Ophthalmologists have developed a formula that slashes by nearly two-thirds the likelihood that patients will need repeat visits to an eye surgeon to adjust their vision after their initial LASIK visit. That's because the formula makes it more likely that surgeons will get it right the first time.
The new results, presented at the European Society of Cataract and Refractive Surgery meeting in London, are the result of a complex computer formula compiled by doctors and scientists at the University of Rochester Medical Center that takes into account myriad imperfections within the eye that weren’t even known to exist a decade ago.
Even though most patients come out of refractive surgery with vision that is 20/20 or better, doctors have noticed that some patients exit the surgery slightly farsighted – not enough to seriously degrade their quality of vision or to require contact lenses or reading glasses, but enough to be a leading reason why people complain about the results of the surgery. A few others end up slightly nearsighted. While many of these patients still see at a level around 20/20, the slight farsightedness or nearsightednessis is one of the chief barriers preventing them from seeing even better, at a level around 20/16.
Eye surgeon Scott MacRae, M.D., of the University of Rochester Eye Institute presented the results showing a dramatic drop in farsightedness among LASIK patients. In a recent study where MacRae and colleagues used the formula, known as the University of Rochester Nomogram, during surgery, just six of 445 eyes or 1.3 percent were slightly farsighted after LASIK. He compared this to results from a previous study five years ago without the formula. In that study of 340 eyes, even though 91 percent of patients had 20/20 vision or better – the highest known percentage of any large study in the world at the time – 74 of the 340 eyes treated, or 21.8 percent, were slightly farsighted.
"Though those results were among the best anyone had gotten to date, we thought we could do better," said MacRae, who worked for two years with post-doctoral associate Manoj Venkiteshwar, Ph.D., to develop the formula.
While some doctors have noticed that patients are more likely to be slightly farsighted than nearsighted after LASIK, doctors have had no way to predict which patients would be affected, MacRae said. If a doctor adjusted all of his or her surgeries to avoid the problem, then the other 80 percent of patients would wind up slightly nearsighted.
The new formula takes the guesswork out of the picture and establishes a scientific basis for the phenomenon.
The software developed by Venkiteshwar and MacRae controls how the laser beam dances around the surface of the cornea during a LASIK procedure, allowing the surgeon to sculpt the cornea into just the right shape so that it produces as flawless an image as possible. During a procedure that typically might last anywhere from 15 to 60 seconds, the laser beam hits the cornea about 50 times per second, with generally 750 to 3,000 pulses. The timing and aim, controlled by both the surgeon and the software, have to be precise.
By taking into account the unique anomalies in each person’s eye, the formula predicts which patients are most likely to be slightly farsighted after a LASIK procedure, then adjusts the laser to avoid that outcome.
Ironically, Venkiteshwar and MacRae found that the cause of the shift was the new capability doctors have to fix subtle visual imperfections that weren’t even known to exist until David Williams, Ph.D., at the University of Rochester developed a system to see them.
Williams' system opened the door, for the first time in history, to the possibility of fixing not only the three major flaws in the eye that reading glasses and contact lenses have corrected for decades, but also approximately 60 additional imperfections that were never known before. Nearly everyone has these flaws in their eyes to some extent; while most people don’t notice them, they hurt our quality of vision in subtle ways. Since Williams' discovery, several companies have introduced technology that makes possible a technique known as customized ablation, a form of LASIK that corrects these imperfections, bringing about a super-crisp quality of eyesight. Beyond making vision on the order of 20/15 or 20/16 possible or even commonplace in some groups of patients, the technology also increases the eye’s ability to see in situations where there is low light or little contrast.
MacRae and Venkiteshwar were surprised to find that fixing these subtle imperfections affects vision in unexpected ways. They found that some of the improvements make an eye undergoing LASIK more prone to becoming slightly farsighted in some patients, and slightly nearsighted in a few patients. They’ve found the relationship in at least three different laser systems used in LASIK procedures.
"This is not something anyone would have predicted," said MacRae, who is a professor of Ophthalmology and of Visual Science. "When you fix these flaws, it can affect vision in ways that were previously unpredictable."
For instance, the team found that treating coma – a subtle imperfection where a point of light looks like it has the tale of a comet – affects a patient’s astigmatism as well as his or her degree of nearsightedness or farsightedness. Other common flaws that can now be fixed, but which also affect a person’s vision more broadly, include spherical aberration, where a point of light appears to have several rings of light around it; trefoil, where a point of light seems to be surrounded by three other points; and others such as secondary astigmatism, quadrafoil and pentafoil.
MacRae credits the new formula, part of a procedure he calls second-generation customized ablation, with slashing the need for repeat treatments in patients from about 8 percent to 3 percent.
The latest results are part of an ongoing program by MacRae, a pioneer in the field of customized ablation, to bring Williams' findings to the clinic and improve patients’ vision to unprecedented levels. Each year, MacRae says, scientists and physicians learn new things that help future patients.
"We’ve taken a very good procedure and made it even better. I am extremely confident in this technology, which I’ve even had done on myself. A conscientious, systematic approach to evaluating patients is key. Not everyone is a good candidate for LASIK. Surgeons need to be extremely diligent about their pre-operative evaluations to maximize safety and the outcomes for their patients," said MacRae, the author of the best-selling book on customized ablation, Customized Corneal Ablation: The Quest for Supervision.
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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