Abstract
Purpose :
Eye models generally represent the lens as a homogenous thick lens with an equivalent index calculated to match the known lens power. Studies have found that the equivalent index decreases with age. In parallel, lens growth is associated with the progressive formation of a central plateau in the refractive index gradient profile. The purpose of this study is to determine the relation between lens gradient parameters and equivalent index and evaluate the impact of the plateau formation on lens optics.
Methods :
A paraxial model of the lens with gradient was developed. The gradient is characterized by the axial variation of the index, n(z), and radius of curvature of isoindicial surfaces, R(z). n(z) was a power function with three parameters: surface and equatorial indices and power coefficient (p). The surface (1.371) and equatorial (1.418) indices were independent of age. The power coefficient was age-dependent. A linear variation was assumed for R(z) in the anterior and posterior lens. An expression of lens power in terms of gradient parameters was derived by integrating the incremental gradient power over the thickness of the lens. Comparison with the formula for the equivalent lens power produced a relation between gradient parameters and equivalent index. The model was used to predict the age-dependence of p corresponding to equivalent index data from three studies: 1.441-0.00039*Age (Dubbleman et al, Vis Res 2001), 1.4506-0.00035*Age (Atchison et al, J Vis, 2008) and 1.4483-0.0007*Age (Chang et al, Biom Opt Exp, 2018). The calculated value of p was compared with values obtained from MRI (Kasthurirangan et al, IOVS 2008) and OCT (De Castro et al, J Mod Opt 2011).
Results :
Power coefficient (p) and equivalent index (neq) are related by p = (neq - 1.371) / (neq - 1.418). At age 20, p ranged from 2.8 to 4.1, consistent with MRI and OCT data (Figure 1). With all three datasets, the decrease in neq with age is correlated with an increase in p, consistent with the formation of a refractive index plateau.
Conclusions :
The model produces results that are consistent with experimental data. The formation of a central refractive index plateau is associated with a decrease in lens equivalent index.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.