Corneal topography and elasticity in normal and keratoconic eyes. A methodological study concerning the pathogenesis of keratoconus.

Abstract

One eye of 27 patients with keratoconus and 37 normals were examined by photokeratoscopy and topographic pachometry to obtain informations about the corneal shape and thickness profile. The progressive alteration of corneal topography in keratoconus may be based on elastic deformation. Through a distention, predominantly of the central area, increasing large differences between central and peripheral thickness appear. Simultaneously the area of the corneal surface increases into a more elliptical corneal shape. In keratoconus relative to normals, the study demonstrates, an alteration of, as well as a correlation between, the corneal shape and thickness profile in accordance with the elastic hypothesis. Based on corneal topography determination, intraocular pressure measurements and a derived elastic theory the corneal tissue mass (M), tissue strength (Young's Modulus in steady state Y(s) , and the ocular rigidity (E) were determined. Y(s) and E was found to be significantly decreased, whereas M seems to be equal in keratoconus compared with normals. Histologically cornea consists of an amorphous matrix in which the collagen fibers are embedded. E seems to reflect the immediate elastic response of the collagen fibers (Y(i], which, however, may be relaxed in steady state. Y(s) seems to reflect the elastic response of the matrix, which consists of matrix compression and sliding of collagen fibers. Thus the biomechanical alteration of cornea in keratoconus may be introduced by increased sliding fo collagen fibers due to reduced attachment to Bowman's layer and altered synthesis of the matrix substance.