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S. Jouzdani, J. E. Whitcomb, R. Amini, V. H. Barocas; Detailed Assessment of the Mechanics of the Multilayer Iris by Indentation and Finite Element Analysis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5551.
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To assess the mechanical properties of different constituent components of the iris via indentation testing and a simplified finite-element model.
Indentation and histological analysis were done in combination with a finite element axisymmetric 2D bilayer model using the software ABAQUS. Indentation experiments were done on both surfaces of the porcine iris: anterior to emphasize the stroma, and posterior to emphasize the sphincter and dilator. Effective moduli and viscoelastic parameters were calculated from the experiments and compared to the model. In the model, the dilator and sphincter were represented as neo-Hookean solids, and the stroma was represented as a viscoelastic solid defined by a two-exponential Prony series. An iterative method was applied to identify finite-element model parameters that minimized the difference between the simulation and experimental results.
The experimental posterior indentation gave slightly but significantly higher forces, corresponding to an effective modulus of 5.91 ± 0.56 kPa (mean ± 95%CI, n = 45) versus 3.97 ± 0.48 kPa for the anterior surface. To achieve the same effective moduli in a bilayer finite-element simulation, the anterior portion of the iris (representing the stroma) required modulus 1.25 ± 0.10, and the posterior portion (representing the muscles) required modulus 4.75 ± 0.47. For the anterior-surface indentation, the characteristic relaxation times were 1.58 ± 0.09s and 12.42 ± 0.83 s; for posterior-surface indentation, they were 2.64 ± 0.19 s and 17.76 ± 1.56 s. The fraction of instantaneous response attributable to viscoelasticity was 2.45 ± 1.47 for the anterior-surface indentation and 1.70 ± 0.12 for the posterior-surface indentation (p = 0.04).
The posterior tissues in the iris are stiffer than the anterior tissues; this is taken to mean that the dilator and sphincter are stiffer than the stroma, but it must be noted that our model is a simplification. For example, it might be possible that the sphincter is extremely compliant, but the dilator is very stiff, so the net result is a greater stiffness. The anterior-surface indentation showed slightly more viscoelastic relaxation, which may be due to interstitial flow in the stroma.
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