Single-value keratometric index may lead to clinical errors

March 7, 2014

For the calculation of corneal power, using a single value for a keratometric index (nk) is imprecise and may lead to significant clinical errors, warn researchers of a recent study.

For the calculation of corneal power, using a single value for a keratometric index (nk) 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 nk is used and to clinically confirm the errors induced using this approach.

They simulated and evaluated theoretically the differences ΔPc between central corneal power estimation with the classical nk (Pk) and the Gaussian equations in eyes with keratoconus. They considered the potential range of variation of the central radius of curvature of the anterior (r1c) and posterior (r2c) 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 Pk and true net power obtained with a Scheimpflug photography-based topographer was evaluated.

For nk = 1.3375, an overestimation was seen in most cases in the theoretical simulations, with differences ranging from an underestimation of –0.1 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 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 ΔPc and r2c (r = –0.93; P

To access the abstract of this study, click here.