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Dan Midgett, Thao D Nguyen, Harry A Quigley, Mary Ellen Pease, Christian Franck, Mohak Patel; Regional Variations in the Strains of the Human Optic Nerve Head. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3564.
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© ARVO (1962-2015); The Authors (2016-present)
The mechanical behavior of the lamina cribrosa (LC) plays an important role in the development glaucomatous optic neuropathy. We developed an in vitro inflation method to measure global and regional LC strains caused by controlled pressurization.
Six human eyes from 5 donors (ages 26-71) obtained within 48 hours post-mortem had the choroid and retina removed, the optic nerve removed at the myelin line, and the posterior sclera mounted on a custom inflation chamber. The pressure was raised to 5, 10, and 45 mmHg. A Zeiss LSM 710 NLO microscope, tuned to 790nm with a band pass 390-410nm filter, was used to acquire two duplicate stacks of the LC structure using second harmonic generation (SHG) imaging at each pressure. The SHG stacks were treated with Huygens Essential deconvolution algorithm using a theoretical point spread function (SVI) and local adaptive histogram equalization (FIJI) to enhance contrast. The Fast Iterative Digital Volume Correlation (DVC) algorithm (Bar-Kochba et al. 2014) was used to post-process the SHG stacks to calculate the 3D displacement fields and correlation error. The correlation coefficient was used to remove poorly correlating subsets and Gaussian filtering was used to remove local error spots. Strains were calculated by fitting a high order polynomial function to the displacement field and taking the gradient. The LC was divided into 4 quadrants surrounding the central retinal artery and vein (inferior-nasal (IS), inferior-temporal (IT), superior-nasal (SN), superior-temporal (ST). The principal strains were averaged and analyzed for regional differences.
Displacements and strains in the LC exhibited substantial variation in the plane but not through the thickness (Figure 1). The average shear strains (in plane: -0.15±0.14%, out-of-plane: -0.22±0.23%, 0.19±0.34%) were significantly smaller than the normal strains. For nearly all eyes, the strain at 45mmHg along the inferior-superior axis (3.02±1.09%) was larger than along the nasal-temporal axis(2.03±0.75%). The maximum principal strain was largest in the IT quadrant and smallest in the IN quadrant. In paired t-tests, the maximum principal strain in IT was significantly larger than in IN (p=0.026) and nearly significantly larger than in SN (p=0.067) and IT (p=0.076).
Regional variations in pressure-derived strains in the LC may help to explain the progression of axonal damage in glaucoma.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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