Corrective contact lenses
Corrective contact lenses are designed to improve vision, most commonly by correcting refractive error. This is done by directly focusing the light so that it enters the eye with the proper power for clear vision. Recently, there has been renewed interest in orthokeratology, the correction of myopia by deliberate overnight flattening of the corneal epithelium, leaving the eye without a refractive error during the day.
A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct myopia and hyperopia. A toric contact lens has a different focusing power horizontally than it does vertically, and as a result can correct for astigmatism. Some spherical rigid lenses can also correct for astigmatism. (See below.) Because a toric lens must have the proper orientation to correct for a person's astigmatism, a toric contact lens must have additional design characteristics to prevent the lens from rotating out of the ideal alignment. This can be done by weighting the bottom of the lens or by using other physical characteristics to rotate the lens back into position. Some toric contact lenses have marks or etchings that can assist the eye doctor in fitting the lens. The first disposable toric lenses were introduced in 2000 by Vistakon.
The correction of presbyopia (a need for a reading prescription that is different from the prescription needed for distance) presents an additional challenge in the fitting of contact lenses. Two main strategies exist: multifocal contact lenses and monovision. Multifocal contact lenses are comparable to bifocals 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 do incorporate a shift in lens position to view through the reading power (similar to bifocal glasses). Monovision[21] is the use single vision lenses (one focal point per lens) to focus one eye for distance vision (typically the person's dominant eye) and the other eye for near work. The brain then learns to use this setup to see clearly at all distances. A technique called modified monovision uses multifocal lenses and also specializes one eye for distance and one eye for near. Alternatively, a person may simply wear reading glasses over their distance contact lenses. Other types of vision correction
For those with certain color deficiencies, a red-tinted "X-Chrom" contact lens may be used. Although the lens does not restore normal color vision, it allows some colorblind individuals to distinguish colors better.[22][23]
ChromaGen lenses have been used and these have been shown to have some limitations with vision at night although otherwise producing significant improvements in color vision.[24] An earlier study showed very significant improvements in color vision and patient satisfaction.[25]
Later work that used these ChromaGen lenses with dyslexics in a randomised, double-blind, placebo controlled trial showed highly significant improvements in reading ability over reading without the lenses[26] This system has been granted FDA approval in the USA. Cosmetic contact lenses A woman wearing a cosmetic type of contact lenses; the enlarged section of the image shows the grain produced during the manufacturing process. Curving of the lines of printed dots suggests these lenses were manufactured by printing onto a flat sheet then shaping it.
A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct refractive error. Although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change the color of the eye are far less common, accounting for only 3% of contact lens fits in 2004.[27]
In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection.[28] For this reason all contact lenses, even purely cosmetic ones, are classified as medical devices in many countries (USA, UK). All individuals who would like to wear cosmetic lenses should have a contact lens examination with an eye doctor prior to first use, and if used long-term, regular aftercare examinations, in order to avoid potentially blinding complications.
Cosmetic lenses can be used to drastically alter the appearance of the eye, as seen in the entertainment industry. Scleral lenses that cover the white part of the eye (i.e., sclera) are used in many theatrical applications. These lenses are typically custom made for a specific production and as a result have very limited availability to the general public. As with any cosmetic lens, if the design changes the clarity of the center of the lens, the lens may interfere with vision.
A new trend in Japan, South Korea and China is the circle contact lens. Circle lenses extend the appearance of the iris onto the sclera. The result is the appearance of a bigger, wider iris.
Cosmetic lenses can have more direct medical applications. For example, some lenses can restore the appearance and, to some extent the function, of a damaged or missing iris.