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Andrew Shin, Lawrence Yoo, Hansang Kim, Joseph L. Demer; Micro-CT During Tensile Loading: Accurate Determination of the Poisson Ratio (PR) of Passive Bovine Extraocular Muscles (EOMs). Invest. Ophthalmol. Vis. Sci. 2011;52(14):1558.
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© ARVO (1962-2015); The Authors (2016-present)
The PR is a fundamental mechanical parameter relating transversely contracting areal strain to longitudinal-extension strain of a material. For example, the PR approximates the ratio of relative change in cross sectional area to the amount of relative longitudinal stretching of a long, thin specimen. However, the PR of biological materials such as EOMs is almost never measured because of experimental constraints. We overcame this problem by determining changes in EOM dimensions using computed x-ray tomography with microscopic resolution (Micro-CT) during tensile elongation (longitudinal strain) to determine the change in cross section, which represents transverse strain. The PR ratio is then calculated from two transverse strains in orthogonal directions.
EOM specimens were prepared from fresh bovine orbits obtained from a local abattoir. Preliminary experiments suggested that all 6 anatomical EOMs would have the same PR. Specimens were clamped in a tensile fixture within the Micro-CT scanner (Skyscan, Belgium) with temperature and humidity control, and stretched up to 30% of initial length. Sets of 500 to 800 contiguous CT images were obtained at 10 micron resolution for 3D reconstruction before and after imposition of tensile loading (stretching). 3-D models were discretized into 5 to 8 microns thick elements. Changes in longitudinal thickness of each microscopic element were determined to calculate strain in stretching direction. Green’s theorem was used to calculate areal strain in transverse directions orthogonal to the stretching direction. The PR for the EOM was determined as the mean of PRs for all microscopic elements.
PRs of all 6 anatomical EOMs were similar. The mean PR for every microscopic element in each EOM was 0.46 ± 0.02 (SD), giving a 4.73% coefficient of variation.
An ideal incompressible material has a PR of 0.5. As determined using microscopic resolution CT, the PR of passive bovine EOM is nearly ideal. The measured PR can be used to enhance the accuracy of finite element analysis models to improve accurate and pragmatic characteristics of simulation of biomechanics.
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