The Future of Reading Vision Restoration

Accommodation, the process affording shape change of the natural human lens, is what allows adjustability for close focus in the young eye. This is an elegant and complex function that depends upon linkage and interaction between the ciliary muscle (the "focusing muscles" surrounding the lens, inside the eye), the zonules (tiny fibers that connect the ciliary muscle to the lens capsule), and the lens itself.

The lens is a unique organ. It maintains transparency through a combination of remarkable design, molecular biology and physiology. An excellent article on lens physiology appeared in the October, '04 Scientific American authored by Rolf Dahm, entitled Dying To See . The lens consists of layers of cells containing proteins called crystallines, arranged concentrically. This protein center is surrounded by and contained within a cellophane-thin membranous sheath called the lens capsule . In youth, the lens is pliable and elastic, allowing the pull of muscle tension (from the ciliary body, transmitted via the zonules) to bring about a shape change.

The lens of the human eye grows throughout life. Layers of clear protein are laid down just inside the lens capsule, but outside the existing layers of lens protein. As the lens grows through life it increases in thickness, diameter, density and stiffness. Thus the very process of maturing steadily reduces the ability to accommodate.

Accommodative amplitude is a measure of the amount of focusing adjustability of the eye. The eye of an infant has exceptional accommodative amplitude, allowing it to focus clearly up to a few inches away. This is important, as the infant's world depends on close vision -- it needs to recognize its mother (facial recognition and maternal feature recognition have been demonstrated at several weeks of age), and recognize a food source (bottle or breast).

Accommodative amplitude can be measured in diopters (focusing units, mathematically the inverse of the focal length in meters). An infant's eye may be capable of about 16 diopters of accommodation (allowing focus as close as 1/16th of a meter, or about 2.5 inches). As we mature, the lens grows, and lamentably, accommodative amplitude slowly decreases. The figure below shows average accommodative amplitude of the human lens as a function of age.

Accommodative Amplitude and Hardness of the Human Lens as a function of Age © All Rights Reserved.

Shape change of the lens (which allows accommodation) occurs by a complex process. In the resting state (with no "pull" from the ciliary muscle) the zonules are under tension as the ciliary muscle body is positioned farthest away from the equator of the lens. This tension on the zonules is transmitted to the lens, which encourages flattening of the central curvature of the lens, and optimum distance clarity. When close focus is desired, voluntary nerve input creates contraction of the ciliary muscle, bringing it more forward in position (front-to-back in the eye) and also closer to the equator of the lens. When this occurs, the zonules are under reduced tension, and the lens is allowed to "round up" or assume a more curved configuration, thus enabling close focus.

Current treatments to restore accommodation are a very good start, but are not *perfect* solutions that can restore complete accommodative facility and perfect clarity at all distances.

As anyone knows from their own personal experience, holding something too close can cause eye strain. This typically occurs when the pull of the ciliary muscle (focusing effort) exceeds about half of the available accommodative amplitude of the lens. So, for a person (with optimum or fully corrected distance vision) to comfortably focus at 19 inches (0.5 meters or 2.0 diopters of accommodation), the lens needs to have an accommodative amplitude of about 4.0 diopters. From the graph above you can see that the average 40-year-old person's lens has barely 4.0 diopters of accommodative capacity. This is why people start to feel some eye strain after their 40th birthday, even if they do not need to start wearing reading glasses until about 45 or so.

The CrystaLens HD-500 has at best about 2.5 diopters of accommodative capacity. Therefore, if distance vision is optimally corrected in the relaxed state (no focusing effort), the point of closest clear focus is about 16". While this is much better than any monofocal lens implant, it is still shy of a perfect goal for restoration of complete accommodative capability to the eye. For some people this is a very acceptable option (for this group, "the glass is half full"), while for others it may not be quite good enough (for this group, "the glass is half empty").

A multifocal lens implant such as the ReStor (made by Alcon) or the ReZoom (made by AMO) has different focal zones for different distance ranges. If you have ever worn a multifocal lens in a pair of glasses, you know that this also is not a perfect solution akin to restoring complete focus adjustability. Multifocal lenses force a compromise as some portions of the lens focus at far distance, and other portions focus at an intermediate range, and still other portions focus at close range. These lenses can cause some glare, halo, and slight reduction in maximum clarity at any distance compared to a single-focus lens intended to focus at the same distance. So, the multifocal lens is also not *perfect,* but for many is a significant improvement over conventional single focus implants and mandatory bifocals or monovision.

In the future it may be possible to use new and unique polymers to restore even more natural vision (and accommodation) than available at present.