Ocular Tissues Biomechanics

The transparent cornea and opaque sclera together form the tough outer layer of the eye-wall.  These tissues serve a multitudes of function.  The sclera protects the more delicate internal components of the eye, such as the lens, from external injuries and provides mechanical support to the retina. The tissue also functions to maintain an optimal shape for vision in the presence of physiological fluctuations in the intraocular pressure. Myopia and hyperopia, also known as nearsightedness and farsightedness, are caused by axial lengthening and shortening of the sclera.  The mechanical properties of the cornea and sclera arise from the anisotropic fibrous microstructure of the stroma, which contains densely stacked lamellae primarily of type I collagen fibrils  embedded in a  hydrated matrix of proteoglycans.

 Collaborators

  • Kristin Myers, PhD -Assistant Professor, Department of Mechanical Engineering, Columbia University
  • Craig Boote, PhD – Lecturer, School of Optometry and Vision Sciences, Cardiff University
  • Harry A.  Quigley, MD – A. Edward Maumenee Professor of Ophthalmology, Director, Glaucoma Center of Excellence, Johns Hopkins Medical School

Biomechanics of the Human Sclera

droppedImagedroppedImage_2The sclera is a dynamic biological tissue that undergoes physiological changes with age and pathological changes with diseases,  such as  glaucoma.  Studies have shown that the human and monkey sclera stiffen and creep more slowly with age.  Glaucoma  is a blinding disease characterized by progressive degeneration of the axons of retinal ganglion cells. The degree of fiber alignment is significantly lower in the superior-temporal and inferior-nasal quadrants in the peripapillary region, immediately adjacent to the optic nerve head,  of glaucoma eyes.  Uniaxial strip tests and inflation tests of the sclera consistently measure a stiffer mechanical response and altered viscoelastic properties for glaucoma eyes in humans and animal models of the disease.  Dramatic alterations in the mechanical behavior and structure have also been measured in myopia, where the sclera thins and elongates in response to the quality of the image focus on the retina. The  sclera of induced-myopia chick and tree shrew eyes are significantly more compliant and exhibit faster creep than the sclera of contralateral control eyes.

Our research aims are:

    • Measure the mechanical properties and collagen structure of the human posterior sclera using inflation test with digital image correlation (DIC) and wide angle x-ray scattering (WAXS).
    • Compare the effects of age and glaucoma.
    • Determine how the extracellular matrix structure influence the mechanical properties
    • Develop microstruture-based constitutive model for the sclera that incorporates explicitly the WAXS measurements of collagen structure
    • Computationally model he effect age and disease induced variation in scleral properties and structure on the deformation of the tissues of the optic nerve head
    • Develop micromechanism-based constitutive models for the growth and remodeling of the tissues in response to mechanical loadi

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Biomechanics of the Mouse Sclera

droppedImage_1Experiments have shown that the sclera of human glaucoma eyes show measurable and statistically significant differences in the mechanical properties and collagen structure from that of age-match glaucoma eyes.  Whether these differences are natural variations that predisposes an individual to the development of glaucoma damage or the product of tissue remodeling induced by glaucoma damage cannot be determined in cross-sectional study of human eyes.  To understand the effect of these material variations on the development of glaucoma, we are using mouse models to study the relationship between the collagen/elastin structure and the mechanical properties of the sclera and their associations with glaucomatous optic neuropathy.  Mouse models allows for a longitudinal study of the effects of scleral biomechanics on the development of glaucoma damage.  Furthermore, they provide important opportunities to manipulate the composition and structure of the ocular connective tissues by chemically treating the animals in vivo or by gene knockout techniques. 

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Our research aims are

  • Measure the mechanical properties and collagen structure of the mouse posterior sclera using inflation test with digital image correlation (DIC) and wide angle x-ray scattering (WAXS).   
  • Compare the effects of induced chronic IOP elevation on mechanical properties of the sclera and optic nerve damage.
  • Perform experiments on multiple mouse strains to compare the effects of baseline properties on scleral remodeling and optic nerve damage.
  • Develop micromechanism-based constitutive models for the growth and remodeling of the tissues in response to mechanical loading.

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Biomechanics of the Cornea

A unique combination of mechanical strength, stiffness, and optical transparency enables the cornea to serve as both a protective barrier and the primary refractive component of the eye. These properties are derived from the fibrous microstructure of the corneal stroma, which in humans constitutes 90% of the cornea thickness. The stroma is formed by approximately 200 lamellar sheets of collagen fibrils embedded in a hydrated matrix of proteoglycans, glycoproteins, and keratocytes

The project seeks to develop experiments to characterize the collagen structure and mechanical properties of the cornea.  These will be used to develop constitutive models for the anisotropic nonlinear viscoelastic behavior that incorporate the details of the collagen structure.  The goal of the project is to investigate the relationship between the collagen structure, mechanical properties, and physiological biomechanical function of the cornea and its alteration with the development of diseases like keratoconus.

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Biomechanics of the Optic Nerve Head

lamina The optic nerve head describes the opening in the posterior sclera through which the axons of the retinal ganglion cells exit the eye.  The main connective tissue structure of the optic nerve head is the lamina cribrosa,  delicate lacework of collagen and elastin beams that guides the retinal ganglion cell axons out of the optic nerve head and structurally supports the nourishing capillaries, astrocytes, lamina cribrosa cells.  Mechanically, the optic nerve head acts as weak spot in posterior eye-wall.  Increases in pressure will cause the scleral canal opening to widen circumferentially and the lamina cribrosa to bow posteriorly. The degree of posterior bowing to circumferential opening is determined by the material properties of sclera and lamina cribrosa. It is hypothesized that the resulting strain and stress state in the lamina cribrosa can cause axon dysfunction and death in glaucoma both directly or indirectly by disrupting the capillary and astrocyte function.

The goal of our work is to develop methods to measure the full-field strain response of the lamina cribrosa volume to inflation.  We will also develop computational micro mechanical models to determine the material properties and stress response to inflation of the lamina cribrosa. We will then alter the biomechanical environment of the lamina cribrosa by collagen cross linking or enzymatic degradation of the human posterior eye-wall in vitro and of the mouse posterior eye-wall in vivo and measure the effects of chemical treatment.

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