Algorithm for Correcting the Keratometric Estimation Error in Normal Eyes

Abstract

The generalization of the keratometric index (n(k)) value is not an appropriate approximation for the estimation of the corneal power and it can lead to significant errors. We proposed a new algorithm depending on r(1c), with a maximal associated error in the calculation of the corneal power of 0.5 D and without requiring knowledge of the posterior corneal curvature.

Clinical validation of an algorithm to correct the error in the keratometric estimation of corneal power in normal eyes

“…1 Specifically, the following expression was defined assuming the ocular conditions of the Gullstrand eye model and the range of anterior and posterior curvature for the normal healthy population 1 :…”

“…However, the total P c is usually calculated considering only the radius of curvature of the anterior corneal surface measured using a keratometer or a topography system. 1 This simplification arises because in the past, it has been difficult to measure the posterior corneal surface because of technological limitations. In addition, there was an assumption that this surface contributes little to the ocular refractive power because of the small difference in the refractive index at this surface.…”

“…This approach provides P c estimations differing from the Gaussian calculation a maximum of 0.70 diopter (D) and less than 0.50 D in most of the probable combinations of r 1c and r 2c . 1 The aim of the current study was to clinically validate this algorithm based on the use of a variable n k developed to correct the error in the keratometric estimation of P c in a normal healthy population.…”

“…Our research group recently proposed the use of a variable n k (n kadj ) that depends on the radius of curvature of the anterior corneal surface. 1 Specifically, the most appropriate value of n k to use in each specific case is derived from a simple linear equation that requires only the r 1c value in millimeters. This approach provides P c estimations differing from the Gaussian calculation a maximum of 0.70 diopter (D) and less than 0.50 D in most of the probable combinations of r 1c and r 2c .…”

“…In addition, there was an assumption that this surface contributes little to the ocular refractive power because of the small difference in the refractive index at this surface. 1 In an attempt to minimize the error caused by this simplification, the keratometric index of refraction (n k ) was defined. The most commonly used approach in a clinical setting is to estimate P c by considering only the radius of curvature of the anterior corneal surface and the n k value of 1.3375, which is used by most commercially available keratometers and topography systems.…”