Corneal collagen cross–linking (CXL) with riboflavin and ultraviolet–A light is a method used to stop the progression of keratectasia. The currently accepted treatment parameters induce collagen cross–linking in the anterior 250 to 350 μm of corneal stroma. To protect the endothelium, CXL inclusion criteria require a minimum corneal thickness of 400 μm after removal of the epithelium. In some keratoconus eyes, however, progressive corneal thinning often leads to a remaining stromal thickness of less than 400 μm after epithelial removal.