Few of us who have reached the half century mark can forget the sight of our grandparents wearing their Coke-bottle eyeglasses after having surgery to remove a cataract. Those extremely thick glasses were a feeble attempt to replace the natural lens.
Because eyeglasses are worn in front of the eye, the corrective lenses had to be very thick in order to adequately bend images for the retina. These thick lenses had huge disadvantages and at best produced poor overall visual acuity. Later, intraocular lens (IOL) implants were developed, a remarkable advancement.
Interestingly, the inert material initially used for IOL implants was the same material (polymethylmethacrylate) used in WWII warplane windshields. Howard Ridley, a Royal Air Force ophthalmologist, observed no patient rejection or reaction related to fragments of this material in pilots who had pieces of the windshield penetrate the eye after combat. He implanted a PMMA lens in 1949 after removing a cataract, but it took several decades for this knowledge to develop into the practice of implanting lenses in patients after cataract removal.
But as revolutionary as the original intraocular lens implants were, there were two major flaws: implantation required making a large incision in the patient's eye, and once the lens had been implanted, the patient could see clearly only at far distances.
The patient required glasses to see near and intermediate distances.
Two different technologies exist that allow for patients undergoing cataract surgery to avoid eyeglasses and still be able to focus in all three important distances — near, intermediate, and far. Part of this advancement uses the same approach contact lens wearers have used for years — namely the ability of the brain to "choose" the best image that either the right or left eye presents. This adaptive talent allows a person to use one pair of eyeglasses for reading and distance, even though each lens prescription is different.
This adaptation also helps us in managed care to understand why the outcome measures used for IOL implants use "binocular" measures. Each IOL is "powered" differently to allow for overall focused vision when viewing the world with both eyes.
A focusing lens
The first product that allows for focusing is manufactured by Eyeonics and is called the CrystaLens Accommodating IOL. This lens changes shape when the patient attempts to focus. It is a biconvex lens made with polyimide loops and hinges. It is designed for placement into the capsular bag — that space left after the surgeon has removed the diseased lens from the eye. This lens is connected to the same muscles within the eye that contract to change the shape of a natural lens. This lens changes shape — and hence is able to focus in much the same manner as a natural lens — allowing approximately one diopter of accommodation.
The surgery takes about 15 to 20 minutes per eye and is conducted under local anesthesia on an outpatient basis. Interestingly, the ability to focus using the CrystaLens improves over time as the muscles that control it regain strength and allow for more focusing of the artificial lens. Cataracts cause eye muscles to weaken due to the inability of those muscles to expand and contract the rigid cataract.
Clinical studies demonstrated that over 93 percent of subjects achieved bilateral uncorrected visual acuities of 20/32 or better at near, intermediate, or far distances one year after surgery. Other data demonstrated that nearly 94 percent of patients receiving bilateral replacements were able to perform most visual functions, and nearly 26 percent reported never wearing glasses for any activity day or night. Nearly 50 percent reported almost never needing to wear glasses.
The second approach to allow for "focusing" in post cataract removal patients involves a totally different technology. There is no mechanical movement of the lens to produce the desired focus of near, intermediate and far. This IOL implant by Alcon (called AcrySof Restor) uses apodized diffractive and refractive technologies to provide a full range of vision.
Apodization is a gradual bending of the diffractive step heights. This biconvex lens has a series of small, soft, concentric steps on its surface. Each concentric step diffracts light in a slightly different manner which allows the focusing of images from different distances on the retina. It is like having several different lenses in one. But it also projects numerous images to the retina requiring the brain to process the images to create the final visual acuity.
For this reason, patients with bilateral cataracts should have the second eye operated on shortly after the first to enable the brain to learn and adapt to the images being presented to the retina.
Some visual effects may occur due to the superposition of both focused and unfocused multiple images. These include perception of halos or radial lines around point sources of light under night-time conditions.
There is also some decrease in contrast resulting in some difficulty in low-light activities compared to a traditional monofocal IOL. The manufacturer cautions against implanting this lens in professional drivers and those patients who have a lifelong complaint about night-time glare.
Both lenses faced several FDA approval hurdles. Obviously efficacy was important. In addition, each had to pass a battery of in vivo and in vitro acute and chronic toxicity tests to establish biocompatibility.
There are some general IOL contraindications such as presence of inflammation or infection within the eye, especially difficult cataract removal, or a defect in the eye that precludes normal placement of the lens. In general, safety and effectiveness has not been established in potential IOL patients with amblyopia, diabetic retinopathy, prior corneal transplant, history of retinal detachment or congenital bilateral cataracts.
With these conditions, the surgeon should weigh the risks and benefits. Each patient offers a unique combination of prior medical and vision needs, as well as unique and complex eye measures. Physician judgment is required in selecting the proper lens for each patient.
Obviously, the majority of people requiring cataract surgery are over 65. Medicare has traditionally and still covers the cost of cataract removal and the traditional monofocal lenses. In the past, Medicare did not pay for these new presbyopia-correcting lenses, and as a result, patients wishing to take advantage were locked out. This changed in May of last year.
Patients wishing to receive the newer IOLs may do so, but must pay additional amounts that vary widely by provider and type of extra surgery needed (such as LASIK to correct other visual problems such as astigmatism, which incidentally is not correctable with any IOL technology).
Gone are the Coke-bottle glasses. Now, most people can benefit from the remarkable advances in intraocular lens technology. Tomorrow's Medicine promises even more choices as research in vision science advances at an ever increasing pace!
Dr. Morrow discloses that he has received honoraria or other financial benefits during the last three years from the following commercial companies: Amgen, Amylin Pharmaceuticals, AstraZeneca, Biogen Idec, Centocor, Galderma, Genentech, GlaxoSmithKline, Johnson & Johnson, Merck, Novartis, Novo Nordisk, Pfizer, Procter & Gamble, Q-Med, Sanofi-Aventis, Teva Pharmaceuticals Industries, UCB, and Wyeth.