Computational Mechanics of the Sclera and Optic Nerve Head (ONH): Effects of ONH Size and Pressure Range

2008 ◽  
Author(s):  
Lewis Waldman ◽  
Crystal Cunanan ◽  
Sanjay Asrani ◽  
Roy Kerckhoffs ◽  
Andrew McCulloch
Keyword(s):  
Optic Nerve ◽  
Optic Nerve Head ◽  
Material Properties ◽  
Pressure Range ◽  
High Pressures ◽  
Strain Differences ◽  
Temporal Variations ◽  
Nonlinear Material ◽  
Stress And Strain ◽  
Strain Components

Computational modeling was performed to study how loss of compliance of the eye and abnormally high pressures result in changes in stresses and strains that may impact the optic nerve in diseases such as glaucoma. Hemispherical finite element models of the eye were created in which scleral thickness varied from the equatorial region to the optic nerve head (ONH). Nonhomogeneous material properties were used to model the ONH as a continuous region softer than the adjacent sclera. The ONH and an adjacent buffer zone in the sclera were modeled with enough detail that the size of the ONH could be changed to account for variations observed in humans. The model was provided with appropriate dimensions typical of patients and nonlinear material properties with decreased compliance. Models with different ONH sizes were inflated in small steps to 55 mmHg (7.33 kPa), providing deformed configurations at intermediate pressures of 15, 30 and 45 mmHg, respectively. Color-coded maps of stress and strain components were rendered directly on deformed configurations of the eye model; and animations were produced that show both spatial and temporal variations of stresses and strains as internal pressure increases. Three-dimensional stresses and accompanying finite strains were similar for ONH sizes ranging form 1.5 to 2.5 mm in diameter. Stress and strain differences were estimated as pressure was increased from 15 to 25 mmHg, 30 to 40 mmHg, and 45 to 55 mmHg. Substantial changes occurred in stress and strain differences as the pressure range was varied with large changes occurring in the lowest pressure range for strain components and moderate increases in stress differences as pressures increase.

The Influence of Material Properties and Geometry on Optic Nerve Head Biomechanics

2009 ◽  
Author(s):  
Hongli Yang ◽  
Ian A. Sigal ◽  
Michael D. Roberts ◽  
Claude F. Burgoyne ◽  
J. Crawford Downs
Keyword(s):  
Optic Nerve ◽  
Optic Nerve Head ◽  
Material Properties ◽  
Lamina Cribrosa ◽  
Stress And Strain ◽  
Retinal Ganglion ◽  
Biological Mechanisms ◽  
The World ◽  
Related Stress ◽  
Pass Through

Glaucoma is the second leading cause of irreversible blindness in the world. The biological mechanisms of this disease are not well understood, and the factors contributing to its progression are not well characterized. It is generally accepted that the retinal ganglion cell axons are damaged in glaucoma as they pass through the optic nerve head (ONH). The ONH contains the lamina cribrosa (LC), a fenestrated connective tissue network that spans the scleral canal, through which the axons pass as they leave the eye. It has been postulated that intraocular pressure (IOP)-related stress and strain plays a central role in the pathophysiology of glaucoma [1], so the study of LC biomechanics is important in developing an understanding of the disease.

scholarly journals Viscoelastic Characterization of Peripapillary Sclera: Material Properties by Quadrant in Rabbit and Monkey Eyes

2003 ◽  
Vol 125 (1) ◽  
pp. 124-131 ◽  
Author(s):  
J. Crawford Downs ◽  
J-K. Francis Suh ◽  
Kevin A. Thomas ◽  
Anthony J. Bellezza ◽  
Claude F. Burgoyne ◽  
...  
Keyword(s):  
Optic Nerve ◽  
Viscoelastic Material ◽  
Optic Nerve Head ◽  
Material Properties ◽  
Uniaxial Stress ◽  
Species Group ◽  
Linear Viscoelastic ◽  
Viscoelastic Theory ◽  
Linear Viscoelastic Theory

In this report we characterize the viscoelastic material properties of peripapillary sclera from the four quadrants surrounding the optic nerve head in both rabbit and monkey eyes. Scleral tensile specimens harvested from each quadrant were subjected to uniaxial stress relaxation and tensile ramp to failure tests. Linear viscoelastic theory, coupled with a spectral reduced relaxation function, was employed to characterize the viscoelastic properties of the tissues. We detected no differences in the stress-strain curves of specimens from the four quadrants surrounding the optic nerve head (ONH) below a strain of 4 percent in either the rabbit or monkey. While the peripapillary sclera from monkey eyes is significantly stiffer (both instantaneously and in equilibrium) and relaxes more slowly than that from rabbits, we detected no differences in the viscoelastic material properties (tested at strains of 0–1 percent) of sclera from the four quadrants surrounding the ONH within either species group.

An Analytical Model for the Unbonded Flexible Pipe Stress Analysis With Consideration of Nonlinear Material Properties for Metal Layers

2010 ◽  
Author(s):  
Jian Liu ◽  
Zhimin Tan ◽  
Terry Sheldrake
Keyword(s):  
Stress Analysis ◽  
Analytical Model ◽  
Material Properties ◽  
Failure Modes ◽  
Nonlinear Material ◽  
Von Mises Stress ◽  
Stress And Strain ◽  
Flexible Pipe ◽  
Metal Layers ◽  
Plastic Stress

This paper presents an improved analytical model for the unbonded flexible pipe stress analysis with consideration of nonlinear material properties for metal layers. Analytical methods have often been used to analyse the stress and strain of flexible pipe systems because of their low cost and efficiency compared with detailed finite element modeling. Most of these kinds of models only consider the deformation of pipes within the elastic region. Such linear models can not be used directly to assess pipe failure modes such as the pipe burst strength, where the nonlinearity of the metallic material plays an important role in governing the pipe deformation and pipe structural capacity. The improved analytical model presented in this paper has fully considered the nonlinearity of metal layers such as the pressure armour and tensile armour layers because of their importance in resisting internal pressure and tension loads. Non-associative elasto-plastic stress strain curves obtained from experiments are used to simulate the metal layers. Von Mises stress is adopted in the model as the yield criterion of the metal layers. Radial return method (Simo and Taylor 1985 [1], Simo and Hughes 1998 [2]) is used to solve the plastic stress and strain of metal layers beyond the yield point. Due to its high nonlinearity from both system equations and material properties, Newton-Raphson method is adopted in the model as the solving method. The proposed study here considers tension, torque and pressure loads only for a straight pipe. The model predictions have been compared against measurements from Wellstream burst tests and failure tension tests performed over the full scale pipe samples. The prediction and experiment results agree.

scholarly journals Integrins in Trabecular Meshwork and Optic Nerve Head: Possible Association with the Pathogenesis of Glaucoma

2013 ◽  
Vol 2013 ◽  
pp. 1-8 ◽  
Author(s):  
Yisheng Zhong ◽  
Jing Wang ◽  
Xunda Luo
Keyword(s):  
Optic Nerve ◽  
Optic Nerve Head ◽  
Trabecular Meshwork ◽  
Intracellular Signaling ◽  
Open Angle Glaucoma ◽  
Glaucomatous Optic Neuropathy ◽  
Stress And Strain ◽  
Membrane Spanning

Integrins are a family of membrane-spanning proteins that are important receptors for cell adhesion to extracellular matrix proteins. They also provide connections between the extracellular environment and intracellular cytoskeletons and are responsible for activation of many intracellular signaling pathways. In vitro and in vivo data strongly indicate that integrin-mediated signaling events can modulate the organization of the actin cytoskeleton in trabecular meshwork (TM) cells and are associated with astrocyte migration and microglia activation of the optic nerve head in patients with primary open angle glaucoma. Consequently, increase in resistance in the TM outflow pathways and remodeling of the optic nerve head occur, which in turn increases intraocular pressure (IOP), adds additional mechanical stress and strain to optic nerve axons, and accelerates damage of axons initially caused by optic nerve head remodeling. Integrins appear to be ideal candidates for translating physical stress and strain into cellular responses known to occur in glaucomatous optic neuropathy.

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