Schachar's theory of accomodation: illustration

Summary of Schachar's theory of accomodation

1. There are 3 sets of zonules: anterior, equatorial, posterior, whose role and attachement to the ciliary body are different. The equatorial zonules uniquelly insert in the valleys of the pars plicata (anterior ciliary body) while the anterior and posterior zonules attach to the pars plana (posterior ciliary body)
2. Accomodation, contrary to Helmholtz theory, is the result of increased tension on the equatorial zonules, with decreased tension in the anterior and posterior zonules. In the unaccomodated state, the anterior and posterior zonules are under increased tension while the equatorial zonules are under decreased tension. By this arrangement of zonules, the lens is stable at all times.
3. Increased equatorial tension produces a flattening of the anterior periphery of the lens and an increased curvature and thickness in the anterior central part of the lens. The posterior lens is practically inert during accomodation.
4. Contraction of the ciliary muscles (meridional, radial, circular fibers) results in pulling the equatorial zonules and releasing the anterior and posterior zonules. The equatorial zonules uniquely insert into the valleys of the pars plicata. Modelling the shape of the ciliary muscle during contraction demonstrates that the anterior portion of the ciliary, the region in line with the equatorial zonules, curls towards the sclera; while the posterior portion of the ciliary muscle, in the region where the anterior and posterior zonules insert, moves forward.
5. The lens growth (~0.02mm/year) reduces the distance between the ciliary body and the lens. This results in an overall decreased of tension in the zonules. The range of action of the ciliary muscle on the equatorial zonules is decreased, which reduces the range of accomodation (presbyopia).
6. By stretching the sclera in the region of the ciliary body, accomodation was restored in presbyopic patients.

Illustration

In the following figures, the cornea would be on the left-hand side.

In the unaccomodated state, the lens is hold in place by the anterior and posterior zonules. The ciliary muscle is relaxed.

In the accomodated state, the ciliary muscle contracts. The portion where anterior and posterior zonules are attached moves forward, decreasing the tension, while the portion where the equatorial zonules are attached, moves closer to the sclera, increasing the tension. The pull of the equatorial zonules increases the equatorial diameter of the lens. The lens periphery is flattened, and the central curvature increases.