Contact Lens Materials
The first choice when considering contact lenses is which lens material will best satisfy your needs. There are five types of contact lenses, based on type of lens material they are made of:
Soft lenses - are made from gel-like, water-containing plastics called hydrogels. These lenses are very thin and pliable and conform to the front surface of the eye. Introduced in the early 1970s, hydrogel lenses made contact lens wear much more popular because they typically are immediately comfortable. Traditional hydrogel materials are still used for many daily lenses (eg Acuvue Moist) and some toric lenses – however frequent replacement “soft” lenses are now mostly made from silicone hydrogel.
Silicone hydrogel lenses - allow much more oxygen to reach the cornea than traditional hydrogels and are much safer for longer wearing times as a result. They also tend to lose less water by evaporation and can be good for people with dry eye. Introduced in 2002, silicone hydrogel contact lenses are now the most popular lenses prescribed in the UK.
Gas permeable (RGP) lenses — are rigid contact lenses that look and feel like the original PMMA lenses (see below) but are porous and allow oxygen to pass through them. It usually takes some time for your eyes to adjust to gas permeable lenses when you first start wearing them, but after this initial adaptation period, most people find GP lenses are as comfortable as hydrogel lenses, and they can give better quality vision especially for people with astigmatism or needing multifocal lenses.
PMMA lenses - are made from a transparent rigid plastic material called polymethyl methacrylate (PMMA), which is also used as a substitute for glass in shatterproof windows ( Perspex/ Plexiglas). They have excellent optics but do not transmit oxygen to the eye and can be difficult to adapt to. These (now old-fashioned) "hard contacts" have virtually been replaced by GP lenses and are rarely prescribed today.
Hybrid contact lenses - are designed to provide wearing comfort that rivals soft or silicone hydrogel lenses, combined with the crystal-clear optics of gas permeable lenses. Hybrid lenses have a rigid gas permeable central zone, surrounded by a "skirt" of hydrogel or silicone hydrogel material. Despite these features, only a small percentage of people in the UK wear hybrid contact lenses, perhaps because these lenses are more difficult to fit and are more expensive to replace than soft and silicone hydrogel lenses.
In 2017, 64 percent of contact lenses prescribed were silicone hydrogel lenses, followed by soft (hydrogel) lenses (22 percent), gas permeable lenses (11 percent), hybrid lenses (2 percent) and PMMA lenses (1 percent).
Contact Lens Wearing Time
Until 1979, everyone who wore contact lenses removed and cleaned them nightly. The introduction of "extended wear" enabled wearers to sleep in their contacts. Now, two types of lenses are classified by wearing time:
Daily wear - must be removed nightly
Extended wear - can be worn overnight, usually for seven days consecutively without removal
"Continuous wear" - is a term that's sometimes used to describe 30 consecutive nights of lens wear — the maximum wearing time approved for certain brands of extended wear lenses.
When To Replace Your Contact Lenses
Even with proper care, contact lenses (especially soft contacts) should be replaced frequently to prevent the build-up of lens deposits and contamination that increase the risk of eye infections.
Soft lenses have these general classifications, based on how frequently they should be discarded:
Daily disposable lenses - Discard after a single day of wear
Frequent replacement lenses - Discard fortnightly or monthly, occasionally quarterly.
Traditional (reusable) lenses - Discard every six months or longer
Gas permeable contact lenses are more resistant to lens deposits and don't need to be discarded as frequently as soft lenses. Often, GP lenses can last a year or longer before they need to be replaced.
The most frequently prescribed contact lens replacement schedule in 2017 was monthly (40 percent), followed by daily (35 percent), every one to two weeks (24 percent) and annually (1 percent).
Contact Lens Designs
Soft contact lenses (both standard hydrogel and silicone hydrogel lenses) are available in a variety of designs, depending on their intended purpose:
Spherical contact lenses - have the same lens power throughout the entire optical part of the lens to correct myopia (nearsightedness) or hyperopia (farsightedness).
Toric soft contact lenses - have different powers in different meridians of the lens to correct astigmatism as well as nearsightedness or farsightedness. [Read more about toric contact lenses.]
Multifocal contact lenses - (including bifocal contacts) contain different power zones for near and far vision to correct presbyopia as well as nearsightedness or farsightedness. Some multifocal lenses also can correct astigmatism. [Read more about bifocal contacts.]
RGP contact lenses are also available in the above designs but can be manufactured in many more sizes and curvatures offering greater customisation for an individual patient, including lenses fabricated for use in special situations, such as correcting for keratoconus.
More Contact Lens Features
Bifocal contacts for astigmatism - these are advanced soft contacts that correct both presbyopia and astigmatism, so you can remain glasses-free after age 40 even if you have astigmatism.
Contacts for dry eyes - are your contacts uncomfortably dry? Certain soft contact lenses are specially made to reduce the risk of contact lens-related dry eye symptoms.
Cosmetic contact lenses - include colour contacts designed to change or intensify your eye colour. A contact lens prescription is required for cosmetic contacts even if you have no refractive errors that need correction.
Special-effect lenses - also called theatrical, novelty, or costume lenses, special-effect contacts take coloration one step further to make you look like a cat, a vampire, or another alter-ego of your choice. Halloween, theatrical and other special-effect contacts are also considered cosmetic lenses.
Prosthetic lenses - coloured contact lenses also can be used for more medically oriented purposes. Opaque soft lenses called prosthetic contacts can be custom-designed for an eye that has been disfigured by injury or disease to mask the disfigurement and match the appearance of the other, unaffected eye.
Custom lenses - if conventional contact lenses don't seem to work for you, you might be a candidate for custom contact lenses that are made-to-order for your individual eye shape and visual needs.
UV-inhibiting lenses - some soft contact lenses help protect your eyes from the sun's ultraviolet rays that can cause cataracts and other eye problems. But because contacts don't cover your entire eye, you still should wear UV-blocking sunglasses outdoors for the best protection from the sun.
Scleral lenses - large-diameter gas permeable lenses called scleral contacts are specially designed to treat keratoconus and other corneal irregularities, as well as presbyopia.
Semi-scleral lenses - are a relatively new form of lens larger than a traditional RGP lens and covering more than the cornea. They are often more comfortable than RGP lenses and can be used to correct more unusual corneas and prescriptions.
Myopia control contact lenses - special contact lenses are being developed to slow or stop the progression of nearsightedness in children.
Ortho-keratology contact lenses – RGP lenses that are worn overnight to reshape your cornea and temporarily correct your prescription. They are removed during the day.