Introduction

Corneal collagen crosslinking (CXL) is a minimally invasive procedure used to prevent progression of corneal ectasia such as keratoconus and post-LASIK ectasia.

Background

Cross-linking of collagen refers to the ability of collagen fibrils to form strong chemical bonds with adjacent fibrils. In the cornea, collagen cross-linking occurs naturally with aging due to an oxidative deamination reaction that takes place within the end chains of the collagen. It has been hypothesized that this natural cross-linkage of collagen explains why keratectasia (corneal ectasia) often progresses most rapidly in adolescence or early adulthood but tends to stabilize in patients after middle-age.

While crosslinking tends to occur naturally over time, there are other pathways that mean lead to premature crosslinking. Glycation refers to a reaction seen predominantly in diabetics that can lead to the formation of additional bonds between collagen. In the pathway most relevant to this topic, oxidation has been shown to be able to trigger corneal crosslinkage through the release of oxygen free radicals. 

The bases for the currently employed corneal collagen cross-linking techniques were developed in Europe by researchers at the University of Dresden in the late 1990s.  UV light was used to induce collagen cross-linking in riboflavin soaked porcine and rabbit corneas via the oxidation pathway. The resultant corneas were shown to be stiffer and more resistant to enzymatic digestion. The investigation also proved that treated corneas contained higher molecular weight polymers of collagen due to fibril crosslinking. Safety studies showed that the endothelium was not damaged by the treatment if proper UV irradiance was maintained and if the corneal thickness exceeded 400 microns.

Human studies of UV-induced corneal cross-linking began in 2003 in Dresden, and early results were promising. The initial pilot study enrolled 16 patients with rapidly progressing keratoconus and all of the patients stopped progressing after treatment. Additionally, 70% had flattening of their steep anterior corneal curvatures (decreases in average and maximum keratometric values), and 65% had an improvement in visual acuity. There were no reported complications. 

In late 2011, orphan drug status was awarded by the FDA to Avedro for its formulation of riboflavin ophthalmic solution to be used in conjunction with the company's particular UVA irradiation system. Corneal collagen cross-linking using riboflavin and UV received FDA approval on April 18, 2016.

In 2015, a Cochrane systemic review analyzing CXL for treating keratoconus revealed that the evidence for the use of CXL in the management of keratoconus is limited due the lack of properly conducted Randomized Controlled Trials.

Basic Concepts

The main components of CXL are a photosensitizer, a UV light source, and the resulting photochemical reaction.

Riboflavin

A photosensitizer is a molecule that absorbs light energy and produces a chemical change in another molecule.

In CXL, Riboflavin is used as the photosensitizer. It is safe systemically and can be adequately absorbed by the corneal stroma topically. It has an absorption peak at 370 nm.

UV Light

As the absorption peak of riboflavin was noted to be 370 nm, UV-A light was found to be ideal for CXL, while at the same time protecting the other ocular structures. The total fluence required was found to be 5.4J/cm2.

The Bunsen Roscoe law states that the photochemical effect should be similar if the total fluence remains constant. Based on this, various protocols have been devised with different combinations of the intensity and duration of UV-A exposure. However, it has been noted that CXL fails to be effective once the energy intensity exceeds 45mW/cm2.

Photochemical Reaction

Once exposed to UV-A light, the riboflavin generates Reactive oxygen species, which induce the formation of covalent bonds both between collagen molecules and between collagen molecules and proteoglycans.