Sequential-Digital Image Correlation for Mapping Human Posterior Sclera and Optic Nerve Head Deformation

Abstract

Optic nerve head (ONH) deformations may be involved in the onset or further development of glaucoma, including in patients with relatively normal intraocular pressures (IOPs). Characterizing posterior scleral deformations over physiological pressures may provide a better understanding of how changes in IOP lead to changes in the mechanical environment of the ONH and possibly retinal ganglion cell death. Pressure inflation measurement test protocols are commonly used to measure deformation of the peripapillary sclera with full-field noncontact optical methods. The purpose of this work was to develop and validate a new sequential 3D digital image correlation (S-DIC) approach for quantification of posterior scleral pressure induced deformation that improves z (in-depth) resolution of the DIC measurement without losing in-plane sensitivity, while also being able to contour and map deformations of the complex-shaped ONH. Our approach combines two orthogonal axes of parallax with standard 3D DIC methods using a single high-resolution camera. The enhanced capabilities of S-DIC with respect to standard 3D DIC has been demonstrated by carrying out a complete benchmark for shape, deformation, and strain measurement on an object of known complex geometry. Our S-DIC method provided a reconstruction accuracy of 0.17% and an uncertainty in z-position measurement of 8 μm. The developed methodology has also been applied to a human posterior scleral shell, including the full peripapillary sclera and optic nerve. The relatively inexpensive S-DIC approach may provide new information on the biomechanical deformations of the optic nerve head and, thus, the death of retinal ganglion cells in primary open angle glaucoma.

The effects of glycosaminoglycan degradation on the mechanical behavior of the posterior porcine sclera

“…These areas are a small proportion of the sclera and their effect was minimized by averaging the strains over a whole quadrant of the sclera. The sclera is known to exhibit large local variations in strains during inflation testing [20,65,66] and averaging the strains over a large region does not allow the measurement of the effects of s-GAG removal on local variations of the mechanical behavior. The spherical thin-walled membrane model was used for the stress analysis.…”

Biomechanical assessment in models of glaucomatous optic neuropathy

“…Girard et al (2009a) used 5 mmHg for a baseline pressure for the posterior cup of monkey eyes, while Coudrillier et al (2012) used a lower 1.5 mmHg baseline pressure for the thicker human posterior sclera. A number of other works have also used 5 mmHg as the reference pressure for the human sclera (Fazio et al, 2012b; Pyne et al, 2014; Tang and Liu, 2012). …”

“…To overcome this limitation, Pyne et al (2014) developed the sequential digital image correlation method that used a single stationary camera to image a specimen mounted on a stage that can rotate 90° about 2 orthogonal axes. In the method, the specimen is subjected to a sequence of rotations about the 2 axes.…”

Quantitative confocal optical coherence elastography for evaluating biomechanics of optic nerve head using Lamb wave model

“…12 Characterizing elasticity of the ONH over physiological pressures may provide a better understanding of how changes in IOP lead to changes in the mechanical environment of the ONH. [13][14][15] Optical coherence elastography (OCE) is an emerging technique, which can detect tissue biomechanics noninvasively with both high temporal and spatial resolution compared with conventional ultrasonic elastography. 16 The axial displacement range is typically in the micrometer to nanometer level.…”