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A look at the science behind accommodation and how it must be understood properly in order to create effective lenses
With age, lens hardening and other factors result in the loss of accommodation-the condition we know as presbyopia. Presbyopic patients are understandably quite interested in overcoming this loss of accommodation to once again achieve unaided distance and near vision, and surgeons and IOL manufacturers have been eager to meet this need. In recent years, intraocular lenses to correct presbyopia have been marketed aggressively.
However, we strongly believe that all presbyopia-correcting IOLs should be based on scientific approaches and verified by sound, independent, peer-reviewed studies.
Restoring true accommodation, as in the young eye, would seem to be the ideal approach to presbyopia. However, duplicating the complex natural process of accommodation with a simple lens implant turns out to be quite difficult.
Most accommodative IOLs have been designed to physically move in the eye. In theory, therefore, the axial shift of the lens optic and resulting reduction in anterior chamber depth provides sufficient refractive change to allow for near vision. Although this is a compelling principle, one must be able to explain how the lens moves and whether there is sufficient space in the eye for the degree of lens movement required.
To date, we have not seen published, well-documented evidence to support this principle for any implant. To the contrary, in fact, it seems clear that the human eye does not have enough space to allow the 2.0 mm axial shift that would be necessary to achieve a change in dioptric power sufficient for near reading vision.
Other possibilities for correction of presbyopia are lenses with an adjustable surface curvature or dual optic lenses using axial or radial displacements.
Lens refilling may hold the key to restoring accommodation. The idea of removing the hard lens contents and refilling the lens capsule with a soft material that would behave like a young eye is straightforward, but the particulars are not so simple. This principle, first proposed many years ago, has been tested in animal eyes somewhat successfully,1-7 but many challenges remain before lens refilling can be performed successfully in human eyes. For example, researchers must figure out how to determine the appropriate degree of filling and to improve the refilling material and surgical technique. But the first priority will be to find methods to prevent stiffening of the lens capsule that loses its elasticity and clarity because of secondary cataract formation.
Even more theoretical at this point is the potential for micro- or nanotechnology electronic devices that would be able to control eye movements to simulate accommodation.
Of course, lens multifocality, rather than true accommodation, is another alternative, and one that has already been implemented commercially. Multifocality has some limitations, but at least there is a scientific basis for success with this approach that is in accordance with the laws of optics. 8-11