Estimation of the Central Corneal Power in Keratoconus: Theoretical and Clinical Assessment of the Error of the Keratometric Approach.
ABSTRACT 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 (nk) is used and to clinically confirm the errors induced by this approach.
Differences (ΔPc) between central corneal power estimation with the classical nk (Pk) 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 (r1c) and posterior (r2c) 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 Pk and (true net power) obtained with a Scheimpflug photography-based topographer was evaluated in such eyes.
For nk = 1.3375, an overestimation was observed in most cases in the theoretical simulations, with ΔPc ranging from an underestimation of -0.1 diopters (D) (r1c = 7.9 mm and r2c = 8.2 mm) to an overestimation of 4.3 D (r1c = 4.7 mm and r2c = 3.1 mm). Clinically, Pk always overestimated the given by the topography system in a range between 0.5 and 2.5 D (P < 0.01). The mean clinical ΔPc was 1.48 D, with limits of agreement of 0.71 and 2.25 D. A very strong statistically significant correlation was found between ΔPc and r2c (r = -0.93, P < 0.01).
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.
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ABSTRACT: To evaluate the effects of posterior corneal astigmatism and the absolute flat meridian difference between anterior and posterior corneal surfaces (AMD(Ant-Post)) on the estimation of total corneal astigmatism using anterior corneal measurements (simulated keratometry [K]). Ninety-nine eyes of 99 healthy participants were enrolled. Anterior, posterior, and total mean corneal power, cylinder power, flat meridian, and vector components J(0), and J(45) measured by a dual Scheimpflug camera were analyzed. The correlation between the posterior corneal cylinder power, AMD(Ant-Post), and the difference in the cylinder power between simulated K and total cornea (cylinder power difference(SimK-Tot)) were evaluated. The cylinder power difference(SimK-Tot) was positively correlated with the posterior corneal cylinder power (rho = 0.704 and P < 0.001) and negatively correlated with AMD(Ant-Post) (rho = -0.717 and P < 0.001). In the multivariate linear regression analysis, anterior corneal J(0) was strongly associated with the posterior corneal cylinder power and the AMD(Ant-Post). When corneal J(0) had a positive value, the cylinder power of simulated K tended to be larger than the total corneal cylinder power. In comparison, the opposite trend was presented in eyes with negative anterior corneal J(0). When anterior corneal J(0) was larger than 1.0 or smaller than -0.9, the errors from estimating the total corneal cylinder power using anterior corneal measurements tended to be larger than 0.25 D. Posterior corneal astigmatism should be considered for more accurate corneal astigmatism predictions, especially in eyes with anterior corneal astigmatism greater than 2.0 D of with-the-rule astigmatism or greater than 1.8 D of against-the-rule astigmatism.Albrecht von Graæes Archiv für Ophthalmologie 07/2014; 252(11). DOI:10.1007/s00417-014-2737-9 · 2.33 Impact Factor
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ABSTRACT: 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.Cornea 07/2014; 33(9). DOI:10.1097/ICO.0000000000000190 · 2.36 Impact Factor