Efficacy and Safety of Contact Lens-assisted Corneal Cross- Linking in the Treatment of Keratoconus With Thin Corneas

Corneal collagen cross linking showed effective results in stabilizing progressive keratoconic corneas. It is the only treatment that halts progression of keratoconus, It was first described in 1998 by Spoerl et al . A major limitation of the procedure that it cannot be used in corneas with pachymetry less than400 micrometers based on the fact that an irradiance of 0.37 mW/cm2 has been found to be cytotoxic for the endothelial cell layer. Since the absorption coefficient for the human corneas is 70 cm-1 and the intended surface irradiance is 3.0 mW/cm2, the 0.37 mW/cm2 irradiance is reached at 300 microns depth. In a 400 microns thick cornea saturated with riboflavin, the irradiance at the endothelial level is 0.18 mW/cm2, which is a factor of 2 smaller than the damage threshold. Therefore, the 400 microns limit is considered to be a safe limit to protect the endothelium and intraocular structures from the hazards of UV irradiation 12 However , the patients with advanced ectasia who are desperately in need for cxl , are the same ones who have thin corneas often below the threshold which is considered to be safe for CXL treatment However further studies with longer follow up and larger numbers of cases are needed to confirm our findings.