Use of Corneal Power-Specific Constants to Improve the Accuracy of the SRK/T Formula

Abstract

Purpose: To evaluate the effect of average corneal power (K) and axial length (AL) in a data-adjusted A-constant for improving the refractive outcome in the Sanders-Retzlaff-Kraff (SRK)/T formula. Design: Retrospective, consecutive, case series. Participants: A total of 637 patients (637 eyes) who underwent uncomplicated phacoemulsification with implantation of the Acrysof IQ (IQ; Alcon, Fort Worth, TX; 314 eyes) or Akreos AO (AO; Bausch & Lomb, Rochester, NY; 323 eyes) intraocular lens (IOL) by a single surgeon. Methods: The correlation among the K, AL, and predicted refractive error in the SRK/T formula was analyzed. Patients were divided into 2 subgroups, the first to calculate the different data-adjusted A-constants based on the K and the second to compare the median absolute error (MedAE) based on different A-constants with the traditional A-constant in the SRK/T formula. Main Outcome Measures: The data-adjusted A-constant and the MedAE (diopters [D]). Results: The data-adjusted A-constant showed a decreasing trend as K increased. The data-adjusted A-constant was 119.04 in the IQ group and 118.27 in the AO group. The calculated A-constant was 119.33 in the IQ group and 118.57 in the AO group when the cornea was flatter than 43.0 D and 43.2 D, respectively. The A-constant was 118.71 in the IQ group and 117.96 in the AO group when the cornea was steeper than or equal to 44.7 D and 45.0 D, respectively. The MedAE decreased from 0.29 D to 0.23 D in the IQ group (P = 0.001) and from 0.44 D to 0.38 D in the AO group (P < 0.001) when different A-constants were used. The MedAE further decreased from 0.36 D to 0.24 D in the IQ group (P = 0.005) and from 0.58 D to 0.37 D in the AO group (P < 0.001) when subjects with K 1.00 D or more above or 1.00 D below the most accurate K in each group were compared. Conclusions: For a steep cornea, the calculated A-constant was smaller than that of the entire K, but for a flat cornea, a larger A-constant was calculated. Using different A-constants based on the K improved the refraction outcomes relying on the SRK/T formula. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. Ophthalmology 2013;120:477-481 (C) 2013 by the American Academy of Ophthalmology.