Model of zonular forces on the lens capsule during accommodation

Abstract

How the human eye focuses for near; i.e. accommodates, is still being evaluated after more than 165 years. The mechanism of accommodation is essential for understanding the etiology and potential treatments for myopia, glaucoma and presbyopia. Presbyopia affects 100% of the population in the fifth decade of life. The lens is encased in a semi-elastic capsule with attached ligaments called zonules that mediate ciliary muscle forces to alter lens shape. The zonules are attached at the lens capsule equator. The fundamental issue is whether during accommodation all the zonules relax causing the central and peripheral lens surfaces to steepen, or the equatorial zonules are under increased tension while the anterior and posterior zonules relax causing the lens surface to peripherally flatten and centrally steepen while maintaining lens stability. Here we show with a balloon capsule zonular force model that increased equatorial zonular tension with relaxation of the anterior and posterior zonules replicates the topographical changes observed during in vivo rhesus and human accommodation of the lens capsule without lens stroma. The zonular forces required to simulate lens capsule configuration during in vivo accommodation are inconsistent with the general belief that all the zonules relax during accommodation.

Figure 1

Schematics of the lens. (a) At far (unaccommodated), all the zonules are under tension. At near (accommodated) according to (b) Helmholtz, all the zonules are relaxed; and therefore, the isolated lens should be maximally accommodated and according to (c) Schachar, equatorial zonular tension is increased and the anterior and posterior zonules relax; and therefore, the isolated lens without zonular tension should be unaccommodated. Superimposed representation of lens sagittal profiles when (d) unaccommodated (black) and accommodated as predicted by Helmholtz (red) and Schachar (blue).

Figure 2

Lens capsule topography. Graves’ drawing of an intact in vivo human lens capsule with no vestige of lens stroma in the (a) unaccommodated and (b) accommodated states and how they should appear during accommodation according to (c) Helmholtz and (d) Schachar.

Figure 3

A rhesus monkey lens capsule following extracapsular lens extraction during Edinger Westphal electrically stimulated accommodation (reproduction of from Fig. 6 of Croft et al.). Note the fine capsule wrinkles identified by the added white arrows.

Figure 4

The change in lens capsule shape from a baseline capsule containing aqueous humor when equatorial zonular tension is increased (horizontal red arrows) and anterior and posterior zonular is tension relaxed (green arrows). The equatorial zonular forces are distributed along the curved lens capsule surface inducing component anterior and posterior forces towards the equatorial lens axis (vertical red arrows). Since aqueous humor is incompressible, aqueous humor is forced to move towards the center (large double blue arrows) inducing outward forces causing the central anterior and posterior capsules to move apart (single blue arrows) as aqueous humor exits the anterior capsulotomy and the posterior capsule bows posteriorly.

Figure 5

Latex balloon zonular force lens capsule model following an extracapsular lens extraction (anterior capsule up). (a) Unaccommodated lens capsule with equal force of 7 g on each of the black 6–0 silk sutures representing the anterior, equatorial and posterior zonules. The accommodated lens capsule according to (b) Helmholtz with all the zonules relaxed and according to (c) Schachar with 22 g force on each of the equatorial zonules with the anterior and posterior zonules relaxed and (d) A rhesus monkey lens capsule following extracapsular lens extraction during Edinger Westphal electrically stimulated accommodation (reproduction from Croft, et al.). Note the similarity in topography of (c) and (d) with the anterior and posterior peripheral lens capsules close together while the anterior and posterior central capsules are apart.

Figure 6

Photograph of the balloon lens capsule model with the sutures representing the equatorial zonules under tension and the sutures representing the anterior and posterior zonules totally relaxed superimposed (a) on the schematic of the force analysis when equatorial zonular tension is increased and the anterior and posterior zonules are relaxed (Fig. 4) and (b) on the B-scan image of the rhesus lens capsule without lens stroma during accommodation (Fig. 3).