Accommodation is a dynamic change in the dioptric power of the eye that allows a change in focus from distant to near objects. This process is achieved by changing the shape of the lens, following contraction of the ciliary muscle, thereby releasing the zonular tension around the lens equator, according to the widely accepted accommodation theory of Helmholtz. 1 Numerous authors have confirmed this theory over time, including Fincham, 2 who demonstrated the ability of the elastic lens to round up after the release of zonular tension, and Glasser and Kaufmann, 3 who showed relaxation of the zonules due to ciliary muscle contraction along with other investigators. 4 The ciliary muscle is the engine that drives the accommodation process. It consists of three groups of muscle fibres (i.e., longitudinal, radial and circular), although their

The purpose of this study was to investigate how the mechanical properties and geometry of the lens influence the changes in lens shape during accommodation. To do so, ex vivo stretching tests of the isolated lens were simulated via finite element analysis. In these tests, the lens is stretched from the accommodated state to the non-accommodated state. Several key characteristics of the lens were studied: the stiffness gradient of the lens material, the distribution of the capsule thickness, the mechanical properties of the capsule and the material comprising the lens, nucleus and cortex, and the influence of two different age-related lens geometries (17 and 29 y/o subjects). To determine the effects on the changes in lens shape during accommodation, changes in the anterior and posterior radius, the lens and nucleus thicknesses and the equatorial lens diameter were analysed. The results suggest that multiple factors exert statistically significant influences on how the lens changes its shape, but two factors predominate over the rest: the stiffness ratio between the nucleus and cortex and the stiffness of the capsule, specifically the posterior surface.

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