Abstract
Corneal collagen cross-linking (CXL) is a novel technology that utilizes a combination of riboflavin (vitamin B2) eye drops, to saturate the cornea, and UV-A light which sets off a chemical reaction to shorten the cross links between and within collagen fibers thereby increasing the biomechanical stability of the corneal stroma (1–4). The objective of this procedure is to halt progression of keratoconus, post-LASIK ectasia, and other corneal ectasias, such as pellucid marginal degeneration. In some cases, there has been improvement in visual acuity, reduction of corneal curvature and improvement in quality of life (5). In the 1970’s Siegel discussed cross-linking reactions whereby lysyl oxidase catalyzed the formation of cross-linking aldehydes in collagen and elastin (6–7). Cross-linking is well known in material sciences and dentistry where this enzymatic process increases molecular bonds to increase the mechanical strength of tissue. Cross-linking can be induced enzymatically by means of aldehydes, chemical fixatives or by photosensitizing radiation. Of the three methods the photosensitizing radiation has been shown to be most effective. In corneal clinical practice, CXL utilizes this method where the riboflavin acts as a photosensitizer for the production of free radicals produced by the interaction of the riboflavin and UV-A light. The production of these reactive oxygen species (singlet oxygen) causes the formation of chemical bonds within the corneal stroma resulting in corneal stiffening (3).
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