking (PiXL) for the last few years as a noninvasive method of topography-guided, accelerated crosslinking using riboflavin and UVA light for correction of low myopia. Another technique that had recently published results in an animal model, though clinical trials have not yet been conducted, is femtosecond laser crosslinking. The research on the latter front, led by Sinisa Vukelic, PhD, Fu Foundation School of Engineering and Applied Science, Columbia University, New York, uses a femtosecond laser at low- energy pulses at a high-pulse rate to produce a low-density plasma that interacts with water molecules that are then broken, creating reactive oxygen species (ROS), or oxygen radicals, Dr. Vukelic explained to EyeWorld . These radicals interact with the collagen fibrils and form crosslinks, which leads to the known phenomenon of stiffening. Dr. Vukelic thinks this technique could be used to induce customized myopic and hyperopic correction even to the depth of the posterior stroma, without having to remove the epithelium. Dr. Vukelic and coinvestigators published their results of this technique on ex vivo and in vivo animal models in Nature Photonics . 1 Fifteen ex vivo porcine eyes received a flattening treatment, which would be used to correct myopia, and 13 porcine eyes received a steepening treatment, as would correct hyperopia. Both groups were paired with control eyes, and a separate control study was conducted to evaluate the experimental setup. According to the paper, the depth of the treatment was about 220 μm. In terms of the flattening effect, a 12% change in effective refractive power was seen within 8 hours after treatment. This lessened afterward as the cornea stabilized to a refractive power that was about 92% of the initial level, which Wang et al. equated to a 3.45 D shift. With the corneal steepening treatment, Wang et al. observed a steepening effect that increased gradually over 12 hours before stabilizing at a significantly higher effective power than baseline. Histological examinations showed no laser-induced damage, and the crosslinking density after 1 week appeared stable, the researchers stated in the paper. In vivo experiments on a rabbit model assessed effective refractive power at 24 hours, 7 days, and out to 3 months postop, using a similar treatment protocol as that performed on porcine eyes. At 48 hours postop, the myopic treatment group saw a mean 1.74 D change in effective refractive power, and a mean 1.64 D change in effective refractive power was observed by 7 days in the hyperopic treatment group. The study authors wrote that these changes remained stable out to 3 months postop and that there was “no wound or wound-healing response resembling that observed after other refractive surgery … New, noninvasive surgeries, such as epi-on PiXL (Avedro) shown here, hope to capitalize on corneal collagen crosslinking to correct refractive error. Source: Anders Behndig, MD