A contact lens is a small lens worn directly on the surface of the eye to aid or correct defective vision. Modern contact lenses are of two main types – soft and rigid gas permeable (RGP). Both types are made of materials that allow oxygen to pass through to the cornea, which is vitally important for the health of the eye.
Earlier "hard" contact lenses prevented the passage of oxygen. This meant they had to be made small so that the cornea could still get oxygen from the part that wasn't covered by the lens. The old hard lenses made blinking uncomfortable and had a habit of popping out. By contrast, today's lenses are larger in diameter, which enhances comfort and peripheral vision while at the same time reducing the image distortion that can be caused by some spectacles.
History of contact lenses
In 1508 Leonardo da Vinci composed a journal, Codex of the Eye, Manual D, in which he described how the perception of the eye alters when opened in the water. Da Vinci was not interested in how to correct faulty vision, however; what concerned him was how the eye alters its optics to maintain clarity of vision when viewing an object at varying distances. For the artist, the eye was key to everything, but it never occurred to him, apparently, that a lens could be placed over the eye to correct faulty vision.
This idea would have to wait another 128 years. In 1636, French thinker and father of modern philosophy René Descartes pondered whether, if a person were to fill a glass tube with water and then place that tube over the eye's cornea, it might have the effect of correcting that person's vision. Descartes's idea was innovative but impractical, as the glass tube he proposed would be to thick to allow blinking.
In 1801 English scientist and founder of physiological optics Thomas Young coined the term "astigmatism" and constructed a rudimentary set of contact lenses following Descartes's design principle. However, it would not be until 1887 that the first functioning set of contact lenses was made and fitted, in Zurich, Switzerland, by the German-born ophthalmologist Adolf Fick (1829–1901). Made from heavy blown glass, Fick's lenses ranged in diameter from 19mm to 21mm and were shaped after first taking casts of rabbit eyes and those of deceased humans. Instead of resting directly on the cornea, these lenses were placed on the eye around it.
Soft contact lenses
Soft contact lenses are made of flexible, oxygen-permeable plastic that contains between 30% and 80% water. For most people, soft contact lenses are easy to get used to, and are comfortable and secure to wear. Soft lenses can correct not only myopia (near-sightedness) and hyperopia (far-sightedness) but also many types of astigmatism and presbyopia. Soft contact lenses are available in daily wear (including daily disposable) and extended wear forms. See below for more details.
Rigid gas permeable (RGP) contact lenses
Rigid gas permeable contact lenses (RGPs) combine some of the qualities of soft and hard lenses. Made of special firmer plastics, which are permeable to oxygen, these lenses are more durable, generally give a clearer, crisper vision, and usually have a longer life span than soft lenses. Many people find them easier to handle than soft lenses and they're less prone to tear. However, they are not as comfortable initially as soft contacts and it may take a few weeks to get used to wearing RGPs, compared to several days for soft contacts. RGPs may last for several years without the need for replacement. They are often prescribed for people who have high degrees of astigmatism.
Daily wear contact lenses
Most contact lenses are worn on a "daily-wear" basis. They are taken out in the evening, stored in a special solution, and put back in the next morning.
Daily disposables are worn only once and thrown away each night. They are especially handy for people who use contacts infrequently, or for activities, such as swimming and various sports, where losing a lens is more likely. Day disposables are also often recommended for patients with ocular allergies or other conditions, because this type of lens limits deposits of antigens and protein.
The commoner type of daily wear lenses are those prescribed to be disposed of on a two-week or monthly basis.
Extended wear contact lenses
Extended wear contact lenses are designed for overnight or continuous wear, ranging from one to six nights or even longer. The latest extended wear contacts use silicone hydrogels, with 5 to 6 times the oxygen permeability of conventional soft lenses, and can be worn continuously for up to 30 days. However, extended lens wearers may have an increased risk for corneal infections and corneal ulcers. The length of recommended continuous wear depends both on lens type and your eye care professional's evaluation of your tolerance for overnight wear. It's important for the eyes to have a rest without lenses for at least one night following each scheduled removal.
Toric contact lenses
Specialized uses of contact lenses
Conventional contact lenses correct vision in the same way that glasses do, but do so by being in direct contact with the eye. Two types of lenses that serve a different purpose are orthokeratology lenses and decorative (plano) lenses.
Orthokeratology, or Ortho-K, is a lens fitting procedure that uses specially designed rigid gas permeable (RGP) contact lenses to change the curvature of the cornea to temporarily improve the eye's ability to focus on objects. This procedure is primarily used for the correction of myopia (nearsightedness).
Overnight Ortho-K lenses are the most common type of Ortho-K. There are some Ortho-K lenses that are prescribed only for daytime wear. Overnight Ortho-K lenses are commonly prescribed to be worn while sleeping for at least eight hours each night. They are removed upon awakening and not worn during the day. Some people can go all day without their glasses or contact lenses. Others will find that their vision correction will wear off during the day.
The vision correction effect is temporary. If Ortho-K is discontinued, the corneas will return to their original curvature and the eye to its original amount of nearsightedness. Ortho-K lenses must continue to be worn every night or on some other prescribed maintenance schedule in order to maintain the treatment effect. Your eye care professional will determine the best maintenance schedule for you.
Currently, FDA requires that eye care professionals be trained and certified before using overnight Ortho-K lenses in their practice. You should ask your eye care professional about what lenses he or she is certified to fit if you are considering this procedure.
Decorative (plano) contact lenses
Some contact lenses do not correct vision and are intended solely to change the appearance of the eye. These are sometimes called plano, zero-powered or non-corrective lenses. For example, they can temporarily change a brown-eyed person's eye color to blue, or make a person's eyes look "weird" by portraying Halloween themes. Even though these decorative lenses don't correct vision, they're regulated by the FDA, just like corrective contact lenses. They also carry the same risks to the eye. These risks include: