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L. H. Yoo, J. Reed, J. L. Demer; Micro-Indentation Characterization of Bovine Extraocular Tissues (EOTs) in Ocular Motility: Hertzian Viscoelastic Models. Invest. Ophthalmol. Vis. Sci. 2010;51(13):20. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The Hertzian viscoelastic model derives properties of a material from the force exerted when a harder body indents the material. This permits the material to be characterized in situ in certain instances. We performed in vitro testing to determine if micro-indentation analyzed using the Hertzian formulation can provide a practical characterization of mechanical properties of bovine EOTs.
Specimens of sclera, pulley tissue, and orbital fat were rapidly removed from fresh adult bovine orbits obtained from a local abbatoir. We assembled a micro-indentation test cell incorporating a 100 nm precision linear stepper motor to indent biological specimens with hard spherical probes (diameters of 2, 3, 4, and 5 mm) during force monitoring by a vibration-isolated force transducer having 100 µg precision. Stress relaxation testing was using indentations imposed at five different rates from 0.1 - 2 mm/s, and subsequently maintained. Maximum specimen indentation was limited to the spherical indenter radius. Initial force data permitted determination of Hertzian elasticity. Relaxation after 10 s indicated effects of viscosity.
Pulley tissue, fatty tissue and sclera exhibited markedly different elastic and viscous behaviors, suggesting different biomechanical properties. Stiffness was lowest in orbital fat tissue and highest in sclera. Asymptotic forces during maintained indentation were reproducible within tissues, but varied widely among the three EOTs, indicating differences in viscosity. For pulley tissue, mean force after 10 s indentation was reduced to 41.7%, but this value was 51.5% for orbital fatty tissue, and 57.8% for sclera.
Microindentation permits biomechanical characterization of EOTs important to ocular motility, and illustrates markedly different viscoelastic properties among bovine pulley tissue, fatty tissue and sclera. Such properties might eventually be implemented into models of the ocular motor system used to simulate strabismus. The microindentation approach is advantageous because it can characterize biomechanics without invasive manipulations necessary to stretch specimens in a conventional load cell.
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