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research-article

Finite Element Biomechanics of Optic Nerve Sheath Traction in Adduction

[+] Author and Article Information
Andrew Shin

Department of Ophthalmology, Jules Stein Eye Institute
ksshin0711@gmail.com

Lawrence H. Yoo

Department of Ophthalmology, Jules Stein Eye Institute; Intelon Optics Inc., Cambridge, MA
yoolawrence@hotmail.com

Joseph Park

Department of Ophthalmology, Jules Stein Eye Institute; Department of Mechanical Engineering
JPark@jsei.ucla.edu

Joseph L. Demer

Department of Ophthalmology, Jules Stein Eye Institute; Biomedical Engineering Interdepartmental Program; Neuroscience Interdepartmental Program; Department of Neurology, University of California, Los Angeles; Intelon Optics Inc., Cambridge, MA
jld@jsei.ucla.edu

1Corresponding author.

ASME doi:10.1115/1.4037562 History: Received March 28, 2017; Revised July 28, 2017

Abstract

Historical emphasis on increased intraocular pressure (IOP) in glaucoma pathogenesis has been challenged by recognition that many patients lack abnormally elevated IOP. We employed finite element analysis (FEA) to infer contribution to optic neuropathy from tractional deformation of the optic nerve head (ONH) and lamina cribrosa (LC) by extraocular muscle counterforce exerted when optic nerve (ON) redundancy becomes exhausted in adduction. We characterized Young's modulus of fresh adult bovine ON, ON sheath, and peripapillary and peripheral sclera by tensile elongation to failure under physiological temperature and humidity. Physical dimensions of the FEA were scaled to human histological and MRI data, and used to predict stress and strain during adduction 6° beyond ON straightening at multiple levels of IOP. Young's modulus of ON sheath of 44.6±5.6 MPa (SEM) greatly exceeded that of ON at 5.2±0.4, peripapillary sclera at 5.5±0.8, and peripheral sclera at 14.0±2.3 MPa. FEA indicated that adduction induced maximum stress and strain in the temporal ONH. In the temporal LC, maximum stress was 180 kPa, and maximum strain 9-fold larger than produced by IOP elevation to 45 mmHg. Simulation suggests that ON sheath traction by adduction concentrates far greater mechanical stress and strain in the ONH region than does elevated IOP, supporting the novel concept that glaucomatous optic neuropathy may result at least partly from external ON traction, rather than exclusively on pressure on the ON exerted from within the eye.

Copyright (c) 2017 by ASME
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