Contact lenses and solutions

General information

There are two types of contact lenses, soft and hard; most are manufactured from methacrylate polymer [ ].

Soft lenses are made of hydrophilic hydrogel polymers that contain 36–74% water. Other plastics and co-polymers are added to alter the physical characteristics of the lens. The diameter is 10.5–15.5 mm and the thickness at the center 0.03–20 mm. Soft lenses can correct most optical defects, including myopia, hyperopia, and astigmatism. Bifocal lenses are also available. They can be colored with either transparent hues or opaque patterns to change apparent eye color or to mask malformations of the cornea or iris. They are available for daily, weekly, and twice-weekly disposable use, 1–3 months frequent replacement, and annual replacement.

Hard lenses are made from gas-permeable materials that contain silicon and fluorine. They can correct most optical defects and have diameters of 8–10 mm.

Some lenses combine soft and hard materials, including hard lenses with soft surfaces and lenses with a hard center and a soft periphery.

The ocular risks related to the use of contact lenses are due not only to the lenses themselves, but also to the toxic or allergic effects of cleaning solutions [ ] and the preservatives that they contain. The latter are also found in various eye-drops. Preservatives have adverse effects on the corneal epithelium and endothelium. Eye-drops that contain preservatives should not be used during surgery or in patients with ocular surface disease, in cases of perforating injury.