The last frontier of corrective refractive surgery is to create a surgical procedure that corrects human accommodative loss. To understand this procedure some basic anatomy and physiology (function) of the human eye is necessary.
Inside the eye, just behind the iris (the part of eye that is colored) is located a clear disc shaped structure called the lens. The human lens is about the size of an M & M candy. Its function is to focus and present a clear image to the retina. Although the lens is made of living cells, it is unique because it is crystal clear and is soft and flexible. The lens is surrounded by a thin capsule, which looks exactly like saran wrap. This capsule is attached around its equator for 360° by tiny “strings” or zonules. These in turn attach to a circular series of miniature muscles called the ciliary muscles. By contracting and relaxing, these muscles tighten or loosen the zonules and change the tension on the capsule that surrounds the lens. This results in a change in the shape of the lens. It turns out that when the muscles relax the lens thickens. This allows one to shorten the focal length and focus on a near object. Conversely, when the ciliary muscles tighten, the lens gets thinner and focuses at distance. The process of focusing from distant to near objects is known as “accommodation”.
Accommodation is part of a reflex and requires no conscious effort. The lens is soft in young people and reading at near is no problem. However as we age the natural lens becomes less elastic and more rigid. By about age 40 people began to notice this and have increasing problems focusing on near objects. By the mid 40’s most people require aids for reading and submit to reading glasses or “cheaters”. This condition of loosing accommodative ability is known as presbyopia (Latin for old eyes). Unfortunately there is no escape from this natural aging process.
As we grow older still other changes take place in the lens. The living cells begin to die off and the clarity of the lens changes. It changes color and becomes cloudy. This is the process of developing a cataract. By the six or seventh decade, cataracts began to interfere with vision and must be removed surgically to restore natural vision. Today, when we remove a cataract, we take the cloudy lens out of the capsular bag. Once removed, we replace the cloudy lens with a clear plastic “lens implant”. Typically this greatly enhances the patient’s vision. In general, patients desire to restore their vision for distance. With rare exceptions, patients still need to wear glasses for near vision. Why?
The plastic lens implant we insert is not soft and cannot “accommodate”. As noted above, the holy grail of refractive surgery is to develop a procedure where the patient can accommodate after cataract surgery or to correct presbyopia.
Today we have made great strides to enhance patient’s vision. For example, LASIK surgery has been a revolution for nearsighted patients. Now with LASIK we can correct astigmatism and farsightedness, as I noted in earlier articles. But those LASIK patients over age 45 still need to wear reading glasses for near. LASIK surgery is performed on the front part of the eye known as the cornea. By changing its shape with a laser we can correct the patient’s refractive error. But LASIK does not restore accommodation, which is an activity of the lens.
Much research has been done to resolve this accommodative dilemma. Most doctors believe that surgery on the natural lens is the best way to solve the accommodative problem. How could this be done? One solution would be to surgically remove the cataract (or the clear lens in a patient over 45) and replace it with a flexible soft plastic lens that would change shape just as the lens in a young patient. The problem is that there must be no reaction by the body against the material, it must last for many years, it must remain in the capsule bag, and it must have no irregular surfaces, which could distort vision. This is a tall order, but investigators are close.
Another idea is to develop an implant, which has one or more hinges on it. This would allow the implant to move to and fro and duplicate accommodation. Several companies are working this angle. One company has gotten approval by the FDA for such a hinged implant. It is called the “Crystalens”. In the FDA study, a large number of patients obtained 20/20 for distance and near vision without the aid of glasses. This implant is the first to solve the problem of accommodation with an implant that can change shape in the eye.
I have been performing this surgery on a select group of cataract patients since July 2004. Anyone with an interest in discussing this surgery can arrange for a consult by calling (410) 224-4550.