New options for IOL calculations after refractive surgery (2006)

Here's my ophthalmic trivia question of the month: How many ways are there to calculate intraocular lens (IOL) power in eyes that have had photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK)? The list is long, over 10 by my count, and growing. As is always the case when there are several solutions to a problem, none is perfect. Fortunately, new methods continue to improve accuracy and, in some cases, reduce the complexity of these calculations. In this issue, 3 methods are described. Walter et al.1 describe an approach in which one simply calculates IOL power as if the patient had not had LASIK, targeting the patient's original pre-LASIK refractive error. Therefore, if the patient was −7.00 before LASIK, simply use −7.00 as the refractive target. The results in their small series are excellent. The limitations of their approach include (1) pre-LASIK data may not be available or accurate and (2) the patient's outcome from LASIK prior to developing the cataract may not be known or accurate. In Masket's formula,2 one calculates IOL power in a standard fashion and simply modifies the final value of the IOL as a function of the LASIK-induced refractive change. An advantage of this method is that there is less reliance on historical data as the LASIK-induced change is multiplied by 0.323. Therefore, if there is a 1.0 diopter (D) error in the historical data regarding the refractive change, this translates to only a 0.32 D error in IOL selection. The Masket formula is similar in approach and structure to 2 formulas devised by Latkany et al.3 They describe 2 methods: (1) Calculate IOL power using mean keratometry readings obtained using the Javal keratometer and modify it by −0.46x + 0.21, where x equals the surgically induced change in refraction, or (2) calculate IOL power using the flat K and modify it by −0.47x + 0.85. The differences between these 3 formulas are relatively small, and comparisons are subject to error since K-readings were obtained differently in each (IOLMaster for Masket and Javal keratometer for Latkany et al.). However, it is interesting to look at the values they calculate. For example, for a 3.0 D LASIK-induced change, the modification of IOL power would be 0.87 D with Masket's formula, 1.17 D with Latkany et al.'s average-K formula, and 0.56 D with Latkany's flat-K formula. For 8.0 D, the respective values are 2.48, 2.91, and 3.47. Finally, Mackool et al.4 describe a technique in which the cataract is removed and the patient is refracted 30 minutes later. Using a nomogram developed by the authors, the power of the IOL is calculated and the IOL is then implanted. Again, the results are excellent. This technique was first described by Mackool in 1998.5 Recently, Ianchulev et al.6 described a similar approach using intraoperative automated refraction. Both Mackool and Ianchulev et al. multiplied the measured refractive error by a fixed, albeit different, value—1.75 for Mackool and 2.01 for Ianchulev et al. A disadvantage of Mackool's approach, of course, is the need to return to the operating room for IOL implantation. What should the clinician do when faced with the daunting problem of post-refractive-surgery IOL power calculation? In my clinical practice, I continue to use several approaches, including the modified topographic approach described by us,7 the modified Maloney method,7 clinical history (which we often find to be the least accurate), and Masket and Latkany et al.'s formulas. We also plan to try the method of Walter et al. Ideally, it would be optimal to have a spreadsheet that allows the clinician to insert all values available. The various formulas would then be calculated automatically. When faced with a range of values for the IOL power, we look for values that are consistent with at least 1 other reading. We also prefer values that select higher IOL powers, preferring to leave the patient slightly myopic rather than hyperopic. Resolution of this problem will require a method for accurately measuring posterior corneal power or a technique for adjusting IOL power after implantation. Until then, surgeons are faced with performing multiple calculations to “guesstimate” the correct IOL for patients who, by their original decision to have PRK or LASIK, have demonstrated that they have above-average refractive demands.