The cornea, the clear dome-shaped window on the front of the eye, consists of 5 different layers of tissue. The stroma accounts for 90% of the thickness of the cornea and is made up of about 300 layers of collagen fibrils that run parallel to each other in a “jelly-like” substance . It was originally thought that Corneal Collagen Cross Linking (CXL) strengthened or increased the bonds between the collagen fibres. According to the latest research, it is now believed that it may actually be a stiffening of the “jelly-like” proteoglycans, similar to adding pectin to jelly to make it stiffer. The effect is that the structural strength of the cornea is increased.
For therapeutic use, there are several corneal conditions that CXL is being used for. Keratoconus, Pellucid Marginal Degeneration and post-laser refractive surgery ectasia are the most common conditions that are treated with CXL. Keratoconus is a progressive corneal disease whereby the central cornea gradually bulges forward in a cone shape, with corneal thinning and distortion of vision. Pellucid Marginal Degeneration involves thinning and bulging of the inferior portion of the cornea. Post-laser refractive surgery ectasia is a bulging forward of the thinned, central portion of the cornea after Myopic laser refractive surgery correction.
For prophylactic use, virtually any patient can be treated with cross-linking to reduce the chance of future development of the above conditions, especially patients with thinner than normal corneas, irregular corneal astigmatism, asymmetry on corneal topography, against-the-rule astigmatism or steeper than normal corneas.
The 5 layers of the cornea include the epithelium (the outer corneal surface layer), Bowman’s membrane, stroma, Descemet’s membrane and endothelium. The objective is to apply a solution of Riboflavin to the stromal layer and then apply UVA light. Current Riboflavin solutions are typically not able to penetrate the epithelium so the epithelium needs to be removed. There are new Riboflavin solutions being developed that can penetrate the epithelium and this procedure is called Transepithelial Corneal Collagen Cross-linking. There is a 30 minute Riboflavin soak time and then 15 minutes of UVA exposure. Once the UVA exposure is complete, if the epithelium has been removed, a bandage contact lens is applied.