Scleral anisotropy and its effects on the optic nerve head biomechanics

Abstract

 

Purpose

 

To determine the effects of the collagen fiber structure of the sclera on the mechanical response of the sclera and the optic nerve head (ONH)

 

Methods

 

Specimen-specific inverse finite element models were developed to calculate the material properties of two normal human sclera (age 67, 71) subjected to inflation testing. A distributed fiber model incorporating wide-angle X-ray scattering (WAXS) measurements of the collagen structure was applied to describe the anisotropic elastic behavior of the sclera. The material parameters were used for micromechanical studies of the mechanical anisotropy at various length scales (0.5, 2, and 4 mm). The effects of the sclera fiber structure on the ONH mechanical response were evaluated by progressively filtering out local anisotropic features (Fig. 2).

 

Results

 

The collagen structure of the midposterior sclera (MPS) showed large variations in the preferred fiber direction and degree of fiber alignment. The mechanical anisotropy of the MPS decreased with increasing length scale (Fig. 1). At the 4 mm length scale, the MPS was nearly isotropic. The anisotropy of the MPS had little effect on the mechanical response of the ONH. Modeling 71% of the MPS as isotropic caused minimal changes (< 1%) to the scleral canal expansion and posterior lamina bowing (Fig. 2, second row). In the peripapillary sclera (PPS), collagen fibers were circumferentially aligned with spatially heterogeneous degree of alignment. The deformation of the ONH was acutely sensitive to the spatially heterogeneous anisotropy of the PPS.

 

Conclusions

 

Alterations in the degree of fiber alignment within the PPS observed in glaucoma eyes (Pijanka et al., IOVS, 2012) may significantly impact the biomechanical environment of the ONH.

 

 

Map of the anisotropic ratio at the 0.5 mm (a) and the 2 mm (b) length scale. The anisotropic ratio (AR) was calculated in finite element (FE) simulations of the strain response to an applied 21 kPa equibiaxial tension for a model of square section. AR=1 for an isotropic tissue.

 

 

First row: A white square represents a WAXS measurement for which the anisotropic ratio was below the threshold indicated on top of the figure. The corresponding area of the sclera was modeled as isotropic in the FE model of an inflation at 30 mmHg. Second row: Evolution of the posterior deformation of the LC and scleral canal expansion as the sclera becomes more isotropic. The star corresponds to the isotropic model.

 

Keywords: 708 sclera • 568 intraocular pressure