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R. Regueiro, A. Blanchard, K. Constancio, L. Williams; Experiments and Computations for Simulating Unconfined Compression of Porcine Ocular Lenses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3831.
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© ARVO (1962-2015); The Authors (2016-present)
Understanding the mechanics of lens accommodation can assist in the diagnosis of early presbyopia as well as identify potential clinical treatments and lens prosthetic implantation strategies. Presbyopia is attributed to changes in ciliary muscle function, as well as changes in the mechanical properties of the lens substance, lens capsule, and zonules, presumably. The precise relationship of these changes, however, is not well described.
Fresh 6-9 month porcine eyes obtained one day after slaughter are dissected to extract the lens, which is then immersed for testing in a cup full of Balanced Salt Solution (BSS) warmed to 39.2°C (pig body temperature) to reduce as much as possible the non-physiological effects of testing in-vitro. A loading ram then cyclically compresses the lens along its anterior-posterior axis at various displacement rates up to 30% nominal strain. Force is measured, and digital photographs are taken at various stages of deformation to obtain a crude measure of cross-sectional deformation. Parameters for a large deformation hyper-viscoelastic constitutive model are fit to experimental data by optimizing an axisymmetric finite element model against experimental data. The lens capsule and substance are assumed to be nearly incompressible (negligible volume change), and constitutive parameters are fit separately for the capsule and substance.
The force-displacement curves at certain displacement rates for lenses extracted from pairs of eyes can be fit by the optimized parameter-fitting finite element analysis. It was found that lenses needed to be tested immediately upon receipt as even one day later it was observed that the lens substance degraded. Freezing did not preserve the mechanical properties of the lens substance.
Unconfined compression loading is not physiological, such that the lens changes curvature through relaxation and tension in the zonules attached to the equatorial region of the lens capsule, but it provides a simple means for measuring the mechanical properties of the whole lens. Once confidence is gained in modeling the whole lens--which involves higher resolution mechanical testing of the lens’ individual components (capsule and substance)--as well as the zonules, a mechanical model can be loaded in a physiological manner using finite element analysis to attempt to predict the mechanics of lens accommodation, and in turn any influence by change in mechanical properties of the individual components and zonules.
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