contact lens

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.[29] 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.[30] Contact lenses that deliver drugs to the eye have also been developed.[31] Materials 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. Rigid lenses

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. Soft lenses

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" daily 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 biggest improvements to soft lens polymers have been increasing oxygen permeability, lens wetability, and overall comfort.

In 1998, 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. 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.[32]

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. Hybrid

A small number of hybrid lenses exist. Typically these lenses consist of a rigid center and a soft "skirt". A similar technique is "piggybacking" of a smaller, rigid lens on the surface of a larger, soft lens. These techniques give the vision corrections benefits of a rigid lens and the comfort benefits of a soft lens. Wear schedule

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 allow for the transfer of 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. Replacement schedule

The contact lenses are often categorized by their replacement schedule. Single use lenses (typically called 1-day or daily disposable) are discarded after one use. Because they do not have to stand up to the wear and tear of repeated uses, single use lenses can be made thinner and lighter. This can improve the comfort of the lens. Lenses replaced frequently gather less deposits of allergens and germs, making these lenses preferable for patients that have ocular allergies or are prone to infection. Single use lenses are also useful for people who use contacts infrequently, or when losing a lens is likely or not easily replaced (like on vacation).

Other disposable contact lenses are designed for two-week or 4-week replacement. Quarterly or annual lenses, which used to be very common, have lost favor because 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, but had other drawbacks.

Lenses with different replacement schedules can be made of the same material. Although the materials are the same, differences in the manufacturing processes determines if the resulting lens will be a "daily disposable" lens or a lens recommended for two-week or 4-week replacement.

In the United States, contact lens manufacturers must prove to the Food & Drug Administration (FDA) that a lens is safe before it can be sold to the public. Typically, the prescribing doctor instructs patients to follow the same limitations used during those studies. This included wear time and replacement schedule.