Engineering Mechanics Institute Conference 2013

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Experiments and Finite Element Analysis of Stress Relaxation upon Compression of Whole Porcine Lenses

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Richard Regueiro
University of Colorado, Boulder
United States

Christopher Bay
Texas A&M University
United States

Chad Hotimsky
University of Colorado, Boulder
United States

Vicky Nguyen
Johns Hopkins University
United States

Abstract:
Traumatic cataract can result in a partially or fully clouded lens, complete dislocation of the lens (floating between aqueous and vitreous humors), or zonule rupture such that partial or full vision loss may occur. The mechanisms of traumatic cataract formation that may require cataract surgery (implantation of an intraocular lens (IOL)) are not well understood in comparison to the mature and ever-improving surgical technology and procedures. Traumatic cataract under blast conditions, potentially involving Intra-Ocular Foreign Bodies (IOFBs), is also not well understood.
Fresh 6-9 month and 2+ year old 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 degrees C (pig body temperature) to reduce as much as possible the non-physiological effects of testing in-vitro. A loading platen then cyclically compresses the lens along its anterior-posterior axis at displacement rates 0.1mm/s and 0.3mm/s (0.01/s to 0.03/s strain rate) up to 10% and 20% 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 isotropic hyper-viscoelastic constitutive model are fit to experimental data by optimizing an axisymmetric finite element model against the data.
The force-displacement curves are calibrated by the optimized parameter-fitting finite element analysis. A range of parameters for various conditions (age, strain, and strain rate) are determined given the natural variability of the extracted pig lenses.
Unconfined compression provides a simple means for measuring the mechanical properties of the whole lens at various strain rates. Methods are being investigated to extend to higher rates and thus determine potential change in mechanical properties upon traumatic cataract formation. An ultrastructurally-based nonlinear finite element model of the lens, in conjunction with experiments, is being developed to better understand formation of traumatic cataract under blast loading.

 

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