Mechanical basis of cortical cataract.

PURPOSE: Accommodation, the ability of the human eye to adjust focus, declines with age due to progressive lens stiffening. This study hypothesizes that cycles of attempted accommodation and relaxation in presbyopic eyes (≥ 50 years) induces internal deformations in the equatorial cortex of the lens, contributing to cortical cataract formation. METHODS: Finite element simulations were performed to evaluate internal strains and stresses in the equatorial cortex of lenses of different ages caused by ciliary body movements during accommodation or attempted accommodation. The models incorporated previously published data on spatial variations in lens stiffness, quantified by shear modulus, and its age-related changes. RESULTS: In the 55-year lens model, relaxation after attempted accommodation produced significant shear strain and radial stress concentrations in the equatorial cortex. The locations of these stress and strain concentrations corresponded with cortical cataract sites reported in the literature. The simulations also demonstrated local separations at the cortex-capsule and cortex-nucleus boundaries, consistent with prior in vitro findings. These separations reduced local stress and strain concentrations but may increase cataract risk through damage-repair and scarring. CONCLUSIONS: Finite element modeling suggests the possibility that internal deformations generated during attempted accommodation in aging lenses may play a role in cortical cataract development. This suggests a complex interplay between mechanical strain and biological repair mechanisms in age-related lens pathology.