The proposed biomechanical studies test the following hypotheses regarding the effects of intraocular pressure (IOP) within the load bearing connective tissues of the optic nerve head (ONH): 1) The lamina cribrosa and scleral canal of the normal ONH deform under the effects of an acute increase in IOP related stress (force/cross-sectional area). 2) An increase in this deformation underlies the onset of ONH surface hypercompliance in experimental glaucoma. 3) The magnitude of IOP-related stress within the ONH connective tissues is substantial at all levels of IOP. and, 4) for a given level of IOP, the magnitude of IOP-related stress is principally influenced by the size, shape, and three-dimensional (3-D) anatomy of the scleral canal.
The specific aims are : 1) to measure and compare the position of the lamina cribrosa and scleral canal diameter within serial Sagittal and transverse sections of both eyes of normal monkeys in which one eye was perfusion fixed at low and the other at high IOP; 2) to make similar comparisons between both eyes of a monkey after one ONH has been made hypercompliant by the induction of unilateral experimental glaucoma; and 3) to build and test a series of finite element models (FEMs) of the load bearing connective tissues of the normal and hypercompliant monkey ONH so as to characterize the distribution and magnitude of IOP-related stress. The stress-strain testing of posterior monkey sclera to determine its viscoelastic material properties. The methodology includes compliance testing of the ONH (to detect the onset of hypercompliance in experimental glaucoma); uniaxial stress-strain testing (to determine the material properties of posterior sclera); 3-D reconstruction of digitized histologic sections (to construct the first digital, 3-D geometry's of the ONH load bearing tissues at physiologic and pathophysiologic levels of IOP); and finite element modeling of the load bearing tissues of the ONH to characterize the distribution of IOP-related stress within the ONH. These studies lay the foundation for the clinical estimation of an individual patient's susceptibility to glaucomatous damage at a given level of IOP.
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