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A.J. Bellezza, R.T. Hart, J.C. Downs, J.F. Reynaud, B.A. Hirons, C.F. Burgoyne; Continuum Finite Element Modeling of the Normal and Early Glaucomatous Monkey Optic Nerve Head . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1094.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To build the first continuum finite element models (FEMs) of the load-bearing connective tissues of a normal and early glaucomatous monkey optic nerve head (ONH) in order to predict the overall and regional levels of maximum stress and strain. Methods: Both eyes of one early glaucoma (EG) monkey (one eye given a laser-induced IOP elevation then followed to the onset of CSLT-detected ONH surface change) were cannulated, and IOP was set to 10 mm Hg in the normal (N) and 30 mm Hg in the EG eye. Following perfusion fixation, the ONH from each eye was serially sectioned (3 um) for reconstruction into a digital 3D geometry of the connective tissues. Each geometry was then discretized into an FEM consisting of 168 finite elements and 1102 nodes. Anisotropic material properties were assigned to each element within the scleral canal based on the tissue volume fraction (% connective tissue) and predominant tissue orientation within that element, and were then refined so that laminar position at IOP 10 and 30 in each model matched previously reported values (Bellezza et al., IOVS, in press). Maximum stresses and strains overall and for each region were then characterized within the N and EG models at IOP 30 mm Hg. Results: Within the FEM of the normal ONH, maximum stresses were largest in the temporal, nasal, and central regions, and noticeably smaller in the superior and inferior regions. Maximum strain was largest in the superior and inferior regions, and approximately 50% higher than in the central and temporal regions. Within the elements of the FEM of the early glaucoma ONH, tissue volume fractions were not significantly different from normal. Maximum stresses were largest in the central region, and smallest in the superior region. Maximum strains were 2 to 5 times those in the normal ONH in all regions, with the inferior and nasal regions showing the largest increases. Conclusions: In these first models, ONH connective tissue mechanical behavior was reasonably described by elastic moduli derived from tissue volume fraction and degree of anisotropy. Regional structural variability in the normal ONH leads to larger maximum stresses in the central, nasal, and temporal regions, and larger maximum strains in the superior, inferior, and nasal regions. In the early glaucoma ONH, while there was no gross change in regional structure, larger strains, particularly inferiorly and nasally, suggest that early connective tissue damage may be greatest within these regions.
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