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Cataract Surgery

2008-06-20 Corneal refractive surgery (LASIK, PRK, RK) for myopia were analyzed retrospectively. Keratometry measurements, ie, autokeratometry readings ks, simulated keratotopography readings, and topographically measured average central corneal power in a 3-mm zone (ACP) were compared with calculated refraction-derived keratometric value. Correction factors based on the difference between measured and calculated keratometric powers were rated. Measured and calculated keratometric values. Direct power measurements with standard keratometers and planokeratotopography systems overestimate corneal power after myopic PRK and LASIK. The average ks were significantly greater than the average calculated refraction-derived keratometric values. Corneal power overestimation correlated significantly with the spherical equivalent change after refractive surgery. After RK, there is no significant correlation of the difference between all measured K values and refraction-derived power with the spherical equivalent change. In these cases, the Sim-K value seems the most accurate among measured keratometric powers. The precision of measurement significantly depends on the parameters of the autokeratometer (ie, measurement place, number of measurement points, keratometric index of refraction). To avoid underestimation of intraocular lens (IOL) power after cataract surgery in eyes that had previously undergone myopic corneal refractive surgery, the measured corneal power must be corrected. Although correction factors may be calculated for eyes after PRK and LASIK, there are no universal and absolutely reliable methods to determine corneal power in these cases. More than 1 accessible method should be used, and the lowest, most reliable data should be used. RK becomes moderately popular in the United States in the 1980s. PRK and LASIK become very popular in the 1990s. Most of these patients underwent refractive surgery with the goal of eliminating the need for distance correction. Many of these patients are beginning to need cataract surgery. The problem: These patients have high expectations for excellent uncorrected distance vision. It is difficult to accurately determine corneal power for the IOL calculation after refractive surgery. Current technology tends to overestimate central power (eg, keratometry readings) in patients who have undergone myopic correction. If these measurements are used, patients become hyperopic. The solution: To more accurately determine corneal power to ensure better IOL calculations They propose that the gold standard method of determining corneal power after refractive surgery is to subtract the change in manifest refraction after the refractive surgery from the preoperative keratometry reading. This calculation is only possible when both the preoperative refraction and preoperative keratometry readings are known. How is corneal power best determined with the gold standard calculated keratometry reading. In eyes after RK, they did not find a consistent under- or overestimation with the current keratometry readings. For eyes after PRK and LASIK, they did find a consistent overestimation of the current keratometry readings. They provide a table (Table 6) with correction factors to avoid this overestimation. For example, eyes that achieved a 4-diopter (D) myopic correction after PRK or LASIK would need 0.930 D subtracted from the current keratometry reading to give the actual keratometry reading to be used for the IOL calculation. Feiz et al 1 performed similar analyses and also came up with a correction table. However, they suggested that about twice as much power be subtracted. For example, for a 4-D myope, they suggest subtracting 2.15 D from the current keratometry reading. Having performed numerous cataract surgeries after refractive surgery, I believe that there are several problems with these 2 articles. The first is that the calculated keratometry reading, ie, subtracting the change in refraction from the original keratometry reading, is not perfect and cannot be used a gold standard. Second, there are other factors that influence postcataract refractive error including the type of IOL used, anterior chamber depth, and possibly even surgical technique. The best way to answer the question is to perform cataract surgery on all these patients and then determine which calculation would have resulted in emmetropia. I believe the best current method is to perform multiple measurements (eg, standard keratometry, corneal topographic central power) and calculations and err on the side of making the patients slightly myopic (by selecting a keratometry reading on the low end). I also inform patients that they may require a lens exchange, piggyback lens, or additional refractive surgery to achieve emmetropia. Surgeons should remember that RK corneas tend to be flat centrally for days to weeks after cataract surgery, so do not rush to modify the cataract surgical refractive result until the refraction has stabilized. For eyes after PRK and LASIK, there are no universal and absolutely reliable methods to determine corneal power to allow the calculation of the proper IOL power.