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Hyperopic LASIK appears safe, effective using different ablation zone diameters
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ORLANDO, Fla. — Hyperopic laser in situ keratomileusis (LASIK) appears safe and effective using different ablation zone diameters, according to the results of 49 eyes presented here at the American Academy of Ophthalmology (AAO) meeting. However, there is an increased tendency toward overcorrection with a 6 mm-by-9 mm zone using the current laser algorithms.
“With the new laser parameters of the Visx (Santa Clara, Calif.) Star S2 excimer laser, we’re now able to adjust both the repetition rate and ablation zone sizes,” said senior author Jonathan M. Davidorf, MD, a refractive surgeon in private group practice in West Hills, Calif.
The study evaluated 49 eyes with spherical hyperopia, ranging up to + 5.5 D of spherical equivalent refraction, and less than or equal to 1.5 D of astigmatism. The laser’s repetition rate was 10 Hz, “which we had determined from previous work to be just as good as 6 Hz or 8 Hz but a faster treatment,” Dr. Davidorf said.
Eyes were divided into three groups: an ablation zone size of 5 mm by 9 mm, 5.5 mm by 8.5 mm and 6 mm by 9 mm. “The spherical equivalent refraction was greatly reduced in all cases,” Dr. Davidorf said. However, “there was a tendency toward a larger effect on the 6 mm-by-9 mm group, compared with the other two groups.” In addition, “stability was good in all three groups, but we only followed up 3 to 6 months, and we did not calculate 1-day refractions.”
Predictability was best for the 5.5-mm-by-8.5 mm ablation zone group using the current nomograms. For all three groups, though, approximately 90% of eyes had uncorrected visual acuity of at least 20/40 at the last examination. But for best spectacle corrected visual acuity, the 6 mm-by-9 mm group had the lowest percentage of eyes that lost 1 line of vision and the greatest percentage that gained 1 line or more.
“These results are not statistically significant, however, because of the small number of eyes treated in the three groups,” Dr. Davidorf said. Still, no eyes lost more than one line of vision.
Slope of the line important
Nonetheless, there was a statistically significant difference between the 5 mm-by-9 mm group and the 6 mm-by-9 mm group regarding the slope of the line of best fit, despite the same outer zone size. “As we move from a 5-mm inner zone diameter to a 6-mm inner zone diameter, each 0.5-mm incremental increase in the inner zone size yields a 7.5% incremental increase in the amount of effect for each diopter entered into the laser,” Dr. Davidorf explained. The 5 mm-by-9 mm group had a 0.97 slope, compared with a 1.12 slope for the 6 mm-by-9 mm group.
“For each entered diopter correction, we achieved more effect with the current laser algorithms,” Dr. Davidorf said. Furthermore, “it may prove safer to use larger inner zones, which makes some conceptual sense. Additionally, there are reports not documented in this study that indicate a larger outer zone may be more stable.”
Currently, Dr. Davidorf uses a 5.5 mm-by-8.5 mm ablation zone diameter. “I like being able to maintain a slightly larger inner zone size,” he said. “It doesn’t appeal to me to go all the way out to 9 mm with the current microkeratome designs, because you rarely obtain a complete and perfectly centered 9-mm diameter stromal bed. Whether it’s the hinge or epithelium, the ablation will likely not be confined to the stromal bed. So until we’re able to predictably create perfectly centered 9-mm stromal beds, I would prefer to somewhat limit the outer zone size.”
Limitations of study
Roy S. Rubinfeld, MD, who critiqued the study at the AAO meeting, applauded the presentation, despite “significant limitations.” In addition to a relatively short follow-up period and relatively few eyes, “there appeared to be more higher hyperopes in the 6 mm-by-9 mm ablation zone group, and so the results are relatively preliminary. It’s a bit of a pilot study.”
Another limitation of the study is that it can take a full 6 to 12 months with hyperopes before true refractive stability can be determined, Dr. Rubinfeld conveyed. Nonetheless, “this is a very important study because we all want to know the best way to perform hyperopic LASIK. We want to know which zone size will provide the optimum efficacy, safety and predictability,” he said. Other issues to consider, though, include stability, visual recovery rate, contrast sensitivity and quality of vision. “Answers to these questions will determine the best flap size, for example, and hence, keratome designs,” Dr. Rubinfeld noted.
Furthermore, Dr. Rubinfeld pondered the limits of hyperopic LASIK in general. “How high can we go, and does changing the optical treatment area affect how high we can safely go?” he said. The repetition rate may still also prove significant.
For Your Information:
- Jonathan M. Davidorf, MD, can be reached at 7230 Medical Center Drive, Ste. 201, West Hills, CA 91307; (818) 883-0112; fax: (818) 883-2767. Dr. Davidorf has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Roy S. Rubinfeld, MD, can be reached at 5454 Wisconsin Ave., Ste. 950, Chevy Chase, MD 20815; (301) 654-5114; fax: (301) 654-9132. Dr. Rubinfeld has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.