Measuring the cornea: the latest developments in corneal topography

Abstract

Corneal measurement uses Placido-disc topographers and tomographers, creating three-dimensional corneal models from cross-sectional images. Technology includes slit scanning, Scheimpflug imaging, very high frequency ultrasound, and high-speed anterior segment optical coherence tomography. Normative data in the Asian population using the Orbscan, and repeatability information for the 3 and 5-mm zone were reported. Best fit sphere and the thinnest point were not significantly different, but posterior surface elevation was higher using the Orbscan in keratoconic eyes. Pachymetry data from the Pentacam show pattern differences used as keratoconus indices. Scheimpflug imaging identified changes in the posterior surface that were not mirrored by the anterior surface with aging, and may be better for surgical planning than conventional keratometry following excimer treatments. Optical coherence tomography mean central corneal thickness measurements were repeatable; however, mean central epithelial thickness measurements were not. Very high frequency ultrasound can be used successfully to create epithelial and flap thickness maps. Studies reveal specular microscopy measures thinner and less reliable readings than Pentacam and Orbscan. New corneal topographers, such as Scheimpflug imaging, ultrasound and optical coherence tomography have expanded capabilities and precision in measuring the structure of the cornea.