title: Corneal biomechanical properties at different corneal cross-linking (CXL) irradiances.
abstract: OBJECTIVE
  New corneal cross-linking (CXL) devices are capable of using higher UV-A light irradiances than used in original CXL protocols. The Bunsen-Roscoe law states that a photochemical reaction should stay constant if the delivered total energy is kept constant; however, little clinical data are available to support this hypothesis.
  
  
  METHODS
  We investigated the biomechanical properties of four groups (n = 50 each) of porcine corneas. Three groups were exposed to riboflavin 0.1 % and UV-A irradiation of equal total energy (3 mW/cm(2) for 30 minutes, 9 mW/cm(2) for 10 minutes, and 18 mW/cm(2) for 5 minutes). Controls were exposed to riboflavin 0.1% without irradiation. Young's modulus of 5-mm wide corneal strips was used as an indicator of corneal stiffness.
  
  
  RESULTS
  We observed a decreased stiffening effect with increasing UV-A intensity. Young's modulus at 10% strain showed significant differences between 3 mW/cm(2) and 9 mW/cm(2) (P = 0.002), 3 mW/cm(2) and 18 mW/cm(2) (P = 0.0002), 3 mW/cm(2) and the control group (P < 0.0001), and 9 mW/cm(2) and the control group (P = 0.015). There was no difference between 18 mW/cm(2) and the control group (P = 0.064) and between 9 mW/cm(2) and 18 mW/cm(2) (P = 0.503).
  
  
  CONCLUSIONS
  The biomechanical effect of CXL decreased significantly when using high irradiance/short irradiation time settings. Intrastromal oxygen diffusion capacity and increased oxygen consumption associated with higher irradiances may be a limiting factor leading to reduced treatment efficiency. Our results regarding the efficiency of high-irradiance collagen cross-linking (CXL) raise concerns about the clinical efficiency of the new high-irradiance CXL devices already used in clinical practice without proper validation.
authors.affiliation: Department of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland.
authors.forename: Arthur
authors.initials: A
authors.lastname: Hammer
authors.forename: Olivier
authors.initials: O
authors.lastname: Richoz
authors.forename: Samuel
authors.initials: S
authors.lastname: Arba Mosquera
authors.forename: David
authors.initials: D
authors.lastname: Tabibian
authors.forename: Florence
authors.initials: F
authors.lastname: Hoogewoud
authors.forename: Farhad
authors.initials: F
authors.lastname: Hafezi
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D003094:Collagen / Q000378:metabolism*; D003315:Cornea / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D003432:Cross-Linking Reagents / Q000494:pharmacology*; D004307:Dose-Response Relationship, Radiation; D004548:Elasticity / Q000502:physiology; D006801:Humans; D023421:Models, Animal; D017319:Photosensitizing Agents / Q000494:pharmacology; D012256:Riboflavin / Q000494:pharmacology; D013552:Swine; D014466:Ultraviolet Rays
chemical_list: D003432:Cross-Linking Reagents; D017319:Photosensitizing Agents; D003094:Collagen; D012256:Riboflavin
keywords: biomechanics; corneal cross-linking; efficiency; high irradiance; oxygen