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I. A. Sigal, H. Yang, M. D. Roberts, J. L. Grimm, C. F. Burgoyne, J. C. Downs; The Effect of Lamina Cribrosa Thickness and Position on Optic Nerve Head Biomechanics. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3668.
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It has been suggested that susceptibility to glaucoma depends on the specific mechanical response of an individual’s lamina cribrosa (LC) to increases in IOP, but the nature of this dependence is still not clear. Our objective was to estimate the effects of two aspects of LC geometry, thickness and position (relative to the anterior laminar insertion), on its mechanical response.
A specimen-specific 3D finite element model of the LC of a normal monkey eye was reconstructed from high resolution histology images using techniques previously described [IOVS, 2004, 45(12) and IOVS, 2007, 48(5)]. The geometry of the LC was then modified using newly developed morphing techniques to produce a series of models with varying degrees of LC thickness and position. LC position ranged from flat to deep and thickness from one third to three times baseline. The mechanical response of each model to an increase in IOP from 10 to 45 mmHg was simulated and characterized by tensile strain and von Mises stress.
LC thickness and position affected LC mechanical response, such that on the geometries studied median magnitudes of tensile strain ranged from 2 to 6%, and of von Mises stress from 5 to 22 x IOP. Everything else being equal, either a thinner or a flatter LC led to increased levels of strain and stress. Moreover the dependence of stress and strain on thickness and position were non-linear, and non-additive. For example, thinning the LC to 1/3 baseline thickness increased the median von Mises stress by 25% in a flat LC and 64% in a deep LC (34% and 64%, respectively for tensile strain). These results extend to specimen-specific geometries previous findings obtained with generic models.
Both thinning and flattening of the LC individually and together led to increased levels of IOP-induced stress and strain within the LC for a given IOP. These data represent the first application of novel morphing techniques to 3D reconstructions of the laminar cribrosa, and suggest how laminar thickness and position may influence the biomechanical environment of the optic nerve head at all levels of IOP.
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