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Increasing treatment in periphery improved hyperopic LASIK results
By increasing the amount of time the laser beam spent on peripheral ablation during hyperopic LASIK, Nidek investigators improved refractive results.
Increasing the amount of laser treatment in the periphery with an empirically derived algorithm for hyperopic LASIK resulted in improved refractive results, according to one investigator.
![George O. Waring III, MD [photo]](/images/OSN/200609b/waring.jpg) George O. Waring III |
George O. Waring III, MD, presented the results of hyperopic LASIK performed with the Nidek EC-5000 laser at the American Society of Cataract and Refractive Surgery meeting earlier this year.
“What we’ve tried to do in this series is to identify the need for a radial ablation compensation,” Dr. Waring said in his presentation. “We got results that are comparable to the standards in the field at this time.”
Dr. Waring explained that excimer laser ablation decreases in efficacy as the laser moves toward the periphery of the cornea because the beam strikes the cornea at an oblique angle rather than directly, and the small spot widens into an oval. In hyperopic LASIK, in which much of the treatment is delivered to the paracentral and peripheral cornea, this radial ablation fall-off makes planning ablations “a little bit trickier,” he said.
Using a new algorithm, the Nidek laser was able to produce the desired ablation profile without increasing the number of pulses in the periphery, Dr. Waring said. Removing less tissue resulted in rapid refractive stability and little hyperopic regression, likely due to a reduction in the amount of remodeling in the peripheral cornea compared to traditional hyperopic LASIK, he said.
Flat versus curved plane
Dr. Waring explained the rationale for this study using a modified algorithm.
“The idea is that the lasers that we use have been calibrated on a flat plate, but the cornea’s not a flat plate, it’s a curved surface,” he said. “So as the beam that comes further and further out on the cornea, the angle of incidence changes. The shape of the beam on the cornea becomes more oval, and the energy that’s delivered in each pulse to the area diminishes. This gives us less efficient ablation in the peripheral part of our ablation than in the central part.”
This phenomenon is common to all lasers, Dr. Waring noted, not only the Nidek laser with which he and his colleagues were working.
“When we’re working with hyperopia, it gets a little bit trickier because we’re working in the paracentral and peripheral cornea – that’s our target area,” he explained. “If we’re going to get a proper profile, we have to be aware of this variability in the ablation efficiency as we go out.”
Dr. Waring and his colleagues quantified the differences in laser efficiency in the periphery by performing ablations on plates held at different angles.
“As the angle increases, we got a larger decrease in the efficiency of each pulse,” he said. “When we apply that information to the curved surface of the cornea, and we get out to the radius of about 1.5 mm or a diameter of 3 mm, then we begin to get the fall-off.”
At a 9-mm radius, the efficiency decreased by 20%, he said, creating a difference between what was theoretically calculated and what was actually ablated.
Implementation
“Our problem then is, how do we compensate for this?’ Dr. Waring said. “Well, you can change the energy per pulse or you can change the number of pulses delivered. Since the Nidek laser is a time-based laser, … we were able then to deal with the beam offsets: that is, as we went further and further out, to increase the amount of time that each pulse remained out there and the number of pulses.”
In the clinical trial for Food and Drug Administration for approval of the Nidek laser for hyperopia, Dr. Waring and colleagues evaluated their empirically derived algorithm in 300 eyes with up +6 D of spherical error and 3 D of cylinder. Nidek has not yet received approval for the treatment of hyperopia in the United States.
The results showed a tendency toward undercorrection, Dr. Waring said, but because the series was part of a clinical trial the investigators were not able to make a nomogram adjustment, as they would have in clinical practice.
“Even with that tendency toward undercorrection, we did reasonably well,” he said. About 70% of eyes were within ±0.5 D of target, and 94% were within ±1 D. Dr. Waring said these figures are comparable to the results of hyperopic LASIK achieved with other companies’ lasers.
For more information:
- George O. Waring III, MD, can be reached at InView, 301 Perimeter Center North, Suite 600, Atlanta, GA 30346; 678-222-5102; fax: 404-250-9006; e-mail: drgeorge@georgewaring.com. Dr. Waring is a paid consultant for Nidek.
- Nidek, maker of the EC-5000 excimer laser, can be reached at info@nidek.co.jp; Web site: www.nidek.com.
- Katrina Altersitz is an OSN Staff Writer who covers all aspects of ophthalmology.