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Joel Palko, Jun Liu; Dynamic Testing of Regional Viscoelastic Behavior of Canine Sclera. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6247.
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Intraocular pressure (IOP) fluctuations (e.g., ocular pulse) have gained recent clinical interest and thus warrant an understanding of how the sclera responds to dynamic stress insults. The objective of this study was to characterize the regional dynamic viscoelastic properties of canine sclera under physiological loadings.
Three strips from each of ten canine eyes were excised from the anterior, equatorial, and posterior scleral shell in the superior-inferior direction. The dimensions of each strip were measured using high frequency B-mode ultrasound (Vevo660, VisualSonics, Toronto). The strips were then mounted to a Rheometrics Systems Analyzer (RSA III, TA Instruments, New Castle) and allowed to equilibrate in a humid environmental chamber at approximately 37°C. A cyclic strain input at 1 Hz and amplitude of 0.25% was applied on the strips, superimposed upon static prestresses corresponding to an IOP of 15, 25, and 45 mmHg. The cyclic stress output at each prestress level was recorded and the complex modulus, dynamic viscosity, and tan(delta) were calculated assuming linear viscoelasticity.
The complex modulus and dynamic viscosity increased significantly (p<0.001) while the tan(delta), a measure of the tissue’s ability to dampen dynamic loads, decreased significantly (p<0.02) with increasing prestresses. There were no significant differences between the complex modulus and dynamic viscosity among sclera regions at any levels of prestress. A significant difference was found between tan(delta) of the anterior sclera compared to that of the equator at all prestress levels.
To our knowledge this is the first investigation of the dynamic viscoelastic properties of the sclera. All sclera regions exhibited an increased ability to resist cyclic stress insults at increasing prestress (i.e., steady-state IOP) levels. However, the ability of the sclera to dampen the dynamic insults appeared to decrease at higher IOPs.
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