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Anthony J. Bellezza, Richard T. Hart, Claude F. Burgoyne; The Optic Nerve Head as a Biomechanical Structure: Initial Finite Element Modeling. Invest. Ophthalmol. Vis. Sci. 2000;41(10):2991-3000.
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purpose. To study the relationship between intraocular pressure (IOP) and the
IOP-related stress (force/cross-sectional area) it generates within the
load-bearing connective tissues of the optic nerve head.
methods. Thirteen digital, three-dimensional geometries were created
representing the posterior scleral shell of 13 idealized human eyes.
Each three-dimensional geometry was then discretized into a
finite element model consisting of 900 constituent finite elements. In
five models, the scleral canal was circular (diameters of 0.50, 1.50,
1.75, 2.00, and 2.56 mm), with scleral wall thickness (0.8 mm) and
inner radius (12.0 mm) held constant. In three models, the canal was
elliptical (vertical-to-horizontal ratios of 2:1 [2.50 × 1.25
mm], 1.5:1 [2.1 × 1.4 mm], and 1.15:1 [1.92 × 1.67
mm]), with the same constant scleral wall thickness and inner radius.
In five additional models, scleral canal size was held constant
(1.92 × 1.67 mm), and either scleral wall thickness (three
models, 0.5, 1.0, and 1.5 mm) or inner radius (two models, 13.0 and
14.0 mm) was varied. In all models, each finite element was assigned a
single isotropic material property, either scleral (modulus of
elasticity, 5500 kPa) or axonal (modulus of elasticity, 55 kPa).
Maximum stresses within specific regions were calculated at an IOP of
15 mm Hg (2000 Pa).
results. Larger scleral canal diameter, elongation of the canal, and thinning of
the sclera increased IOP-related stress for a given level of IOP. For
all models, maximum IOP-related stress ranged from 6 × IOP
(posterior sclera) to 122 × IOP (laminar trabeculae). For each
model, maximum IOP-related stress was highest within the laminar
trabecular region and decreased progressively through the laminar
insertion, peripapillary scleral, and posterior scleral regions.
Varying the inner radius had little effect on the maximum IOP-related
stress within the scleral canal.
conclusions. Initial finite element models show that IOP-related stress within the
load-bearing connective tissues of the optic nerve head is substantial
even at low levels of IOP. Although the data suggest that scleral canal
size and shape and scleral thickness are principal determinants of the
magnitude of IOP-related stress within the optic nerve head, models
that incorporate physiologic scleral canal and laminar geometries, a
more refined finite element model meshwork, and nonisotropic material
properties will be required to confirm these
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