As long ago as 1912, a glass contact lens was being produced, but because of the manufacturing difficulties and wearing problems, the widespread use of this type of optical aid was delayed until the introduction of plastic scleral lenses in 1937. The obvious advantage of placing a lens directly on the cornea over the wearing of spectacles is the cosmetic one, but the system also has optical advantages. Because the lens moves with the eye, there are none of the problems associated with looking through the edge of the lens experienced by the wearer of spectacles. In addition, a more subtle effect is the more accurate representation of image size on the retina in subjects with high degrees of refractive error.
Although the original type of moulded scleral contact lenses are still occasionally used, they have been largely replaced by the modern rigid and soft lenses, which are much smaller and thinner and hence cause less interference with corneal physiology. Rigid lenses are made from gas-permeable plastics and have generally replaced the early "hard" lenses, which were impermeable to oxygen. In 1960, the hydrophilic soft contact lens was introduced. This had the great advantage of being soft and malleable and hence more comfortable to wear, but optically it has never been quite as good as the rigid lens, especially when the patient has high degrees of astigmatism. Several different materials have now been used in the production of soft lenses, although the basic material used is hydroxy-ethylmethacrylate. The different types of soft lenses differ in their ability to take up water and transmit oxygen. Lenses are now being made that can be worn for long periods without needing to be removed and cleaned. Similarly, disposable and "planned-replacement" contact lenses are now widely available. Care should be taken that such lenses are used under professional care.
Soft contact lenses tend to absorb and adsorb material from the tear film. It is particularly important to ensure that a patient is not wearing a soft lens before fluorescein dye is instilled into the eye.
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