The Structural Layers of the Porcine Iris Exhibit Inherently Different Biomechanical Properties

PURPOSE. The purpose of this study was to isolate the structural components of the ex vivo porcine iris tissue and to determine their biomechanical properties. METHODS. The porcine stroma and dilator tissues were separated, and their dimensions were assessed using optical coherence tomography (OCT). The stroma underwent flow test (n = 32) to evaluate for permeability using Darcy's Law ( Delta P = 2000 Pa, A = 0.0391 mm(2)), and both tissues underwent stress relaxation experiments ( epsilon = 0.5 with initial ramp of delta epsilon = 0.1) to evaluate for their viscoelastic behaviours (n = 28). Viscoelasticity was characterized by the parameters beta (half width of the Gaussian distribution), tm (mean relaxation time constant), E-0 (instantaneous modulus), and E8 (equilibrium modulus). RESULTS. For the stroma, the hydraulic permeability was 9.49 +/- 3.05 x 10(-6) mm(2)/Pa center dot s, and the viscoelastic parameters were beta = 2.50 +/- 1.40, and iota(m) = 7.43 +/- 4.96 s, with the 2 moduli calculated to be E-0 = 14.14 +/- 6.44 kPa and E-infinity = 6.08 +/- 2.74 kPa. For the dilator tissue, the viscoelastic parameters were beta = 2.06 +/- 1.33 and iota(m) = 1.28 +/- 1.27 seconds, with the 2 moduli calculated to be E-0 = 9.16 +/- 3.03 kPa and E-infinity = 5.54 +/- 1.98 kPa. CONCLUSIONS. We have established a new protocol to evaluate the biomechanical properties of the structural layers of the iris. Overall, the stroma was permeable and exhibited smaller moduli than those of the dilator muscle. An improved characterization of iris biomechanics may form the basis to further our understanding of angle closure glaucoma.