Eyeworld

FEB 2012

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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82 82 EW REFRACTIVE SURGERY February 2012 The blunt truth about corneal strength after refractive surgery by Maxine Lipner Senior EyeWorld Contributing Editor While other studies had consid- ered factors such as airbag deploy- ment 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 for- eign body projectile on the human cornea after LASIK and PRK," Dr. Peyman said. Blunt impact computer model 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 pho- torefractive keratectomy (PRK) than LASIK, right? Not accord- ing 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 sur- gery. Results showed that paradoxi- cally rather than weakening the cornea, the LASIK flap served to protect it, according to Gholam A. Peyman, M.D., emeritus professor of ophthalmology, Tulane Univer- sity, New Orleans; professor of basic medical science, University of Ari- zona, Phoenix; and professor of optical sciences, College of Optical Sciences, University of Arizona, Tucson. Operation Eyesight 2012 marks 10-year milestone O n Saturday, February 4, and Saturday, March 10, the surgeons, physicians, anesthesiologists, and clinic professionals of Minnesota Eye Consultants, various locations, will donate their time and skills to perform eye surgery, at no cost, to patients from the Twin Cities, greater Minnesota, and Wisconsin. For the 10th year running, Op- eration Eyesight addresses the urgent need of patients facing dire financial burdens, unemployment, bankruptcy, or lack of insurance or disability benefits. Their stories are compelling and all too relatable in the present economy. Without insurance, Medicaid, or other support, surgery is just not an option for these people. This year, family, friends, and doctors referred over 50 applicants to the program, more than any year in the past. A vetting process takes place during which 10-20 of these patients are chosen to receive surgeries in- cluding cataract, glaucoma, and corneal transplants. Many patients will have two surgeries, after which they'll be cared for post-op, at no cost, for a period of 6 months. In addition, all supplies, equipment, and medica- tions are donated to the program. Elizabeth A. Davis, M.D., managing partner, Minnesota Eye Consult- ants, helped launch Operation Eyesight as a means to give back to the community. Since its inception, the program has become an invaluable service for patients who would not otherwise receive this specialized care. Dr. Davis' goal is to inspire other practices, medical providers, and volun- teers to help serve the growing needs of these patients. Testing for this multicenter trial, conducted at Pennsylvania State University, Shahid Beheshti Univer- sity 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 sec- ond; 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 cre- ating rupture of corneal thickness was lower after PRK than LASIK," Dr. Peyman said. "For example, if there was a corneal thickness of 500 mi- crons, 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 be- cause we didn't expect it," Dr. Peyman said. "Generally people think that after PRK the cornea bet- ter resists these kind of projectile in- juries, 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 ve- locity 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 con- cerned, 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 for- eign body projectiles as well as after PRK, if not better," Dr. Peyman said. Still, he urged caution in extrap- olating the data to what may occur to LASIK or PRK patients embroiled in athletic or other potentially haz- ardous pursuits without proper pro- tective 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 person, even those who have not undergone any kind of re- fractive surgery, can be in jeopardy of blunt trauma, he pointed out. Overall, in addition to a new un- derstanding of the strength of the LASIK cornea, Dr. Peyman hopes that the computer simulation used here will be a useful prototype for new in- vestigations. "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 in- juries. "They give us good informa- tion about what we can expect when we encounter trauma." EW Editors' note: Dr. Peyman owns the LASIK procedure patent but receives no financial remuneration. Contact information Peyman: gpeyman1@yahoo.com

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