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Wavefront technology wave of future for refractive surgery


Wavefront technology is the wave of the future for refractive surgery, according to Marguerite B. McDonald, MD, Ophthalmic Consultants of Long Island, New York, United States.

Wavefront technology is the wave of the future for refractive surgery, according to Marguerite B. McDonald, MD, Ophthalmic Consultants of Long Island, New York, United States.

"Iris registration wavefront-guided laser vision correction provides clearly better clinical outcomes than phoropter-guided, or conventional, laser vision correction," Dr. McDonald said. "This is the result of improved technology, use of a more robust, high-fidelity algorithm, the Fourier and Zernike, because of pupil centroid shift compensation and because of iris registration."

Just in the United States, Dr. McDonald said, there are 54 million patients with naturally occurring refractive errors and 80 million patients with presbyopia.

"And secondary cases-re-treatment of LASIK, PRK, and RK-represent over a million eyes," she added. "There also are 2 million to 3 million eyes per year that require a reduction of their residual pseudophakic error, and no one knows how many highly fimbriated corneas require rehabilitation after disease or injury."

Conventional laser vision correction produces excellent results, and customized procedures can cost twice as much, Dr. McDonald said, but recent research has found that wavefront-guided procedures are superior.

Dr. McDonald cited a study by Lt. Col. Scott D. Barnes, MD, of Fort Bragg soldiers.

"The initial experience with contralateral wavefront in the Army was mixed, a bit disappointing," Dr. McDonald said. "But there's been progress with wavefront technology. We have upgraded software, advanced internal nomograms that most of us have developed, and, of course, there's iris registration [and] pupil centroid shift compensation, and perhaps it is time for more doctors to reevaluate wavefront technology."

A subsequent contralateral study conducted by Dr. Barnes at Fort Bragg has enrolled 225 patients to have one eye treated with wavefront technology and one eye treated with conventional technology, she said. A proprietary laser (VISX Star S4 IR, Advanced Medical Optics) was used in the prospective study, and follow-up data are available for the first 50 patients.

The two treatment options have yielded similar results for visual acuities of 20/40, 20/25, and 20/20.

"However, at the highest level of vision, 20/15, there was a statistically significant bias toward the wavefront-guided group," Dr. McDonald said. "They were clearly superior. So he concluded that conventional PRK is great, but wavefront-guided PRK is better."

Wavefront-guided PRK is better because lower- and higher-order aberrations can be corrected, and the technology offers iris registration and pupil centroid compensation, she said.

"Iris registration finds multiple matching reference points for each iris section in the [wavefront] image and the laser image to compensate for cyclotorsion, and pupil centroid shift compensation makes up for the fact that many pupils decenter under the laser," Dr. McDonald said. "As the pupil constricts under the bright lights in the laser suite, the center of the pupil is in a completely different position than it was during wavefront capture."

Dr. McDonald also described a study of the effect of cyclotorsion and pupil centroid shift on excimer laser photoablation, which was performed by colleague Eric D. Donnenfeld, MD, and presented recently at the American Society of Cataract and Refractive Surgery annual meeting.

Dr. Donnenfeld performed an analysis of 1,193 eyes between January and June 2006.

"He was able to achieve iris registration in 93% of the eyes," she said. "He measured cyclotorsion, pupil centroid shift, and pupil size at the wavefront capture in total darkness and at the time of surgery under minimal illumination.

"He found that some eyes, of course, turn counterclockwise-the average was 3.6°, and some turn clockwise-the average was 2.3°," Dr. McDonald continued. "He also found in the x axis the mean shift was 0.26 µm, ranging up to 500 µm, and in the y axis, the mean shift was 0.13 µm, once again ranging up to 500 µm."

In a typical patient, she said, there is "no centroid shift and no cyclotorsion, the wavefront is perfect, the residual refraction is plano, the point spread function is perfect, and the Snellen 20/20 E[-chart] is perfect. But if you use the average values for centroid shift, of 0.26 in the x axis and 0.23 in the y axis, and you include the average amount of cyclotorsion that Dr. Donnenfeld found in his study, now you see that the wavefront is no longer perfect, the point spread function is very poor, [and] the Snellen 20/20 E[-chart] is very blurry."

Residual refraction, coma, and spherical aberration are troubling as well, Dr. McDonald said.

"Now if you take the maximum amount of x and y centroid shift that Dr. Donnenfeld found in his study, and the maximum amount of cyclotorsion that he found in his study, now you see that the wavefront map is truly distorted, the point spread function is horrible, and the 20/20 E[-chart] is no longer recognizable," she said. "There is a very significant residual refraction, horrible coma, and a startling amount of spherical aberration."

She concluded, "Is wavefront technology the future? Absolutely yes."

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