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

Abstract

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.

Errors Associated to Keratoconus Grading using Systems based on Corneal Power

“…In the current study, we have tried to confirm if the use of the adjusted keratometric corneal power (P kadj ), a concept developed by our research group in previous studies, 1,2 could affect significantly the grading of keratoconus severity using classification systems based on the use of corneal power. For such purpose, we have compared the result obtained with the adjusted keratometric approach with that obtained using the classical keratometric 1,2 Likewise, the use of a P k(1.3375) value higher than 55 D as a limit for defining a severe keratoconus would be associated to potential overestimations between 1.8 and 4.0 D. 1,2 These errors in using the classical keratometric powers as valid estimators of corneal power in keratoconus are the reasons for the findings of the current study.…”

“…2 Besides the calculation of P kadj , all cases included in the study were classified according to five different classification systems: Alió-Shabayek, 6 Amsler-Krumeich, 6 Rabinowitz-McDonnell, 7 collaborative longitudinal evaluation of keratoconus (CLEK), 12 and McMahon 13 classification systems ( Table 2). The specific details of such classification or grading systems for keratoconus are summarized in Table 2.…”

“…2 Current classification and detection tools for keratoconus are based on different criteria, but most of them still consider the optical power of the cornea as one of the most relevant parameters to consider. 3 As there is no uniform classification for the severity of keratoconus to date, different approaches have been reported in the literature using a combination of objective and subjective parameters.…”

Preliminary validation of an optimized algorithm for intraocular lens power calculation in keratoconus

“…It has been demonstrated theoretically and clinically that differences (Δ P c ) between the central corneal power( P c ) calculated with the classical keratometric approach (assumption of only one corneal surface and a fictitious index of refraction, keratometric index, ( n k ) ( P k ) and that considering the curvature of both corneal surfaces and the Gaussian equation ( P c Gauss ) can be significant and lead to errors in clinical practice. [ 1 2 3 4 5 ] Specifically, the keratometric approach for estimating the P c has been shown to be able to induce over- and under-estimations of intraocular lens power ( P IOL ) in a range between +0.14 D and −3.01 D.[ 6 ]…”

“…In keratoconus, the use of the classical keratometric index of 1.3375 has shown to produce an overestimation of P c in theoretical simulations and clinical measurements, with a range of overestimation among 0.5 and 2.5 D found in a sample of 44 keratoconic corneas evaluated with a Scheimpflug imaging-based system. [ 1 ] As the use of a single value of n k for the calculation of P c has been demonstrated to be also imprecise in keratoconus, our research group developed eight different algorithms according to the severity of keratoconus to also obtain a variable called keratometric index ( n kadj ) and a calculation of P kadj . This adjusted P c minimized the error associated to the use of the keratometric approach for P c calculation to a range of ±0.7 D.[ 1 ] However, the impact of the use of the classical and adjusted keratometric approach for P c estimation has not been evaluated in keratoconus.…”