Wavefront surface ablation good choice for unusual topography

October 1, 2006
Lynda Charters

Minneapolis-Wavefront surface ablation seemed to be a safe and effective treatment for patients with substantial degrees of coma or trefoil in the early follow-up period. The 1-year data in a small number of eyes showed that the results remained stable, according to David R. Hardten, MD.

In these patients with unusual topography, surgeons still are not achieving results that are as good as in patients with normal topography, but it is hoped that iris registration technology will improve their outcomes, explained Dr. Hardten, director, refractive surgery services, Minnesota Eye Consultants, and clinical associate professor of ophthalmology, University of Minnesota, Minneapolis.

In patients with unusual topography or corneal curvature, surgeons have to decide between LASIK or a surface procedure such as PRK or LASEK.

Surgeons generally opt for either LASIK or PRK based upon corneal thickness, the ablation level needed to achieve the desired refractive result, and corneal topography, he added.

"PRK may be a better choice for some patients with very small eyes in which access may not be available to the entire ablation area when performing LASIK, or because of unusual topography, thin corneas, or anterior basement membrane dystrophy," Dr. Hardten said.

In a retrospective study, Dr. Hardten and his colleagues included 92 eyes of 59 patients who had high degrees of coma or trefoil. High levels in this study were defined arbitrarily as 0.15 μm of coma or trefoil. All patients had undergone either PRK or LASEK performed with the CustomVue wavefront system (VISX/Advanced Medical Optics).

The mean spherical equivalent in the study eyes was about –3 D, the mean astigmatism was 0.7 D, and the mean coma was 0.31 μm. Some patients had a combination of trefoil and coma.

In addition to higher aberrations, most of the corneal topographies were atypical, Dr. Hardten pointed out. The mean depth of the treatment was consistent with spherical equivalent and the astigmatism, that is, an average of 60 μm. Mitomycin-C was used in all procedures.

Reasons for PRK

"The clinical reason that the surgeon chose to perform PRK in 62% of the patients was because of an atypical topography alone; 16% had thin corneas alone; 16% had a combination of anterior basement membrane dystrophy and atypical topography, and some patients had deep-set eyes with atypical topography," Dr. Hardten said.

Eighty-four eyes were followed for 1 month or longer; the mean follow-up was 8.4 months.

"The spherical equivalent at the last follow-up visit was about plano (–0.04 D) with a standard deviation of about 10% of the original spherical equivalent," he said. "The mean astigmatism was 0.29 D."

At the final follow-up visit, 69% of patients had 20/20 or better visual acuity and 93% had 20/30 or better vision. Considering that PRK may not be stable 1 month after treatment, Dr. Hardten showed that the visual acuity results improved slightly in those patients with 1-year follow-up data; 74% of patients had 20/20 or better vision; and 93% had 20/30 or better vision.

"The results from both time points are very similar," he noted.

When Dr. Hardten and colleagues compared these results with other series of patients with normal topography who underwent wavefront-guided LASIK from their practice, they found that 90% of those patients had 20/20 or better vision in 2004.

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