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Lawrence Hyun Yoo, Robert A Clark, Andrew Shin, Joseph L Demer; Novel Reverse Compressibility of Contracting Human Extra-ocular Muscles (EOMs) indicated By High Poisson Ratio (PR). Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):556.
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© ARVO (1962-2015); The Authors (2016-present)
A fundamental descriptor of the mechanical behavior of a material is its PR, the ratio of transverse to axial strain, i.e., the ratio of change in cross sectional area to change length during uniaxial loading. A material having a PR <0.5 is said to be compressible. Measurement of PR requires accurate 3-D determination of specimen dimensions, typically by quantitative imaging during mechanical loading. During passive, ex vivo tensile elongation, computed x-ray tomography showed the PR of bovine EOM to be ~0.45 (Kim et al., BioMed. Res. International, 2013), but optical coherence tomography demonstrated the PR of extraocular tendon to slightly exceed the ideal incompressible value of 0.5 (Shin et al, ARVO, 2013). Since the PR of contracting EOM is unknown, we used magnetic resonance imaging (MRI) to determine the PR of all 4 rectus EOMs whose axial dimensions change physiologically during horizontal & vertical duction.
Surface coil MRI of 40 orbits of 20 normal adults was performed at 312 micron resolution in ~20° target-controlled horizontal and vertical eccentric gazes. Lengths were measured in axial or quasi-sagittal images parallel to EOM long axes. Cross sections were measured in quasi-coronal images perpendicular to the long axes of each orbit. EOMs were outlined in coronal planes to obtain area centroids cross sectional areas, and areal strain by Green’s theorem. To correct for path curvature, centroids were sequentially aligned to straighten each EOM for analysis. EOMs were then discretized into elements 10-20 microns long. Changes in longitudinal thickness of each element were determined to calculate strain.
Mean (±SD) PR values from discretized 3-D models for the 20 superior, inferior, medial and lateral rectus muscles were 0.87±0.06, 0.79±0.03, 0.75±0.04 and 0.78±0.02 respectively.
PR values markedly exceeding the ideal compressible value of 0.5 for contracting rectus EOMs imply that total volume in the active contraction significantly exceeds that in relaxation, a behavior termed reverse compressibility. Heretofore demonstrated for tendons, reverse compressibility of EOMs would strongly impact accuracy of finite element analysis simulations of EOM cooperative biomechanics, and also provides a strong rationale for use of EOM volume metrics as functional indices of contractility.
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