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W. H. Chan, A. A. Hussain, J. Marshall; Youngs Modulus of Bruchs Membrane: Implications for AMD. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2187.
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© ARVO (1962-2015); The Authors (2016-present)
To determine Youngs’ modulus of elasticity of Bruchs membrane and assess its implications for developing AMD
Young’s modulus of elasticity was derived from the induced deformation on application of a hydrostatic pressure head (500-3000Pa) to a membrane clamped in an open-type Ussing chamber. The applied stress (f) to the exposed segment was calculated as f = RP / 2tm , R was calculated using scans from an OCT based tracking technique. The corresponding circumferential strain in the sample, [l-lo/lo, lo and l being initial and final arc-lengths] was due to two principal stresses acting at right angles, the first acting along the circumference and the other at right angles to the first, such that : (l-lo/lo) = f/E - σf/E (2), giving (l-lo/lo) = [RP(1-σ) / 2tmE] (3)[R = radius of curvature of sample, P = applied pressure, tm= the thickness of the membrane, E = Young’s modulus of elasticity, σ = Poisson’s ratio for the membrane] Thickness of Bruch’s (tm) was assumed to be constant under the pressures examined and its value obtained from glutaraldehyde fixed sections by transmission electron microscopy. 10-12 locations were averaged to obtain thickness. Since Bruch’s and the lens capsule consist primarily of a collagenous network, the experimentally evaluated Poisson’s ratio for the capsule (σ = 0.47) was also applied to Bruch’s membrane.Using equation 3 and the constants mentioned above, applied stress was plotted against the induced circumferential strain and Young’s modulus E obtained as the gradient of this linear relationship by regression analysis. Four donor preparations, age range 22-83 years were assessed.
4 samples from four donors aged 22, 31, 75 and 83 years were processed for transmission electron microscopy to determine the average thickness of Bruch’s membrane. The thickness values obtained for the preceding donors were 2.3, 2.57, 3.2 and 3.29µm respectively. Assuming a Poisson ratio σ for Bruch’s to be equivalent to 0.47, rearrangement of equation (3) allowed a plot of stress vs. strain, the gradient, obtained by linear regression, representing the Young’s modulus of elasticity. For the four donors aged 22, 31,75 and 83 years, the corresponding Young’s moduli were computed as 6.93, 3.65, 13.78 and 18.8 MPa respectively
The age-related loss in the elasticity of Bruch’s is expected to influence the functional properties of the membrane. Elasticity maybe pivotal in maintaining patent pathways for nutritional transport and its decline with age may contribute to the pathogenesis of age-related macular degeneration.
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