Rochester, NY--Ophthalmologists at the University of Rochester Medical Center have developed a laser-guiding computer algorithm 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 (laser-assisted in-situ keratomileusis) visit.
The approach, presented at the XXIV Congress of the European Society of Cataract and Refractive Surgery (September 9 to 13, 2006; London, England) takes into account myriad imperfections within the eye that weren't even measurable a decade ago.
Although 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 nearsightedness is is one of the chief barriers preventing them from seeing even better, at a level around 20/16.
Eye surgeon Scott MacRae 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 )1.3%) 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% 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%, 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 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% 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.
A LASIK procedure typically lasts anywhere from 15 to 60 seconds; the excimer-laser beam hits the cornea with about 50 pulses per second, with generally 750 to 3,000 pulses. The timing and aim are controlled by both the surgeon and the software. By taking into account anomalies in each person's eye, the University of Rochester 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 wavefront aberrations in the eye beyond just power (nearsightedness or farsightedness); these aberrations were first seen through an adaptive-optics system developed by David Williams at the University of Rochester.
For instance, the team found that treating coma (an off-axis aberration) affects a patient's astigmatism (another off-axis aberration) 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.
Williams' system opened the door to the possibility of fixing not only the major flaws in the eye that reading glasses and contact lenses have corrected for decades, but also approximately 60 additional imperfections. 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."
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% to 3%.
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.