Contact lenses - a brief description
Contact lenses are thin lenses placed directly on the surface of the eyes. Contact lenses are eye prosthetic devices used by over 150 million people worldwide, and can be worn for vision correction or for cosmetic or therapeutic reasons. In 2010, the global contact lens market was valued at $6.1 billion, while the US soft lens market was valued at $2.1 billion. Several analysts have estimated that the global contact lens market will reach $11.7 billion by 2015. As of 2010, the average age of contact lens wearers globally was 31, and two-thirds of wearers were women.
Types of contact lenses
Contact lenses are classified in different ways: according to their primary function, material, wear schedule (how long the lens can be worn), and replacement schedule (how long before the lens needs to be discarded).
Refractive error correction
Corrective contact lenses are designed to improve vision, usually by correcting refractive error. This is done by directly focusing the light so that it enters the eye with the proper strength for clear vision. Spherical contact lenses bend light equally in every direction (horizontally, vertically, etc.). They are usually used to correct myopia and hypermetropia. There are two ways that contact lenses can correct astigmatism. One way is with toric soft lenses that work in the same way as cylindrical prescription glasses; A toric lens has a different focusing power horizontally than vertically, and as a result can correct astigmatism. Another way is to use rigid gas-permeable lenses; since most astigmatism is caused by the shape of the cornea, rigid lenses can improve vision because the front surface of the optical system is a perfectly spherical lens. Both approaches have advantages and disadvantages. Toric lenses must have the proper orientation to correct astigmatism, so such lenses must have additional design features to prevent them from rotating out of alignment. This can be done by weighting the bottom of the lens or using other physical features to push the lens back into position, but these mechanisms rarely work perfectly, so some misalignment is common and results in somewhat imperfect correction and blurring of vision after rotating the lens blink.
Correction of presbyopia
Correction of presbyopia (the need for a prescription for reading different from the prescription needed for distance) presents an additional challenge in contact lens fitting. There are two main strategies: multifocal lenses and monovision. Multifocal contact lenses (eg, bifocal or progressive) are comparable to glasses with bifocal or progressive lenses because they have multiple focal points. Multifocal contact lenses are typically designed for constant viewing through the center of the lens, but some designs include shifting the position of the lens to see through the reading power (similar to bifocal glasses).
Hard contact lenses
Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do this were those made of polymethyl methacrylate (PMMA or Perspex/Plexiglas), now commonly referred to as "hard" lenses. Their main disadvantage is that they do not allow oxygen to reach the cornea, which can cause numerous unwanted, and often serious, clinical events. Beginning in the late 1970s, improved solid materials were developed that were permeable to oxygen. Contact lenses made from these materials are called rigid gas permeable lenses or 'RGP'. A solid lens is able to cover the natural shape of the cornea with a new refractive surface. This means that spherical rigid contact lenses can correct corneal astigmatism. Rigid lenses can also be made as front-toric, rear-toric or bi-toric. Rigid lenses can also correct corneas with irregular geometries, such as those with keratoconus or post-surgical ectasias. In most cases, patients with keratoconus see better through rigid lenses than through glasses. Hard lenses are more chemically inert, allowing them to be worn in more challenging environments where chemical inertness is important compared to soft lenses.
Soft contact lenses
Soft lenses are more flexible than hard lenses and can be rolled or bent slightly without damaging the lens. While rigid lenses require an adjustment period before comfort is achieved, new soft lens wearers typically report lens awareness rather than pain or discomfort. Hydrogel lenses rely on their water content to transport oxygen through the lens to the cornea. As a result, lenses with a higher water content allowed more oxygen to the cornea. In 1998, silicone hydrogel or Si-hi lenses became available. These materials have the extremely high oxygen permeability of silicone and the comfort and clinical performance of conventional hydrogels. Since silicone allows greater oxygen permeability than water, the oxygen permeability of silicone hydrogels is not related to the water content of the lenses. Lenses have now been developed with such oxygen permeability that they are approved for overnight wear (extended wear). Lenses approved for daily wear are also available in silicone hydrogel materials. Current brands of soft lenses are traditional hydrogel or silicone hydrogel. Due to the drastic differences in oxygen permeability, replacement schedule and other design features, it is very important to follow the instructions of the ophthalmologist who prescribes the lenses. When we compare traditional hydrogel soft contact lenses with silicone hydrogel versions, there is no clear evidence to recommend which lenses are superior. The disadvantages of silicone hydrogels are that they are a little harder and the surface of the lens can be hydrophobic, and therefore less "clutch" - factors that can affect the comfort of using the lens. New production techniques and changes in multipurpose solutions have reduced these effects to a minimum.
Hybrid contact lenses
There are a small number of hybrid lenses. Typically, these contact lenses consist of a rigid center and a soft "skirt". A similar technique is to "mount" a smaller, rigid lens on the surface of a larger, soft lens. These techniques are often chosen to provide the vision correction benefits of hard lenses and the comfort of soft lenses.
Carrying schedule
"Daily wear" contact lenses are designed to be worn for one day and removed at bedtime. Extended wear contact lenses are designed to be worn continuously overnight, usually up to 6 consecutive nights. Newer materials, such as silicone hydrogels, allow even longer wear periods of up to 30 consecutive nights. These extended wear lenses are often called "continuous wear". Such contact lenses can be worn overnight due to their high oxygen permeability. While awake, the eyes are mostly open, allowing oxygen from the air to dissolve in the tears and pass through the lens to the cornea. While you sleep, oxygen is supplied from the blood vessels in the back of the eyelid. A lens that interferes with the passage of oxygen to the cornea causes hypoxia of the cornea that can lead to serious complications, such as a corneal ulcer that, if left untreated, can permanently reduce vision.
Replacement schedule
Contact lenses are often categorized by replacement schedule. Disposable lenses (called daily or daily disposable lenses) are discarded after one use. Because they don't have to withstand the wear and tear of repeated use, these lenses can be thinner and lighter, greatly improving their comfort. Frequently replaced lenses collect less allergen and germ deposits, making these lenses preferable for patients with eye allergies or those prone to infections. Disposable lenses are also useful for people who rarely wear contact lenses or when lens loss is likely or not easy to replace (such as when on holiday). They are also considered beneficial for children as no cleaning or disinfection is required, resulting in improved compliance. Other disposable contact lenses are designed to be replaced every two or four weeks. Quarterly or annual lenses, which were once very common, are now much rarer. Rigid gas permeable lenses are very durable and can last for several years without needing to be replaced. PMMA hard materials were very durable and typically lasted 5 to 10 years, but they had several drawbacks.