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Algorithm for Correcting the Keratometric Error in the Estimation of the Corneal Power in Eyes With Previous Myopic Laser Refractive Surgery

Camps, Vicente J. PhD; Piñero, David P. PhD; Mateo, Veronica MSc; Ribera, David MSc; de Fez, Dolores PhD; Blanes-Mompó, Francisco J. MD; Alzamora-Rodríguez, Antonio MD

doi: 10.1097/ICO.0b013e31829e1eb5
Clinical Science

Purpose: To calculate theoretically the errors in the estimation of corneal power when using the keratometric index (nk) in eyes that underwent laser refractive surgery for the correction of myopia and to define and validate clinically an algorithm for minimizing such errors.

Methods: Differences between corneal power estimation by using the classical nk and by using the Gaussian equation in eyes that underwent laser myopic refractive surgery were simulated and evaluated theoretically. Additionally, an adjusted keratometric index (nkadj) model dependent on r1c was developed for minimizing these differences. The model was validated clinically by retrospectively using the data from 32 myopic eyes [range, −1.00 to −6.00 diopters (D)] that had undergone laser in situ keratomileusis using a solid-state laser platform. The agreement between Gaussian (PGaussc) and adjusted keratometric (Pkadj) corneal powers in such eyes was evaluated.

Results: It was found that overestimations of corneal power up to 3.5 D were possible for nk = 1.3375 according to our simulations. The nk value to avoid the keratometric error ranged between 1.2984 and 1.3297. The following nkadj models were obtained: nkadj= −0.0064286r1c + 1.37688 (Gullstrand eye model) and nkadj = −0.0063804r1c + 1.37806 (Le Grand). The mean difference between Pkadj and PGaussc was 0.00 D, with limits of agreement of −0.45 and +0.46 D. This difference correlated significantly with the posterior corneal radius (r = −0.94, P < 0.01).

Conclusions: The use of a single nk for estimating the corneal power in eyes that underwent a laser myopic refractive surgery can lead to significant errors. These errors can be minimized by using a variable nk dependent on r1c.

*Grupo de Óptica y Percepción Visual, Department of Optics, Pharmacology and Anatomy, University of Alicante, Alicante, Spain;

Foundation for Visual Quality (Fundación para la Calidad Visual), Alicante, Spain; and

Department of Ophthalmology (Oftalmar), Medimar International Hospital, Alicante, Spain.

Reprints: Vicente J. Camps, Departamento de Óptica, Farmacología y Anatomía, Universidad de Alicante, Crta San Vicente del Raspeig s/n, 03690 San Vicente del Raspeig, Alicante, Spain (e-mail:

The authors have no funding or conflicts of interest to disclose.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (

Received May 01, 2013

Accepted May 29, 2013

Copyright © 2013 Wolters Kluwer Health, Inc. All rights reserved.