Estimation of the Central Corneal Power in Keratoconus

Abstract

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

Algorithm for Correcting the Keratometric Error in the Estimation of the Corneal Power in Keratoconus Eyes after Accelerated Corneal Collagen Crosslinking

“…In keratoconus eyes, theoretical and clinical errors associated to the calculation of central corneal power considering P k have been also analyzed and compared with P c Gauss [ 4 ]. In theoretical simulations, an overestimation of P k (1.3375) was observed in most of cases, with differences among Gaussian and keratometric calculations (Δ P c = P k (1.3375) − P c Gauss ) ranging from −0.1 to 4.3 D, depending on the r 1c / r 2c combination and the theoretical eye model considered.…”

“…In theoretical simulations, an overestimation of P k (1.3375) was observed in most of cases, with differences among Gaussian and keratometric calculations (Δ P c = P k (1.3375) − P c Gauss ) ranging from −0.1 to 4.3 D, depending on the r 1c / r 2c combination and the theoretical eye model considered. Clinically, P k (1.3375) was found to overestimate corneal power in a range between 0.5 and 2.5 D, with a mean clinical difference (Δ P c ) of 1.48 D [ 4 , 5 ]. The clinical value of the keratometric corneal refractive index matching P k and P c Gauss ( n k exact ) ranged from 1.3225 to 1.3314 in a keratoconus population evaluated in a previous study [ 4 ].…”

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

“…According to the peer-reviewed literature, we considered that the anterior corneal radius in the keratoconus population ranged between 4.2 and 8.5 mm, whereas the posterior corneal radius ranged between 3.1 and 8.2 mm. [ 1 2 ] Therefore, we assumed k ratio values ranging from 0.96 to 1.56 in our theoretical calculations. [ 2 ] It should be considered that differences among keratometric and Gaussian P c are commonly zeroed by constant optimization in the range of corneal curvature of the normal healthy eyes, but not for eyes with significantly higher corneal curvature, as in keratoconus.…”

“…[ 1 2 ] Therefore, we assumed k ratio values ranging from 0.96 to 1.56 in our theoretical calculations. [ 2 ] It should be considered that differences among keratometric and Gaussian P c are commonly zeroed by constant optimization in the range of corneal curvature of the normal healthy eyes, but not for eyes with significantly higher corneal curvature, as in keratoconus. In addition, we considered that ELP could vary between 2 and 6 mm in the calculations performed in the current study according to previous authors dealing with this issue.…”

“…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 ]…”