A numerical investigation of changes in lens shape during accommodation

Abstract

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.

Regulation of the Membrane Trafficking of the Mechanosensitive Ion Channels TRPV1 and TRPV4 by Zonular Tension, Osmotic Stress and Activators in the Mouse Lens

“…This ability of lens pressure to respond to changes in zonular tension induced by muscarinic modulation of the ciliary muscle implies that the transport of water through the mouse lens driven by the microcirculation system can be dynamically regulated by stimuli external to the lens. In this regard, it is well known that the processes of accommodation can rapidly change the shape of and overall optical power of primate lenses [ 36 ], but the results from our current and previous studies suggest that changes in zonular tension may also have effects on water transport and therefore the optical properties of the non-accommodating mouse lens. Not only have changes in zonular tension induced by pilocarpine been shown to remove TRPV4 from the membrane ( Figure 4 and Figure 5 ) and increase the hydrostatic pressure gradient [ 11 ], they have also been shown to induce the removal of the water channel aquaporin 5 (AQP5) from the membranes of peripheral fiber cells located in the anterior influx pathway and equatorial efflux zones, but not in the posterior influx pathway [ 22 ].…”

A validated finite element model to reproduce Helmholtz’s theory of accommodation: a powerful tool to investigate presbyopia

“…To achieve this aim, we previously reported that the mechanical properties of the crystalline lens are key to how the lens shape changes. 15 The presented model indicates that several factors such as lens mechanical properties, the zonular arrangement and capsule thickness distribution must be included to simulate the accommodation of humans.…”

“…The mechanical properties of the lens are fundamentally related to how it changes its shape, 8,10,26 above other factors such as the lens geometry. 15 Therefore, we analysed the decrease in accommodation amplitude by varying the mechanical properties, i.e., the lens capsule, nucleus and cortex, while we kept the remaining tissues of the FE model proposed for the 29-year-old subject constant. Thus, in this hypothetical case, we kept the same lens geometry and the initial stresses of the lens capsule calculated for the validated FE model, as reflected in Figure 4a.…”

“…Moreover, the thickness of the lens nucleus increases, but without a significant change in thickness of the cortex 4,7 . Several authors have designed finite element (FE) models to explain and understand accommodation 8‐15 . However, most of these biomechanical models are based on accommodated geometry, and reproduced the accommodation process in reverse by applying the forces exerted by the relaxation of the ciliary muscle to reach the non‐accommodated state.…”

“…4,7 Several authors have designed finite element (FE) models to explain and understand accommodation. [8][9][10][11][12][13][14][15] However, most of these biomechanical models are based on accommodated geometry, and reproduced the accommodation process in reverse by applying the forces exerted by the relaxation of the ciliary muscle to reach the non-accommodated state. These FE models are a powerful tool to understand the opto-mechanical crystalline lens changes.…”