Abstract
Purpose :
To investigate the influence of the lens shape and gradient refractive index on the age-related changes of the isolated human crystalline lens power and spherical aberration.
Methods :
Isolated human donor crystalline lenses (n=42, 3 to 56 years old, Ramayamma International Eye bank) were imaged with an Optical Coherence Tomography (OCT, λ=880 nm) system in two orientations. An integrated laser ray tracing (LRT) measured the OCT beam refracted by the lens at different axial positions. The lens anterior and posterior radius, asphericity, thickness and average refractive index (Ra, Rp, Qa, Qp, LT, nAV) were obtained from the OCT images using custom algorithms for image segmentation and distortion correction. The lens focal length (FL) and spherical aberration (SA) for a 6-mm pupil diameter were calculated from LRT, using the slope of the rays refracted by the lens. Computational ray tracing with a homogeneous refractive index (n) or gradient refractive index (GRIN) models was performed using the reconstructed lens geometry. The GRIN profiles were defined with a power law with 1 (1 exponent constant with age and equal in all directions, GRIN1), 2 (2 exponents, P1 and P2, to model the axial and meridional decay, GRIN2) or 4 variables (P1 and P2 linearly dependent with age, GRIN3). The variables were optimized to match the experimental nAV, FL and SA.
Results :
In the first two decades Ra and Rp were constant (p>0.05) and Qa, Qp and LT decreased (slopes -0.39yr-1, -0.10yr-1 and -0.04mm/yr, p<0.05). nAV increased (2E-4yr-1), FL was constant and SA decreased (-0.10μm/yr). After age 20, Ra, Rp and LT increased (0.08 and 0.028 and 0.018 mm/yr), Qa and Qp were constant, nAV decreased (-2E-4yr-1), and FL and SA increased (0.59mm/yr and 0.05μm/yr). With an age-dependent n, the SA residuals were highest in young lenses (<20 yrs, rms 1.7μm) and decreased with age (-0.03 μm/yr). With GRIN1 and GRIN 2 the change in nAV and SA was reproduced (age-independent residuals, p>0.05) but not in FL (residuals slope -0.28 and -0.20 mm/yr, respectively). GRIN3 with a constant P1 and an increasing P2 could replicate the age dependent change in nAV, FL and SA (age-independent residuals, p>0.05).
Conclusions :
Measurements of lens geometry and optical properties allow to evaluate different refractive index models. Age-dependent GRIN parameters are needed to reproduce the changes in lens power and spherical aberration.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.