EyeWorld Asia-Pacific June 2012 Issue

38 June 2012 EWAP REFRACTIVE A recent study examined corneal strength following PRK and LASIK Source: National Eye Institute, National Institutes of Health How LASIK may help shield the eye from some trauma I t’s common knowledge— the cornea is stronger after photorefractive keratectomy (PRK) than LASIK, right? Not according to study results published in the September 2011 issue of the Journal of Refractive Surgery, which looked at the effect of blunt impact on the cornea after refractive surgery. Results showed that paradoxically rather than weakening the cornea, the LASIK flap served to protect it, according to Gholam A. Peyman, MD , emeritus professor of ophthalmology, Tulane University, New Orleans, USA; professor of basic medical science, University of Arizona, Phoenix, Ariz., USA; and professor of optical sciences, College of Optical Sciences, University of Arizona, Tucson, Ariz., USA. While other studies had considered factors such as airbag deployment or overinflation of a LASIK or PRK eye, none had specifically looked at blunt trauma. “We wanted to find out the effect of a blunt foreign body projectile on the human cornea after LASIK and PRK,” Dr. Peyman said. Blunt impact computer model Testing for this multicenter trial, conducted at Pennsylvania State University, Shahid Beheshti University in Tehran, Iran, the University of Illinois at Chicago, and other leading institutions, was done using computer models. “It was not quite possible for us to do that physically on the cadaver eye or the animal eye,” Dr. Peyman said. Using the computer model, investigators were able to show what happened to the cornea after PRK or LASIK when struck with blunt projectiles. “The projectile that we had was launched at different velocities,” he said. “It was about 20 to 60 meters per second; we wondered at what velocity these corneas would rupture with PRK or LASIK.” The key findings for the varying corneal thicknesses, which ranged from 300 to 500 microns, was the antithesis of what might have been expected. “It was interesting because we found that the threshold for creating rupture of corneal thickness was lower after PRK than LASIK,” Dr. Peyman said. “For example, if there was a corneal thickness of 500 microns, the velocity needed to rupture it was 33 meters per second for PRK versus LASIK, which was 40 meters per second.” For the thin 300-micron cornea, the PRK rupture threshold was just 22.8 meters per second compared with 26.7 meters per second for LASIK. “We were quite surprised because we didn’t expect it,” Dr. Peyman said. “Generally people think that after PRK the cornea better resists these kind of projectile injuries, but it didn’t turn out that way.” Instead it was PRK cases that were more prone to blunt injury. “It seemed that it required lowered velocity to have the cornea rupture after PRK than LASIK.” Dr. Peyman sees the outcomes as good news for refractive surgery as a rule. “Our results show that the laser procedures we are doing have little effect on the corneal resistance toward small foreign bodies,” he said. “But as far as LASIK is concerned, in this experimental model it showed a better result than PRK.” LASIK in a new light Despite prevailing wisdom, Dr. Peyman thinks that it is the LASIK flap that played a protective role. “At least in this experiment, the corneal flap acted as a kind of shield to protect the rest of the stroma against a small foreign body,” he said. “In other words, the flap reduced the impact of the force reaching the stroma.” He hopes that practitioners come away from the study with a new appreciation of the strength of the LASIK cornea. “I would say that the take-home message is that the cornea after LASIK can absorb foreign body projectiles as well as after PRK, if not better,” Dr. Peyman said. Still, he urged caution in extrapolating the data to what may occur to LASIK or PRK patients embroiled in athletic or other potentially hazardous pursuits without proper protective eye gear. “I would definitely not recommend the way of thinking that if [patients] do this or that [they] will be protected,” he said. While the study can be predictive of what might happen to the cornea at certain velocities, without protective goggles, any The blunt truth about corneal strength after refractive surgery by Maxine Lipner Senior EyeWorld Contributing Editor person, even those who have not undergone any kind of refractive surgery, can be in jeopardy of blunt trauma, he pointed out. Overall, in addition to a new understanding of the strength of the LASIK cornea, Dr. Peyman hopes that the computer simulation used here will be a useful prototype for new investigations. “As for the future, I think that we probably have a model against which other similar studies can be performed and compared with each other,” he said. “I think it’s helpful to look at these models for eye injuries, not only in the front part of the eye but also in the back.” He sees the simulation as proving helpful for a variety of traumatic injuries. “They give us good information about what we can expect when we encounter trauma.” EWAP Editors’ note: Dr. Peyman owns the LASIK procedure patent but receives no financial remuneration. Contact information Peyman: gpeyman1@yahoo.com