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Ahmed Elsheikh, Charles Whitford, Sherif Hassaan, Ashkan Mohammadvali, Ricardo Magalhaes, Craig Boote; Numerical Representation of Collagen Fibril Anisotropic and Density Related Stiffness: The Intact Human Eye Globe. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3713.
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© ARVO (1962-2015); The Authors (2016-present)
Develop reliable numerical representation of regional and directional variation in mechanical stiffness across the whole surface of the human cornea and sclera.
Two intact eyes were tested using an inflation rig previously presented at ARVO; providing relationships between intraocular pressure (IOP) and deformation; while also providing topography maps. The eyes were fixed and analyzed by wide angle X-ray scattering (WAXS) to determine the density, isotropic and anisotropic distribution of collagen fibrils within the stroma. Material laws were developed representing: stroma matrix stiffness; dilation; collagen density; and both isotropic and anisotropic collagen distribution. Bespoke software was developed to analyze the data from WAXS analysis. This software was able to match the experimental data to numerically accurate geometric models of the specific eyes in the form of finite element models (FEM). The software used processes of noise filtration, Zernike surface fitting and spatial interpolation to define microstructure relationships for each element within the FEM. Once geometry and microstructure was numerically defined; mechanical stiffness was obtained through an inverse modelling process. This combined regional IOP-deformation relationships for the specific eyes with the numerical representations.
Results revealed stiffness variation consistent with variation in collagen density and arrangement. Stiffness (measured using tangent modulus) was highest at the limbus in the annular direction where collagen arrangement is responsible for preserving the bi-radial topography. The area around the optic nerve was also found to have relatively high annular stiffness able to restrain excessive deformation of the lamina cribrosa despite the low stiffness of this tissue. Lower stiffness was observed in the peripheral corneas and the posterior sclera.
Representing collagen arrangement from WAXS within FEMs has produced an important advance in accurate simulations of global and microstructure behavior of the intact human eye globe. For the first time it has been possible to compare the effects of microstructural arrangement in both the cornea and sclera for the same eye under the same IOP and IOP history. These advances are expected to lead to improvement in developments of diagnostic and treatment processes for numerous degenerative ocular conditions.
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