Extended wear lenses generally have a higher water content or thinner center thickness than other lenses and permit more oxygen to reach the eye. However, their use has been linked to a higher incidence of eye problems. Extended wear lenses need to be cleaned and disinfected at recommended intervals or discarded after use.

By Replacement Period
Contact lens are often prescribed with a specific replacement schedule suitable to your specific needs. Planned (or Frequent) Replacement contacts are disposed of and replaced with a new pair according to a planned schedule. Unplanned replacement lenses (often called conventional lenses) are not replaced according to a pre-determined schedule. They are typically used for as long as they remain undamaged, usually around 12 months for soft lenses.

Why replace lenses frequently?
Almost immediately after they are inserted, contact lenses begin attracting deposits of proteins and lipids. Accumulated deposits, even with routine lens care, begin to erode the performance of your contacts and create a situation that presents a greater risk to your eye health.

A specific replacement schedule helps to prevent problems before they might occur. Contact lens wearers, in turn, enjoy the added comfort, convenience and health benefits of a planned replacement program. Planned replacement lenses are generally a thinner design or are made of different, more fragile materials with a higher water content than unplanned replacement or conventional contact lenses.

Based on a complete assessment of your needs, a prescription for planned replacement lenses may call for replacement:

• Quarterly,
• Monthly
• Every 1-2 weeks
• Daily

Except for daily disposables, planned replacement lenses require cleaning and disinfection after each period of wear unless they are discarded immediately upon removal. Planned replacement lenses can be worn as daily wear -- removed before sleep -- or as extended wear, if recommended by your practitioner.

By Type of Vision Correction Required
Contact lenses may be identified by the type of refractive error they are designed to correct.

• Spherical contact lenses for nearsightedness (myopia) and farsightedness (hypermetropia);
• Toric contact lenses for astigmatism;
• Bifocal lenses for presbyopia, the loss of ability to focus on reading or close-up activities.

As an alternative to special bifocal contact lenses, many practitioners use a system called monovision where one eye is fitted with a distance lens and the other with a reading lens. Approximately two-thirds of patients adapt to this type of contact lens wear.

By Type of Tint
Contact lenses may be described as clear or tinted. Tints are used to make lenses more visible during handling, or for therapeutic or cosmetic reasons. Tints can enhance eye color, or change it altogether.

Three categories of tinted contact lenses are available.

• Cosmetic enhancement tints are translucent and are designed to enhance your natural eye color. They are best for light-colored eyes (blues, greens, light hazel or grays). When wearing these tints, the color of your eye is a blend of the lens tint and your natural eye color and iris pattern.
• Opaque or "cosmetic" tints change the color of your eyes whether they are dark or light. The pattern on the lens, which is colored, overlies the colored part of your eye, resulting in a color with a natural look.
• Visibility tints are very pale, colored just enough to make the contact lens visible while you are handling it. They usually have no effect on eye color.