Contact lenses are classified in many different manners.
Functions
Corrective contact lenses
A corrective contact lens are designed to improve vision. For many people, there is a mismatch between the refractive power of the eye and the length of the eye, leading to a refraction error. A contact lens neutralizes this mismatch and allows for correct focusing of light onto the retina. Conditions correctable with contact lenses include myopia (near or short sightedness), hypermetropia (far or long sightedness), astigmatism and presbyopia. Contact wearers must usually take their contact lenses out every night or every few days, depending on the brand and style of the contact. Recently, there has been renewed interest in orthokeratology, the correction of myopia by deliberate overnight flattening of the cornea, leaving the eye without contact lens or eyeglasses correction during the day.
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.
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. An earlier study showed very significant improvements in color vision and patient satisfaction.
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 This system has been granted FDA approval in the USA.
Cosmetic contact lenses
A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the color or design. In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. These types of lenses tend to cause mild irritation on insertion, but after accustoming to the lenses, the eyes are typically well tolerated. As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection. All individuals who decide to wear cosmetic lenses should check with an eye care provider prior to first use, and periodically over long term use in order to avoid potentially blinding complications.
Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the entertainment industry to make the eye appear confusing and arousing in appearance, most often in horror film and zombie movies, where lenses can make one's eyes appear demonic, cloudy and lifeless, or even to make the pupils of the wearer appear dilated to simulate the natural appearance of the pupils under the influence of various illicit drugs.
Scleral lenses cover the white part of the eye (i.e. sclera) and are used in many theatrical lenses. Due to their size, these lenses are difficult to insert and do not move very well within the eye. They may also hamper the vision as the lens has a small area for the user to see through. As a result they generally cannot be worn for more than 3 hours as they can cause temporary vision disturbances.
Similar lenses have more direct medical applications. For example, some lenses can give the iris an enlarged appearance, or mask defects such as absence of (aniridia) or damage to (dyscoria) the iris.
A new trend in Japan, South Korea and China is the circle contact lens. Circle lenses appear to be bigger because they are not only tinted in areas that cover the iris of the eye, but tinted prominently in the extra-wide outer ring of the lens. The result is the appearance of a bigger, wider iris.
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.
As a specialist's tool, in the hands of the untrained general public, non-prescription cosmetic contact lenses may represent a health risk.
Therapeutic contact lenses
Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lens protects an injured or diseased cornea from the constant rubbing of blinking eyelids thereby allowing it to heal. They are used in the treatment of conditions including bullous keratopathy, dry eyes, corneal ulcers and erosion, keratitis, corneal edema, descemetocele, corneal ectasis, Mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis. Contact lenses that deliver drugs to the eye have also been developed.
Constructional material
The first contact lenses were made of glass, which caused eye irritation, and were not wearable for extended periods of time. But when William Feinbloom introduced lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas), contact lenses became much more convenient. These PMMA lenses are commonly referred to as "hard" lenses (this term is not used for other types of contact lens).
However, PMMA lenses have their own side effects: no oxygen is transmitted through the lens to the cornea, which can cause a number of adverse clinical events. In the late 1970s, and through the 1980s and 1990s, improved rigid materials—which were also oxygen-permeable—were developed. Collectively, these polymers are referred to as rigid gas permeable or 'RGP' materials or lenses. One advantage of hard lenses is that, due to their non-porous nature, they do not absorb chemicals or fumes. The absorption of such compounds by other types of contacts can be a problem for those who are routinely exposed to painting or other chemical processes.
Rigid lenses offer a number of unique properties. In effect, the lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a regular (spherical) rigid contact lens can provide good level of vision in people who have astigmatism or distorted corneal shapes as with keratoconus.
While rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in soft lenses made by Otto Wichterle led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the approval of the 'Soflens' material (polymacon) by the United States FDA in 1971. Soft lenses are immediately comfortable, while rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability by varying the ingredients making up the polymers.
A small number of hybrid rigid/soft lenses exist. An alternative technique is piggybacking of contact lenses, a smaller, rigid lens being mounted atop a larger, soft lens. This is done for a variety of clinical situations where a single lens will not provide the optical power, fitting characteristics, or comfort required.
In 1999, 'silicone hydrogels' became available. Silicone hydrogels have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. These lenses were initially advocated primarily for extended (overnight) wear, although more recently daily (no overnight) wear silicone hydrogels have been approved and launched.
While it provides the oxygen permeability, the silicone also makes the lens surface highly hydrophobic and less "wettable." This frequently results in discomfort and dryness during lens wear. In order to compensate for the hydrophobicity, hydrogels are added (hence the name "silicone hydrogels") to make the lenses more hydrophilic. However the lens surface may still remain hydrophobic. Hence some of the lenses undergo surface modification processes by plasma treatments which alter the hydrophobic nature of the lens surface. Other lens types incorporate 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.
Wear Schedule/Wear Indicator
A daily wear (DW) contact lens is designed to be removed prior to sleeping. An extended wear (EW) contact lens is designed for continuous overnight wear, typically for 6 or more consecutive nights. Newer materials, such as silicone hydrogels, allow for even longer wear periods of up to 30 consecutive nights; these longer-wear lenses are often referred to as continuous wear (CW). Generally, extended wear lenses are discarded after the specified length of time, according to the replacement schedule (see next section). Extended- and continuous-wear contact lenses can be worn for such long periods of time because of their high oxygen permeability to the cornea (typically 5–6 times greater than conventional soft lenses), which allows the eye to remain healthy even when the eyelid is closed.
Extended lens wearers may have an increased risk for corneal infections and corneal ulcers, primarily due to poor care and cleaning of the lenses, tear film instability, and bacterial stagnation. Corneal neovascularization has historically also been a common complication of extended lens wear, though this does not appear to be a problem with silicone hydrogel extended wear. The most common complication of extended lens use is conjunctivitis, usually allergic or giant papillary conjunctivitis (GPC), sometimes associated with a poorly fitting contact lens.
Replacement Schedule
The various soft contact lenses available are often categorized by their replacement schedule. The shortest replacement schedule is single use (1-day or daily disposable) lenses which are disposed of each night. Shorter replacement cycle lenses are commonly thinner and lighter, due to lower requirements for durability against wear and tear, and may be the most comfortable in their respective class and generation. These may be best for patients with ocular allergies or other conditions because it limits deposits of antigens and protein, and is considered the healthiest wear schedule due to the most frequent replacement. Single use lenses are also useful for people who use contacts infrequently, or for purposes (e.g. swimming or other sporting activities) where losing a lens is likely.
More commonly, contact lenses are prescribed to be disposed of on a two-week or 4-week basis. Quarterly or annual lenses, which used to be very common, have lost favor because a more frequent replacement allows for increased comfort and fewer on-lens deposits. Rigid gas permeable lenses are very durable and may last for several years without the need for replacement. PMMA hard lenses were very durable, and were commonly worn for 5 to 10 years. Interestingly, a careful analysis of the materials used to manufacture many 'daily' disposable lenses show that they are often manufactured from the same material as the longer life disposables (4-week replacement for example), from the same company. Although the materials are the same, the manufacturing processes by which the respective contact lenses are made is what differentiates a 'daily disposable' lens from a lens recommended for two-week or 4-week replacement.
Contrary to popular belief, replacement schedule is not determined by the Food & Drug Administration (FDA). Replacement schedule is recommended only by the manufacturer of that contact lens. The only FDA-approved measure of contact lens wear is the 'wear indication' or 'wear schedule' (extended wear or daily wear) as was discussed in the previous section.
Design
A spherical contact lens is one in which both the inner and outer optical surfaces are portions of a sphere. A toric lens is one in which either or both of the optical surfaces have the effect of a cylindrical lens, usually in combination with the effect of a spherical lens. Myopic (nearsighted) and hypermetropic (farsighted) people who also have astigmatism and who have been told they are not suitable for regular contact lenses may be able to use toric lenses. If one eye has astigmatism and the other does not, the patient may be told to use a spherical lens in one eye and a toric lens in the other. Toric lenses are made from the same materials as regular contact lenses but have a few extra characteristics:
• They correct for both spherical and cylindrical aberration.
• They may have a specific 'top' and 'bottom', as they are not symmetrical around their centre and must not be rotated. Lenses must be designed to maintain their orientation regardless of eye movement. Often lenses are thicker at the bottom and this thicker zone is pushed down by the upper eyelid during blinking to allow the lens to rotate into the correct position (with this thicker zone at the 6 o'clock position on the eye). Toric lenses are usually marked with tiny striations to assist their fitting.
• They are usually more expensive to produce than non-toric lenses; therefore they are usually meant for extended wear. The first disposable toric lenses were introduced in 2000 by Vistakon.
Like eyeglasses, contact lenses can have one (single vision) or more (multifocal) focal points.
For correction of presbyopia or accommodative insufficiency multifocal contact lenses are almost always used; however, single vision lenses may also be used in a process known as monovision: single vision lenses are used to correct one eye's far vision and the other eye's near vision. Alternatively, a person may wear single vision contact lenses to improve distance vision and reading glasses to improve near vision.
Some contact lenses have small text written around the edge, such as 123 or AV. These are used to identify if the contact lens in put on correctly or not.
Rigid gas permeable bifocal contact lenses most commonly have a small lens on the bottom for the near correction, when the eyes are lowered to read, this lens comes into the optical path. RGPs must translate (move vertically) to work properly, and thus the gaze of the eye can change from the near to the distant sections, much like bifocal eyeglasses.
Multifocal soft contact lenses are more complex to manufacture and require more skill to fit. All soft bifocal contact lenses are considered "simultaneous vision" because both far and near vision corrections are presented simultaneously to the retina, regardless of the position of the eye. Of course, only one correction is correct, the incorrect correction causes blur. Commonly these are designed with distance correction in the center of the lens and near correction in the periphery, or vice versa.
Implantation
Intraocular lenses, also known as an implantable contact lenses, are special small corrective lenses surgically implanted in the eye's posterior chamber behind the iris and in front of the lens to correct higher degrees of myopia and hyperopia.
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