Mechanical characterization of the bovine iris

Area of Science:

• Ophthalmology
• Biomechanics
• Materials Science

Background:

• Passive iris deformation is implicated in certain forms of glaucoma.
• Quantifying iris mechanical properties is essential for understanding its clinical significance.
• The iris comprises two main muscles: the sphincter iridis and the dilator pupillae.

Purpose of the Study:

• To determine the passive mechanical behavior of the iris.
• To quantify the contribution of the sphincter iridis and dilator pupillae muscles to iris mechanics.
• To assess the anisotropy and elasticity of iris tissue.

Main Methods:

• Extension tests performed on isolated bovine irises.
• A "loop" experiment utilized intact iris tissue, stretched by hooking and pulling.
• Radial extension experiments conducted on dissected iris samples.
• A mathematical model was employed to account for experimental geometry and tissue recruitment.

Main Results:

• The iris exhibits anisotropic, elastic, and incompressible properties.
• The average azimuthal Young's modulus of the sphincter iridis was 340 kPa.
• The average azimuthal Young's modulus of the dilator pupillae was 890 kPa (significantly higher, p < 0.01).
• The radial Young's modulus of the dilator pupillae (9.6 kPa) was substantially lower than its azimuthal value.

Conclusions:

• The dilator pupillae muscle is significantly stiffer in the azimuthal direction compared to the sphincter iridis.
• Iris tissue exhibits directional mechanical properties, with radial stiffness being much lower than azimuthal stiffness.
• These findings provide crucial data for understanding iris biomechanics and its role in ocular conditions like glaucoma.