Estimation of the Central Corneal Power in Keratoconus

Theoretical and Clinical Assessment of the Error of the Keratometric Approach

Abstract

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.