Single-value keratometric index may lead to clinical errors

For the calculation of corneal power, using a single value for a keratometric index (n_k) is imprecise and may lead to significant clinical errors, warned researchers of a recent study published in the journal Cornea.

These researchers sought to theoretically analyse the errors in the central corneal power calculation in eyes with keratoconus when an n_k is used and to clinically confirm the errors induced using this approach.

They simulated and evaluated theoretically the differences ΔP_c between central corneal power estimation with the classical n_k (P_k) and the Gaussian equations in eyes with keratoconus. They considered the potential range of variation of the central radius of curvature of the anterior (r^1c) and posterior (r^2c) corneal surfaces, and studied these differences in a clinical sample that included 44 keratoconic eyes (27 patients aged 14 to 73 years). Clinical agreement between P_k and true net power obtained with a Scheimpflug photography-based topographer was evaluated.

For n_k = 1.3375, an overestimation was seen in most cases in the theoretical simulations, with differences ranging from an underestimation of –0.1 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 Gaussian equation given by the topography system in a range between 0.5 and 2.5 D (P P _c was 1.48 D, with limits of agreement of 0.71 and 2.25 D. A very strong statistically significant correlation was bound between ΔP_c and r^2c (r = –0.93; P

To access the abstract of this study, click here.