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    New approach to corneal power estimation in eyes with previous corneal refractive surgery and keratoconus eyes


    As devices measuring the posterior corneal curvature are not available in all clinical settings, we developed the concept of adjusted keratometric index (nkadj), which is a clinically valid method for estimating an appropriate keratometric index, allowing the corneal power to be calculated using the keratometric approach, but with enough accuracy.3 This method requires postoperative measurement of the anterior corneal radius only, which can be easily obtained in any clinical setting.

    In eyes that have had previous keratorefractive surgery for the correction of myopia, several methods have been proposed in recent years for the estimation of corneal power, some of which require historical clinical data that are not always available. Our research group recently proposed a new approach based on the concept previously mentioned, adjusted keratometric index.4 The corneal power calculated with this keratometric index (adjusted corneal power) has been clinically validated in 62 eyes of 62 patients with previous myopic LASIK surgery, comparing it with Gaussian corneal power and other corneal power values calculated with other previously described methods (clinical history, Awwad, Savini, Camellin, Jarade, Shammas, Seitz and Haigis-L).5 The mean (standard deviation) difference between the adjusted corneal power and the Gaussian value in this sample was 0.14 (0.24) D, with a range of agreement from –0.33 to 0.60 D, with only 4 out of 66 cases showing differences of more than ±0.5 D between adjusted and Gaussian power. Compared with the other methods of corneal power estimation, the adjusted corneal power was found to be interchangeable with the estimation obtained using the Haigis-L method, whereas the other methods were observed to provide corneal power values that were significantly different from the adjusted and Gaussian values (overestimation).5 Therefore, the adjusted keratometric index is a valid and easy method to estimate the central corneal power in corneas with previous myopic laser refractive surgery, improving the accuracy of most methods described previously.

    Keratometric approach

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