Effects of Gradient Mechanical Properties on Human Visual Accommodation studied using Nonlinear Finite Element Method (FEM)

摘要

Studies have confirmed that the primary cause of 'old sightedness' (presbyopia; the inability to focus up close for reading) is gradual lens stiffening. It has been suggested that refilling the contents of a presbyopic crystalline lens could restore the accommodative ability of the lens. Phaco-ersatz replaces the hardened content of the presbyopic lens with a flexible polymer to restore the ability of the lens to change shape. For this surgery to be successful, a thorough understanding of the effects of substituting the natural lens material with a gel is critical. In particular, the natural lens possesses a gradient in its mechanical property that cannot be reproduced by an isotropic gel. The impact on accommodation of a loss in mechanical gradient in phaco-ersatz has not been studied. We utilized the Finite Element Method (FEM) to build an axisymmetric model of the accommodative system and to investigate the contribution of gradient mechanical properties to the accommodative response of the lens and to predict the outcome of refilling the lens with a material of isotropic elasticity and refractive index. Using the lens aspect ratio as the criterion, an equivalent isotropic elasticity is found. Results show that the equivalent elasticity, to some extent, depends on the gradient elasticity profile implemented into the model. We showed that this method yields an equivalent elasticity in agreement with previous empirical studies. Numerical ray-tracing using a custom-made MATLAB code to assess the accommodative response of both models reveals that lens refilling is capable of restoring a major portion of accommodation.