First, your contacts must address the problem that is prompting you to wear lenses in the first place. Your contact lenses must provide good vision by correcting your myopia, hyperopia, astigmatism, or some combination of those vision problems.

Second, the lens must fit your eye. To do that, lenses come in tens of thousands of combinations of diameter and curvature. Of course, not every lens brand comes in every “size.”

Your optometrist is skilled in evaluating your eye’s physiology, and your eyesight, to determine which lens best satisfies the two criteria above.
Third, you may have another medical need that drives the choice of lens. For example, your optometrist might pick a particular lens if your eyes tend to be dry.

Finally, consider your “wish list” of contact lens features — colours, for example, or overnight wear.

Contact Lens Materials

Your contact lens practitioner will decide which lens material will best satisfy your needs. There are no blood vessels in the cornea to carry oxygen like in the rest of the body. The cornea gets its oxygen from the atmosphere. An important goal in developing new materials was to get more oxygen to pass through the lens. Another goal was to find a material that was comfortable on the eye.

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. The only alternative at the time was hard contact lenses made of PMMA plastic (see below). PMMA lenses typically took weeks to adapt to and many people couldn’t wear them successfully.
  • Silicone hydrogel lenses are an advanced type of soft contact lenses that are more porous than regular hydrogel lenses and allow even more oxygen to reach the cornea. Introduced in 2002, silicone hydrogel contact lenses are now the most popular lenses prescribed.
  • Gas permeable lenses — also called GP or RGP lenses — are rigid contact lenses that look and feel like PMMA lenses (see below) but are porous and allow oxygen to pass through them. Because they are permeable to oxygen, GP lenses can be fit closer to the eye than PMMA lenses, making them more comfortable than conventional hard lenses. Since their introduction in 1978, gas permeable contact lenses have essentially replaced nonporous PMMA contact lenses. GP contacts often provide sharper vision than soft and silicone hydrogel contacts — especially if you have astigmatism. 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.
  • 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 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.
  • PMMA lenses are made from a transparent rigid plastic material called polymethyl methacrylate (PMMA), which also is used as a substitute for glass in shatterproof windows and is sold under the trademarks Lucite, Perspex and Plexiglas. PMMA lenses have excellent optics, but they 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.

Silicone hydrogel is probably the material of choice for most practitioners today.

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:

SOFT CONTACT LENS DESIGNS

  • 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.
  • 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.
  • Cosmetic contact lenses include colour contacts designed to change or intensify your colour. Halloween, theatrical and other special-effect contacts also are considered cosmetic lenses. A contact lens prescription is required for cosmetic contacts even if you have no refractive errors that need correction.
Share this on Facebook: