Abstract
Knowledge of the biomechanical properties of the human cornea is crucial for understanding the development of corneal diseases and impact of surgical treatments (e.g., corneal laser surgery, corneal cross-linking). Using a Surface Force Apparatus we investigated the transient viscous response of the anterior cornea from donor human eyes compressed between macroscopic crossed cylinders. Corneal biomechanics was analyzed using linear viscoelastic theory and interpreted in the framework of a biphasic model of soft hydrated porous tissues, including a significant contribution from the pressurization and viscous flow of fluid within the corneal tissue. Time-resolved measurements of tissue deformation and careful determination of the relaxation time provided an elastic modulus in the range between 0.17 and 1.43 MPa, and fluid permeability of the order of 10−13 m4/(N∙s). The permeability decreased as the deformation was increased above a strain level of about 10%, indicating that the interstitial space between fibrils of the corneal stromal matrix was reduced under the effect of strong compression. This effect may play a major role in determining the observed rate-dependent non-linear stress-strain response of the anterior cornea, which underlies the shape and optical properties of the tissue.
Highlights
The human cornea is a layer of transparent tissue with the optical function of focusing light onto the retina and the mechanical function of preserving the integrity of the eye
The presence of a short response time was due to small mechanical drifts of the micrometer as well as viscous forces generated by the dextran solution as it flowed out of the contact during surface approach or into the contact region during separation
The response time was much longer than the interval between consecutive cantilever step displacements and we used the cumulative exponential fit (Eq 4) to determine the equilibrium amplitude Ak, viscous factor λk and relaxation time τk
Summary
The human cornea is a layer of transparent tissue with the optical function of focusing light onto the retina and the mechanical function of preserving the integrity of the eye. The tissue has an anisotropic microstructure [1,2,3,4,5,6] mainly comprising a network of collagen fibrils that are intertwined in the anterior stroma (200–250 μm anterior depth) while those in the posterior thirds lie in stacked parallel layers. Knowledge of the native biomechanical properties of human cornea as a function of stress levels and loading rates is crucial to assess the impact of diseases (e.g., keratoconus) or surgery (e.g., riboflavin/UV-A cross-linking; LASIK) [7], and optimize the design of corneal prosthetic devices.
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