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Andrew Shin, Lawrence H. Yoo, Zia Chaudhuri, Joseph L. Demer; Highly Independent Passive Mechanical Behavior of Bovine Extraocular Muscle (EOM) Compartments. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1008.
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Intramuscular innervation of horizontal rectus EOMs is segregated into superior and inferior compartments (vertical compartments), while all EOMs are also divided into global (GL) and orbital (OL) layers with scleral and pulley insertions, respectively. We tested passive mechanical coupling between EOM compartments.
Specimens of the 6 different EOMs were rapidly removed from fresh adult bovine orbits, and each whole EOM was clamped to a temperature- and humidity-regulated, dual channel micro tensile load cell consisting of two independent linear motors capable of high speed (≤100 mm/sec) and fine resolution (20 nm). Only one channel a time was extended 3 or 5 mm, while the other channel was stationary, as forces in both channels were monitored. Fine metal bead fiducials were distributed on the EOM surface for high definition video tracking to visualize local deformation. Variable loading rates (5, 20, and 100 mm/sec) were applied to explore speeds from slow vergence to saccades. Control loadings were performed using isotropic latex of stiffness comparable to EOM.
Regardless of whether compartmentalization was parsed by pulley (OL) vs. scleral (GL) insertion, or parsed in any proportion from 20 - 80% of the scleral insertional, all rectus and both oblique EOMs exhibited a high degree of compartmental independence during tensile loading. Intercompartmental force coupling was ≤10% in all 6 EOMS even for saccadic loading rates up to 100 mm/s and for extreme loading to EOM rupture. In contrast, isotropic latex exhibited ~50% coupling. Optical tracking of fiduicials demonstrated independent strain distribution between EOM compartments.
EOMs behave as if composed of parallel fiber bundles with robust mechanical independence. Within each EOM, fiber bundles have different insertional targets resulting in differing mechanical actions. This implies that the mechanical repertoire of each individual EOM can be multi-faceted, and separately controllable by motor axon projections and innervational coordination as proposed in the active pulley hypothesis, and suggested by functional MRI of human EOMs during vergence and ocular counter-rolling.
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