Keratometry and the Central Radius with Aspheric Corneal Surfaces

To develop a simple concept for deriving the central corneal radius (RC) from the keratometry (K) or simulated keratometry (SimK) radius (RS) and corneal asphericity (QC), and to show the differences RC-RS based on a large dataset of measurements from a modern anterior segment tomographer.

Comparing the local slopes of surface representations with a conoid and a reference sphere at the location of a keratometry measurement (diameter KD), RC and RC-RS can be derived as a function of RS, QC and KD. The differences RC-RS were evaluated for a large dataset containing measurements from the Casia 2 tomographer made before cataract surgery.

Depending on QC and KD, RC could deviate from the measured RS by up to 0.1 mm. With prolate corneas RS overestimates RC, and with oblate corneas RS underestimates RC. For example, with a typical cornea with RS = 7.7 mm and QC = -0.22 and a keratometer measuring at KD = 3 mm the central cornea is 0.0322 mm steeper (RC = 7.6678 mm) compared to keratometric measurement. Based on the dataset, the 95% confidence interval of RC-RS with KD = 3 mm was -0.0976 to +0.0265 mm.

For corneal representations with a conoid surface or where central corneal radius is required e.g. for paraxial calculations, the keratometric radius of curvature could easily be converted to the central radius based on corneal asphericity and the keratometric zone diameter. With large positive or negative values of QC the differences RC-RS could be clinically relevant.