Methods for IOL power calculation after radial keratotomy
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First Author: A.Turnbull UK
Co Author(s): G. Crawford G. Barrett
To compare IOL power calculation methods for patients undergoing cataract surgery after radial keratotomy (RK), including the 2016 update of the True K formula.
Lions Eye Institute, Perth, Australia
52 eyes of 34 patients who had sequential RK (4-16 cut) and cataract surgery performed in the same institution were retrospectively studied. Seven IOL calculation formulae were evaluated: True K [History], True K [Partial History], True K [No History], Double-K Holladay 1 (DK-Holladay-IOLM), Potvin-Hill, Haigis and Haigis with a -0.50D offset. Subjective refraction was performed at 4-6 weeks postoperatively. Median and mean absolute error (MedAE and MAE), and percentage of patients within +/-0.50, +/-0.75 and +/-1.00D of refractive target were calculated. Mean error (ME) was calculated to demonstrate whether a formula tended towards more myopic or hyperopic outcomes.
Best results were achieved with the True K [History]. MedAE was higher (0.382 vs. 0.275) with the True K [Partial History], but a similar percentage of patients (75.0 - 76.6%) achieved within +/-0.50D of target. Of the methods that require no refractive history, the True K [No History] and unadjusted Haigis were most accurate (69.2% within +/-0.50D of target), with the True K [No History] returning the lowest MedAE but also more of a tendency towards hyperopia (ME +0.269 vs. -0.006 for Haigis). The DK-Holladay-IOLM and Potvin-Hill methods were the least accurate.
Knowledge of the refractive history significantly improves the accuracy of IOL calculations in patients undergoing cataract surgery after previous RK. The post-RK refraction appears to be the most important parameter, with inclusion of the pre-RK refraction offering a further slight improvement in MedAE. When no refractive history is available, the True K [No History] and Haigis formulae both perform well.
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