Deformations and Ruptures in Human Lenses With Cortical Cataract Subjected to Ex Vivo Simulated Accommodation

Abstract

Purpose: Human cortical opacities are most commonly accompanied by changes in lens fiber structure in the equatorial region at the lens nucleus-cortex interface. Cortex and nucleus have different elastic properties, which change with age. We therefore subjected ex vivo lenses to simulated accommodation and studied the internal deformations to better understand the mechanism of cortical cataract formation.

Methods: Nine human donor lenses (33-88 years old) were tested using a bespoke radial stretching device for anterior eye segments. Seven of the lenses exhibited cortical cataracts. The other two lenses, without cataract, were used as controls. Frontal and cross-sectional images of the lens obtained during stretching facilitated measurements on equatorial lens diameter and central lens thickness in the stretched and unstretched states.

Results: Stretching caused the lens equatorial diameter to increase in all cases. Conversely, the lens central thickness showed no systematic variation during stretching. For four of the lenses with cortical cataract, ruptures were observed during stretching at the nucleus-cortex boundary adjacent to the cortical cataracts. Ruptures were not observed in the control lenses or in the three other lenses with cortical cataract.

Conclusions: Internal ruptures can occur in aged ex vivo lenses subjected to simulated disaccommodation. These ruptures occur at the nucleus-cortex interface; at this location, a significant stiffness discontinuity is expected to develop with age. It is hypothesized that ruptures occur in in vivo lenses during accommodation-or attempted accommodation.

Figure 1.

Human donor lens with cortical cataract. Frontal view (top) and cut at the axial plane (center), both imaged with dark-field illumination. Boxed area with groups of opaque fibers is shown below with scanning electron microscopy (bottom). Lens fibers at the border zone between the nuclear and cortical lens regions are broken (arrows), and the broken ends are directed against the nuclear fibers, which maintain a regular, uninterrupted organization. Further, note the curled (asterisk) and folded (arrowheads) fibers in the region adjoining the broken fibers. n, nuclear side; c, cortical side. Reprinted with permission from Michael R, Barraquer RI, Willekens B, van Marle J, Vrensen GFJM. Morphology of age-related cuneiform cortical cataracts: the case for mechanical stress. Vision Res. 2008;48(4):626–634. Copyright 2007 Elsevier Ltd.

Figure 2.

Two sample lenses, one control lens (above; M80, 46 years) and one with cortical cataract and internal ruptures after stretching (below; M69, 86 years). Frontal image on the left shows the first image, before the stretching experiments in the unstretched state. Frontal image on the right shows the last image after two or three cycles of stretching in the stretched state. The lens outline (right) is given for frontal view and cross-sectional view in the unstretched state (black) and with a stretching force of approximately 54 mN applied (red) during the same stretching cycle.

Figure 3.

Equatorial lens diameter (left) and central lens thickness (right) for the total lens (dark color) and subdivided for the lens nucleus (light color) for the unstretched state (green) and stretched state (orange). Results are given as a percentage of the dimension with the total lens unstretched state set to 100%. Below are the specimen identification numbers (M71–M87) and the ages of the donor in years. Ruptures are indicated with a small white rectangle. Lenses are stretched with an average force of 54 mN (10⁻³ Newton). The radial force applied to the lens for specimen M69 was estimated, because the ciliary body was not cut as in all other samples due to an oversight.