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Volume 18, Number 2 |
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Elastohydrodynamics of the Anterior Eye
Aqueous humor, the clear, watery fluid that fills the space between the lens and cornea of the eye, has been known to exist for centuries, but until the 1940s it was not even known for certain that the aqueous humor flows. Since then, researchers have determined: 1) that aqueous humor flows and serves as a nutrient delivery system for the avascular cells in the cornea; 2) that the aqueous humor is pumped into the eye and eventually drains into a vein; and 3) that the resistance of the drainage system leads to an increased intra-ocular pressure (IOP), typically 15 mm Hg gauge (2000 Pa). Higher IOP is necessary to maintain the curvature of the cornea and thus proper eye function, but increased IOP can lead to damage to the optic nerve. The various pathologies with increased IOP and subsequent optic nerve damage, collectively known as glaucoma, afflict three million Americans, 120,000 of whom are now blind from the disease. Glaucoma is the leading cause of blindness in African Americans and the second leading cause of blindness overall in the United States.
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Figure 1: Iris deformation in response to lens motion. An axisymmetric view of the eye containing the cornea (C), iris (I), and lens (L), as well as the aqueous humor (colored portion of each plot) is shown. In all three images, the pressure is scaled so that the posterior chamber (between the iris and the lens) is at P = 0. Before accommodation, the pressure is very slightly lower in the anterior chamber. During accommodation, however, the motion of the lens pushes aqueous humor into the anterior chamber (between the cornea and the lens), leading to a dramatic increase in anterior chamber pressure. Long after accommodation, the pressure relaxes, and, in fact, the pressure drop between chambers is slightly higher than before because of the reshaping of the lens. |
The challenge to the biomedical engineer is to use fundamental engineering concepts of transport and mechanics to understand how different types of glaucoma arise, why different demographics are more prone to different types of glaucoma, and generally to understand the behavior of the components of the anterior eye. Former graduate student Jeff Heys, current student Eric Huang, and Professor Victor Barocas of the Department of Biomedical Engineering have been using University of Minnesota Supercomputing Institute resources to explore these basic questions by developing a computational model of the iris and the aqueous humor. They have developed a coupled-flow model that accounts for the passive deformation of the solid iris in response to flow of the fluid aqueous humor.
The problem is fully coupled because the location of the iris depends on the stress from the flow, but the fluid flow pattern depends on the location of the iris. For example, the model has been used to resolve a paradox observed in pigment dispersion syndrome. The syndrome is characterized by, among other symptoms, severe posterior bowing of the iris, suggesting that the aqueous humor pressure is higher on the anterior than the posterior surface of the iris. This observation is inconsistent, however, with our knowledge that aqueous humor enters the eye posterior to the iris and exits anterior to it.
Figure 1 shows that, as suggested by clinical ultrasound studies, motion of the lens during near-object focusing activities (such as reading) traps fl uid on the anterior surface of the iris, leading to a temporary "pressure inversion" (Figure 2) in which the anterior pressure is higher than the posterior pressure f or about two minutes. This transient inversion pins the iris against the lens and is believed to cause the erosion of pigment from the posterior iris, for which pigme nt dispersion syndrome is named. Therefore, the iris acts like a valve, preventing posterior flow during the pressure inversion and allowing net anterior flow over ti me in spite of fluctuations in pressure difference due to eye behavior.
This relatively simple model is already large enough to require considerable computational resources because of the fluid-solid coupling, and the rapidity of ocular events (the lens accommodates from far-viewing to near-viewing in a few tenths of a second).
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Figure 2: Pressure in the anterior and posterior chambers. When the lens moves, the pressure in the anterior chamber rises abruptly. After about two minutes, however, the pressure difference relaxes back to zero, and after an additional three minutes a new steady state is reached. |
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This information is available in alternative formats upon request by
individuals with disabilities. Please send email to
alt-format@msi.umn.edu
or call 612-624-0528.
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