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Fabrice Manns, Bianca M. Maceo, Arthur Ho, Jean-Marie A. Parel; Age-dependence Of The Paraxial Refractive Index Gradient And Its Power In The Baboon Lens. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2231.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify the age-dependence of the axial refractive index gradient profile of the baboon lens and its contribution to total lens power.
Measurements were made on 25 lenses from 20 baboons (age: 1.8 to 28 years, PMT: 22+/-13h). Lens back vertex power was measured using an optical system based on the Scheiner principle. The lens curvatures and thickness were measured using a custom-built optical coherence tomography (OCT) system (Uhlhorn et al, Vis Res 2008). Optical distortions were corrected using published methods (Borja et al, Biomedical. Opt. Exp, 2010). The contribution of the gradient to lens back vertex power was defined as the difference between the measured back vertex power and the power of a homogeneous thick lens with refractive index equal to the surface index (n=1.365). The paraxial refractive index gradient was modeled as a set of spherical iso-indicial surfaces, with radius of curvature R(z) that vary linearly from R=0 at the lens equator (z=0) to the surface radius value. The axial refractive index follows a power-dependence in each half of the lens, with core index equal to 1.410, surface index equal to 1.365 and axial variation defined as: 1.410-0.045xzb/tb, where t is the anterior or posterior half-thickness (measured from the equator) of the lens. For each lens, a numerical method was used to find the value of the power coefficient, b, and of the equivalent uniform index, which produce a back vertex power that matches the measured value.
The power coefficient first increases with age from a minimum of b=2.0 (parabolic profile) at 1.8 years, to b=4.0 at 5 years, and remains constant after 5 years of age, with an average of b=3.9+/-0.8. The equivalent index first decreases with age from 1.8 to 5 years, with a maximum of 1.457 at age 1.8 years, and remains constant after 5 years of age, with an average value of 1.428+/-0.006. The contribution of the gradient to total back vertex lens power decreases with age in a biphasic fashion, with a decrease of -7.1D/year from 1.8 to 5 years, followed by a slower rate of decrease (-0.6D/years) from 5 to 28 years.
The results suggest that, unlike in human lenses, the formation of a refractive index plateau is not a requirement to explain the age-dependent changes in baboon lens power. The decrease in the gradient power of baboon lenses is not due to a flattening of the axial gradient, but to changes in the shape of the outer and iso-indicial lens surfaces.
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