Numerical simulation of the human eye accommodation

The structure of the human eye is almost spherical, elastic and the properties of the components are considerably age-related. The main parts of the eye are the sclera, the cornea, the ciliary muscle, the zonular fibers and the crystalline lens. We have built a complex three dimensional finite element model which contains the biologically and mechanically required parts for biomechanical analysis. We have examined the accommodation problem based on the classical Helmholtz theory according to the age-related changes in geometry and material. Based on mechanical and optical consideration, in this paper we would like to give answers why our accommodation width decreases with increasing age. Introduction The eye is one of the most important sensory organ of the human body. There are several studies and numerical investigations which dealt with this subject. The widespread fields of research in this topic are the following: accommodation of the human eye, reasons of presbyopia, effect of eye surgeries and so on. Many studies dealt with these parts separately ([1], [4], [11]) but the complex modeling of the human eye is rare, so we tried to develop a numerical model to analyse several problems. In this article we introduce our three dimensional finite element model and its application to examine the accommodation process. Accommodation First of all very shortly we have to discuss what the accommodation of the human eye is. To answer this question we have to look over the main parts of the eye. These parts are the sclera, the cornea, the ciliary muscle, the zonula fibers and the crystalline lens with nucleus, cortex and capsule (see Figure 1). Fig. 1. Section of the human eye and the accommodation process (www.micrographia.com; aglasser.opt.uh.edu) The ciliary muscle contraction causes the lens to thicken and change shape. It allows us to see objects closely (see Figure 1). This is the accommodation process. When we are young our crystalline lenses are soft and flexible so they are able to change their shape easily, allowing focusing on objects both close and far away. Around the age of 40 our crystalline lens becomes less flexible (more rigid, higher Young's modulus) what makes it more difficult to see close. We call this presbyopia ("aging eye"). The accommodation width is the difference between the lens diopter when seeing near and the lens diopter when seeing far. The central optical power (COP) is the diopter of the lens in its axis. We calculated the optical power based on the conventional lens formula: