Contact lenses can be classified in many different manners. Contact lenses can be separated by their primary function, material, wear schedule (how long a lens can be worn before removing it), and replacement schedule (how long before a lens needs to be discarded).
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. 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 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.
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.
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. 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.
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.
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 abrasions 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.
Contact lenses, other than the cosmetic variety, become almost invisible once inserted in the eye. Most corrective contact lenses come with a light "handling tint" that may render the lens slightly more visible on the eye. Soft contact lenses extend beyond the cornea, and the border is sometimes visible against the sclera.
Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do so were lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas). PMMA lenses are commonly referred to as "hard" lenses. A disadvantage of these lenses is that they do not allow oxygen to pass through to the cornea, which can cause a number of adverse clinical events.
Starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed. Lenses made from these materials are called rigid gas permeable or 'RGP' lenses.
A rigid lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a spherical rigid contact lens can correct for astigmatism. Rigid lenses can also be made as a front-toric, back-toric, or bitoric. This is different from a spherical lens in that one or both surfaces of the lens deliver a toric correction. Rigid lenses can also correct for corneal irregularities, such as keratoconus. In most cases, patients with keratoconus see better through rigid contact lenses than through glasses. Rigid lenses are more chemically inert, allowing them to be worn in more challenging environments than soft lenses.
Silicone hydrogels 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, the oxygen permeability of silicone hydrogels is not tied to the water content of the lens. 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 and less "wettable." These factors can influence the comfort of the lens. New manufacturing techniques and changes to multipurpose solutions have minimized these effects. A surface modification processes called plasma coating alters the hydrophobic nature of the lens surface. 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 "daily wear" (DW) contact lens is designed to be worn for one day and removed prior to sleeping. An "extended wear" (EW) contact lens is designed for continuous overnight wear, typically for up to 6 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). Extended and continuous wear contact lenses can be worn overnight because of their high oxygen permeability. While awake, the eyes are typically open, allowing oxygen from the air to dissolve into the tears and pass through the lens to the cornea. While asleep, 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 can cause corneal hypoxia which can result in many complications, including a corneal ulcer, which has the potential to permanently decrease vision. Extended and continuous wear contact lenses typically transfer 5–6 times more oxygen than conventional soft lenses, allowing the eye to remain healthy, even when the eyelid is closed.
Wearing lenses designed for daily wear overnight has an increased risk for corneal infections, corneal ulcers, and corneal neovascularization. The most common complication of extended wear lenses is giant papillary conjunctivitis (GPC), sometimes associated with a poorly fitting contact lens.
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.