Abstract
The accommodating volume-constant age-dependent optical (AVOCADO) model of the crystalline lens is used to explore the age-related changes in ocular power and spherical aberration. The additional parameter m in the GRIN lens model allows decoupling of the axial and radial GRIN profiles, and is used to stabilise the age-related change in ocular power. Data for age-related changes in ocular geometry and lens parameter P in the axial GRIN profile were taken from published experimental data. In our age-dependent eye model, the ocular refractive power shows behaviour similar to the previously unexplained "lens paradox". Furthermore, ocular spherical aberration agrees with the data average, in contrast to the proposed "spherical aberration paradox". The additional flexibility afforded by parameter m, which controls the ratio of the axial and radial GRIN profile exponents, has allowed us to study the restructuring of the lens GRIN medium with age, resulting in a new interpretation of the origin of the power and spherical aberration paradoxes. Our findings also contradict the conceptual idea that the ageing eye is similar to the accommodating eye.
Highlights
The human eye has been studied extensively over the past century or so
We see that optimisation of the lens parameters becomes much more difficult and requires models with sufficient flexibility to account for age-related restructuring of the gradient index (GRIN) medium
Our goal is to illustrate that the lens paradox can be resolved, by showing how a restructuring of the GRIN medium can match the power of the ageing eye to a published longitudinal study
Summary
More advanced measurements are becoming available, yet we still do not fully understand the structure of the human lens. The usefulness of models lies in their ability to be optimised to provide results for different scenarios that cannot be observed experimentally. They can provide information on the aged eye, of which measurements cannot be made due to the presence of severe cataract. In the case of the human eye, much the same procedure is followed, but the human lens is not a simple homogeneous element; rather, the lens consists of a gradient index (GRIN) medium. We see that optimisation of the lens parameters becomes much more difficult and requires models with sufficient flexibility to account for age-related restructuring of the GRIN medium
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