Abstract
PurposeTo examine the effect of riboflavin/UVA corneal crosslinking on stromal ultrastructure and hydrodynamic behaviour.MethodsOne hundred and seventeen enucleated ungulate eyes (112 pig and 5 sheep) and 3 pairs of rabbit eyes, with corneal epithelium removed, were divided into four treatment groups: Group 1 (28 pig, 2 sheep and 3 rabbits) were untreated; Group 2 (24 pig) were exposed to UVA light (3.04 mW/cm2) for 30 minutes and Group 3 (29 pig) and Group 4 (31 pig, 3 sheep and 3 rabbits) had riboflavin eye drops applied to the corneal surface every 5 minutes for 35 minutes. Five minutes after the initial riboflavin instillation, the corneas in Group 4 experienced a 30 minute exposure to UVA light (3.04 mW/cm2). X-ray scattering was used to obtain measurements of collagen interfibrillar spacing, spatial order, fibril diameter, D-periodicity and intermolecular spacing throughout the whole tissue thickness and as a function of tissue depth in the treated and untreated corneas. The effect of each treatment on the hydrodynamic behaviour of the cornea (its ability to swell in saline solution) and its resistance to enzymatic digestion were assessed using in vitro laboratory techniques.ResultsCorneal thickness decreased significantly following riboflavin application (p<0.01) and also to a lesser extent after UVA exposure (p<0.05). With the exception of the spatial order factor, which was higher in Group 4 than Group 1 (p<0.01), all other measured collagen parameters were unaltered by cross-linking, even within the most anterior 300 microns of the cornea. The cross-linking treatment had no effect on the hydrodynamic behaviour of the cornea but did cause a significant increase in its resistance to enzymatic digestion.ConclusionsIt seems likely that cross-links formed during riboflavin/UVA therapy occur predominantly at the collagen fibril surface and in the protein network surrounding the collagen.
Highlights
The most anterior part of the ocular system, the cornea, is tough and transparent
It is generally believed that proteoglycans act as interfibrillar spacers via the attachment of their core proteins to collagen fibrils [3] and the lateral projection of their highly sulphated glycosaminoglycan side chains which form a hydrophillic coating around the fibrils
At the same time-point, the average diameter of the riboflavin/ultraviolet A light (UVA) cross-linked specimens had decreased by only 21% and the anterior curvature of the cornea remained visible
Summary
It offers protection to the inner contents of the eye and facilitates the passage of light onto the retina. The cornea is a powerful refracting surface and provides the eye with up to 75% of its focussing power. The arrangement of corneal collagen is such that, uniformly narrow fibrils lie parallel to each other in layers (lamellae) which are themselves organized in an ordered, lattice-like configuration [1,2]. The transparency of the cornea is largely dependent on the narrow diameter and shortrange order of the collagen fibrils which in turn is regulated by close interactions with proteoglycans. The fibril coating is thought to counteract the attractive force caused by the thermal motion of the proteoglycan-glycosaminoglycan complex [4]
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