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A contact lens is a thin lens placed directly on the surface of the eye.
Contact lenses are considered medical devices that can be worn to correct vision, or for cosmetic or therapeutic reasons.
When compared with spectacles, contact lenses typically provide better peripheral vision, and do not collect moisture such as rain, snow, condensation, or sweat. This makes them ideal for sports and other outdoor activities.
Contact lens 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.
Additionally, there are conditions such as keratoconus and aniseikonia that are typically corrected better by contacts than by glasses.
Artist's impression of Da Vinci's method for neutralizing the refractive power of the cornea.
Leonardo da Vinci is frequently credited with introducing the idea of contact lenses. He described a method of directly altering corneal power by either submerging the head in a bowl of water, or wearing a water-filled glass hemisphere over the eye. Neither idea was practically implementable in Da Vinci's time.
In 1888, Adolf Fick was the first to successfully fit contact lenses, which were made from blown glass.
In 1888, the German ophthalmologist Adolf Gaston Eugen Fick constructed and fitted the first successful contact lens.
In 1949, the first "corneal" lenses were developed.
PMMA corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing (lathe) technology.
One important disadvantage of PMMA lenses is that no oxygen is transmitted through the lens to the conjunctiva and cornea, which can cause a number of adverse clinical effects.
Occasionally, the term "gas permeable" is used to describe RGP lenses, but this is potentially misleading, as soft lenses are also gas permeable in that they allow oxygen to move through the lens to the ocular surface.
In 1998, an important development was the launch of the first silicone hydrogels.
These new materials encapsulated the benefits of silicone - which has extremely high oxygen permeability - with the comfort and clinical performance of the conventional hydrogels that had been used for the previous 30 years.
Contact lenses can be classified in many different ways:
Corrective contact lenses are designed to improve vision, most commonly by correcting refractive error.
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. 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.
Alternatively, a person may simply wear reading glasses over their distance contact lenses.
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.Red-filtering contact lenses can also be an option for the extreme light sensitivity in some visual deficiences such as
Magnification is another area being researched for future contact lens applications. The embedding of telescopic lenses and electronic components suggests that future uses of contact lenses may become extremely diverse. However, there are still barriers that prevent the transition of these technologies from research and development to practical application and commercial availability.
A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct refractive error.
As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection.
For this reason all individuals who would like to wear cosmetic contact lenses should have a contact lens examination with an contact lens specialist eye doctor prior to first use, and if used long-term, regular aftercare examinations, in order to avoid potentially blinding complications.
Cosmetic contact 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.
A scleral contact lenses is a large, firm, oxygen permeable lens 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 may be 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 growing number of disorders or injuries to the eye, such as severe microphthalmia, keratoconus, corneal ectasia, Stevens-Johnson syndrome, Sjögren's syndrome, aniridia, neurotrophic keratitis (aneasthetic corneas).
Injuries to the eye such as surgical complications, distorted corneal implants, as well as chemical and burn injuries also may be treated by the use of scleral contact lenses.
Soft contact 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.
Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do so were lenses made from polymethyl methacrylate (PMMA). 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.
While rigid lenses have been around for about 120 years, soft contact lenses are a much more recent development. Soft lenses are usually comfortable shortly after insertion, while rigid lenses require a period of adaptation before full comfort is achieved. The biggest improvements to soft contact 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.
Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic, and thus, less "wettable."
A small number of hybrid contact 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 are often chosen to give the vision correction benefits of a rigid lens and the comfort benefits of a soft lens.
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.
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.
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 fewer deposits of allergens and germs, making these lenses preferable for patients that have ocular allergies or are prone to infection.
Other disposable contact lenses are designed for two-week or 4-week replacement. Quarterly or annual lenses, which used to be very common, have been discontinued by manufacturers..
Typically, soft contact lenses are mass-produced, while rigid lenses are custom-made to exact specifications for specific individual patients.
An eye examination is needed to determine an individual's suitability for contact lenses. This typically includes a refraction to determine the proper power of the lens and an assessment of the health of the anterior segment of the eye. Many eye diseases can prohibit contact lens wear, such as active infections, allergies, and dry eye. Keratometry is especially important in the fitting of rigid lenses.
Contact lens prescriptions expire yearly. This is to ensure that the patient's eyes are still healthy enough to support contact lens wear and that the current lenses are not causing any adverse effects. However, the policies of some vendors make it possible for expired and fraudulent prescriptions to be filled without verification by the prescribing doctor. This can be very unsafe and potentially cause permanent damage to the eye.
CLARE, or contact lens associated red eye, is a group of inflammatory complications from contact lens wear.
Contact lenses are generally safe as long as they are used correctly. Complications due to contact lens wear affect roughly 5% of contact lens wearers each year.Improper use of contact lenses may affect the eyelid, the conjunctiva, and the various layers of the . Poor lens care can lead to infections by various microorganisms including bacteria, fungi, and Acanthamoeba.
Many complications arise when lenses are worn not as prescribed (improper wear schedule or lens replacement). Sleeping in lenses not designed or approved for extended wear is a common cause of complications. Many people go too long before replacing their lenses, wearing lenses designed for 1, 14, or 30 days of wear for multiple months or years. While this does save on the cost of lenses, it risks permanent damage to the eye and loss of sight.
One of the major factors that cause contact lens complications is that the lens is a barrier to oxygen. The cornea needs a supply of oxygen to function and it normally gets that oxygen from the surrounding air while awake and from the blood vessels in the back of the eyelid while asleep. The most prominent risks associated with long-term, chronic low oxygen to the cornea include corneal neovascularization, increased epithelial permeability, bacterial adherence, microcysts, corneal edema, endothelial polymegathism and potential increase in myopia. That is why much of the research into the latest soft and rigid contact lens materials has centered around improving oxygen transmission through the lens.
Mishandling of contact lenses can also cause problems. Corneal abrasions can increase the chances of infection. When combined with improper cleaning and disinfection of the lens, the risk of infection further increases. Decreased corneal sensitivity following extended contact lens wear may cause a patient to miss some of the earliest symptoms of such complications. The way contact lenses interact with the natural tear layer is a major factor in determining lens comfort and visual clarity. People that suffer from dry eyes are particularly vulnerable to discomfort and episodes of brief blurry vision. Proper lens selection can minimize these effects for some patients.
Long-term wear (over 5 years) of contact lenses may "decrease the entire corneal thickness and increase the corneal curvature and surface irregularity." Long-term wear of rigid contact lens is associated with decreased corneal keratocyte density and increased number of epithelial Langerhans cells.
Before touching the contact lens or the eye, it is important to wash hands thoroughly with soap and rinse well with water.
Soaps that contain moisturizers or potential allergens should be avoided as these can cause irritation of the eye.
Drying of hands using hand towels or tissues, prior to handling the lenses, can transfer lint ('fluff') to the users hands and, subsequently, to the lenses, causing irritation upon insertion.
Rinsing the case under a source of clean running water, prior to opening it, can help alleviate this problem.
Next the lenses should be removed from their case and inspected for defects (e.g. splits, folds, lint). A 'gritty' or rough appearance to the lens surface may indicate that a considerable quantity of proteins, lipids and debris has built up on the lens and that additional cleaning is required; this is often accompanied by unusually high irritation upon insertion.
Contact lenses are typically inserted into the eye by placing them on the index finger with the concave side upward and then using the index finger to place the lens on the eye. Rigid lenses should be placed directly on the cornea. Soft lenses may be placed on the sclera (the white part of the eye) and slid into place. The other hand may be used to keep the eye open. Alternatively, the user may close their eye and then look towards their nose, sliding the lens into place over the cornea.
Problems may arise if the lens folds, turns inside-out, slides off the finger prematurely, or adheres more tightly to the finger than the surface of the eye. A drop of solution may help the lens adhere to the eye.
When the lens first contacts the eye, it should be comfortable. A brief period of irritation may be caused by a difference in pH and/or salinity between the lens solution and the tears. This discomfort fades quickly as the solution drains away and is replaced by the natural tears. If the irritation persists, the cause could be a dirty, damaged, or inside-out lens. Removing the lens, cleaning it, and inspecting it again for damage and proper orientation should correct the problem. If discomfort continues, the contact lens should not be worn. In some cases, taking a break from lens wear for a day may correct the problem. If the discomfort is severe, or does not resolve the next day, the person should be seen as soon as possible by an eye doctor to rule out potentially serious complications.
Removing contact lens incorrectly could result in damage to the lens and injury to the eye, so care must be taken during removal. Rigid contact lenses may be removed by pulling the eyelid tight and then blinking. With one finger on the outer corner of the eyelids, or lateral canthus, the person stretches the eyelids towards the ear. The increased tension of the eyelid margins against the edge of the lens allows the blink to break the capillary action that adheres the lens to the eye. The other hand is typically cupped underneath the eye to catch the lens. This technique can also be used for soft lenses.
A soft lens may be removed by pinching the edge between the thumb and index finger. Moving the lens off the cornea first can improve comfort during removal and reduce the risk of scratching the cornea with a fingernail. It is also possible to push a soft lens far enough to the side or bottom of the eye to get it to fold out of the eye without pinching it. Using these techniques on a rigid lens will likely scratch the cornea.
There are also small tools specifically for removing lenses. Usually made of flexible plastic, these tools can resemble small tweezers, or plungers that suction onto the front of the lens. Typically these tools are only used with rigid lenses. Extreme care must be exercised when using mechanical tools or finger nails to insert or remove contact lenses.
Lens care varies depending on material and wear schedule. Daily disposable lenses are discarded after a single use and thus require no cleaning. Other lenses require regular cleaning and disinfecting to prevent surface coating and infections.
There are many ways to clean and care for contact lenses, typically called care systems or lens solutions:
Multipurpose SolutionsMultipurpose solutions are the most common method for rinsing, disinfecting, cleaning, and storing soft lenses. Newer generations of multipurpose solutions are effective against bacteria, fungi, and acanthamoeba. The latest multipurpose solutions also contain ingredients that improve the surface wetability and comfort of silicone hydrogel lenses.
Saline SolutionSterile saline is used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect, so it must be used in conjunction with some type of disinfection system. One advantage to saline is that it can not cause an allergic response, so it is well suited for individuals with sensitive eyes and/or strong allergies.
Daily CleaningClean lenses on a daily basis. A few drops of cleaner are applied to the lens while it rests in the palm of the hand, and the lens is rubbed for about 20 seconds with a fingertip (with washed hands) on each side. The lens must then be rinsed.
Some products must only be used with certain types of contact lenses. Water alone will not adequately disinfect the lens, and can lead to lens contamination and has been known in some cases to cause irreparable harm to the eye.
Contact lens solutions often contain preservatives such as thiomersal, benzalkonium chloride, and benzyl alcohol. Preservative-free products usually have shorter shelf lives, but are better suited for individuals with an allergy or sensitivity to one or more preservatives.
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Refractive Errors
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