Encyclopedia - Intraocular Lenses

Intraocular lenses (IOLs) are permanent, plastic lenses that are surgically implanted to replace or supplement the eye's natural crystalline lens. They have been used in the United States since the late 1960s to restore vision to cataract patients, and more recently are being used in several types of refractive eye surgery. To understand how an intraocular lens works, you need to understand how the eye's natural lenses function.

The eye works like a camera with two lenses. One lens is the cornea, a clear membrane that covers the front of the eye. The other lens is the eye's natural crystalline lens, which is located behind the pupil. The cornea is responsible for about 70 percent of the eye's focusing power, while the natural lens "fine-tunes" the image before it is focused on the retina at the back of the eye. The retina works like the film in a camera, receiving light images and sending them through the optic nerve to the brain (See Anatomy of the Eye). If both lenses are working properly, the image is focused precisely on the surface of the retina, and the result is perfect "20/20" vision.

A cataract occurs when the eye's natural crystalline lens becomes cloudy, usually because of the natural process of aging. The only way to treat a cataract is to remove the cloudy lens. Because the eye cannot focus properly without this lens, most modern cataract surgery includes the implantation of a permanent prescription intraocular lens. Most intraocular lenses used in cataract surgery can be folded and inserted through the same tiny opening that was used to remove the natural lens. Once in the eye, the lens unfolds to its full size. The opening in the eye is so small that it heals itself quickly without stitches. The intraocular lenses are made of inert materials that do not trigger rejection responses by the body.

IOLs are permanent. They do not get lost nor do they have to be replaced. Careful measurement of the eye prior to surgery can determine the focusing power of the IOL to correct nearsightedness or farsightedness.

Most intraocular lenses implanted during cataract surgery are monofocal lenses, meaning the prescription is set for one distance -- usually for distance vision. Therefore, you will probably still need reading glasses after surgery. A new intraocular lens implant, the multifocal lens, functions more like the eye's natural lens by providing clear vision at a distance and good focus for a range of near distances. Although not all patients are good candidates for the multifocal lens, those who can use the lens are generally pleased with the results.

Several new refractive eye surgery procedures now rely on intraocular lenses rather than laser surgery to correct vision deficiencies. Unlike laser surgery, which sculpts the cornea to correct refractive disorders, these new procedures replace or supplement the eye's natural crystalline lens with an intraocular lens. Clear Lens Extraction (CLE) works like cataract surgery in that the eye's natural lens is removed and replaced with a prescription intraocular lens. The only difference is that the natural lens being replaced by the CLE procedure is, as the name implies, clear. Studies indicate that CLE may eventually be the procedure of choice for patients with farsightedness.

Another refractive surgery procedure (currently investigational) that uses an intraocular lens is referred to as Implantable Contact Lens (ICL) surgery. With this procedure, a prescription implantable lens is inserted inside the eye between the iris and the natural lens of the eye. This intraocular lens is prescription-engineered to correct refractive error by complementing the focusing powers of the natural inner lens of the eye.

A different refractive surgery procedure implants a tiny plastic lens, similar to a contact lens, within the corneal tissue to correct problems of nearsightedness. These implants, called Intracorneal Lens Implants, are under FDA investigational protocol, as are the ICL lenses.

Also under FDA investigation is an intraocular lens that holds promise for correcting presbyopia, the vision problem that occurs when the natural lens of the eye becomes less flexible. This condition inhibits the reflex ability of the lens to contract or expand in order to focus properly on nearby objects. Because this condition comes about as a person matures, most people over the age of 45 need reading glasses. Known as a Small Diameter Corneal Inlay (SDCI), this prescription lens is inserted into the corneal tissue to create an effect similar to a bifocal contact lens.