Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy

Purpose: To compare methods of calculating the required intraocular lens (IOL) power for patients undergoing cataract surgery after radial keratotomy (RK), including the 2016 update of the True K formula. Design: Retrospective case series. Participants: A total of 52 eyes of 34 patients who had sequential RK and cataract surgery performed in the same institution by 1 of 2 surgeons. Methods: 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 −0.50 diopter (D) offset. Biometry was obtained with the IOLMaster 500 (Carl Zeiss Meditec AG, Jena, Germany) and Pentacam (OCULUS Inc, Arlington, WA) devices. Subjective refraction was performed at 4 to 6 weeks postoperatively. The achieved spherical equivalent outcome was compared with the target outcome to calculate the absolute error for each eye with each formula. Main Outcome Measures: Median absolute error (MedAE) and mean absolute error (MAE), and percentage of patients within ±0.50 D, ±0.75 D, and ±1.00 D of refractive target. Mean error (ME) was also calculated to demonstrate whether a formula tended toward more myopic or hyperopic outcomes. Results: Best results were achieved with the True K [History]. The 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 ±0.50 D of target. Of the methods that do not require refractive history, the True K [No History] and unadjusted Haigis were most accurate (69.2% within ±0.50 D of target), with the True K [No History] returning the lowest MedAE but also more of a tendency toward hyperopia (ME +0.269 vs. −0.006 for Haigis). The DK-Holladay-IOLM and Potvin-Hill methods were the least accurate. Conclusions: 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, with the added advantage of not requiring data from separate biometric devices.