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Intraocular lens power calculation after corneal refractive surgery remains challenging

2000; Elsevier BV; Volume: 107; Issue: 2 Linguagem: Inglês

10.1016/s0161-6420(99)00107-4

1549-4713

Ran Sun, Howard V. Gimbel, Ellen E. Anderson Penno,

Ocular Infections and Treatments

In the article, “Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy”, published in Ophthalmology 1999;106:693–702, Dr. Seitz et al found that direct corneal power measurements underestimate corneal flattening after PRK by an average of 24%. When these values are entered into IOL power formulas, theoretically hyperopic shift after cataract surgery will occur. We believe that the authors raised an important issue because the number of refractive surgeries is increasing rapidly, and after 20 or 30 years many of these patients will need cataract surgery. Currently, many cataract surgeons have been using refraction-derived keratometric values instead of the measured keratometric values for IOL power calculation in postoperative radial keratotomy (RK) eyes and PRK eyes to avoid or reduce hyperopic shift after cataract surgery. The challenge still remains. We have several questions for the authors regarding this article. The first question is about the study design. The authors mentioned in the Abstract, and Patients and Methods sections that “this is nonrandomized, prospective study.” However, in the conclusion of the abstract and text the authors stated that “because this study is retrospective and theoretical, there is still need for a large prospective investigation to validate our findings.” Furthermore, we do not feel that a randomized prospective study is practical because it may take many years for the study patients to develop cataracts after PRK surgery, making it be difficult to follow these patients to collect follow-up data. To avoid hyperopia after cataract surgery following PRK, the authors suggested that “the calculation method using spherical equivalent change of refraction at corneal plane seems to be the most appropriate method.” Our question is: if calculated corneal powers (according to spherical equivalent change of refraction at the corneal plane and spectacle plane) are so close in Dr. Seitz’s article (Table 1) and no patient had cataract surgery following PRK, how did the authors know that one method is better than the other? We have experienced clinically that the incidence of unplanned hyperopia after cataract surgery in postoperative PRK eyes can be reduced by using the refraction derived-keratometric value for IOL power calculation (corneal power calculation according to spherical equivalent change of refraction at the spectacle plane rather than at the corneal plane). In the Result section, the authors stated that “keratometric corneal power overestimation (ΔKcalc-co) and IOLP underestimation (ΔIOLPH, Kcalc-co) correlated significantly with actual difference between preoperative and postoperative pachymetric corneal thickness (P = 0.05).” We could not find data in the text, Table, or Figure to support this result. In this article, the authors used calculated keratometric values as a gold standard to predict IOL power calculation. Clinically, the calculated keratometric values are not always gold standard. They may be influenced by index myopic changes. When calculated keratometric values are greater than measured keratometric values, our outcomes suggest that selecting the least keratometric value for IOL power calculation gives more accurate and predictable results (unpublished data). Normally, the calculated keratometric values are smaller than the measured keratometric values in postoperative RK or PRK eyes, but in most cases of an advancing nuclear sclerotic cataract, a myopic shift in the refraction is seen (index myopia). Ideally, when using calculated keratometric values for IOL power calculation, one should try to obtain the early postoperative PRK refraction to be able to take into account the amount of myopic shift that was induced by the cataract progression. For example, if the refractive procedure caused 4.00 D of corneal flattening and then a progressive cataract reversed 2.00 D of this effect through index myopia, the unadjusted calculated keratometric value would overestimate the ‘steepness’ of the cornea as used in the lens power calculation formula. Thus, a less powerful IOL would be chosen resulting in postoperative hyperopia. Why do we often see hyperopic shift after cataract surgery in postoperative RK or PRK eyes? Why could we not obtain accurate results with our instruments and IOL power formulas in postoperative RK or PRK eyes? Possibly, the answer is that the instruments and IOL power formulas are designed based on normally shaped corneas. When we perform corneal refractive surgery, a differently shaped cornea is produced, thereby limiting the ability of the instruments and IOL power formulas to provide accurate results. Therefore, in addition to the authors’ suggestions regarding modified IOL power formulas and corneal power values, designing instruments to more accurately measure the effective refracting power of the cornea would also help to more accurately calculate appropriate IOL powers for post-corneal refractive surgery eyes. Author’s replyOphthalmologyVol. 107Issue 2Preview Full-Text PDF