There are several structures in the human eye that contribute to the process of accommodation. Cramer
9 studied the lens Purkinje images and showed that accommodation is associated with an increase in anterior lens curvature. Helmholtz
10 showed that this increase in curvature is achieved by contraction of the ciliary muscle. His explanation of the accommodative mechanism is still generally accepted today.
11 According to his theory, contraction of the ciliary muscle releases the circumferential tension on the zonules, keeping the lens in suspension. The elasticity of the lens capsule and the lens nucleus and cortex enable the lens curvature to increase. During disaccommodation the tension of the zonular fibers, which insert into the ciliary body and choroid, increases, thus pulling the lens back into its disaccommodated, flattened state. All the structures involved in accommodation (lens capsule, lens nucleus and cortex, zonula, ciliary muscle, and choroid) show age-related changes that may explain the onset of presbyopia at the approximate age of 45 years. However, many investigators consider hardening of the lens nucleus and cortex to be an important factor. It seems to provide a logical explanation of presbyopia, because the lens changes its shape during accommodation. Pau and Kranz
12 described the simultaneous increase of lens sclerosis and decrease of accommodative ability. Fisher,
13 by placing lenses on a rapidly rotating table, demonstrated that older lenses are more resistant to deformation than younger lenses. Glasser and Campbell
14 established that older lenses, when exposed to equatorial stretching forces, show less change in focal length than younger lenses. If the lens nucleus and cortex are responsible for presbyopia, replacement of the hardened lens substance by a suitable soft, transparent polymer may restore the accommodative range. Such a replacement is certainly conceivable in combination with cataract surgery. Several investigators
15 16 17 18 19 have refilled the lens capsule in animal eyes. A number of publications
18 20 21 22 describe experiments to establish accommodative changes in young monkeys’ eyes. Presbyopia tends to develop in these monkeys at 25 years of age.
23 Haefliger and Parel
24 found evidence of accommodative changes in response to intracameral pilocarpine treatment of presbyopic monkeys’ eyes after refilling the lens capsule, which confirms that lens capsule refilling can restore accommodation. With their indirect lens change measurements, they indicated the limitations of their conclusion (they measured a decrease in anterior chamber depth instead of refractive changes). Although promising, these results pertaining to monkeys’ eyes do not necessarily show that accommodation can be restored to the human lens by lens refilling. We therefore used a technique described by Glasser and Campbell
14 to study the effect of refilling the human lens in vitro on induced changes in focal length. A series of reference measurements were obtained in a set of human lenses in their natural state.