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M. Mattson, D.M. Schwartz, J.A. Kornfield; Mechanical Measurements of Sclera for Screening Myopia Treatments . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1991.
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Purpose: The physical properties of the sclera are important in maintaining the structure and function of the eye. If the sclera has inadequate strength, it may lead to diseases such as progressive myopia. We present a quantitative method for comparing the mechanical properties of healthy and diseased tissue, and for tracking changes in individual tissue samples. Using this method we have studied the stability of the sclera in various solutions and evaluated candidates for treatment of progressive myopia. Methods: Fresh 8mm diameter tissue sections from porcine (<36 hrs post mortem) and human donor (<72 hrs p.m.) eyes were placed in Dulbecco’s phosphate buffered saline (DPBS) for 24 hours at 22°C prior to initial measurements of the storage modulus G', a measure of the elastic properties of the sample. G' was measured by oscillatory shear tests on a TA Instruments AR2000 rheometer fitted with a novel cleat tool which reduces wall slip, common in biological samples. The tests were run at a constant amplitude stress (5 Pa) and angular frequency (1 rad/sec) with the sclera in a 37°C solution bath of DPBS. Next, the samples were immersed for 24 hours in the following solutions: DPBS, NaCl in water (0–2.0% w/w), phosphate buffered saline (10mM, pH 5–9), and 2% glyceraldehyde (GA) in DPBS – a potential candidate for myopia treatments. G' measurements were then repeated with each scleral section in its respective solution and repeated again after a 24 hour DPBS rinse. Results: Initial measurements show that porcine sclera is significantly stiffer than human sclera (G' = 4800 ± 1800 Pa and 2200 ± 2700 Pa respectively). The modulus increases as salt concentration decreases from 2% to 0%: for porcine sclera G' increases 10–fold over this range; for human sclera G' increases 8–fold. The modulus also increases as pH decreases from 8 to 5 (porcine G' increases 250% over this range; human G' increases 150%). Salt and buffer effects are reversed by the DPBS rinse. Treatment with 2% GA increases the modulus (porcine: 380% increase; human: 170% increase), and G' remains high after the DPBS rinse. Conclusions: There is a clear and predictable dependence of the mechanical properties of the sclera on the solution in which it is equilibrated. This predictability 1) indicates the range in which to maintain the tissue to best represent in vivo behavior of the sclera, 2) enables the detection of deviations from normal behavior, and 3) enables screening for the effectiveness of potential myopia treatments.
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