title: The location- and depth-dependent mechanical response of the human cornea under shear loading.
abstract: OBJECTIVE
  To characterize the depth-dependent shear modulus of the central and peripheral human cornea along the superior-inferior and nasal-temporal directions with a high spatial resolution.
  
  
  METHODS
  Cylindrical explants from the central and peripheral corneas of 10 human donors were subjected to a 5% shear strain along the superior-inferior and nasal-temporal directions using a microscope-mounted mechanical testing device. Depth-dependent shear strain and shear modulus were computed through force measurements and displacement tracking.
  
  
  RESULTS
  The shear modulus G of the human cornea varied continuously with depth, with a maximum occurring roughly 25% of the way from the anterior surface to the posterior surface. G also varied with direction in the superior region and (at some depths) was significantly higher for superior-inferior shear loading. In the anterior half of the cornea, the shear modulus along the nasal-temporal direction (GNT) did not vary with location; however, the superior region had significantly higher GNT in posterior cornea. In contrast, the shear modulus along the superior-inferior direction (GSI) was independent of location at all depths.
  
  
  CONCLUSIONS
  This study demonstrates that the peak shear modulus of the human cornea occurs at a substantial distance within the corneal stroma. Depth-dependent differences between central and peripheral cornea possibly reflect the location-dependent mechanical environment of the cornea. Moreover, the cornea is not a transverse isotropic material, and must be characterized by more than a single shear modulus due to its dependence on loading direction. The material properties measured in this study are critical for developing accurate mechanical models to predict the vision-threatening morphological changes that can occur in the cornea.
authors.affiliation: Department of Biomedical Engineering, University of Rochester, Rochester, New York, United States.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Sloan
authors.affiliation: School of Medicine, Emory University, Atlanta, Georgia, United States.
authors.forename: Yousuf M
authors.initials: YM
authors.lastname: Khalifa
authors.affiliation: Department of Biomedical Engineering, University of Rochester, Rochester, New York, United States.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Buckley
mesh_terms: D000368:Aged; D000704:Analysis of Variance; D002102:Cadaver; D003315:Cornea / Q000502:physiology*; D003319:Corneal Stroma / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D033081:Shear Strength / Q000502:physiology*; D013314:Stress, Mechanical; D014019:Tissue Donors
keywords: biomechanics; cornea; keratoconus; shear