Abstract
Corneal collagen cross-linking (CXL) is a therapeutic procedure aiming at increasing the corneal stiffness in the keratoconus eyes by induction of cross-links within the extracellular matrix. It is achieved by ultraviolet-A (370 nm) irradiation of the cornea after saturation with the photosensitizer riboflavin. In the conventional CXL protocol, a minimum de-epithelialized corneal thickness of 400 μm is recommended to avoid potential irradiation damage to the corneal endothelium. In advanced keratoconus, however, stromal thickness is often lower than 400 μm, which limits the application of CXL in that category. Efforts have been undertaken to modify the conventional CXL procedure to be applicable in thin corneas. The current review discusses different techniques employed to achieve this end and their results. The overall safety and efficacy of the modified CXL protocols are good, as most of them managed to halt the progression of keratectasia without postoperative complications. However, the evidence of safety and efficacy in the use of modified CXL protocols is still limited to few studies with few patients involved. Controlled studies with long-term follow-up are required to confirm the safety and efficacy of the modified protocols.
Highlights
Keratoconus is a degenerative disorder of the cornea, characterized by progressive stromal thinning and conical ectasia that result in irregular astigmatism and associated vision loss [1, 2]
It was estimated that the stiffness of a keratoconic cornea is only 60 % of that of the normal cornea, and that the development of conical shape in keratoconus is the result of decreased biomechanical stability [3]
A minimum corneal thickness of 400 μm is recommended in conventional Corneal collagen cross-linking (CXL) treatment
Summary
Keratoconus is a degenerative disorder of the cornea, characterized by progressive stromal thinning and conical ectasia that result in irregular astigmatism and associated vision loss [1, 2]. After a quick consumption of oxygen, which occurs only within several seconds, depending on UV-power, temperature, amongst other factors, it is suggested that the main photochemical kinetics mechanism is the direct interaction between the riboflavin triplets and reactive groups of corneal proteins, which leads to the cross-linking of the proteins mainly through radical reactions [19] These induce formation of new covalent bonds between the amino acids among the neighboring collagen molecules [17, 20] and among proteoglycan (PG) core proteins, as well as limited linkages between collagen and PG core proteins [21]. Conventional collagen cross-linking The conventional CXL procedure as described in the Dresden protocol in 2003 [17], its modified version in 2008 [32], and the Siena protocol [33] applies to corneas with minimal
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