Q: I have several patients who underwent either radial keratotomy (RK), photorefractive keratectomy (PRK) or LASIK several years ago and now require cataract surgery. Is there a difference in how the corneal power differs among the procedures in determining intraocular lens (IOL) power?

A: Yes. Patients who undergo refractive sur-gery require a modified method of determining the corneal refractive power because a keratometer and topographer read the curvature of the cornea as if it has never undergone either procedure, says ophthalmologist Douglas D. Koch, of the Cullen Eye Institute, a part of Baylor College of Medicine in Houston.

Specifically, keratometers and topographers employ a standardized value for the refractive index of the cornea to estimate the total cor-neal power. But, refractive procedures that remove corneal tissue (e.g., PRK and LASIK) alter this index because they change the relationship between the front and back surfaces of the cornea, says Li Wang, M.D., Ph.D., also of the Cullen Eye Institute.

Another consideration: Post-surgical corneas have wider ranges of curvature within the central 3mm. These are missed on standard or simulated keratometry, which measures only four points, Dr. Wang says.

Both doctors have done extensive research on this topic. Based on this research and their clinical experience, they suggest determining the corneal refractive power in these patients using one of the following:

A modified Maloney method. Use the Humphrey Atlas axial
topographical map to measure the central corneal power. Then use a modified form of the Maloney methoda way to modify the cor-neal power at the center of the topography map, created by Robert K. Maloney, M.D.¹ (If you do not use this device, contact the manufacturer of your own topographer to find out how to use this method.)


Drs. Koch and Wang created a modified version of the Maloney method based on their own retrospective data. This data showed that the mean IOL prediction error with the Maloney method was larger than that with the clinical history method (see below); however, the variance (square of standard deviation) was significantly smaller with the Maloney method. This finding indi- cated that with appropriate modification, this method might provide more consistent results.

Their modified Maloney method converts the central corneal power obtained from topography back to the anterior corneal power, then subtracts the posterior corneal power from the anterior cor-neal power. Their formula: Central power (as displayed with the cursor in the center of the topographic map) = [central topographic power x (376/337.5)] - 6.1.

Effective refractive power (EffRP) method. Measure the EffRP with the EyeSys Corneal Analysis System (use any model). Adjust the EffRP according to the amount of the refractive change caused by PRK or LASIK. To obtain the adjusted EffRP, subtract 0.15D for every diopter of surgically induced refractive correction from the measured EffRP.² (Again, if you do not use the EyeSys device, contact the manufacturer of your own topographer to find out how to use this method in determining corneal refractive power.) In hyperopic LASIK, since ablation takes place outside the central cornea, EffRP measures generally work well for obtaining IOL calculations.

The clinical history method. This corneal power estimation method requires keratometry values from before and after the refractive surgery. To obtain the correct cor-neal refractive power, subtract the change in manifest refraction at the corneal plane from the corneal power values derived before the refractive procedure.

If both the keratometry values prior to refractive surgery and the extent of refractive correction caused by surgery are available, calculate the corneal power using the clinical history method, Drs. Koch and Wang say. If the preoperative data are not available, the modified Maloney method is the right choice.

To determine the corneal refractive power in RK patients, use the average central topographic values obtained from the EffRP, as shown on the Holladay Diagnostic Summary of the EyeSys Corneal Analysis Topographer, or follow the instructions from the manufacturer of the topographer you do use, both doctors. say. The Holladay 2 formula, invented by Jack T. Holladay, M.D., is available at: www.docholladay.com/iolprogram.cfm.

Vol. No: 142:7Issue: 7/15/2005