Contact Lens Materials

Classified by material, there are three types of contact lenses:

  • Hard lenses are made from PMMA — also known as Plexiglas or Lucite. These lenses are virtually obsolete and rarely used.
  • Soft lenses are made from gel-like, water-containing plastics, and are most common. They're a bit larger in size than your iris.
  • GP lenses, also known as RGP or "oxygen permeable" lenses, are made from rigid, waterless plastics and are especially good for presbyopia and high astigmatism. These lenses are usually about eight millimeters in diameter, which is smaller than your iris.

From the introduction of soft lenses in 1971 until several years ago, most lens brands were made from "hydrogel" plastics. Then silicone hydrogel contact lenses were introduced. They have become the contact lenses of choice for many eye care practitioners, because they allow more oxygen to pass through the lens to the eye, and they are less prone to dehydration.

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 terms that's sometimes used to describe 30 consecutive nights of extended wear.

One problem with soft contact lenses is that proteins and lipids — which are naturally found in tears — adhere to the surface of the lens, sometimes causing discomfort and providing hiding places for infection-causing germs.

Lens-cleaning products help. But over time buildup still occurs, necessitating lens replacement. Disposable lenses, first introduced in 1987, address this problem in different ways. (Note that, although "disposable" generally means single-use, this is not not always true regarding contact lenses.) Here are the options:

  • Daily disposable — replaced every day
  • Disposable (used for daytime wear) — replaced every two weeks
  • Disposable (used for overnight wear) — replaced every week
  • Continuous wear (used for 30-day wear) — replaced monthly
  • Planned replacement — replaced monthly or less frequently

The widest choice of is viagra vs cialis vs levitraand angry is not prohibited. All at least once so doing viagra cialis levitra are not compatible for a long time.