Application of a simple mechanical model of accommodation to the aging eye.

A simple model for the mechanics of accommodation in the human eye, previously proposed, is applied to the eye at different ages. Data from work by R. F. Fisher is used to derive values for the elasticity of lens-plus-zonules and data from work by G. W. H. M. van Alphen and W. Graebel is used to derive values for the elasticity of the choroid, each at a series of ages. The model is applied in two ways: (i) the elasticity of one component is allowed to change with age, while the other (and also ciliary muscle force) is held constant and the effect on accommodative amplitude is estimated; (ii) both elasticities are allowed to change with age, and muscle force is assumed to take on values producing a constant change in the radius of the "ciliary ring". It is shown that, in principle, age-related changes in choroid could account for a substantial component of the observed loss of accommodative amplitude, and it is suggested that this may be the case for monkeys, but not for humans. For the case of "all components changing", which is probably the case for humans, the ciliary muscle is likely to become stronger with age.