The posterior eye with age-related macular degeneration has isotropic and nonlinear viscoelastic properties

Here we characterize and compare the anisotropic and nonlinear viscoelastic properties of the posterior eye of advanced dry age-related macular degeneration (AMD) patients and age-matched normal subjects. Ten normal horizontal, ten normal vertical, ten AMD horizontal, and ten AMD vertical strips of the macular retina and the underlying choroid and sclera were preloaded, preconditioned, and subjected to incremental stress-relaxation tests in body-temperature saline. The stress-relaxation response was characterized by a fully nonlinear viscoelastic formulation in which the relaxation modulus was approximated by a Prony series and a second-order polynomial using the comprehensive viscoelastic characterization method. Normal retina, choroid, and sclera were found to be anisotropic, whereas AMD tissues were isotropic. AMD retina and choroid showed greater stress-relaxation response than normal tissues (p < 0.05), whereas AMD sclera had smaller stress-relaxation response than normal tissue (p < 0.05). The nonlinear viscoelastic stress-strain relationship of the posterior eye is hence describable for the first time.