Figure 4. The material properties of the peripapillary sclera are influenced by nonlinearity and collagen fiber orientation (anisotropy).

Separate from its thickness, the behavior of the sclera is governed by its material properties, which in turn are influenced by nonlinearity and fiber orientation. (A) Nonlinearity is an engineering term for tissues or structures whose material properties are altered by loading. Figure A demonstrates that the sclera becomes stiffer as it is loaded uniaxially (in one direction). In the case of the sclera, this is likely due to the fact collagen fibers embedded within the surrounding ground matrix start out crimped and progressively straighten as the load is increased. This conformational change in the fibrils accounts for the transition from an initially compliant, nonlinear response to a stiffened linear response as IOP increases. (B) Apart from nonlinearity, collagen fiber orientation (anisotropy) within the sclera strongly influences its mechanical behavior. Fiber orientation can be totally random (isotropic - not shown) or have a principal direction (anisotropic – 3 idealized cases shown). Finite element (FE) models of an idealized posterior pole with principal collagen fiber orientation in the circumferential, helicoidal, and longitudinal directions are shown. As the displacement plots show, the underlying fiber orientation can have profound effects on the deformation that occurs for a given IOP. Note that the displacement scale is exaggerated for illustrative purposes. (Figure courtesy of Michael Girard)