Strain Analysis

In the Cartesian coordinate system, the inferior–superior axis of the ONH was identified based on the typical location of inferior blood vessels, and the strain tensor component matrix, E, was rotated, such that x = nasal–temporal axis, and y = inferior–superior axis. The components of E greater than the resolution of the DVC (see “Digital Volume Correlation”) were E_xx, E_yy, E_xy, and E_zz. The strain tensor was also transformed to the cylindrical coordinate system with the origin at the geometric center of the AL, such that r indicates radial projections from the geometric center of the AL, and θ indicates circumferential projections around the AL. In the cylindrical coordinate system, the components of E greater than the baseline DVC error (see “Digital Volume Correlation”) were E_rr, E_rθ, E_θθ , and E_zz. The specimen-averaged strain refers to the mean of all points in the AL. In addition, the mean of points in the central region and the mean of points in the peripheral region were computed as follows. To divide the AL into central and peripheral regions, an ellipse was drawn at the external limit of AL, and an inner ellipse was drawn with major and minor axes half of the external ellipse. Sensitivity analysis was used to identify the minimum percentage of DVC-correlated points for a region to be included for statistical analysis.