Overview Several types of hard and soft contact lenses are available. Consult an eye doctor who is willing to work with you to select the best type of lens for your needs and lifestyle.
Hard contact lenses Hard contact lenses correct refractive errors astigmatism (Figure 1) , with no distortion. Most hard (rigid) lenses are made of gas-permeable materials that allow oxygen to reach the cornea (Figure 2) . This reduces problems that may occur when the cornea doesn't get enough oxygen.
Conventional hard lenses (PMMA) Conventional hard contact lenses are made of a fairly stiff plastic (polymethyl methacrylate, or PMMA), which does not mould to the shape of the eye. They are inexpensive and durable, but they are the least comfortable type of contact lens. They reduce the amount of oxygen that reaches the cornea. So people who wear them are at risk of overwearing syndrome and other problems.
Rigid gas-permeable (RGP) lenses Gas-permeable lenses cost more than conventional hard lenses and are somewhat less durable. But they are more comfortable than conventional hard lenses. Some gas-permeable lenses are designed for extended wear (overnight). But many eye specialists advise against wearing them for an extended period of time or while sleeping. RGP lenses usually need to be replaced after 2 to 3 years of use.
Soft contact lenses Soft (silicone hydrogel) lenses are made of soft, very flexible plastics that absorb water (up to 90% of the lens weight). Many people find them more comfortable to wear than hard lenses, but hard lenses usually provide sharper vision. Soft lenses are more fragile than hard lenses and require more intensive cleaning. Soft lenses that can correct astigmatism are called toric lenses.
Daily-wear lenses Daily-wear soft lenses are removed and cleaned at night and reinserted in the morning. They provide sharp vision but do not correct astigmatism as well as hard lenses do. They take less time to get used to than hard lenses, but they are less durable.
Extended-wear lenses Extended-wear lenses can be worn day and night. After several days, you take them out, clean them, and wear them again. But some people's eyes become irritated if the contacts are not routinely removed and cleaned. Extended use may be uncomfortable and increases the risk of damaging the eye. Even though Health Canada has approved some of these lenses for 30-day use, many eye doctors recommended that people remove them every day.
Disposable lenses Disposable soft lenses were designed to be worn for up to several weeks and then discarded. Many new contact lens wearers now choose disposable lenses. Disposable lenses were introduced because it was thought that replacing lenses more often would reduce the risks of infection of the cornea (infectious or bacterial keratitis), a dangerous complication of soft lenses. But later studies have shown that people who wear disposable lenses, especially the extended-wear kind, may have a higher risk of keratitis.
There are two types of disposable lenses:
Daily-wear disposable lenses are removed and cleaned daily, just like conventional lenses. They are worn for 2 to 4 weeks and then discarded. Most extended-wear disposable lenses are designed to be worn day and night for 1 week and then discarded. Health Canada has approved some lenses for 30-day use. But some people cannot tolerate the extended wear. Their eyes become irritated if the contacts are not routinely removed and cleaned. Also, the risk of infection is greater when a person wears the lenses all day and night. Many eye doctors recommend that people remove them every day. Figure 1 - Cornea, Lens, and Iris The iris is the coloured part of the eye. A circular muscle in the iris controls the amount of light entering the eye through the pupil, the black area in the centre of the iris.
The cornea is the outer clear, round structure that covers the iris and the pupil. The cornea directs light rays into the eye and helps focus them on the light-sensitive retina at the back of the eye, providing sharp, clear vision.
The lens is located behind the iris and is normally clear. Light passes through the pupil to the lens. Small muscles attached to the lens can make the lens change shape, allowing the eyes to focus on near or far objects.
Current as of: September 27, 2023
Author: Ignite Healthwise, LLC StaffClinical Review Board
Figure 2 - Cornea, Lens, and Iris The iris is the coloured part of the eye. A circular muscle in the iris controls the amount of light entering the eye through the pupil, the black area in the centre of the iris.
The cornea is the outer clear, round structure that covers the iris and the pupil. The cornea directs light rays into the eye and helps focus them on the light-sensitive retina at the back of the eye, providing sharp, clear vision.
The lens is located behind the iris and is normally clear. Light passes through the pupil to the lens. Small muscles attached to the lens can make the lens change shape, allowing the eyes to focus on near or far objects.
Current as of: September 27, 2023
Author: Ignite Healthwise, LLC StaffClinical Review Board
