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Standard contact lenses often used successfully in post-PRK patients
Fitting with a conventional contact lens: Trying to give a normal peripheral fit will result in steepness centrally, a physiologic compromise unless a very high Dk material is used.
WAILEA, Hawaii - Photorefractive keratectomy (PRK) patients fall into one of four categories postsurgically, according to H. Jonathan Kersley, MD, FRCS. Most fit into the first two categories: ecstatic and happy. The third group consists of those who can be managed and the fourth group, unfortunately, consists of the disasters.
"In the first lot, we had eight [disaster] patients from a group of about 3,000; these were mainly the higher myopes. You can see we're dealing with small numbers," said Dr. Kersley at the Hawaii `98 meeting here.
Management of PRK patients falls into four headings: those not requiring any support, those who need an occasional - usually simple - overcorrection, those who need a rigid lens to treat astigmatism and those complicated patients who get poor results with glasses because of irregular astigmatism.
Standard contact lenses may be used, quite often successfully, although the mechanics of fitting are different because of the shape of the post-PRK cornea - "a relative steepening of the periphery in relation to the flattened central zone," said Dr. Kersley.
Dr. Kersley said he and his colleagues developed a rule of thumb from the evaluation of 10 patients extracted from phase 3 (approximately 1,300 patients), of the ongoing Moorfields Eye Hospital PRK Study. This rule of thumb states that when comparing the pre- and post-keratometry averages, if the change was less than 0.3 mm, a standard contact lens can be used. "Or, looking at it the other way around, if the difference in diopters - pre- and post-PRK - was 2 D or less, you can get away with fitting a standard lens."
When they extended the rule to 3.5 mm they found it would still hold true for the two groups of patients studied. The second group was made up of eight patients taken from phase 4 (approximately 440 patients) of the Moorfields studies.
For those with a greater change, they came up with a design that incorporates reverse geometry.
"If you try to fit these difficult cases with a conventional contact lens and try to fit the central zone you end up with a dark area with no fluorescein and, therefore, no tears in the middle, surrounded by a lot of fluorescein around the edge," Dr. Kersley said. "Or you can try to fit the relatively steep periphery and end up with an enormously steep part in the center that is a physiologic compromise unless you have a material with a very high Dk - not a satisfactory situation."
Because the surgery flattens the central zone, you find a flat zone surrounded by a "junction zone" and then the relatively steep periphery, said Dr. Kersley. The reverse geometry lens design spreads the bearing area over the transition zone with a nearly aligned center and edge lift-off.
Dr. Kersley stressed that presurgical counseling would best be done by someone very familiar with contact lens work. Patients who cannot or will not wear contact lenses may be difficult to manage afterward.
Most successful contact wearers
Post-PRK cornea: PRK will create a flattened central zone surrounded by a "junction zone" and then a relatively steep periphery. The reverse geometry lens design spreads the bearing area over the transition zone with a nearly aligned center and edge lift-off.
He also said many patients were happily wearing contact lenses prior to surgery. Of the second group mentioned above, phase 4 of the Moorfields study, Dr. Kersley said 13% had never worn contact lenses and 80% had tried them. Of those who had tried them, 37% were contact lens failures. Fifty-five percent of the patients were wearing contact lenses when they came in for their pre-PRK assessment.
"More than half had worn them for more than 10 years, more than half were wearing them for more than 11 hours per day and comfort was assessed at `very comfortable' in 84% or more," he said. "So, you're dealing with a successful contact lens group most of the time."
For Your Information:
- H. Jonathan Kersley, MD, FRCS, is in private practice. He can be reached at 143 Harley Street, London W1N 1DJ, England; (44) 171-935-0886; fax: (44) 171-486-1956. Dr. Kersley has no financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Susan E. Marren, OD, FAAO, can be reached at (609) 829-4229; e-mail: Smarren@aol.com. Dr. Marren has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.