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Adjusted formula offers best IOL power calculation after PRK

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THOROFARE, N.J. — Accurate IOL calculations for cataract patients who have previously undergone photorefractive keratectomy can be made by using the Hoffer Q formula with a corrected keratometric value. Surgeons should use pre-PRK keratometric measurements, minus the refractive change due to PRK, when calculating IOL power for the best results, according to surgeons with Amsterdam's Academic Medical Center.

With an increasingly aging population, surgeons in the near future will have an influx of former PRK patients who now need cataract surgery, said Monica Odenthal, MD. Previously, surgeons were not aware of the difficulty of IOL calculations in these patients and measured just the corneal curvature preoperatively as they would in any other cataract case, she said.

“Unfortunately, calculating IOL power from these measurements brought unexpected refractive results and erroneous hyperopic outcomes,” Dr. Odenthal said.

In a retrospective study, Dr. Odenthal and colleagues analyzed 15 eyes of nine patients who underwent cataract extraction and IOL implantation after PRK to correct myopia. Pre-PRK measurements found that patients fell between -6 D and -12 D. Patients ranged in age from 29 to 66 years old at the time of PRK, with an average of 2 years 9 months time between cataract surgery and PRK.

“In six eyes, K value was measured after PRK and before cataract surgery. In the other nine eyes, different methods were used, like the ‘standard IOL’ method (where you simply take a 22-D IOL when an eye is emmetropic), the automated refraction method where you check refraction during the operation with a handheld autorefractometer and other types of calculation and guesswork,” Dr. Odenthal said.

She found corrected, refraction-derived K values are more accurate than the post-PRK or conventional measurement. The most accurate K value was achieved by subtracting the achieved change in spectacle refraction 1 year post-PRK from the K value of the cornea before PRK.

More details of Dr. Odenthal's study and findings will be published in the July 15 print edition of Ocular Surgery News.