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Estimation of the Central Corneal Power in Keratoconus: Theoretical and Clinical Assessment of the Error of the Keratometric Approach

Piñero, David P. PhD; Camps, Vicent J. PhD; Caravaca-Arens, Esteban MSc; Pérez-Cambrodí, Rafael J. OD, PhD; Artola, Alberto MD, PhD

doi: 10.1097/ICO.0000000000000048
Clinical Science

Purpose: The aim of this study was to analyze theoretically the errors in the central corneal power calculation in eyes with keratoconus when a keratometric index (n k) is used and to clinically confirm the errors induced by this approach.

Methods: Differences (ΔP c) between central corneal power estimation with the classical n k (P k) and with the Gaussian equation (



) in eyes with keratoconus were simulated and evaluated theoretically, considering the potential range of variation of the central radius of curvature of the anterior (r 1c) and posterior (r 2c) corneal surfaces. Further, these differences were also studied in a clinical sample including 44 keratoconic eyes (27 patients, age range: 14–73 years). The clinical agreement between P k and



(true net power) obtained with a Scheimpflug photography–based topographer was evaluated in such eyes.

Results: For n k = 1.3375, an overestimation was observed in most cases in the theoretical simulations, with ΔP c ranging from an underestimation of −0.1 diopters (D) (r 1c = 7.9 mm and r 2c = 8.2 mm) to an overestimation of 4.3 D (r 1c = 4.7 mm and r 2c = 3.1 mm). Clinically, P k always overestimated the



given by the topography system in a range between 0.5 and 2.5 D (P < 0.01). The mean clinical ΔP c was 1.48 D, with limits of agreement of 0.71 and 2.25 D. A very strong statistically significant correlation was found between ΔP c and r 2c (r = −0.93, P < 0.01).

Conclusions: The use of a single value for n k for the calculation of corneal power is imprecise in keratoconus and can lead to significant clinical errors.

Supplemental Digital Content is Available in the Text.

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

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

Group of Optics and Visual Perception, Department of Optics, Pharmacology, and Anatomy, University of Alicante, Alicante, Spain.

Reprints: David P. Piñero, Department of Ophthalmology, Oftalmar, Medimar International Hospital, C/Padre Arrupe, 20, 03016 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 August 29, 2013

Accepted October 30, 2013

Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.