Derivation of corneal flattening factor, p-value.

A mathematical equation was derived to calculate the corneal p-value based on the central and peripheral corneal radii and the corresponding semichord diameters. The calculation was simplified by a computer program written using GWBASIC. The accuracy of corneal p-value generated from this program was checked with Douthwaite's modified keratometer with good correlation. This program could be used to find out the corneal p-value and apical radius ro for any topographic keratometer which only provides central and peripheral radii without any information of the p-value and apical radius.