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Judith Birkenfeld, Alberto De Castro, Susana Marcos; Astigmatism of the isolated human crystalline lens: Surface shape and gradient refractive index contributions. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3566.
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© ARVO (1962-2015); The Authors (2016-present)
To estimate the contribution of lens surface shape and lens gradient refractive index (GRIN) to the lens astigmatism in human donor lenses as a function of age.
Thirty-five human donor lenses (age 19-71) were imaged with quantitative custom-developed 3D-spectral optical coherence tomography (840 nm SLD illumination; 12 mm x 12 mm lateral scan, 1668 A-scans x 60 B scans , 6.9 um axial resolution). Laser Ray Tracing was used to measure the lenses’ back focal length (BFL, paraxial and non-paraxial). 3-D lens GRIN (described by a 4-variable model) was reconstructed with an optimization genetic algorithm using OCT optical path differences in 2 lens orientations, using 3-D lens shape, and BFL as input data. Lens surface astigmatism was obtained from ellipsoid fitting to the lens surfaces. Lens astigmatism was estimated by computational ray tracing on the lens, assuming either the reconstructed GRIN lens or an equivalent homogenous Refractive index (EHRIN) lens. Astigmatism magnitude and relative astigmatic angle between lens surfaces, GRIN lens and lens astigmatism were evaluated using power vector notation, and analyzed as a function of age.
The astigmatism magnitude in the anterior lens surface decreased with age (slope= -0.005 D/year; r= 0.397, p=0.018), although the posterior surface astigmatism and the (total) lens astigmatism were not age-dependent. Presence of GRIN altered slightly the magnitude (average GRIN-EHRIN magnitude difference: 0.53 D) and the axis of the lens astigmatism (Average GRIN-EHRIN axis difference: 22.17 deg). The total astigmatism of the GRIN lens and the EHRIN lens correlated to the anterior lens surface astigmatism (GRIN: p=3.9E-6 r=0.693; EHRIN: p=4.1E-4, r=0.565). The average relative angle between anterior and posterior surface was 34.08 deg, and had no significant age dependency (r= 0.172, p= 0.32, slope: -0.293).
The anterior lens surface astigmatism undergoes changes with age, and has the largest impact on the total lens astigmatism . The GRIN index plays only a minor role in lens astigmatism magnitude and axis. Knowledge of the sources of astigmatism in the lens gives insights in the understanding the optical interactions between ocular components, with impact in the understanding of presbyopia, myopia development and design of new IOLs.
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