Clinical Evaluation of a New Approach for IOL Power Calculation in Keratoconus

Purpose: To obtain an expression of the adjusted IOL power (PIOLadj) in keratoconus eyes associated with minimal errors in IOL power calculation. Materials and methods: This retrospective study included a total of 25 eyes of 25 patients with ages ranging from 20 years to 76 years. The following IOLs were implanted: Acrysof IQ Toric, Acrysof SA60AT in 9 eyes, Sensar in 3 eyes, Tecnis 1 in 4 eyes, and Tecnis Toric in 2 eyes. The PIOLadj is based on Gauss equations, using adjusted keratometric index (nkadj) specific to keratoconus eyes. From this nkadj, an adjusted keratometric corneal power is calculated (Pkadj). The PIOLadj calculation was performed after estimating the effective lens position (ELP) using a mathematical expression obtained by multiple regression analysis (named ELPadj). Comparison between the PIOLadj and the real intraocular power implanted in each patient (PIOLreal) was carried out. Results: No significant differences between PIOLreal and PIOLadj were found. However, differences could be clinically relevant up to of 2.54 D as PIOLreal increases. But, in the range of PIOLreal between 0 and 20 D, differences were lower than 1.5 D, being most of them below 1 D. Conclusion: A new formula of IOL power calculation (PIOLadj) based on the use of an adjusted keratometric power (Pkadj) that considers a variable keratometric index due to the influence of the posterior corneal surface (nkadj) and adjusted effective lens position (ELPadj) is useful for estimating IOL power in low-to-moderate keratoconus, with more limitation in the most advanced keratoconus.David P Piñero is supported by the Spanish Ministry of Economy, Industry, and Competitiveness within the program Ramón y Cajal, RYC-2016-20471

Comparison of Four Intraocular Power Calculation Formulas in Keratoconus Eyes

Introduction: This study aimed to evaluate the differences in Intraocular Lens (IOL) power in keratoconus (KC) eyes between calculations obtained clinically with the most commonly used formulas in healthy eyes (SRK T, Holladay 1, Hoffer Q and Haigis) as well as to define predictive factors for such differences. Methods: This retrospective study comprised 43 keratoconus eyes of 22 patients with no previous ocular surgery. IOL powers were calculated with SRK T, Holladay 1, Hoffer Q, and Haigis formulas, considering the Effective Lens Position (ELP) of each formula and the desired refraction of 0 D (Rdes=0 D). Results: All differences between formulas were statistically significant and clinically relevant. Haigis formula always provided higher values compared to the rest of the formulas, with the highest differences observed when comparing Haigis with Hoffer (0.84 D) and Hoffer Q (1.17 D) formulas. The lowest difference was obtained for the comparison between SRK-T and Holladay 1 formulas (0.22 D). Differences of the Haigis formula compared to the rest were higher as the magnitude of the IOL power calculated decreased, becoming the patient more myopic. Increased differences between Haigis and Hoffer formulas were observed in eyes with deep anterior chambers, steeper anterior and posterior corneal surfaces, and high axial lengths. Conclusion: The most comparable results in IOL power in keratoconus are provided by the Holladay 1 and SRK T formulas, whereas the Haigis formula provides the most discrepant outcome. The consideration of the curvature of the second corneal surface in IOL power calculations in keratoconus may decrease the variability between calculation methods. However, other factors as anterior chamber depth or axial length are also relevant.The author David P Piñero has been supported by the Ministry of Economy, Industry, and Competitiveness of Spain within the program Ramón y Cajal, RYC-2016-20471