Contacts are also known as contact lens (single is a contact) is a cosmetic, therapeutic lens or corrective lens usually placed on the cornea of the eye. Modern soft contact lenses were invented by Otto Wichterle and Drahoslav Lim who produced the first gel used for the lenses production. Typically contacts serve the same purposes as prescription glasses but have the advantage of being lightweight and virtually invisible but often have a blue tint to make them visible when stored in a cleaning solution.
It is possible to alter the colour appearance of the eye using contact lenses and newer types have a thin UV coating to reduce the sun’s damage to the natural lens, but do not replace sunglasses. Around 10% of the developed nation use contact lenses with a total of around 125 million worldwide, but only around 2% of the total population. The types of lenses used vary between countries as to if they prefer the rigid pre-scribed lens. While many people choose to wear contact lenses such as practicality or appearance there is a number of medical reasons such as keratoconus and aniseikonia that cannot be accurately corrected with glasses.
The practical benefits of wearing contact lenses are they do not steam up, provide a wider field of vision and are less affected by wet weather conditions and more suitable for sporting activities.
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Classification of contact lenses
Depending on the manufacturer or country there are a number of ways contact lenses are classified typically it is by construction material and lens function.
Functional Contact Lenses
Corrective contact lenses – Typically this lens is used to improve a persons vision, as there is often a mismatch between the refractive power of the eye and the length of the eye which causes a refractive error. The corrective contact lens compensates for this mismatch and allows for the correct focusing of light onto a persons retina. Some of the common conditions that are correctable with contact lenses are:
- Near or short sightedness (Myopia)
- Far or long sightedness (Hypermetropia)
Also patients using contact lenses also must take them out each night or every few days and leave in a cleaning solution, often the only way to save money with contact lenses is to buy in bulk online to get the lowest prices on Acuvue contact lenses. There are even contact lenses that can assist with colour deficiencies where a red tint is added to allow some individuals who are colourblind to distinguish colours better but does not restore normal colour vision. Typically ChromaGen lenses have produced significant improvements in colour vision but have limitations regarding vision at night.
Cosmetic contact lens – These lenses are designed to change the appearance or colour of the wearers eye, but can also correct the vision. As a result of the colour or design some blurring or obstruction of the patients vision may occur, and they are often referred to as decorative contact lenses. Typically they cause mild irritation to the patient on insertion but typically cause no long term damage as long as they are not worn too much, but have no current links to any forms of vision degradation.
The entertainment industry has long used theatrical content lenses to make the actors eye appear confusing and arousing in appearance, often in horror films where the actor’s eye have to appear demonic, cloudy and lifeless. They can also be used to make the actor appear to be under the influence of various illicit drugs and simulate the natural effects had the actor consumed such chemicals without breaking laws. The Scleral lenses which cover the white part of the eye can be difficult to insert and do not move very well within the eye, and typically not be worn for more than 3 hours as they cause temporary vision disturbances.
Circle contact lenses are becoming popular in parts of Asia such as South Korea and Japan where the result is the appearance of a bigger wider iris as the tinting covers the extra-wide ring of the lens.
Therapeutic contact lens – The soft lens material can be used in the management of non-refractive eye disorders and also in treatment of such eye disorders. To prevent an diseased or injured cornea from the patients constant blinking eyelids a bandage contact lens can be used. Recent improvements in pharmaceutical technologies has allowed certain drugs and medicines to be delivered direct to the eye through a contact lens. Contact lenses can be used in the treatment of the following eye conditions:
- bullous keratopathy
- erosion and corneal ulcers
- dry eyes
- anterior corneal dystrophy
- corneal ectasis
- corneal edema
- neurotrophic kertaconjunctivitis
Lens construction material – The first known contact lenses were first constructed of glass, but led to irritation in the patients wearing them for extended periods of time, this was followed with Perspex/Plexiglas (polymethyl methacylate) which allowed contact lenses to be more convenient to wearers. The problem with Plexiglas lenses is that no oxygen can be transmitted through the lens to the cornea which leads to adverse side effects for the wearer. This issue was mostly resolved when the RGP (rigid gas permeable) lens was developed that were oxygen-permeable during the last 20-30 years of the 20th century. The problem with these soft lenses is that they are porous in nature and can absorb chemicals and fumes in time.
Rigid lenses offer a number of unique benefits as they can replace the natural shape of the cornea in effect a new refracting surfer and spherical contact lenses can provide reasonable vision for people who have distorted corneal shapes (keratoconus) or astigmatism.
Soft lenses have a further advantage is that they are immediately comfortable, where rigid contact lenses require an adaption period before full comfort is achieved for the wearer. One of the alternative techniques is to place a smaller rigid lenses atop a larger software lens, where a single lens will not provide the required comfort, optical power or fit.
Silicone Hydrogels released in 1999 benefit the wearer by offering a very high oxygen permeability and all the comfort and clinical performance expected from traditional hydrogels, and where primarily promoted for overnight wear. Following on from this success daily silicone hydrogels have been approved which are not kept in overnight. Because of the nature of the silicone the wearer will still experience discomfort and dryness during lens wear.
Lens by wear time – The daily wear contact lens is designed to be removed by the patient prior to sleeping, where extended wear contact lenses are designed to be warn for 6 or more consecutive nights. By using new high tech materials such as silicone hydrogels they can be worn for periods upto 30 consecutive nights and are often referred to as continuous wear contact lenses, but are often discarded after a set period of time. They also have typically around 5-6 times greater oxygen permeability than conventional soft lenses which allow the eye to remain healthy.
The downside to extended lens wear is an increased risk for corneal ulcers or corneal infections, typically due to bacterial stagnation and poor cleaning and care of the lenses. The most common complication experienced by extended lens wearers is conjunctivitis which is sometimes associated with a poorly fitting contact lens, and is usually an allergic reaction or giant papillary conjunctivitis. If you are considering contact lenses discount Contact Lenses has the exact same brand name contact lenses and should be able to offer the same product you buy in store just at lower prices.
Contact lenses by frequency of replacement – A majority of the soft contact lenses are categorised by their prescribed replacement schedule which start at daily disposable (single use) which are discarded each night, which are lighter and thinner and are the most comfortable for patients to wear. The disposable contacts are usually most suited to those with allergies or similar conditions because it is less likely build up deposits of proteins and antigens. They can also be suitable for sports and activities such as swimming or contact sports like rugby where the wearer doesn’t often wear lenses and losing a contact lens is possible.
A optometrist will likely prescribe a patient contact lenses on a monthly or fortnightly basis, previous quarterly or annual prescriptions are less likely as they allow for deposits to build up and new lenses are much thinner. Typically longer life disposables and daily disposable lenses are made by the same material usually silicon-hydrogel, but the lens only differ in their base curve and diameter.
Contact lenses by design – a spherical contact lens is one that has both outer and inner optical surfers are portions of a sphere, where a toric lens is one in which either or both optical surfaces has a cylindrical lens. Those suffering from Farsighted (hypermetropic) or Nearsighted (myopic) conditions and who also suffer from astigmatism who are not able to use standard contact lenses maybe suitable for toric lenses. If the patient just has one eye affected by astigmatism they may use a spherical lens in one eye and a toric lens in the other, but they have a few extra characteristics from regular contact lenses.
- Toric lenses can have cylindrical and spherical aberration
- Toric lenses have a specific “top” and “bottom” and must be rotated as they are not symmetrical around their center
- Toric lenses are thinker at the bottom and is pushed down by the upper eye lid during blinking
- Toric lenses are more expensive to produce
- Toric lenses are usually meant for extended wear
Contact lenses are similar to eye glasses as they can have single and multi-focal focus points but a person may wear a single contact lens to improve distance vision for driving and reading glasses to improve their near vision for reading books.
Contact lenses by implantation – for patients that have higher degrees of myopia and hyperopia they usually require a intraocular lens that requires a small corrective lens to be implanted in the eye behind the iris and in front of the lens.
Contact lens prescription
The parameters which are specified in your contact lenses prescription will likely include:
- center thickness (CT)
- cylinder axis
- Power in Dioptres
- Diameter (D)
- Base curve radius (BCR)
- Material (Oxygen Permeability/Transmissibility
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