title: Laboratory measurement of the absorption coefficient of riboflavin for ultraviolet light (365 nm).
abstract: OBJECTIVE
  Corneal cross-linking (CXL) is an increasingly used treatment technique for stabilizing the cornea in keratoconus. Cross-linking (polymerization) between collagen fibrils is induced by riboflavin (vitamin B2) and ultraviolet light (365 nm). Although reported to reach a constant value at higher riboflavin concentrations, the Lambert-Beer law predicts a linear increase in the absorption coefficient. This work was carried out to determine absorption behavior at different riboflavin concentrations and to further investigate the purported plateau absorption coefficient value of riboflavin and to identify possible bleaching effects.
  
  
  METHODS
  The Lambert-Beer law was used to calculate the absorption coefficient at various riboflavin concentrations. The following investigated concentrations of riboflavin solutions were prepared using a mixture of 0.5% riboflavin and 20% Dextran T500 dissolved in 0.9% sodium chloride solution: 0%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%, and were investigated with and without aperture plate implementation. An additional test series measured the transmitted power at selected riboflavin concentrations over time.
  
  
  RESULTS
  In diluted solutions, a linear correlation exists between the absorption coefficient and riboflavin concentration. The absorption coefficient reaches a plateau, but this occurs at a higher riboflavin concentration (0.1%) than previously reported (just above 0.04%). Transmitted light power increases over time, indicating a bleaching effect of riboflavin.
  
  
  CONCLUSIONS
  The riboflavin concentration can be effectively varied as a treatment parameter in a considerably broader range than previously thought.
authors.affiliation: Institute of Ophthalmic Surgery, Zurich, Switzerland. hanspeter.iseli@iroc.ch
authors.forename: Hans Peter
authors.initials: HP
authors.lastname: Iseli
authors.forename: Max
authors.initials: M
authors.lastname: Popp
authors.forename: Theo
authors.initials: T
authors.lastname: Seiler
authors.forename: Eberhard
authors.initials: E
authors.lastname: Spoerl
authors.forename: Michael
authors.initials: M
authors.lastname: Mrochen
mesh_terms: D000042:Absorption; D003094:Collagen / Q000378:metabolism; D003319:Corneal Stroma / Q000378:metabolism; D003432:Cross-Linking Reagents; D006801:Humans; D007640:Keratoconus / Q000188:drug therapy / Q000378:metabolism; D017319:Photosensitizing Agents / Q000528:radiation effects* / Q000627:therapeutic use; D012256:Riboflavin / Q000528:radiation effects* / Q000627:therapeutic use; D012542:Scattering, Radiation; D013057:Spectrum Analysis; D014466:Ultraviolet Rays
chemical_list: D003432:Cross-Linking Reagents; D017319:Photosensitizing Agents; D003094:Collagen; D012256:Riboflavin