Glaucoma-related Changes in the Mechanical Properties and Collagen Micro-architecture of the Human Sclera.

The biomechanical behavior of the sclera determines the level of mechanical insult from intraocular pressure to the axons and tissues of the optic nerve head, as is of interest in glaucoma. In this study, we measure the collagen fiber structure and the strain response, and estimate the material properties of glaucomatous and normal human donor scleras.Twenty-two posterior scleras from normal and diagnosed glaucoma donors were obtained from an eyebank. Optic nerve cross-sections were graded to determine the presence of axon loss. The specimens were subjected to pressure-controlled inflation testing. Full-field displacement maps were measured by digital image correlation (DIC) and spatially differentiated to compute surface strains. Maps of the collagen fiber structure across the posterior sclera of each inflated specimen were obtained using synchrotron wide-angle X-ray scattering (WAXS). Finite element (FE) models of the posterior scleras, incorporating a specimen-specific representation of the collagen structure, were constructed from the DIC-measured geometry. An inverse finite element analysis was developed to estimate the stiffness of the collagen fiber and inter-fiber matrix.The differences between glaucoma and non-glaucoma eyes were small in magnitude. Sectorial variations of degree of fiber alignment and peripapillary scleral strain significantly differed between normal and diagnosed glaucoma specimens. Meridional strains were on average larger in diagnosed glaucoma eyes compared with normal specimens. Non-glaucoma specimens had on average the lowest matrix and fiber stiffness, followed by undamaged glaucoma eyes, and damaged glaucoma eyes but the differences in stiffness were not significant.The observed biomechanical and microstructural changes could be the result of tissue remodeling occuring in glaucoma and are likely to alter the mechanical environment of the optic nerve head and contribute to axonal damage

Matrix modulus <i>μ</i>, parameters of the exponential fiber model (<i>α</i>, <i>β</i>) and fiber stiffness (4<i>αβ</i>) obtained by global optimization.

<p>On average, both matrix and fiber stiffness were larger for the damaged glaucoma group compared to the undamaged glaucoma group and larger for the undamaged glaucoma group compared to the normal group. Grade 0 represents ≤ 10% axon loss, 1 means 10–25% axon loss, 2 means 25–50%, and 3 means 50–75% axon loss.</p

Donor information for the normal and diagnosed glaucoma scleras subjected to inflation testing and WAXS measurement of the collagen fiber structure.

<p>Grade 0 corresponded to an optic nerve with less than 10% of axon loss (normal appearance), grade 1 was 10% to 25% axon loss (mild damage), grade 2 was 25% to 50% axon loss (intermediate damage), and grade 3 was 50% to 75% axon loss (severe damage). In the specimen name, F stands for female, M for male, C for Caucasian, AA for African American, r for right, and l for left. Left/right eyes from the same donor are indicated with the same symbol in the specimen name.</p

Material parameters.

<p>Box plot representing the matrix stiffness (a) and fiber stiffness (b) for normal, undamaged glaucoma, and damaged glaucoma specimens. Individual specimens are represented with a dot.</p

Inflation strain response.

<p>a) Pressure/average circumferential strain in the peripapillary sclera, b) pressure/average meridional strain in the peripapillary sclera.</p

Finite strain calculation.

<p>a) Schematic of the scleral surface at the baseline pressure of 1.5 mmHg. The Z-positions of the surface were interpolated at the points of a polar grid, which was defined as a replacement of the DIC cartesian grid. The vector <b>AB</b> is along the circumferential direction, and the vector is <b>AC</b> is along the meridional direction. b) Schematic of the scleral surface in the pressurized configuration. The normal surface strains in the circumferential and meridional directions were calculated from the elongation of the vector <b>AB</b>, and <b>AC</b>, respectively.</p

Results of the general linear mixed model for the peripapillary circumferential and meridional strains with glaucoma diagnosis, peripapillary scleral quadrant, and diabetes history as explanatory variables.

<p>The strains were calculated at 22.5 mmHg. For the two strain components, the variograms indicated that the measurements for a specimen were not spatially correlated. The 74 measurements from each specimen were assumed to have a compound symmetry correlation structure, in which any two measurements have the same correlation regardless of spatial location. S stands for superior, N for nasal, I for inferior, and T for temporal.</p

Results of the general linear mixed model with spatial autocorrelation for the degree of fiber alignment in the peripapillary sclera with glaucoma diagnosis, quadrant, and diabetes history as explanatory variables.

<p>The variogram for the degree of fiber alignment increased and then leveled off as distance between 2 measurements of a same specimen increased, indicating spatial autocorrelation. S stands for superior, N for nasal, I for inferior, and T for temporal.</p

Strain profile in the peripapillary sclera.

<p>Regional variations in circumferential (top row) and meridional (bottom row) peripapillary scleral strains. Strains were calculated at 22 mmHg. Plotted is the mean (central line) and the 95% confidence intervals for the true mean (shaded regions). S stands for superior, N for nasal, I for inferior, and T for temporal. Significant differences in strain magnitude between two quadrants are indicated by connecting brackets (adjusted <i>p</i> ≤ 0.05).</p

Collagen fiber alignment.

<p>Box plot of the average degree of fiber alignment in each of the 4 quadrants of the peripapillary sclera for normal, undamaged glaucoma, and damaged glaucoma specimens. The degree of fiber alignment was defined from the WAXS scatter intensity as the ratio of the aligned scatter to the total scatter (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0131396#pone.0131396.g003" target="_blank">Fig 3</a>). A scatter plot is superimposed to represent individual data points. Significant differences in degree of fiber alignment between two quadrants are indicated by connecting brackets (adjusted <i>p</i> ≤ 0.05).</p