An experimental and theoretical analysis of unconfined compression of corneal stroma

The cornea is a transparent connective tissue with dual functions of protecting the eye (mechanical properties) and refracting the light (optical properties). Both of these properties are derived from the corneal intricate and pseudo regular extracellular matrix, the stroma. From the mechanics point of view, the corneal extracellular matrix is a hydrated structure composed of collagen fibrils, proteoglycans, and the interstitial fluid. The objective of this study was to investigate compressive biomechanical properties of the cornea using an experimental and numerical framework. The unconfined compression stress-relaxation tests were performed to measure the corneal behavior experimentally and the transversely isotropic biphasic theory was used to analyze the experimental data. It was observed that the behavior of the corneal stroma under stepwise stress-relaxation compression is similar to that of the other soft hydrated tissues and is composed of an immediate stiff response, a transient relaxation phase, and a final steady-state stage. Within the range of deformation considered in this study, maximum and equilibrium reaction stresses were linearly dependent on the compressive strain. The linear transversely isotropic biphasic model curve fitted experimental measurements with the coefficient of determination rfit(2)=0.98±0.01. The mechanical parameters of the porcine corneal stroma were calculated as a function of the engineering strain. The corneal out-of-plane modulus was almost independent of the compressive strain, the transverse Young's modulus linearly increased with increasing strain, and the permeability coefficient decayed exponentially with increasing strain. The average mechanical parameters under unconfined compression were found to be: the out-of-plane modulus E¯z=5.61KPa, the transverse Young's modulus E¯r=1.33MPa, and the permeability coefficient κ¯r=2.14×10(-14)m(4)/N.s.