Peripapillary and Posterior Scleral Mechanics—Part II: Experimental and Inverse Finite Element Characterization

Abstract

Background-The posterior sclera likely plays an important role in the development of glaucoma and accurate characterization of its mechanical properties is needed to understand its impact on the more delicate optic nerve head -the primary site of damage in the disease.

On the Validity of the Imbert-Fick Law: Mathematical Modelling of Eye Pressure Measurement

“…Different workers have made fundamentally varying assumptions about the mechanical behaviour of the cornea; Singh et al [16] assumed it to be anisotropic and linear elastic, whereas Pandolfi and Manganiello [17] assumed it to be both nonlinear and anisotropic. There is experimental evidence for nonlinear mechanical behaviour of both cornea and sclera [14] [17], though it has also been reported that nonlinear effects are not observed in the eye below IOP values of 4 kPa [6] [18]. Measurements show that the sclera is highly anisotropic [19] and this is consistent with the fibrous composite material model studied by Girard et al [18].…”

“…Schulz et al [23] presented tensile results from human sclera and arrived at a modulus value 2.6 ± 2 MPa. Girard et al [18] have deduced modulus values for rhesus monkey sclera using a combined experimental and modelling approach, and found evidence of nonlinear behaviour, with stiffness increasing with IOP. They found the stiffness to be in the range 1.9 -3.5 MPa for an IOP of 10mm Hg, and 3.4 -4.7 MPa at 30 mmHg.…”

“…[6] [15]). However, the sclera is a fibre composite that is anisotropic, with fibres aligned preferentially normal to the eye radius [18], giving a low value of stiffness along a radius. Given the experimental methods employed-the stretching of strips excised from the eye-the modulus values summarised so far relate to circumferential material directions normal to a radius of an ideal spherical eye.…”

“…There is experimental evidence for nonlinear mechanical behaviour of both cornea and sclera [14] [17], though it has also been reported that nonlinear effects are not observed in the eye below IOP values of 4 kPa [6] [18]. Measurements show that the sclera is highly anisotropic [19] and this is consistent with the fibrous composite material model studied by Girard et al [18]. The simplest feasible modelling assumption seems to be that of anisotropic linear elasticity, provided that the IOP values are kept below a 4 kPa (=30 mmHg) upper limit: this is how we will proceed below.…”

Polarization microscopy for characterizing fiber orientation of ocular tissues

“…Hence, to preserve vision, it is important to characterize two elements central to connective tissues, namely the orientation and organization of the collagen fibers that form them [5,6]. Several techniques have been developed for measuring collagen fiber orientation and organization, including small angle light scattering [7,8], x-ray scattering [9][10][11][12], non-linear microscopy [7,13,14] and magnetic resonance imaging [15,16], or for estimating them using inverse numerical methods [17,18]. The complexity of the eye calls for a technique that provides data at multiple scales, including high resolution (micron-scale) and broad field of view (several mm to cm) [19].…”