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Andrew Shin, Lawrence Yoo, Joseph Demer; Poisson Ratio Measurements of Extra-ocular Muscle and Tendon. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1925.
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© ARVO (1962-2015); The Authors (2016-present)
Oculorotary force is generated by extraocular muscles (EOMs) and transmitted to the globe via varying lengths of extraocular tendons (EOTs). Potential biomechanical differences between EOM and EOT have been typically assumed but heretofore unmeasured. One such fundamental mechanical parameter is the Poisson ratio (PR), which approximates the ratio of relative change in cross sectional area to tensile elongation. The present study measured the PR for both EOM and EOT by determining morphologic changes using computed x-ray tomography (CT) and optical coherence tomography (OCT) at microscopic resolution.
Fresh bovine EOM specimens were clamped in a tensile fixture within a micro-CT scanner (Skyscan, Belgium) with temperature and humidity control, and stretched up to 35% of initial length. Bovine EOTs were clamped in a load cell with physiologic environmental conditions and were stretched up to 25% of initial length. Sets of 500-800 contiguous CT images and sets of 512 OCT images were obtained at 10 micron resolution before and after tensile loading. Digital 3-D models were then built and discretized into 6-8 and 1-1.25 microns thick elements for EOM and EOT, respectively. Changes in longitudinal thickness of each microscopic element were determined to calculate longitudinal strain. Green’s theorem was used to calculate areal strain transverse to stretching.
The mean PR from discretized 3-D models for every microscopic element in 14 EOM specimens averaged 0.457 ± 0.004 (SD) (P < 0.001) while the mean PR value was computed to be 0.542 ± 0.011 for 7 EOT specimens (P < 0.001). While the PR for EOM is near the condition of incompressibility (PR = 0.5), the PR for EOT was significantly greater than 0.5, indicating that EOT volume decreases during tensile deformation.
The internal structure of EOT is different from that of EOM which causes overall volume to decrease during tensile elongation. Taking mass conservation into account, we can also come to a possible conclusion that EOT density increases during tensile elongation.
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