Abstract
Purpose :
The increased stiffness of crystalline lens material is believed to be the major cause of presbyopia, the loss of focusing ability with age. Most reports of lens material properties come from ex vivo measurements, which present challenges associated to post-mortem degradation, and difficulty of standard testing. We present an approach to estimate the lens’ moduli in vivo from measured lens shape and accommodating response.
Methods :
The accommodative response of 11 subjects (age range 22-30, mean age= 25.8 ± 2.7 yrs) to a 4.5D stimulus from a Badal optometer was measured using a custom swept source OCT system. Lens shape at 4.5D and 0D was reconstructed using custom algorithms, 3D lens volume was estimated from the central pupil data using the Eigenlenses method, and 2D shapes of the lens were obtained as an average of the lens volume cross-sections. The accommodating response was obtained from a Hartmann-Shack aberrometer coupled to the OCT system.
The 2-dimensional shape of the lens in its maximally accommodated state was used as input for a finite element model (FEM) of accommodation with discrete nucleus and moduli. Additional model parameters were adapted from the literature. The lens was subjected to forces of 50 and 60 mN in order to simulate its deformed geometry. A Nelder-mead simplex algorithm was used to find values of nuclear (Gn) and cortical (Gc) shear moduli that would minimize the difference between the simulated deformed geometry, and the experimental geometry at the minimally accommodated state. Simulated accommodation amplitude was calculated by estimating initial and deformed lens power using ray tracing (ZEMAX).
Results :
The estimated mean values of the nuclear and cortical moduli were Gn= 1.47 ± 1.24 kPa and Gc=8.28 ± 5.01 kPa. For all lenses evaluated, the nuclear modulus was found to be lower than the cortical modulus. Correlation was found between age and the average moduli (nuclear and cortical) (0.31 kPa/year, R2=0.77). The average estimated change in power was 3.10 ±1.22 D, compared to the average measured power of 3.33 ± 1.04 D.
Conclusions :
It is possible to reconstruct age-dependent material properties of the crystalline lens from real subjects’ accommodation response in vivo. The results reveal some lag of accommodation, a stiffer cortex than nucleus, and an increase in stiffness with age.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.