Using a single ker­ato­metric value when calculating corneal power in patients with keratoconus yields an imprecise measurement that can potentially lead to clinical errors, according to a recent study published in the March 2014 edition of Cornea.1

A group of researchers in Spain simulated and evaluated the theoretical differences between central power estimation using both the keratometric index (which uses a refraction of 1.3375 instead of the cornea’s actual index of refraction) and the Gaussian equation (a complex formula that accounts for several measures of corneal power, radii and thickness) in eyes with keratoconus. They then analyzed and clinically confirmed the errors induced by these estimations.

Between January 2012 and June 2013, the researchers conducted a clinical study to confirm the theoretical models. Forty-four keratoconic eyes (27 patients, age 14 to 73) underwent extensive examinations, which included refraction, best-corrected distance visual acuity, slit-lamp biomicroscopy, Goldman tonometry, fundus evaluation and an analysis of the corneal structure using a Scheimpflug tomographer (Pentacam).

During the study, the researchers recorded and analyzed anterior and posterior corneal radii, astigmatism and asphericity, and central corneal thickness. Each subject’s keratometric corneal power was calculated and compared to the true net power (determined by the Pentacam system using the Gaussian equation).

The researchers discovered a statistically significant difference between the keratometric power and true net power (P<0.01). In each case, the keratometric approach always overestimated the true net power between 0.5D and 2.5D.
David Geffen, OD, says the study “shows some of the weakness in using typical Scheimpflug imaging—especially in irregular corneas.”

One factor the researchers postulate may account for the significant differences in corneal power between the keratometric and Gaussian estimations is the incorrect assumption that there is a constant, linear relationship between the curvature of the posterior and anterior corneal surfaces in both healthy eyes and post-refractive surgery eyes.

Correctly determining the central corneal power is vital: it is used to fit contact lenses and intraocular lenses, and diagnose keratoconus. The researchers suggest that a more precise model to determine the most appropriate keratometric power in keratoconus to calculate central corneal power with the keratometric approach is needed. “This will hopefully lead to use of newer technology and software to better determine the measurements,” says Dr. Geffen.

1. Pinero D, Camps VJ, Caravaca-Arens E, et al. Estimation of the central corneal power in keratoconus: theoretical and clinical assessment of the error of the keratometric approach. Cornea. 2014 Mar; 33(3):274-9.