History of Corneal Cross-Linking

As far back as 1974, there has been evidence of collagen cross-linking.  Robert C Siegel, PhD at the University of California, San Francisco wrote about the evidence shown that lysyl oxidase has an effect via aldehyde intermediates to create cross-links during collagen fibril formation.  He summarized that this fibril formation may facilitate the biosynthesis of stable collagen fibrils and contribute to increased fibril tensile strength in vivo.
We now know, through the work of Wollensack, Spoerl and Seiler that increasing the amount of collagen cross-links increases the stiffening of the cornea.  This is turn can be used to slow down and in most cases stop the progression of keratoconus and other corneal ectatic diseases.
According to Yaron Rabinowitz, MD, there are 3 methods by which the collagen can be cross-linked:
The first two happen as part of the human maturation and aging process. 

1)  Enzymatic Process: Lysyl Oxidase
One way these cross-links are formed is through the process that starts with the enzyme lysyl oxidase.  The lysyl oxidase (or hydroxylysine) starts a reaction with an adlehyde to help form divalent bonds that link the molecules of collagen fibrils from head to tail.  The collagen fibrils then spontaneously convert from divalent to trivalent cross-links.  These cross-links are stiffer than the original collagen fibrils.

2)  Non-Enzymatic Process: Glycation
Prolonged exposure to monosaccharides result in a spontaneous bond between the reducing sugar and the amino group of a protein.  These bonds have been shown to produce increased stiffness in the cornea with age.  This can be why diabetics show a much lower incidence of keratoconus

3)  Oxidation: Riboflavin and UV-A Light
This is the mechanism that we use today for Corneal Collagen Cross-linking.  It is quite different from the other two mechanisms in that it is UV mediated.  According to Rabinowitz, A monomer substrate in the presence of a photo initiator (riboflavin) can polymerize by way of cross-linking in the presence of a UV-A light source.  This polymerization technique has been used in many applications prior to the use in the eye, such as in the application of dental materials.