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C.R. Ethier, I.A. Sigal, J.G. Flanagan; Predicted Stretching, Compression and Shearing of Optic Nerve Head in Glaucoma . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1228.
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Biomechanical forces acting on optic nerve head (ONH) tissues likely play a role in the pathogenesis of glaucomatous optic neuropathy. Previous work has characterized the magnitude of deformation of ONH tissues but has not focussed specifically on the mode of deformation, i.e. whether ONH cells are stretched, compressed or sheared. Circumstantial evidence indicates that cellular response depends on the mode of deformation. Here we study the relative magnitudes and distribution of the various modes of deformation that occur in an ONH as a result of an increase in IOP.
Eight individual–specific models of the ONHs of 4 donors were reconstructed according to a previously described method. Each individual–specific model consisted of five tissue regions: pre and post–laminar neural tissue, lamina cribrosa, sclera and pia mater. All tissues were assumed to be homogeneous, incompressible and linearly elastic with Young’s moduli of (MPa) 0.03, 0.03, 0.3, 3 and 3 respectively. Commercial finite element modeling software (ANSYS v8) was used to predict the biomechanical response to changes in IOP, and specifically tissue strains (deformations). For each model we computed the three principal components of the strain tensor. The first and third (maximum and minimum) principal strains represent the magnitude and direction of maximum tissue stretching and compression respectively. We also derived three composite measures of strain: the maximum shear strain, the von Mises equivalent strain and the principal strain vector magnitude.
For all ONH models the largest strains occured within the pre–laminar neural tissue. Interestingly, as IOP increased from 5 to 50 mmHg, there were substantial differences in the magnitudes of the various modes of strain. For example, pre–laminar neural tissue showed maximum stretches (peak maximum principal strain), maximum compressions (peak minimum principal strain) and maximum shearing strains of 6.6–8.6%, 11.5–13.9% and 9.0–10.8%, repectively. Here the ranges of values represent different patient–specific models. In the lamina cribrosa the corresponding values were 6.5–7.7%, 10.1–11.4% and 8.4–9.5%.
We predict that cells of individual–specific ONHs are subject to very different modes of deformation as IOP increases. The largest deformations are compressive, with slightly smaller shearing deformations and much smaller stretching deformations. The challenge is now to better characterize cellular responses to these different modes of strain.
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