Mechanical characterization of the bovine iris

Quantification of the mechanical properties of the iris is necessary to assess the clinical significance of passive iris deformation, which has been suggested as a mechanism for certain forms of glaucoma. Extension tests were performed on isolated bovine irises to determine the passive mechanical behavior of the iris and the contribution of each of its two constituent muscles, the sphincter iridis and the dilator pupillae, to the overall properties. Because of the shape of the iris and our desire to use intact tissue, a "loop" experiment was performed in which the iris was stretched by hooking the sample and pulling. A simple mathematical model was used to account for the geometry of the experiment and the progressive recruitment of tissue. Radial extension experiments on samples dissected from the iris were also performed. The iris was found to be anisotropic, elastic, and incompressible. The average azimuthal Young's modulus of the sphincter was found to be 340 kPa; the average azimuthal Young's modulus of the dilator was found to be 890 kPa, which was significantly higher (p < 0.01). The radial Young's modulus of the dilator was found to be 9.6 kPa, much lower than the azimuthal value.