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H.A. Weeber; The Influence of the Changing Stiffness Gradient in the Lens With Age on Presbyopia . Invest. Ophthalmol. Vis. Sci. 2005;46(13):733.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To describe the influence of the stiffness gradient within the human crystalline lens on the development of presbyopia. Methods: In an earlier study, the stiffness gradient of human crystalline lenses has been measured using local Dynamical Mechanical Analysis (DMA): a small piston (0.5–mm diameter) was put into the lens and the resistance to an oscillating movement of the piston was measured. The measurement results were converted to shear modulus. In this study, the results of these measurements were incorporated into a mechanical model of the accommodative apparatus. Two mechanical models were built: model 1 contained a lens having a uniform stiffness and model 2 included the stiffness gradient. In both models, disaccommodation was achieved by stretching the lens through the zonule. The stretching force was equal for both models and was kept constant for all ages. The shape change of the lens was converted to a change in lens power. Results: A stretchting force of ±0.1N was used to stretch the lens in the apparatus. This resulted in an accommodation of ±4 D at 40 years of age. In both models, the very high accommodative amplitudes typically reported for eyes at a young age were not achieved. At older age, model 1 showed a very slow decline in accommodative amplitude of 1 diopter per decade, while model 2 followed the curves of Duane and Ungerer closely. Conclusions: When the lens stiffness is represented as a single uniform modulus, presbyopia cannot be explained by the increasing lens stiffness with age. However, when the stiffness gradient of the lens is taken into account, presbyopia can be fully explained by stiffness changes of the lens with age.
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