Background: The objective of corneal collagen crosslinking (CXL) is to increase the binding of intrafibrillary and interfibrillary covalent bonds to improve the mechanical stability of the cornea and thus to stop the progression of corneal ectasias. Although the vast majority of studies have described pain after photorefractive keratectomy (PRK), the pathophysiological principle of pain is similar in CXL. From the anatomical point of view, the corneal epithelium is the most densely innervated and sensitive surface of the body, being 300-600 times greater than in the skin. The pain after CXL comes from several routes, the process begins with the epithelial rupture that generates exposure of the nerve endings, induces apoptosis and necrosis of the epithelial cells. Subsequently an inflammatory cascade is initiated in which the different cytokines stimulate the nerve terminals. Inflammatory mediators also activate the ion channels in the nerve membrane, and this process continues until the epithelium heals. Additionally, exposure to UVA rays can also cause nerve damage. The effect of local cold for pain management has already been reported in PRK. By cooling the cornea, the release of chemical mediators and inflammation can be reduced. In the CXL radiation is transformed into several forms of energy: fluorescent radiation, chemical energy and, to a small extent, heat. The CXL process is energetically comparable to photosynthesis, in which the radiation energy is transformed into chemical energy (glucose) with the help of pigments (chlorophyll). The thermal effect is negligible in the photochemical method of CXL. Justification: No method for the control of pain after crosslinking is considered ideal or universally accepted, the importance of this study lies in looking for an additional tool to reduce the most common postoperative complaint in a highly performed procedure worldwide. Hypothesis: The application of riboflavin at 4oC reduces the pain assessment after the CXL. Purpose: to evaluate the effect of the application of riboflavin at 4oC in the assessment of postoperative pain in patients undergoing CXL. Materials and methods: Prospective and interventional clinical study in patients older than 18 years with a diagnosis of keratoconus who underwent CXL, in the cornea and refractive surgery service of the Ophthalmology institute Fundación Conde de Valenciana.

Inclusion Criteria:

• patients of any gender
• older than 18 years
• diagnosis of keratoconus who require management with crosslinking in both eyes for evidence of progression.

Exclusion Criteria:

• crosslinking without removal of epithelium or unilateral crosslinking.
• patients with other ocular conditions different from keratoconus.
• cognitive disability that limits the compression of the pain test as Down syndrome, etc.