Contact lens Types
There are two types of material: Rigid lenses and Soft lenses. The best lens material for a patient is dependent upon many factors e.g. spectacle prescription, ocular and medical history, age, recreational and vocational requirements, and the most important: Motivation!
Rigid Gas Permeable (RGP)
Rigid lenses may be Hard or Rigid Gas Permeable (RGP). The degree of gas permeability can vary and new high gas permeable materials are being produced all the time. Hard lenses are not recommended any more due to the physiological advantages of RGP lenses.
These are small lenses about 9 mm in diameter, which float on the eyes tears. RGP lenses are relatively easy to clean and maintain, consequently, the risk of eye infection is slightly lower when compared to soft lenses. They may last longer than soft but are easier to lose due to their size. Adaptation to RGP lenses takes on average 3 to 6 weeks. One needs more motivation to get used to them when compared to soft. Also once adapted, RGP lenses should be worn most days to maintain tolerance. RGP lenses can correct almost all prescriptions including astigmatism.
There are now second generation high gas permeable lenses which may suit patients who previously had problems with lenses or for those patients whose lifestyles require long wearing times.
14 mm in diameter, water permeable, they allow a very high degree of oxygen to reach the eye. The main advantage of soft lenses is comfort. Adaptation is almost immediate. They are ideal for sports, being far less likely to get knocked out of the eye than RGP lenses. Soft Lens materials fall into two groups: Standard Hydrogels and the new Silicon Hydrogels.
Standard Hydrogels have evolved from the original soft lenses, first made in the 1970s. They vary in water content from 38% to 70%. The first soft lenses were 38% but as the need for the eye to receive as much oxygen as possible became understood, lenses either became thinner, or the water content increased. High water content lenses although good for oxygen permeability are very prone to drying out, this is especially true in modern environs such as air-conditioning and when patients are using computer screens. Most standard Hydrogels are now mid water, around 55% to 59% H20, and of thin design. They are either daily or monthly disposable.
Silicon Hydrogels allow four to five times the amount of oxygen to the eye in comparison to standard hydrogels. The permeability is so high that it is almost as if the eye has no lens on it, in terms of oxygen. The water content of a silicon hydrogel is low. They range from 24% to 48% H20 and some materials have had the addition of an internal wetting agent to improve comfort. They have been found to be of benefit to patients who due to life style, demand long wearing times, and also those who have dry eyes. Silicon hydrogels may have a slightly greater stiffness in comparison to standard hydrogels. They are more prone to deposits. They are either daily, two weekly or monthly disposable.