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Andrew Soltisz, Marissa N Ruzga, Matthew Aaron Reilly, Katelyn E Swindle-Reilly; Spatial Variations in Optic Nerve Mechanical Properties. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4317.
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© ARVO (1962-2015); The Authors (2016-present)
Finite element modeling is a powerful tool which can be used to elucidate the mechanisms responsible for optic nerve injury in glaucoma or trauma. These models require an accurate representation of the tissue’s geometry and mechanical properties to produce meaningful results. It is common for researchers to assume that the mechanical properties of the optic nerve are spatially and directionally homogeneous. The purpose of this study is to investigate the validity these assumptions, starting with variations in nerve shear modulus along its length.
Whole bovine eyes were acquired from an abattoir and tested within 4 hours postmortem. The optic nerve was separated from the globe, and the sheath removed using a scalpel blade. Samples of the nerve were taken immediately proximal to the globe, 1 cm from the globe (mid), and 2-3 cm distal from the globe. Using a shear plate rheometer, 2 mm segments from these regions were subjected to 500% strain at a constant strain rate of 0.05s-1 while measuring the stress response. Shear modulus was calculated as the slope of the linear region of the stress strain plots which occurred below 20% strain.
Nerve samples from all regions began to mechanically fail at approximately 20% strain, with the proximal, mid, and distal regions initiating failure at 595±108, 1169±445, and 409±251 Pa respectively (Fig1). Proximal and distal nerve segments were relatively less stiff than samples from the middle of the nerve. Proximal and distal tissues had shear moduli of 22 and 31 Pa respectively, while the middle of the nerve had a modulus of 65 Pa.
This is the first study to investigate spatial variations in mechanical properties of the optic nerve. Preliminary results suggest that stiffness of the tissues which comprise the optic nerve vary along its length, but failure strain remains consistent. This knowledge can be used to inform better models of optic nerve injury and guide strategies for optic nerve repair. Future work will include porcine nerve testing as well as evaluation of anisotropy of tissue mechanical properties with the end of constructing a high fidelity finite element model of the optic nerve to investigate mechanisms of glaucoma or traumatic nerve injury.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
Stress vs strain graph of bovine optic nerve segments from 3 different regions along the nerve’s length. Each point is the average of 4 samples – both eyes from two animals.
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