title: A constant-force technique to measure corneal biomechanical changes after collagen cross-linking.
abstract: OBJECTIVE
  To introduce a constant-force technique for the analysis of corneal biomechanical changes induced after collagen cross-linking (CXL) that is better adapted to the natural loading in the eye than previous methods.
  
  
  METHODS
  For the biomechanical testing, a total of 50 freshly enucleated eyes were obtained and subdivided in groups of 5 eyes each. A Zwicki-Line Testing Machine was used to analyze the strain of 11 mm long and 5 mm wide porcine corneal strips, with and without CXL. Before material testing, the corneal tissues were pre-stressed with 0.02 N until force stabilization. Standard strip extensiometry was performed as control technique. For the constant-force technique, tissue elongation (Δ strain, %) was analyzed for 180 seconds while different constant forces (0.25 N, 0.5 N, 1 N, 5 N) were applied.
  
  
  RESULTS
  Using a constant force of 0.5 N, we observed a significant difference in Δstrain between 0.26±0.01% in controls and 0.12±0.03% in the CXL-treated group (p = 0.003) over baseline. Similarly, using a constant force of 1 N, Δstrain was 0.31±0.03% in controls and 0.19±0.02% after CXL treatment (p = 0.008). No significant differences were observed between CXL-treated groups and controls with 0.25 N or 5 N constant forces. Standard stress-strain extensiometry failed to show significant differences between CXL-treated groups and controls at all percentages of strains tested.
  
  
  CONCLUSIONS
  We propose a constant-force technique to measure corneal biomechanics in a more physiologic way. When compared to standard stress-strain extensiometry, the constant-force technique provides less variability and thus reaches significant results with a lower sample number.
authors.affiliation: Dept. of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland.
authors.forename: Olivier
authors.initials: O
authors.lastname: Richoz
authors.affiliation: Dept. of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland.
authors.forename: Sabine
authors.initials: S
authors.lastname: Kling
authors.affiliation: Dept. of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland.
authors.forename: Souska
authors.initials: S
authors.lastname: Zandi
authors.affiliation: Dept. of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland.
authors.forename: Arthur
authors.initials: A
authors.lastname: Hammer
authors.affiliation: Dept. of Ophthalmology, University Hospital Dresden, Dresden, Germany.
authors.forename: Eberhard
authors.initials: E
authors.lastname: Spoerl
authors.affiliation: Dept. of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland; Dept. of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, California, United States of America.
authors.forename: Farhad
authors.initials: F
authors.lastname: Hafezi
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D003094:Collagen / Q000737:chemistry*; D003315:Cornea / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects*; D004548:Elasticity; D012256:Riboflavin / Q000494:pharmacology; D013552:Swine; D013718:Tensile Strength; D046509:Tissue Culture Techniques; D014466:Ultraviolet Rays
chemical_list: D003094:Collagen; D012256:Riboflavin