The Fibrillar Arrangement and Distribution of Collagen in Keratoconus Corneas.

Abstract

Abstract: : Purpose:To examine the distribution and the preferred fibril orientation of collagen in keratoconus corneal buttons and to relate any observed variations to features on corneal surface topography maps. Methods:Keratoconus corneal buttons of 8mm diameter (n = 3) and normal human corneas (n = 3) were tagged with a nylon suture at the 12 o'clock position, before being preserved in formaldehyde. A videokeratographic image of surface dioptric power was recorded for each cornea (in vivo for keratoconus corneas and in vitro for the normal controls). Wide angle x–ray scattering (WAXS) patterns were obtained at 0.4mm intervals over the entire area of each sample using a computer operated translation stage on Station 14.1 at the Daresbury Synchrotron Radiation Source, UK. Each WAXS pattern was analysed to produce quantitative information regarding the total amount of collagen (aligned and isotropic) and the preferred orientation of aligned collagen at a known corneal location . By arranging the data onto a grid of corneal position various maps were produced to illustrate the distribution and preferential orientation of collagen. The relationship between collagen arrangement and surface topography was examined in detail for both the normal and keratoconus corneas. Results:In the cone region of keratoconus corneas the orthoganol preferred orientation of collagen fibrils that is seen in the normal human cornea, is absent. Also, in stark contrast to the gradual symmetrical increase of collagen from the central region to the periphery in the normal human cornea, maximal thinning occurs in the cone region of keratoconus corneas. Outside the cone region the increase in collagen occurs asymetrically and is less gradual than in the normal cornea. The distribution of aligned collagen is especially altered in keratoconus corneas and appears to correlate closely with cone shape. Conclusion:The results indicate a redistribution of collagen in keratoconus corneas. The present study supports the theory that corneal thinning in keratoconus occurs as result of lamella sliding away from the cone region. The existence of this mechanism would also help to explain the altered orientation of collagen fibrils in this region.

Keywords: keratoconus • extracellular matrix • shape and contour