Dual optic IOLs: accommodating the next generation

Accommodative lenses on the whole are showing significant promise and have certain advantages over multifocal IOLs because they do not have different refractive zones, contrast or glare problems

The accommodative IOL is born

The goal of the new accommodating IOL is to have a similar mechanism of action to the human lens, which retains the ability to focus at all distances ranging from near, far and all points in between. Essentially, the lens would use the eye's natural accommodative mechanism according to the Helmholtz theory, which proposed that during accommodation the ciliary muscle contracts, releasing tension on the zonular fibres at the lens equator, allowing the lens equatorial diameter to decrease, the lens thickness to increase, and the lens anterior surface to become more steeply curved.

Although the mechanism of presbyopia is still incompletely understood, the weight of current evidence seems to suggest that, although some loss of ciliary body action might contribute to reduced accommodation,5 significant ciliary body function persists into advanced maturity and that loss of lens capsule elasticity in combination with changes in the geometry of zonular attachments, are probably largely responsible for producing the distress of presbyopia.6 Using this theory, it seems that replacement of the crystalline lens with a lens that responds to ciliary body contraction should restore accommodative function.

So far, the only accommodating lenses available are the single-optic IOLs Crystalens (Eyeonics), approved for the visual correction of aphakia secondary to the removal of a cataractous lens in adult patients with and without presbyopia and Tekia's Tek-Clear, which recently gained CE Mark approval for the treatment of presbyopia. However, some ophthalmologists believe that true accommodation will always be limited with a moving single-optic IOL because of the large amount of movement that is required to produce a significant power change.

The power of dual-optics

Recognizing these limitations, various researchers proposed the introduction of two, connected, lenses, which would act as a rigid shell, into the capsular bag.

The mechanism of action of this lens is based on a lens complex formed by two optics linked by a spring system that, prior to ocular implantation, are separated by approximately 3.7 mm. When implanted within the capsular bag however, interoptic separation is significantly reduced because of compressive tension exerted by the capsular bag. Characteristic elements incorporated within the lens complex control minimum separation and hence set the resting distance refraction at emmetropia.