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J. E. Whitcomb, R. Amini, N. Simha, V. H. Barocas; Assessment of the Mechanical Properties of the Iris Dilator and Stroma Using Nanoindentation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4907.
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To assess and quantify the relative stiffness of the iris dilator and stroma in the ex vivo porcine model.
Intact porcine irides (4-6 hrs post-mortem) were cut into two equal C-shaped halves. Both halves were glued to a flat, rigid, aluminum surface with one half of the samples anterior face up, and the other half posterior face up. Each sample was then indented using a Nanoindenter XP (MTS; Eden Prairie, MN) with a 30 nm displacement and 0.02 mN force resolution. A stainless steel flat-end cylindrical tip of 1 mm diameter was used. Indentation was performed at a rapid rate between 0.5 - 1.0 mN/s to approximate the instantaneous response for the force-displacement curve at random points on the surface of each C-shaped half. The depth-load curve for each experiment was analyzed to estimate an instaneous modulus (linear fit of data between 0.5 - 1.0 mN load) of the tissue, with the assumption that the thin layer of glue absorbed by the tissue did not affect the properties.
A total of 26 indentations were performed on four tissue samples. The irides ranged from 300 - 500 µm thick, whereas the indentation depth was between 50-60 µm, so the different experiments probed the different components of the tissue. The posterior surface indentation gave slightly higher forces, corresponding to a slightly larger posterior instaneous modulus of 4.10 ± 1.7 kPa (mean ± s.d., n = 18) versus the anterior surface 3.19 ± 1.3 kPa (mean ± s.d., n = 8). Statistical analysis, however, indicated there was not a strong significant difference (p = 0.08).
We have demonstrated that Nanoindentation can provide a spatial map of the properties, which can potentially better explain or model iris behavior. Further study is needed, but these results suggest that the iris dilator and pigment epithelium, which are located on the posterior surface, differs only slightly in stiffness from the iris stroma on the anterior. Dynamic loading and the prospect of poroelastic behavior in the "looser" stromal tissue should be considered, and a slower-rate, equilibrium experiment is necessary.
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