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Elias Pavlatos, Xueliang Pan, Richard T Hart, Paul A. Weber, Jun Liu; ONH Deformation in Porcine Eyes Using Ultrasound Speckle Tracking. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3568.
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© ARVO (1962-2015); The Authors (2016-present)
The mechanical environment of the optic nerve head (ONH) is thought to play an important role in the onset of glaucoma. This study aims to map and quantify the strains within the ONH in response to intraocular pressure (IOP) elevation.
Ten porcine globes were tested within 72 hours postmortem. The optic nerve was trimmed to the outer surface of the peripapillary sclera. A portion of the cornea was removed using a 7.5mm trephine along with the intraocular contents, and the ocular shell was mounted using a custom-built pressurization chamber. The chamber was connected to a programmable syringe pump (PHD Ultra, Harvard Apparatus) and pressure sensor (P75, Harvard Apparatus) to control and monitor IOP. Preconditioning with 20 pressure cycles from 5 to 30 mmHg was followed by equilibration at 5 mmHg for 30 minutes. The globes were inflated by increasing IOP from 5 to 30 mmHg with 0.5 mmHg steps every 15 seconds. 2D cross-sections of radiofrequency data were obtained along the nasal-temporal meridian of the ONH at each pressure step. An ultrasound speckle tracking algorithm was used to calculate the strains within the scanned cross-section (Tang & Liu, J Biomech Eng 2012, 134(9)). The ONH was divided into two equal thickness layers to compare the anterior and posterior response (Fig 1a). Histology was obtained for one eye using periodic acid-Schiff staining (Fig 1b).
The average through-thickness and in-plane strains for all ten eyes at 30 mmHg were -0.052 ± 0.014 and 0.018 ± 0.008 respectively. The through-thickness compressive strains in the anterior ONH were 4.6 times higher than in the posterior ONH at 30 mmHg (0.087 vs. 0.019, p<0.001) while the in-plane tensile strains were slightly but significantly lower in the anterior ONH (0.015 vs. 0.022; p=0.006). The same trend was observed at 15 mmHg (Fig 2).
The anterior ONH (largely the prelaminar neural tissue) appeared to experience large compression during IOP elevation. The more posterior region had significantly reduced compression, likely due to the structural support of the lamina cribrosa. Experimental characterization of ONH deformation will help to better understand the biomechanical etiologies of glaucoma.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Fig 1. B-mode ultrasound image (a) and histological section of porcine ONH (b).
Fig 2. Through-thickness (TT) compressive strains and in-plane (IP) tensile strains in the ONH of porcine eyes (n=10) and strain maps from one eye at 30 mmHg.
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