Accommodative arching explains how IOL, natural crystalline lens work

February 1, 2006

Indianapolis—A process called accommodative arching is what Kevin Waltz, MD, OD, uses to describe how the crystalens IOL (eyeonics Inc.) and the natural crystalline lens work in the eye.

Dr. Waltz theorized that the substantial thinness of the crystalens may account for its ability to change shape, which may result in increased accommodative ability. He described his theory and demonstrated how changes in the crystalens are similar to those in a natural crystalline lens at the annual meeting of the American Academy of Ophthalmology.

The importance of the availability of an IOL that can simulate the movements of a natural crystalline lens is underscored when the presbyopic population is considered, according to Dr. Waltz.

Accommodative arching theory

The accommodative arching theory, Dr. Waltz said, has not been described previously. In line with this theory, similar changes are seen with accommodative effort in both the natural crystalline lens and with the crystalens accommodating IOL.

When the eye accommodates, Dr. Waltz said, there is a nonsymmetrical response within the lens that results in a characteristic wavefront pattern and an increase in higher-order aberrations.

"Most people believe that when the wavefront image shows a uniform green image with distance vision, indicating emmetropia, the image becomes progressively redder, indicating myopia, with accommodation," he said. "However, this is not the case. In actuality, the natural crystalline lens changes shape in a pattern that is very specific to each individual. This was first described in the 1930s in research to determine shape changes in the lens. This information has been lost over the years to most clinicians."

As the lens changes shape, the wavefront image shows patterns of red, blue, and green. "When more red is observed, the more the eye is changing to focus at near. This is the typical change in a young eye," he said. With aging there is more blue and less red in the images. By the time the subject is in his or her seventh decade of life, there is barely any change in color representing accommodation.

The original theory behind how the crystalens provides accommodation is that the lens flexes back and forth, indicating anterior/posterior displacement.

"As the lens is pushed forward, the dioptric power increases. This probably happens," he said.

"However, just as important," Dr. Waltz continued, "the crystalens also probably changes shape because of the force imposed on it by the ciliary body, as the natural crystalline lens does. The process of changing shape by the crystalens is remarkably similar to that of the natural crystalline lens in that there is a bit of anterior/posterior displacement as in the original lens and there is a great deal of change in shape of the radius of curvature of the crystalens.

"The radius of curvature actually decreases, which increases the focusing power of that part of the lens and helps the eye to accommodate," he said.

In discussing his theory, Dr. Waltz contends that the anterior/posterior movement of the lens is insufficient to provide accommodation.

"Not enough power change can be obtained by moving the lens anterior to posterior," he stated.

Interestingly, he noted, if a high-power lens for a patient with hyperopia were moved forward, more effect would be achieved than for a patient who is myopic.