We do not yet have a complete understanding of the relationship between the effect of ciliary muscle contraction and zonular tension on the equatorial lens. In addition, a few markedly different anatomical relationships have been described between the origin of the zonular fibers and the insertion of these fibers into the lens.
The Helmholtz hypothesis, or capsular theory, of accommodation states that during distance vision, the ciliary muscle is relaxed and the zonular fibers that cross the circumlental space between the ciliary body and the lens equator are at a “resting” tension. With accommodative effort, circumferential ciliary muscle contraction releases this tension on the zonules. An anterior movement of the ciliary muscle annular ring also occurs during accommodation. The reduced zonular tension allows the elastic capsule of the lens to contract, causing a decrease in equatorial lens diameter and an increase in the curvatures of the anterior and posterior lens surfaces. This “rounding up” of the lens yields a corresponding increase in its dioptric power, as necessary for near vision (Fig 9-1). When the accommodative effort ceases, the ciliary muscle relaxes and the zonular tension on the lens equator rises to its resting state. This increased tension on the lens equator causes a flattening of the lens, a decrease in the curvature of the anterior and posterior lens surfaces, and a decrease in the dioptric power of the unaccommodated eye.
In the Helmholtz theory, the equatorial edge of the lens moves away from the sclera during accommodation and toward the sclera when accommodation ends. In this theory, all zonular fibers are relaxed during accommodation and all are under tension when the accommodative effort ends. According to Helmholtz, presbyopia results from the loss of lens elasticity with age. When the zonules are relaxed, the lens does not change its shape to the same degree as the young lens; therefore, presbyopia is an aging process that can be reversed only by changing the elasticity of the lens or its capsule.
, ed. Helmholtz’s Treatise on
Physiological Optics. Translated from the 3rd German ed. New York: Dover Publications; 1962.
Diametrically opposed to Helmholtz is the Schachar theory of accommodation. Schachar suggests that during accommodation, ciliary muscle contraction leads to a selective increase in equatorial zonular tension—rather than to the uniform decrease (anterior, equatorial, and posterior) proposed by the Helmholtz theory—with a subsequent pulling of the equatorial lens outward toward the sclera (Fig 9-2). Schachar postulates that accommodation occurs through the direct effect of zonular tension (as opposed to the passive effect proposed by Helmholtz), causing an increase in lens curvature. In this theory, the loss of accommodation with age is a result of the continued growth of the lens, with increasing lens diameter, and a decrease in the lens–ciliary body distance, which results in a loss of zonular tension. Anything that increases resting zonular tension (eg, scleral expansion) should restore accommodation.
Schachar proposes that the mechanism for functional lens shape change is equatorial stretching by the zonules; this decreases the peripheral lens volume and increases the central volume, thus producing the central steepening of the anterior central lens capsule (Fig 9-3). During accommodation and ciliary muscle contraction, tension on the equatorial zonular fibers increases, whereas tension on the anterior and posterior zonules is reduced. These actions allow the lens to maintain a stable position at all times, even as it undergoes changes in shape. Schachar suggests that the anterior and posterior zonules serve as passive support structures for the lens, whereas the equatorial zonules are the active components in determining the optical power of the lens.
. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation.1992;24(12):445–447, 452.
Evidence from recent studies on both human and nonhuman primates disputes Schachar’s theories on accommodation and presbyopia. Investigations in human tissues and with scanning electron microscopy reveal no zonular insertions (equatorial or otherwise) at the iris root or anterior ciliary muscle. Various imaging techniques consistently indicate that the diameter of the crystalline lens decreases with accommodation. In vitro laser scanning imaging shows that the crystalline lens does not change focal length when increasing and decreasing radial stretching forces are applied. This runs contrary to Schachar’s proposal that the lens remains pliable with age and that presbyopia is due solely to lens growth and crowding that prevents optimum ciliary muscle action.
, KaufmanPL. The mechanism of accommodation in primates.1999;106(5):863–872.
, StrenkLM, KoretzJF. The mechanism of presbyopia.2005;24(3):379–393.