Influence of lens geometry on accommodation: Verification with an improved finite element model

Abstract

Purpose : Wang et al (2019) used finite element (FE) modelling to show that accommodation is influenced by age-related changes in lens geometry independently of mechanical properties. Yet, similar to many FE models of accommodation (e.g. Burd et al, 2002), the model zonules were inserted at a single point at the ciliary body (CB). As the distribution and direction of zonules may alter their effects on accommodation, we tested the effect of lens geometry change on accommodation with an improved model that portrays the zonular distribution and CB surface more accurately.

Methods : We based the FE model on Wang et al (2019) but with modifications to the zonules and CB. Images in the literature suggest a wide antero-posterior distribution along the CB while insertion at the lens is confined to narrow annuli on the capsule. Further, the CB slopes inward-posteriorly. These characteristics were modelled with 3 strands each of front and back zonular fibres of 50 um thickness (Figure 1) that were inserted at radial distances from the lens axis between 3.46 mm and 4.10 mm to the anterior capsule and between 3.58 mm and 4.17 mm to the posterior capsule. The CB is sloped 38.95° posteriorly relative to the frontal plane. The attachment points along the CB were distributed from 0.53 mm anterior to 0.23 mm posterior, axially relative to the lens equatorial plane.
We tested the effect of lens geometry change associated with 20, 30 and 40-year-old lens while holding the mechanical properties of all components fixed. A radial stretch of 0.5 mm was applied to the CB to simulate lens disaccommodation. The pre- and post-stretch surface profiles of the lens were fitted to conics equation. From the fitted central radius, conic constants and thickness, minimum RMS spot size by ray-tracing was used to calculate lens power assuming refractive indices of 1.333 and 1.42 for the aqueous and lens core respectively. Accommodation amplitude is the difference between unstretched and stretched powers.

Results : Table 1 shows the radii, conic constants, thickness and power change for the age models.

Conclusions : We verified that lens geometry changes, separate from mechanical property changes, substantially contribute to change in accommodation with aging. Given the good agreement with Wang et al despite the difference in zonular configuration, we speculate that zonular geometry may play only a minor role in influencing accommodation.

This is a 2020 ARVO Annual Meeting abstract.