A telescope and a magnifying glass are useful tools, but not if they are your only means to view the world.

Most cataract patients have a similar plight. Despite cataract surgery's prevalence--it is the most common surgical procedure in the United States-- its best possible outcome still leaves patients seeing clearly only at fixed distances.

Their vision is limited by the artificial intraocular lens (IOL) that replaces the eye's cataract-damaged lens. Patients with conventional IOLs may be able to read a road map, for example, but need help reading a road sign.

Increasing the range of focal distance to that of normal vision is the aim of a new artificial lens being designed with the support of a Whitaker research grant.

"The purpose of our research is to develop a novel intraocular lens that has an adjustable focal length according to changes of the diameter of the lens," says Jin H. Shen, Ph.D., of Vanderbilt University's School of Medicine. "This new IOL may restore the accommodative function."

Accommodation is one of the most important functions of the human lens. It is the ability to focus clearly over a wide range of distances. To do this, small ciliary muscles that ring the lens contract and change the shape of the lens, adjusting its refractive, or dioptric, power. Lens accommodation allows you to quickly and easily read the words on this page, the words on a highway sign, and everything in between.

Unfortunately, accommodation is lost when the lenses must be removed due to damage or disease, the most common by far being cataracts, where lenses gradually become too clouded to see clearly.

During cataract surgery, doctors remove the cloudy fluid from inside each lens, but usually leave the lens membrane or capsule intact. The surgeons then insert the IOL into the empty lens capsule through the same tiny slit they used to remove the contents.

Conventional IOLs use a single, rigid lens made out of a polymer and held in place by small wires. The lens is typically 5 millimeters to 6 millimeters in diameter, or about the size of the top of a pencil eraser. The focal power of the lens is preset with one to five focal zones, arranged like a target on the lens with the "bulls eye" having the greatest focal power. Each focal zone enables the patient to see well at a certain prescribed distance, much like wearing trifocal glasses.

What about seeing at distances between the zones? No matter how many focal zones it has, says Shen, a traditional IOL is still a static device, and, therefore, has no true accommodation. "Accommodation requires adjustment," he says.

Even at the fixed distances of a multifocal IOL, he adds, objects are often blurry and low in color contrast because each focal zone uses only a fraction of the available light hitting the lens.

To achieve accommodation, Shen is arranging six conventional artificial lenses like petals in a flower; each lens overlaps the lens next to it and all the lenses overlap at the center. "The most effective area that contributes to the refractive power is the central portion of the lens," says Shen.

An elastic wire ring holds all the lenses in place and keeps the device properly positioned inside the lens capsule. The entire device fits easily within a space smaller than a dime.

Shen's IOL is designed to work with the eye's normal focusing muscles. As the eye focuses on near objects, ciliary muscles contract and would squeeze the diameter of both the lens capsule and the device inside. As the diameter decreases, the six lenses would slide into a tighter arrangement, one with greater overlap at the center. Because the combined focal power of overlapped lenses is greater than that of single or fewer overlapped lenses, the more the lenses overlap, the greater the refractive power. As the eye focuses on objects farther away, the ciliary muscles relax, the lens arrangement spreads out, decreasing the amount of lens overlap and the combined refractive power.

"Through sophisticated mathematical calculation, we can determine the change in the focal length of the overlapped lenses as the distance of overlap between the individual lenses changes," says Shen. "The change of the focal power at the center of the combined IOL in response to the change in the diameter of the lens capsule will generate the accommodation effect."

The result, he says, is a device that should act more like a normal human lens.

According to Shen's calculations, the accommodation effect should be around 10 diopters. "I could increase the number, theoretically, by more than 10 diopters, but 10 is about the same as for a normal eye," he says. Because the device would use most or all the available light at any given distance, images would be clearer and have greater color contrast than with a standard multifocal IOL, says Shen. It also means that the device should not be susceptible to glare or the halos around lights at night, as conventional IOLs now are.