Corneal collagen cross-linking (CXL) was developed in 1998 by Theo Seiler, MD, and has been shown in numerous clinical trials to strengthen the eye’s clear surface (cornea) through the application of riboflavin, a form of vitamin-B2, followed by treatment with ultraviolet A (UV-A) light.

The two basic types of corneal cross-linking are:

 

Epithelium off, which means the thin layer covering the eye’s surface is removed, allowing for faster penetration with liquid riboflavin.
Transepithelial corneal crosslinking (epithelium on) is where the corneal epithelial surface is left intact, which requires a longer riboflavin loading time.

 

Crosslinking with riboflavin and UV-A light has proven to be a first-line treatment for people with eye conditions such as keratoconus, pellucid marginal degeneration and corneal weakness (ectasia) after LASIK.

Reference : all About Vision