To evaluate theoretically in normal eyes the influence on IOL power (P IOL) calculation of the use of a keratometric index (n k) and to analyze and validate preliminarily the use of an adjusted keratometric index (n kadj) in the IOL power calculation (P IOLadj).
A model of variable keratometric index (n kadj) for corneal power calculation (P c) was used for IOL power calculation (named P IOLadj). Theoretical differences (ΔP IOL) between the new proposed formula (P IOLadj) and which is obtained through Gaussian optics (
) were determined using Gullstrand and Le Grand eye models. The proposed new formula for IOL power calculation (P IOLadj) was prevalidated clinically in 81 eyes of 81 candidates for corneal refractive surgery and compared with Haigis, HofferQ, Holladay, and SRK/T formulas.
A theoretical P IOL underestimation greater than 0.5 diopters was present in most of the cases when n k = 1.3375 was used. If n kadj was used for P c calculation, a maximal calculated error in ΔP IOL of ±0.5 diopters at corneal vertex in most cases was observed independently from the eye model, r 1c, and the desired postoperative refraction. The use of n kadj in IOL power calculation (P IOLadj) could be valid with effective lens position optimization nondependent of the corneal power.
The use of a single value of n k for P c calculation can lead to significant errors in P IOL calculation that may explain some IOL power overestimations with conventional formulas. These inaccuracies can be minimized by using the new P IOLadj based on the algorithm of n kadj.
Grupo de Óptica y Percepción Visual, Department of Optics, Pharmacology and Anatomy, University of Alicante, Alicante, Spain (VJC, DPP, DdF, VM); and Foundation for the Visual Quality (Fundación para la Calidad Visual-FUNCAVIS), Alicante, Spain (DPP).