Dynamic in vivo mapping of the gradient refractive index and strain distribution of the human lens under accommodative stress

The mechanical properties and refractive index (RI) distribution within the human crystalline lens are essential for understanding accommodation and age-related changes in the lens. However, there is limited knowledge about how these properties change in vivo with accommodative demand and aging. Previous methods lacked the ability to simultaneously measure both RI and mechanical strain in the lens, limiting their ability to capture the full complexity of the lens' behavior. To address this gap, we measured the spatial distribution of the RI and mechanical strain in the lenses of six healthy participants (aged 24-45 years) using optical coherence tomography (OCT) under three accommodative demands (0, -2, and -4 diopters). A phase-based signal processing algorithm was developed to compute the RI at each pixel of the OCT B-scans, while instantaneous and accumulated strains were used to assess the lens's mechanical properties during micro-fluctuations. Our results indicated an axial RI gradient, with the highest RI values in the posterior half of the lens. The RI did not significantly change with age or accommodative demand in participants under 45. However, the instantaneous strain, representing the deformation speed, decreased with age, while the accumulated strain during micro-fluctuations increased with higher accommodative demands. No correlation was observed between the RI and mechanical strain distribution within the lens. These findings suggest that OCT is a promising high-resolution tool for in vivo optomechanical characterization of the crystalline lens, offering valuable patient-specific data. OCT could be instrumental in future studies of lenticular changes during emmetropization and myopization.