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J. P. Vande Geest, A. Mortazavi, B. R. Simon; Compressive Mechanical Properties of Human and Porcine Sclera. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4906.
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While it is certain that primary open angle glaucoma results from increased intraocular pressure (IOP), there remain several unanswered questions regarding glaucoma in patients with relatively normal IOP. In this notable case the optic nerve head (ONH) is damaged yet fluid flow within the anterior of the eye remains standard and IOP is not elevated. Various studies have speculated that material properties and behavior of certain ocular tissues, such as the sclera and lamina cribrosa, could contribute to the onset of glaucoma. Consequentially, to understand the eye as a network of interconnected soft tissues, several experimental material tests have been performed, unfortunately resulting in widely varying outcomes. In order to further explore material properties and behaviors in major load baring tissues near the ONH, we performed unconfined compression tests on human and porcine scleral specimens.
Eight pairs of abattoir porcine eyes, ages 6 months to one year were collected from the University of Arizona's Meat Lab approximately 20 minutes post-mortem. In addition, ten human scleral specimens were acquired from seven donors whose age ranged from 78-89 years. In order to determine the drained secant modulus, unconfined compression stress-relaxation tests were performed using step strains of 5% and holding each of these steps for 30 minute intervals. Testing was performed on a dynamic mechanical analyzer whose displacement resolution is 1micron and load resolution is 0.5 micro-Newton. The samples were tested in a custom bath filled with PBS and maintained at 37 degrees Celsius for the duration of each test. Student’s t-tests with a significance level of p=0.05 were used to assess differences in secant modulus between porcine and human samples.
The drained secant compressive modulus of porcine sclera was significantly higher than that for humans (1.1 +/- 0.28 kPa vs 3.9 +/- 2.1 kPa, p<0.05) at 5% step strain. This difference was present at all strains. The secant modulus in both the porcine and human sclera displayed a mild nonlinearity above 10% compressive strain.
Our results indicate that the compressive mechanical behavior of the porcine sclera is significantly stiffer than that of aged humans. Further testing is required to identify what if any are the effects of age on the compressive mechanical behavior of human sclera.
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