Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery

摘要

Until scanning-slit corneal topography and Scheimp-flug imaging became available a few years ago, the posterior corneal curvature could not be measured in a clinical setting. Because manual and automated keratometers as well as computerized videokerato-graphy devices could measure only the anterior corneal curvature, the total corneal power had to be calculated using a fictitious refractive index (ie, kerato-metric index) to convert the measured corneal radius of curvature into diopters (D) by means of a paraxial formula of spherical surfaces. The keratometric index (1.3315 to 1.3375, depending on the manufacturer) aims to calculate the power of a single refractive surface, which is meant to represent the anterior and posterior corneal surfaces, without knowing the actual curvature of the posterior corneal surface.