Contact Lenses to Correct Refractive Errors
Contact lenses can be used to correct any type of refractive error, and in many cases they provide clearer vision than eyeglasses. They rest directly on the cornea and are held in place by the eye's natural tears, which are always present. If fitted and used properly, contacts are effective and safe.
Some patients may wear contact lenses following cataract surgery or if they have keratonaconus (a deformity of the cornea) to see adequately. Most people choose them for cosmetic reasons.
Not everyone is a good candidate for contact lenses due to the type of vision problem, shape of eyes, other eye conditions (e.g., dry eyes, allergies), or certain medical disorders. For example, severe arthritis in the hands makes it extremely difficult to put in or remove contact lenses. Working in a very dry atmosphere or where there is a lot of dust or particles in the air, also may make it difficult to wear contacts.
Wearing contacts requires compliance with specific instructions concerning how many hours they can be worn and how they must be cleaned, handled, and stored. Not adhering to instructions puts the wearer at risk for eye injury and infection. Contacts are also more expensive than eyeglasses. A pair of glasses can last many years, but contact lenses must be replaced more frequently. Contact lenses must be disinfected at regular intervals, requiring solutions and equipment that add considerably to the cost.
There are four main types of contact lenses: hard lenses, soft lenses, extended-wear lenses, and disposable lenses. Contact lenses are available in soft materials and hard gas-permeable varieties. All types of contact lenses, including bifocals and those for astigmatism, are available as disposable and frequent replacement lenses.
Hard Contact Lenses
The first-generation contact lenses were made of a hard, inflexible plastic material called polymethylmethacrylate (PMMA). They were very durable and usually provided the best vision correction. However, PMMA prevented oxygen from entering the cornea and carbon dioxide from leaving it. Because the cornea lacks blood vessels, it receives oxygen from the air. The hard lens interfered significantly with this process. Because of this, hard lenses could not be left in for long periods. Few patients still wear this type and most eye care professionals no longer fit this type.
Today's hard, gas-permeable lenses are similar to the hard lens in design and appearance but allow for the passage of oxygen and carbon dioxide. They are more expensive than hard lenses but more comfortable, can be left in longer, and are easy to clean and healthier for the eye. Hard lenses may provide better vision correction than soft lenses, especially when there is severe astigmatism or an irregularly shaped cornea. Disadvantages of hard lenses are that they are dislodged from the eye easily, debris can get under the lens, and the period of adjustment is long.
Soft Contact Lenses
Soft lenses are constructed of a flexible plastic that easily absorbs water, which allows the passage of oxygen and carbon dioxide to and from the cornea. The amount of water that a lens can absorb varies; low-water content, mid-water content, and high-water content lenses are available. Higher water content allows more oxygen to pass through, but high-water content lenses are fragile and difficult to keep clean.
Soft lenses are larger, more difficult to accidentally dislodge, easier to adjust to, and usually more comfortable than hard lenses. They are available for all types of refractive errors, including astigmatism. They are often difficult to insert, more fragile than hard lenses, and must be cleaned and disinfected daily, unless they are one-day-use disposable lenses. Protein from tears builds up more easily on soft lenses and cannot be removed by regular cleaning. As a result, soft lenses must be replaced every year, sometimes more frequently.
Extended-Wear Contact Lenses
Most extended-wear lenses allow better passage of oxygen to the cornea and can be left in the eyes for longer periods. These lenses are available in hard and soft materials. They can remain in the eyes for up to 7 days, without removal, and then must be cleaned and disinfected.
Extended-wear contacts increase the risk for infection and injury to the eye. The risk for keratitis or cornea infection is 4 to 10 times higher in people wearing extended-wear lenses than in those wearing lenses that are removed daily. Some eye care practitioners do not recommend them.
Disposable and Frequent Replacement Lenses
It is better for the eyes to replace contact lenses frequently because it reduces the risk for infection. Even when properly cleaned, protein deposits build up on the lenses over time, and these deposits attract bacteria that can cause infection and threaten vision.
Disposable lenses are worn for 2 weeks or less and then replaced with a fresh pair of contacts. Frequent replacement lenses are replaced monthly, bimonthly, or quarterly. All types, except daily disposable lenses, require routine cleaning. Daily disposable lenses eliminate the need for cleaning and decrease the risk for infection. These lenses are more expensive than others, but the cost of disinfecting solutions and the time spent cleaning them are eliminated.
Bifocal Contact Lenses
Contact lenses are available as bifocals that have a prescription for near vision and one for distant vision in the same lens. Bifocal contacts are frequently prescribed for patients with presbyopia.
It takes time for the eyes to adjust to bifocal contacts and not everyone is comfortable with the vision while wearing them.