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Christopher J. Bay, Richard A. Regueiro; Experiments and Finite Element Modeling of Indentation and Puncture of the Lens. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1349.
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Mechanical characterization and modeling of the human eye can lead to increased understanding of trauma suffered (e.g., by Intra-Ocular Foreign Body (IOFB) penetration) and effects of surgical procedures (capsulorhexis during cataract surgery) on the eye, thus leading to better treatments and solutions.
Fresh 2+ year porcine eyes obtained one day after slaughter are dissected to extract the lens, and then immersed for testing in a cup full of Balanced Salt Solution (BSS) warmed to 39.2 degrees C (pig body temperature) to reduce the non-physiological effects of testing in-vitro. A puncture tip (6 different geometries) indents the lens (anteriorly and posteriorly) along its anterior-posterior axis at a displacement rate of 0.5 mm/s to 80% nominal strain. The capsule fails, either by puncture at the tip or by bulging rupture along the equatorial region. Force is measured and digital videos are taken of the deformation. Large deformation nonlinear finite element analysis (FEA) using the finite element program ABAQUS simulates the experimental test conditions and is compared to the collected data. The three-dimensional (3D) type-IV collagen meshwork ultrastructure of the capsule is being identified using cryo-electron tomography in support of developing an ultrastructurally-based computational model of the capsule for multiscale modeling of capsule cutting, tearing and rupture.
The force-displacement curves for extracted lenses are fit by an optimized FEA to understand the large strain constitutive response up to rupture. The 3D collagen meshwork has been identified, and observed structure agrees with current studies.
Indentation and puncture loading is meant to mimic perforation by an IOFB. Currently, the experimental method is limited in that it ignores relaxation and tension in the zonules attached to the equatorial region of the lens capsule. The method, however, successfully represents the puncture response of the whole lens. We are developing a test apparatus to measure the mechanical properties of the zonules and ciliary body separately. The preliminary 3D identification of the type IV collagen meshwork of the capsule is promising. The eventual research goal is to better understand the mechanical response of the ocular lens to blast loading, by a combination of mechanical experiments and multiscale nonlinear finite element modeling.
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