I'm going to be away for a few weeks but I did want to post the following article before I leave. This is included in the index of articles in the November '97 issue of Ophthalmology, the peer reviewed journal of the American Academy:
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Hyperopia Correction by Noncontact Holmium: YAG Laser Thermal Keratoplasty: U.S. Phase IIA Clinical Study with 2-year Follow-up
Douglas D. Koch, MD, Thomas Kohnen, MD, Peter J. McDonnell, MD, Richard Menefee, AAS, Michael Berry, PhD
Noncontact holmium: YAG laser thermal keratoplasty reduced hyperopia by 0.5 D with one-ring treatments and by 1.5 D with two-ring treatments, without regression at 1 and 2 years.
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So-we know that LTK is stable for 2 years for at least 1.5 D of hyperopia. The following articles are rather lengthy but full of information on the various procedures. We know that all hyperopic procedures come with varying degrees of regression. PRK may even be able to treat higher degrees of hyperopia, eventually. Remember why SNRS technique is superior: Non-contact shrinking of the cornea (outside of the center area of vision). PRK is a surgical scraping of the epithelium (including the central area of vision). Not only is the safety of LTK superior but also the numerous financial reasons we have discussed before. I am including most of the article. Regression of PRK is addressed in the middle somewhere.......
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The Review of Ophthalmology article 8-96 Update on Hyperopic Refractive Surgery. . . *Walter Bethke Associate Editor
"Holmium Laser Thermokeratoplasty
"With variations in the orientation of the rings relative to one another," says Dr. Koch, "we could probably get 2.0 D to 2.5 D." For the next group, surgeons will treat patients with the spots of the outer ring aligned with those on the inside, rather than offset by 22.5 degrees as they were in his second group. This approach is based on the work of Paolo Vinciguerra of Manza, Italy, who found that surgeons can get higher, more stable corrections with a larger effective treatment zone if they treat in this orientation. Dr. Kock says there is a "moderate" amount of regression in the first month, and a decreasing amount between six months and a year.
Excimer PRK
Surgeons and companies are making progress in this area, especially for low hyperopes. But treating hyperopia is much more challenging than treating myopia, and the procedures are probably some time away from marketability.
Many of the problems relate to the large treatment zone. First, the large zone requires significantly more epithelial removal, significantly lengthening the procedure. In fact, Dr. Jackson says that efficient epithelial debridement is the number one challenge with hyperopic PRK. Dr. Jackson says the Star requires debridement out of 9.5 mm to allow for the blend zone. "If it takes too long," he says, "drying of the stromal bed can lead to uneven ablations." To improve his debridement technique, he's been working with South African surgeon Percy Amoils, who has invented a rotary brush that Dr. Jackson says serves him well in quickly getting off the epithelium.
Harold Johnson, MD, of the Gimbel Eye Center in Edmonton, says that since so much epithelium is removed from the cornea, sometimes completely out of the limbus, the re-epithelialization period can be as much as six days. When he used the Summit Apex for hyperopia, he had to remove epithelium out of 9 mm. When this much epithelium is removed, he says you must watch these patients more closely than post-op myopes, since they're more prone to infection from having their stroma exposed for so long. To help reduce the risk of infiltrates, he prescribed antibiotics, NSAIDs, and a large bandage contact lens, and sees patients frequently post-op until they re-epithelialize. He also suspects this extended healing period to be behind certain physiological changes in the cornea which can affect the refraction, though he's still studying this. He adds that bilateral procedures are probably impossible for hyperopes. "I don't want patients to have to endure two eyes without epithelium for almost a week."
Once the epithelium is removed, the procedure itself is lengthy. "You need at least three times as many spots to correct a 3.0 D hyperope as you do for a 3.0 D myope", says Colorado surgeon Jon Dishler. "This creates greater potential for decentration and dehydration, due to the longer time period involved." He says the dehydration leads to over-corrections, since water on the cornea usually absorbs so much of the laser energy.
The length of the procedure combined with the relatively small optical zones used also poses a significant risk of decentration. Dr. Jackson says hyperopia treatment takes three times longer than myopic treatment, increasing the risk of decentration. He adds that none of the 55 eyes he's treated have had this problem, however. Dr. Dishler says the smaller optical zone is also "less forgiving" when a decentration occurs, since it makes it easier for the zone to miss the visual axis. When Dr. Anschutz began treating hyperopia with the Meditec laser, the machine used a 7 mm treatment zone and a 3.5 mm optical zone. The results were poor. Visual rehabilitation was slow, and there was a "high instance of loss of best corrected visual acuity" due to decentration. Now the machine uses a 9 mm ablation diameter and an effective optical zone of 6 mm, a change which has significantly improved the results, Dr. Anschutz says.
A final problem is regression. Excimers treat hyperopia by creating a doughnut-shaped trough in the mid-periphery of the cornea. This area can fill in, negating the effects of the treatment. Dr. Jackson says he sees regression from one week out to six months, although most eyes appear to stabilize at about four months. Dr. Johnson says he hasn't compiled his results for the Summit Apex. But he believes there is very little regression. "If the post-op refraction wound up at -0.5 D, it didn't fluctuate that much," he says. He says his problem was over-correction. "In the beginning, we were running into over-corrections of almost double. He remedied the problem by cutting the target correction amount in half. Since the Summit hyperopic procedure employs ablatable masks to create the doughnut shaped ablation, he says he simply used masks for lower corrections than he needed. He then finished the procedure normally, using Summit's Axicon lens to create a blend zone out to nine millimeters.
LASIK
Performing hyperopic ablation under a flap may reduce haze and regression of effect. But the width of the ablation makes centering even more critical in these patients. In all, it's too soon to tell whether this procedure will work for large amounts of hyperopia.
Surgeons and companies seem to be making progress, albeit slow progress, toward an effective, lasting accomplishable procedure for hyperopia. But an acceptable solution for farsightd PRK still appears to be fairly far away.
*San Diego surgeon Perry Binder is investigating hyperopic PRK for Summit Technology under the protocol that covers corrections from +1.00 D to +4.00 D.
Dr. Binder says that he and Michael Gordon, MD, will soon be doing the procedure beneath a corneal flap, too. Some surgeons prefer hyperopic LASIK because it avoids the pain and slow visual recovery associated with surface ablations.
Ottawa surgeon Bruce Jackson is performing VISX's Canadian hyperopic trials for up to +4 D. He'll begin the trials for up to +6 D soon.
He explains that the VISX uses an eccentrically rotating lens to distribute laser energy out to 9mm. The treatment also employs the same rectangular slit blades that control the astigmatism treatment.
He says debridement technique seems to make a difference in outcome over the short term. One patient group's debridement was done with the traditional mechanical technique, while the other's was done with a rotating brush. At the four-month mark, he says the brush group had better acuities and better contrast sensitivity. At just over 12 months, however, the gap narrows. At this point, the scrape group, comprised of 11 patients with a mean pre-op error of +2.8 D, had an average refraction of +0.5 D. The brush group, with an average hyperopia of +2.23 D, had an average error of +0.14 D.
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Hope this answers any questions you may have. I'll try to check in with the thread. See you in a couple of weeks.
Nanny |
