Date released: Aug. 10, 1998

New approach widens knowledge of refractive surgery results

MELBOURNE, Australia -- The Alpins astigmatism analysis method, which considers corneal as well as refractive data, cast new light on the results of photorefractive keratectomy (PRK) for myopic astigmatism in a study here.

The study, performed by the Melbourne Excimer Laser Group, University of Melbourne Department of Ophthalmology, included 97 eyes of 79 patients receiving PRK with the VisX 20/20 excimer laser. The study's use of the Alpins method revealed important differences depending on whether results were analyzed using refractive or corneal data.

The Alpins approach, developed by the study's lead author, Noel Alpins, MD, of Melbourne, addresses corneal topography and keratometry data as well as refractive data, allowing parallel analyses to be done. "Objective corneal analyses based on topography and keratometry are important and have been disregarded by doing only subjective refractive astigmatism analyses in the past," Dr. Alpins said.

"The targeting of spherical refractive astigmatism limits the use of corneal measurements for analysis," he said. "Just as there are differences between corneal and refractive measurements prior to surgery, so there are differences after surgery."

A significant finding of the study revealed that astigmatic treatment with the laser was not as successful as spherical treatment using all three measures: topography, keratometry and refraction. The Alpins method also showed that astigmatism correction as measured objectively by topography and keratometry was actually less successful than astigmatism results as measured by refraction.

"There may be 'noise' in topography and keratometry measurements, but at least these tools reduce the human element in the measurement process and will no doubt be improved over time," Dr. Alpins said. "It may be that the subjective measurements of refraction after refractive surgery are less reliable and more flattering to our results than we deserve."

During the course of the study, researchers using the Alpins method found that the laser's programmed astigmatism treatment was undercorrecting patients by 20%. The laser's manufacturer, Visx Inc., of Santa Clara, Calif., later used the finding to revise the treatment algorithm and improve outcomes.

The study was published as the lead article in the July/August 1998 issue of the Journal of Refractive Surgery, a publication of the International Society of Refractive Surgery. The study demonstrates the utility of the Alpins method in handling aggregate astigmatism data for the purpose of nomogram adjustment or scientific presentations.

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