A contact lens, or simply contact or CL, is a thin lens placed directly on the surface of the eye. CLs are considered medical devices and can be worn to correct vision, or for cosmetic or therapeutic reasons.
People choose to wear CLs for many reasons. Aesthetics and cosmetics are the main motivating factors for people who want to avoid wearing glasses or to change the appearance of their eyes. When compared with spectacles, CLs typically provide better peripheral vision, and do not collect moisture (from rain, snow, condensation etc.) or perspiration; this makes them ideal for sports and other outdoor activities. CL wearers can also wear sunglasses, goggles, or other eyewear of their choice without having to fit them with prescription lenses or worry about compatibility with glasses
Corrective CLs are designed to improve vision, most commonly by correcting refractive error.
Multifocal contact lenses (e.g. bifocals or progressives) are comparable to spectacles with bifocals or progressive lenses because they have multiple focal points. Monovision is the use of single vision lenses
A cosmetic CL is designed to change the appearance of the eye. These lenses may, of course, also correct refractive error
Scleral lens, with visible outer edge resting on the sclera of a patient with severe dry eye syndrome
A scleral lens is a large, firm, oxygen permeable CL that rests on the sclera and creates a tear-filled vault over the cornea. The cause of this unique positioning is usually relevant to a specific patient whose cornea is too sensitive to support the lens directly. Scleral lenses may be used to improve vision and reduce pain and light sensitivity for people suffering from a growing number of disorders or injuries to the eye, such as severe dry eye syndrome (Kerotanconjuctivis sicca), microphthalmia, keratoconus, corneal ectasia, Stevens–Johnson syndrome, Sjögren's syndrome, aniridia, neurotrophic keratitis (aneasthetic corneas), complications post-LASIK, high order Aberrations of the eye, complications post-corneal transplant and pellucid degeneration. Injuries to the eye such as surgical complications, distorted corneal implants, as well as chemical and burn injuries also may be treated with scleral lenses
Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage CL protects an injured or diseased cornea from the constant rubbing of blinking eyelids thereby allowing it to heal
Contact lenses, other than the cosmetic variety, become almost invisible once inserted in the eye. Most corrective CLs come with a light "handling tint" that renders the lens slightly more visible on the eye. Soft CLs extend beyond the cornea, their rim sometimes visible against the sclera.
A rigid lens is able to replace the natural shape of the contact (cornea) with a new refracting surface. This means that a spherical rigid CL can correct for astigmatism. Rigids can also be made as a front-toric, back-toric, or bitoric. This is different from a spherical CL in that one or both surfaces of the lens deliver a toric correction. Rigids can also correct for corneal irregularities, such as keratoconus. In most cases, patients with keratoconus see better through rigids than through glasses. Rigids are more chemically inert, allowing them to be worn in more challenging environments than soft lenses.
These materials have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. Because silicone allows more oxygen permeability than water, oxygen permeability of silicone hydrogels is not tied to the CL’s water content. Lenses have now been developed with so much oxygen permeability that they are approved for overnight wear (extended wear). Lenses approved for daily wear are also available in silicone hydrogel materials
Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic, thus less "wettable" – factors that influence comfort of lens use. New manufacturing techniques and changes to multipurpose solutions have minimized these effects. A surface modification process called plasma coating alters the lens surface’s hydrophobic nature. Another technique incorporates internal rewetting agents to make the lens surface hydrophilic. A third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents.
A small number of hybrid lenses exist. Typically these CLs consist of a rigid center and a soft "skirt". Similar technique is "piggybacking" of a smaller, rigid lens on the surface of a larger, soft lens