CONTACT LENSES
Contact lens, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic or therapeutic reasons.
Corneal and rigid lenses (1949–1960s)
In 1949, the first "corneal" lenses were developed. These were much smaller than the original scleral lenses, as they sat only on the cornea rather than across all of the visible ocular surface, and could be worn up to 16 hours a day. PMMA corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing technology. On October 18, 1964, in a television studio in Washington, D.C., Lyndon Baines Johnson became the first President in the history of the United States to appear in public wearing contact lenses, under the supervision of Dr. Alan Isen, who developed the first commercially viable soft-contact lenses in the United States.
Contact lenses are a great option for almost everyone who needs vision correction and doesn't want to wear glasses all the time or have surgery.
Contact lens materials
The first option when considering contact lens wear is which material will best suit your needs. There are five types of contact lenses, depending on the type of material with which they are made:
- Soft lenses are made of gel-like plastics with a water content, called a hydrogel. These lenses are very thin and malleable and conform to the anterior surface of the eye.
- Silicone hydrogel lenses are an advanced type of soft contact lenses that are more porous than common hydrogel lenses and allow even more oxygen to reach the cornea.
- Gas permeable lenses — also called GP or RGP lenses — are rigid lenses that look and feel like PMMA lenses but are porous and allow oxygen to pass through. Because they are permeable to oxygen, GP lenses can fit closer to the eye than PMMA lenses, providing more comfort than conventional hard lenses. Gas permeable lenses have basically replaced PMMA contact lenses, which are non-porous. GP contact lenses tend to offer sharper vision than soft lenses and silicone hydrogel lenses, especially if you have astigmatism.
- Hybrid contact lenses are designed to provide comfortable wear that rivals soft or silicone hydrogel lenses, in combination with the exceptionally clear vision of gas permeable lenses. Hybrid lenses have a rigid gas-permeable central zone surrounded by a "skirt" of hydrogel material or silicone hydrogel material.
- PMMA lenses are made of a rigid, transparent material called polymethyl methacrylate (PMMA), which is also used as a substitute for shatterproof glass in windows. PMMA lenses have excellent optics, but they do not transmit oxygen to the eyes and adjusting to them can be difficult. These "hard lenses" (now outdated) have practically been replaced by GP lenses and are rarely prescribed today.
Contact lens wear time
Until 1979 everyone who wore contact lenses took them off and cleaned them every night. The introduction of "extended wear" allowed wearers to sleep with their glasses on. Now, two types of lenses are classified by time of use:
- Daily use - should be removed at night
- Long-term use - can be used overnight, usually for a period of seven consecutive days without removal.
Contact lens designs
Soft contact lenses (both normal hydrogel and silicone hydrogel) come in a variety of designs, depending on the intended purpose:
- Spherical contact lenses have the same lens power throughout the optical part of the lens to correct myopia (near vision) or hyperopia (distance vision).
- Toric soft contact lenses have different powers in the different meridians of the lens to correct astigmatism, as well as myopia or hyperopia.
- Multifocal contact lenses (even bifocal contact lenses) contain different zones for near and far vision to correct presbyopia, as well as myopia or hyperopia.
- Cosmetic contact lenses include colored contact lenses designed to change or intensify the color of your eyes. Halloween, theatrical effects, and other special effects contact lenses are also considered cosmetic lenses.
