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A. Shin, L. H. Yoo, J. L. Demer; Mechanical Creep in Passive Bovine Extraocular Muscles (EOMs). Invest. Ophthalmol. Vis. Sci. 2010;51(13):22.
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Prior mechanical characterization of EOMs has typically employed brief passive elongations imposed by external stress, followed by almost immediate relaxations. However, in the physiological state, EOMs are under almost continuous stress by their antagonist EOMs, and by the orbital connective tissues. Possible effects on EOMs of sustained stresses are unknown, but are speculated to be causes to strabismus. We aimed to characterize EOM creep, a mechanical concept representing the tendency of a solid material to deform permanently under the influence of stress.
Specimens of the 6 different EOMs were rapidly removed from fresh adult bovine orbits obtained from a local abattoir, and representative sampled from multiple EOM regions were cut longitudinally into strips measuring 25 mm length with a 2 mm x 2 mm cross-section. Samples were clamped into an automated load cell (model 4411; Instron, Norwood, MA) with a 50 N force transducer. Specimen temperature and humidity were maintained physiologically during testing. The load cell imposed a linearly increasing force at one of three rates of 10, 20, or 50 mm/min to a level of 0.2 N that was subsequently maintained over 1500 s by force feedback as specimen length (strain) was monitored to indicate creep.
All rectus and the oblique EOMs exhibited similar creep behavior in both initial slight elongation, and later creep to an asymptotic value close to 90% greater than original length. Specimens reached asymptotic length in less than 1500 s for all three ramp strain rates tested. In the creep region following ramp loading, creep curves were visibly steeper as the ramp strain rate increased. Creep magnitude was an increasing function of initial strain rate. As ramp strain rates increased from 10, 20, and 50 mm/min, the difference between beginning of creep and asymptotic value increased from 1.79, 3.33, to 4.62 mm, respectively.
Bovine EOMs rapidly and substantially deform under sustained, passive stress. This creep behavior of bovine EOMs is significant, but highly uniform. Creep behavior of EOMs should be considered in quantitative modeling of situations, such as strabismus, in which EOMs are subjected to sustained stresses.
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