Abstract
Purpose :
According to the intracapsular accommodation mechanism (IAM) proposed by Gullstrand, an increase in the effective refractive index is required to fully account for the boost in lens power with accommodation. Our goal is to explain the IAM assuming a GRIN accommodating lens based on observed changes in the inner curvature gradient during accommodation.
Methods :
A GRINCU (gradient index and curvature) lens model was fitted to published data on lens geometry changes during accommodation. Assuming a standard profile for the refractive index and a constant curvature gradient for the iso-indicial surfaces, we compute the anterior and posterior curvature gradients, lens power, and equivalent refractive index for each accommodative state. This method was applied to both experimental data based on Scheimpflug imaging and a finite element simulation of accommodation from the literature.
Results :
Geometrical analysis of the finite element simulation with a hyperelastic model of the eye lens yields a sigmoidal accommodation response to the applied force. Besides the well-known increase in lens curvature, the lens surfaces become more hyperbolic (the conic constant becomes more negative), and the inner curvature gradient steepens. These two changes further enhance the paraxial lens power and increase the equivalent refractive index during accommodation, showing a positive IAM. The analysis of experimental data produces similar results. However, different lens geometries could yield zero or even negative IAM.
Conclusions :
The paraxial lens power and equivalent refractive index strongly depend on the lens's internal and external geometry. In the two cases studied, the positive IAM found is linked to the deformation of the iso-indicial surfaces, which become more hyperbolic and augment the curvature gradient during accommodation.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.