The mechanisms that control the iris are remarkable as it routinely undergoes large deformations during pupil constriction and dilation. The iris is a complex tissue embedded in a mechanically active environment and as the iris deforms the contour of the iris changes, which may have a negative clinical impact if the contour is abnormal. In considering the contour of the iris, it is imperative that one considers the internal forces, which arise from the (visco)elastic response of the passive and active components within the iris and the external forces, which arise form the aqueous humor flow in the anterior segment. From a mechanical standpoint, this tissue is very interesting because it is anisotropic, inhomogeneous, and composed of mechanically varying tissues, yet very little has been studied about the iris. The main purpose of this research was to examine how the detailed structures within the iris may affect the iris contour. Mechanical tests and finite element simulations were developed to evaluate the iris by looking at: the active and passive mechanics, the detailed anatomical architecture within the iris (e.g. sphincter, dilator, and stroma), and how the posterior location of the active muscle (dilator) affects the overall curvature.

The active-passive mechanical tests of the porcine iris revealed that an increase of the effective moduli (2-3 times) occurred when either the sphincter (constriction) or dilator (dilation) muscle was activated with pupil-active drugs. Nanoindentation of the iris revealed significant difference between the effective moduli for the anterior and posterior surface indentations, concluding that the posterior components of the iris–the dilator, the pigment epithelial cells, and perhaps the sphincter–are stiffer than the anterior border layer and stroma. Further indentations tests and FEM simulations found that the iris not only varies between the surfaces but throughout its depth. It was found that the dilator and sphincter (posterior layers) were 12-14 times more stiff than the compliant stroma (anterior layer). Our results suggest that the stroma, the largest structure within the iris, plays a small role in the overall structural stiffness. Finally, we concluded that posterior location of the dilator within the iris can cause anterior bowing even in the absence of external pressure from the aqueous humor flow. This suggests the anatomical architecture of the iris has an influence on non-pupillary-block-dependent mechanisms. The main conclusions drawn from this work is that the material properties that define the iris are more complex then previously concluded. The detailed anatomical structure of the iris affects both its passive and active behavior.