SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
progressive ulceration and even globe perforation. This may require tectonic surgery
[grafts] leading to extensive corneal scarring or even enucleation. CXL changes the
molecular structures within the cornea to prevent proteolytic enzymes from binding to
specific cleavage sites, decreasing the efficacy of collagen degrading enzymes.
Thus, through increasing biomechanical strength and decreasing proteolysis, CXL
has been demonstrated to be effective in cases of keratoconus and melting ulcers in
humans. Riboflavin has been used for inactivation of pathogens in blood products,
due to its ability to induce strand cleavage of DNA and RNA, thus interfering with the
replication of pathogenic microorganisms. In its application to the cornea, CXL has
been documented to exert antimicrobial effects, both in vitro and in vivo. The specific
mechanism remain to be elucidated, but it is possible that the increased resistance
of collagen to degradative enzymes, as well as a direct antimicrobial effect of ROS
may be involved.
How does cross linking work?
Photochemical CXL with riboflavin was developed at the University of Dresden and it
was postulated that the procedure induces physical cross-linking of collagen by
riboflavin absorbing UVA to act as a photosensitiser to produce free radicals [oxygen
singlets] that activate the natural lysyl oxidase pathway. By absorbing UVA, the
riboflavin also prevents damage to deeper ocular structures including
theendothelium, lens and retina. Most of the changes in the stroma occur in the
anterior 200µm where most of the UVA is absorbed. One of the fundamental
successes of CXL is the increased resistance to matrix metalloprotease [MMP]
activity that is responsible for stromal melting.
The corneal cross-linking procedure involves application of riboflavin [Vit B2] solution
to the eye [acts as a photosensitiser] that is activated by illumination with UVA light
for approximately 30 or less minutes. This appears to cause improved crosslinks at
the collagen fibril surface, rather than within the fibrils themselves. This results in the
direct increase of corneal stiffness and a reduction of enzymatic collagenolysis. The
corneal epithelial layer is generally removed to increase penetration of the riboflavin
into the stroma. This can be done safely using an Algerbrush [diamond head burr].
Image Ref: http://biomedicalcentral.com/1746-6148/9/128
Besides the treatment of melting corneal ulcers CXL has also been used for the
treatment of bullous keratopathy. A number of conditions can lead to bullous
85