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M. D. Roberts, I. A. Sigal, Y. Liang, C. F. Burgoyne, J. C. Downs; The Effect of Local Microarchitecture on Lamina Cribrosa (LC) Biomechanics. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3669.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the impact of the spatial variation in LC connective tissue (CT) microarchitecture on IOP-related deformation and strain.
3D reconstructions of the optic nerve head were generated for three normal monkey eyes and used to construct continuum finite element models of their posterior poles [IOVS, 2004, 45(12); ARVO 2007, #378-B314]. The LC microarchitecture within each of the 45 LC elements was characterized by (a) connective tissue volume fraction (CTVF), and (b) fabric tensor, a measure of predominant beam orientation and structural anisotropy. These quantities were used to derive direction-dependent material properties (stiffness) for each LC element [JBiomech, 1992, 25(1)].The mechanical effect of LC microarchitecture was studied using three material property schemes (Figure 1): Mapped CTVF + Fabric - each LC element was assigned orthotropic material properties based on local CTVF and fabric; Mapped CTVF - fabric was discarded and each LC element was assigned unique isotropic material properties based on local CTVF only; Uniform CTVF - all LC elements were assigned a common isotropic elastic modulus based on the average CTVF of the entire LC. As a baseline, a global laminar material constant was set to produce an average posterior LC deformation of 16.5 µm for an acute IOP elevation from 10 to 45 mmHg in the Mapped CTVF + Fabric case [IOVS, 2003, 44(42)].
Both CTVF and fabric varied considerably within and across the 3 eyes (Table 1). Mechanically, the Mapped CTVF + Fabric material assignment produced the stiffest LC Structure, with the least LC deformation and the lowest average principal strains (Table 2). Removal of LC beam orientation information dramatically increased the compliance of the LC in two of the eyes and caused a modest increase in the third eye. A uniform isotropic LC was the most compliant, but exhibited more evenly distributed strains.
Our models suggest that the spatial distribution and orientation of the LC beams is an important determinant of the overall LC stiffness. Future advances in clinical imaging of acute IOP-related LC deformations should allow experimental validation of these findings.
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