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I. A. Sigal, H. Yang, M. D. Roberts, J. L. Grimm, C. F. Burgoyne, J. C. Downs; Finite Element Models Predict a Reduction of IOP-Induced Stresses in the Lamina Cribrosa During Progression From Normal to Early Experimental Glaucoma. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4888.
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As glaucoma progresses, the anatomy and mechanical properties of the tissues of the optic nerve head (ONH) change. The purpose of this study was to estimate the roles of these changes on the biomechanical response of the ONH to normal and elevated IOP.
3D finite element models of the lamina cribrosa (LC) and peripapillary sclera of both eyes of a monkey were reconstructed using previously described techniques [IOVS 2007 48(5)]. One of the eyes was normal (N), the other had early experimental glaucoma (EG). Morphing techniques were then used to alter the geometry and tissue mechanical properties of the N model to the EG model. The transformations were parameterized to allow changing canal diameter and eccentricity, LC thickness and position, and LC and sclera stiffness (moduli) independently or in combination. The models were then used to estimate the median levels of stress within the LC at normal (15-20 mmHg) and elevated (45 mmHg) IOP.
The levels of stress predicted at elevated IOP in the EG model were similar to the levels of stress at normal IOP in the N model, both about 75% lower than the stress in the N model at elevated IOP [Figure]. The effects of the parameter changes were nonlinear and not additive. Early in the progression from N to EG, the changes in LC position (deeper in EG) produced the largest reduction in stress, but were later overcome by the effects of changes in the LC stiffness (more compliant in EG). This is important because LC stiffness is not well characterized, and the values used were indirect estimates based on the IOP-induced LC deformation.
Of the changes in tissue anatomy and mechanical properties that occur in the progression from normal to early experimental glaucoma none led to increases in stress, and many led to decreases. Although most of the biological processes involved are still unknown, the results suggest that elevated IOP may trigger remodeling of the ocular tissues that reduces IOP-induced stresses in the LC. The reduction in stress is achieved by a nonlinear combination of changes in the shape and stiffness of the tissues of the ONH.
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