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Susana Marcos, Alberto De Castro, Eduardo Martinez-Enriquez, Pablo Perez-Merino, Miriam Velasco-Ocana, Luis Revuelta, Sally A McFadden; Crystalline lens Gradient Index Profile in the guinea pig myopia model. Invest. Ophthalmol. Vis. Sci. 2019;60(9):610. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The crystalline lens undergoes morphological and functional changes with aging and may also play a role in emmetropization. Both the geometry and the gradient index (GRIN) contribute to the lens optical properties. We estimated the lens GRIN in guinea pig, a common animal model to study myopia.
A custom 3D quantitative sOCT (840 nm, bw=50nm, 25000 Ascans/s, axial pixel size:3.4um; 80 Bscansx1667 Ascans, 8x8x7mm) was used to image enucleated crystalline lenses, acquired at 2 orientations (anterior surface up, posterior surface up). Lenses were extracted from guinea pigs (Cavia porcellus) and tested at 18 days of age (n=4; 3 monolaterally treated with negative lenses & 1 control) or 39 days of age (n=4, untreated). Treated eyes were myopic (-2.07±1.63D) and untreated eyes were hyperopic (+3.3±1.2D), from streak retinoscopy (live, cyclopeged animals). Custom algorithms were used to estimate the lens shape (Anterior and posterior radii, Rant, Rpost; asphericities Qant, Qpost; thickness LT). GRIN was modeled with a 4-variable model (nucleus nn & surface ns refractive indices; axial ρa & meridional ρm decays) and reconstructed from OCT data using a genetic algorithm method. Ray tracing was used to estimate optical power (P) and spherical aberration (SA), assuming a homogeneous index or the estimated GRIN.
Guinea pig lenses exhibited nearly parabolic GRIN profiles: nn:1.371- 1.378, ns:1.451-1.466, ρm:2.367-5.913, ρa:1.912-2.1, and phase refractive index (630 nm):1.431-1.465, across lenses. Older lenses exhibited statistically higher (p<0.05) nn, ns, and ρm. Lenses show steep surfaces (Rant:3.29±0.18; Rpost:2.50±0.13mm) with positive asphericities Qant:2.12±0.20; Qpost:1.22±0.13) & LT: 3.67±0.09mm. Older lenses were significantly thicker (p=0.047). Rpost was correlated with refractive error (0.0317mm/D; r=0.84; p<0.05). Paraxial P ranged from 104.9 to 87.8 D & non-paraxial PA (with GRIN) 93.3 to 78.0 D. The presence of GRIN shifted SA to negative values: +31.58±1.15μm to -4.06±9.99μm in both groups (2 mm pupil). In younger eyes SA was significatly negative (p<0.001).
As found in other species including human, the guinea pig lens exhibits a GRIN profile, with increasing nucleus and surface indices and meridional flattening with age. GRIN plays a major role in the lens optical properties and should be incorporated into computational eye models of emmetropization, myopia development and aging.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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