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New Approach for Correction of Error Associated With Keratometric Estimation of Corneal Power in Keratoconus

Camps, Vicente J. PhD; Piñero, David P. PhD; Caravaca-Arens, Esteban Msc; de Fez, Dolores PhD; Pérez-Cambrodí, Rafael J. OD, PhD; Artola, Alberto MD, PhD

doi: 10.1097/ICO.0000000000000190
Basic Investigation

Purpose: The aim of this study was to obtain the exact value of the keratometric index (nkexact) and to clinically validate a variable keratometric index (nkadj) that minimizes this error.

Methods: The nkexact value was determined by obtaining differences (ΔPc) between keratometric corneal power (Pk) and Gaussian corneal power (



) equal to 0. The nkexact was defined as the value associated with an equivalent difference in the magnitude of ΔPc for extreme values of posterior corneal radius (r2c) for each anterior corneal radius value (r1c). This nkadj was considered for the calculation of the adjusted corneal power (Pkadj). Values of r1c ∈ (4.2, 8.5) mm and r2c ∈ (3.1, 8.2) mm were considered. Differences of True Net Power with



, Pkadj, and Pk(1.3375) were calculated in a clinical sample of 44 eyes with keratoconus.

Results: nkexact ranged from 1.3153 to 1.3396 and nkadj from 1.3190 to 1.3339 depending on the eye model analyzed. All the nkadj values adjusted perfectly to 8 linear algorithms. Differences between Pkadj and



did not exceed ±0.7 D (Diopter). Clinically, nk = 1.3375 was not valid in any case. Pkadj and True Net Power and Pk(1.3375) and Pkadj were statistically different (P < 0.01), whereas no differences were found between



and Pkadj (P > 0.01).

Conclusions: The use of a single value of nk for the calculation of the total corneal power in keratoconus has been shown to be imprecise, leading to inaccuracies in the detection and classification of this corneal condition. Furthermore, our study shows the relevance of corneal thickness in corneal power calculations in keratoconus.

Supplemental Digital Content is Available in the Text.

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

Department of Ophthalmology (OFTALMAR), Medimar International Hospital, Alicante, Spain; and

Fundación para la Calidad Visual (FUNCAVIS), Alicante, Spain.

Reprints: Vicente J. Camps, PhD, Department of Optics, Pharmacology and Anatomy, University of 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 April 04, 2014

Accepted May 20, 2014

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