Negative spherical aberration after CK is desirable

October 1, 2004

San Diego-Inducing negative spherical aberration may be desirable after conductive keratoplasty (CK). Results of a study showed that patients with negative spherical aberration after CK met the target refractive error and target vision compared with patients with positive spherical aberration, according to Stephen Pascucci, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

San Diego-Inducing negative spherical aberration may be desirable after conductive keratoplasty (CK). Results of a study showed that patients with negative spherical aberration after CK met the target refractive error and target vision compared with patients with positive spherical aberration, according to Stephen Pascucci, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

In patients who underwent CK, the degree of negative spherical aberration showed a trend toward significance. In light of this, Dr. Pascucci and his colleagues conducted a pilot study of 12 patients who underwent CK primarily to correct their near vision, he said.

"There is a lot to learn about CK, because these corneas that are reshaped and steepened by the procedure are unique in shape and they function in a way that is unique compared with what we have created previously. As a result, it is important for us to understand why patients who undergo CK achieve much better than expected uncorrected near vision and why distance vision is preserved to a considerable degree. The spherical aberration may partially explain some of what is occurring in these patients," said Dr. Pascucci, a private practitioner in Scranton, PA.

The investigators used the WaveFront Analyzer (VISX) in the study. Most of the patients had low hyperopia, i.e., 0.75 D and less. The patients were stratified to three treatments: groups 1 (n = 6) and 2 (n = 2) were on-label treatments; group 1 had 16-spot treatments, and group 2 had 24-spot treatments in 6-, 7-, and 8-mm patterns. Group 3 (n = 4) had an off-label 24-spot treatment in a 6-, 6.5-, and 7-mm pattern.

"The refractive effect was similar among the three groups. With 24 versus 16 spots, the refractive effect was not very different," Dr. Pascucci reported.

However, he pointed out the refraction is not the only factor to consider with CK. Other important factors are visual acuity, both distance and near, and the spherical aberration value.

The mean preoperative spherical aberration value in these patients was positive, 0.11218 µm (range, 0.25017 to -0.00883 µm). However, the postoperative spherical aberration was negative (mean, -0.02139 µm; range, 0.16185 to -0.14700 µm).

"This is one factor to be considered. Spherical aberration values can change with CK and the shift occurs in a negative direction," he stated.

The second factor to consider as a result of this study, Dr. Pascucci pointed out, is that the degree by which negative spherical aberration is induced is greater with a greater number of spots applied, and that there is a difference between the on-label 24-spot treatment nomogram spot treatment compared with the off-label 24-spot treatment nomogram.

"Regarding the on-label and the off-label 24-spot treatments, there is greater negative spherical aberration. When I compared the preoperative data from the two groups and considered if the group that received the off-label 24-spot treatment had less positive spherical aberration, I found that was not the case; that group actually had greater positive spherical aberration and wound up with more negative spherical aberration," he said.

Target refraction achieved in 67% The authors achieved their target outcome (defined as the range expected in terms of induced myopia as the result of a 16-spot treatment and the range of myopia expected from a 24-spot treatment) in 66.7% of patients. Importantly, those patients in whom the target refraction was achieved had greater induced negative spherical aberration compared with those in whom the refractive target was not achieved, he explained.

When the authors evaluated the patients with postoperative negative spherical aberration, i.e., those in whom the desired refractive error was achieved, they found that the target was achieved in 87.5%.

"The percentage of patients in whom the desired refractive error was achieved mimicked the percentage of patients in whom both good distance vision and J3 uncorrected near visual acuity were achieved in the FDA clinical trials that examined the use of CK to correct presbyopia," he said.