Anatomically-accurate paraxial optical model of cynomolgus lens accommodation with continuous gradient

Abstract

Purpose: To develop an anatomically-accurate model of the primate crystalline lens with continuous refractive index gradient that predicts the accommodative response.

Methods: We used data acquired on 6-year old cynomolgus monkey lenses (n=7, age=6.0 to 6.8 years) during simulation of accommodation in a lens stretcher (Ehrmann et al, Clin Exp Opt, 2008). During stretching, lens power was measured with a system based on the Scheiner principle. Anterior and posterior curvature and thickness were obtained from cross-sectional optical coherence tomography images (Uhlhorn et al, Vis Res 2008). An anatomical model of the accommodating lens was created by averaging the unstretched radii of curvature, unstretched thickness, unstretched power, accommodation, curvature-power slope, and thickness-power slope of the 7 lenses. The values were entered in a paraxial lens model with continuous index gradient (Manns et al, ARVO 2008). The gradient is modeled as a set of spherical iso-indicials with radius that varies linearly from the equator to the surface. The refractive index at the lens center and surface are set to be equal to 1.429 and 1.375 independent of accommodation (De Castro et al, IOVS, 2013). A power-dependence is assumed for the axial index variation: n(z)=1.429-0.054 z^b/t^b, where t is the anterior or posterior half-thickness. The power coefficient b was calculated so that the accommodative response predicted by the optical model matches that of the anatomical model.

Results: Unstretched radius: Anterior = 4.28+/-0.43 mm and posterior = 3.23+/-0.11 mm; unstretched thickness = 3.91+/-0.12 mm; unstretched power = 49.2+/-3.6D; accommodation = 20.1+/-6.1D; curvature-power slope: Anterior = 0.00646+/-0.00102 mm^-1/D and posterior = 0.00525+/-0.00097mm^-1/D; thickness-power slope = 0.042+/-0.002 mm/D. The power coefficient increased non-linearly from b=5.0 in the relaxed state to b=5.8 in the accommodated state. The corresponding average/equivalent indices are approximately independent of accommodation: 1.420/1.440 for the relaxed and 1.421/1.438 for the accommodated lens. The power coefficient, average and equivalent index and their changes with accommodation are consistent with results obtained using a gradient reconstruction algorithm (De Castro et al, IOVS 2013).

Conclusions: The model with continuous gradient accurately predicts the anatomical and optical accommodative response of cynomolgus lenses.