Modeling the Effect of the Experimentally-Derived Collagen Structure on the Mechanical Anisotropy of the Human Sclera

The sclera is the main load-bearing structure of the eye. It must be sufficiently stiff to maintain the shape and dimensions of the eye under acute elevation of intraocular pressure (IOP). These properties stem from the fiber-reinforced structure of the sclera, which contains dense superimposed lamellae of type I collagen fibrils embedded in matrix of proteoglycans and elastin. Recently, wide-angle X-ray diffraction [1] (WAXS) was used to map the fibrillar arrangement and distribution of collagen over posterior human sclera [2]. The results showed that the peripapillary region, immediately adjacent to the optic nerve head (ONH) had a larger amount of collagen and a circumferential collagen structure. The collagen structure in the mid-posterior region was more heterogeneous. The collagen structure of the sclera directly influences its material stiffness properties and therefore the level of strain transmitted to the tissues of the optic nerve head, which is the primary site of damage in glaucoma. Models inspired from the microstructure are needed to evaluate the contribution of the collagen structure on the mechanical properties. Earlier modeling efforts have treated the sclera as a homogenous, isotropic, linear elastic [3] or hyperelastic material [4, 5]. Girard et al. recently added the effect of the collagen structure using a nonlinear anisotropic model [6]. The authors fit their model for the collagen orientation and distribution to mechanical inflation data of the posterior sclera.