Nonlinear material properties of intact cornea and sclera

Abstract

Enucleated human eyes, 1–3 days post mortem, were sectioned equatorially and the anterior or posterior segment of the corneoscleral shell was clamped on a metal chamber for pressurization. A flying spot scanner system was used to measure horizontal surface deformations while the pressure in the chamber was changed stepwise. Lateral deformations in directions 90° and 45° to the original horizontal direction were also measured to establish surface isotropy of the material when the eye specimens were pressurized in their natural geometries. The finite element numerical method was then used to construct a mathematical model of the posterior segment of the eye mounted on the metal chamber. This model uses trilinear stress-strain relationships which were adjusted until the predicted surface deformations agreed with the experimental results. Using a similar procedure on the anterior segment, a trilinear stress-strain relation was obtained for the corneal stroma. The trilinear relations were then converted to exponential form by a least square method. The finite element, axisymmetric mathematical model can assign these nonlinear material properties for different portions of the eye and specific variations in geometry such as central corneal or equatorial scleral thinning. Such geometric complexity and material heterogeneity and nonlinearity could not be modeled conveniently by previous mathematical models of the corneoscleral shell. The model will be useful in the analysis of stress and strain in portions of the eye, pressure-volume relations and tonometry.