28 May 2009
The invention: Small plastic devices that fit under the eyelids, contact lenses, or “contacts,” frequently replace the more familiar eyeglasses that many people wear to correct vision problems. The people behind the invention: Leonardo da Vinci (1452-1519), an Italian artist and scientist Adolf Eugen Fick (1829-1901), a German glassblower Kevin Tuohy, an American optician Otto Wichterle (1913- ), a Czech chemist William Feinbloom (1904-1985), an American optometrist An Old Idea There are two main types of contact lenses: hard and soft. Both types are made of synthetic polymers (plastics). The basic concept of the contact lens was conceived by Leonardo da Vinci in 1508. He proposed that vision could be improved if small glass ampules filled with water were placed in front of each eye. Nothing came of the idea until glass scleral lenses were invented by the German glassblower Adolf Fick. Fick’s large, heavy lenses covered the pupil of the eye, its colored iris, and part of the sclera (the white of the eye). Fick’s lenses were not useful, since they were painful to wear. In the mid-1930’s, however, plastic scleral lenses were developed by various organizations and people, including the German company I. G. Farben and the American optometrist William Feinbloom. These lenses were light and relatively comfortable; they could be worn for several hours at a time. In 1945, the American optician Kevin Tuohy developed corneal lenses, which covered only the cornea of the eye. Reportedly, Tuohy’s invention was inspired by the fact that his nearsighted wife could not bear scleral lenses but hated to wear eyeglasses. Tuohy’s lenses were hard contact lenses made of rigid plastic, but they were much more comfortable than scleral lenses and could be worn for longer periods of time. Soon after, other people developed soft contact lenses, which cover both the cornea and the iris. At present,many kinds of contact lenses are available. Both hard and soft contact lenses have advantages for particular uses. Eyes, Tears, and Contact Lenses The camera-like human eye automatically focuses itself and adjusts to the prevailing light intensity. In addition, it never runs out of “film” and makes a continuous series of visual images. In the process of seeing, light enters the eye and passes through the clear, dome-shaped cornea, through the hole (the pupil) in the colored iris, and through the clear eye lens, which can change shape by means of muscle contraction. The lens focuses the light, which next passes across the jellylike “vitreous humor” and hits the retina. There, light-sensitive retinal cells send visual images to the optic nerve, which transmits them to the brain for interpretation. Many people have 20/20 (normal) vision, which means that they can clearly see letters on a designated line of a standard eye chart placed 20 feet away. Nearsighted (myopic) people have vision of 20/40 or worse. This means that, 20 feet from the eye chart, they see clearly what people with 20/20 vision can see clearly at a greater distance. Myopia (nearsightedness) is one of the four most common visual defects. The others are hyperopia, astigmatism, and presbyopia. All are called “refractive errors” and are corrected with appropriate eyeglasses or contact lenses. Myopia, which occurs in 30 percent of humans, occurs when the eyeball is too long for the lens’s focusing ability and images of distant objects focus before they reach the retina, causing blurry vision. Hyperopia, or farsightedness, occurs when the eyeballs are too short. In hyperopia, the eye’s lenses cannot focus images of nearby objects by the time those images reach the retina, resulting in blurry vision. A more common condition is astigmatism, in which incorrectly shaped corneas make all objects appear blurred. Finally, presbyopia, part of the aging process, causes the lens of the eye to lose its elasticity. It causes progressive difficulty in seeing nearby objects. In myopic, hyperopic, or astigmatic people, bifocal (two-lens) systems are used to correct presbyopia, whereas monofocal systems are used to correct presbyopia in people whose vision is otherwise normal.Modern contact lenses, which many people prefer to eyeglasses, are used to correct all common eye defects as well as many others not mentioned here. The lenses float on the layer of tears that is made continuously to nourish the eye and keep it moist. They fit under the eyelids and either over the cornea or over both the cornea and the iris, and they correct visual errors by altering the eye’s focal length enough to produce 20/20 vision. In addition to being more attractive than eyeglasses, contact lenses correct visual defects more effectively than eyeglasses can. Some soft contact lenses (all are made of flexible plastics) can be worn almost continuously. Hard lenses are made of more rigid plastic and last longer, though they can usually be worn only for six to nine hours at a time. The choice of hard or soft lenses must be made on an individual basis. The disadvantages of contact lenses include the fact that they must be cleaned frequently to prevent eye irritation. Furthermore, people who do not produce adequate amounts of tears (a condition called “dry eyes”) cannot wear them. Also, arthritis, many allergies, and poor manual dexterity caused by old age or physical problems make many people poor candidates for contact lenses.Impact The invention of Plexiglas hard scleral contact lenses set the stage for the development of the widely used corneal hard lenses by Tuohy. The development of soft contact lenses available to the general public began in Czechoslovakia in the 1960’s. It led to the sale, starting in the 1970’s, of the popular, soft contact lenses pioneered by Otto Wichterle. The Wichterle lenses, which cover both the cornea and the iris, are made of a plastic called HEMA (short for hydroxyethylmethylmethacrylate). These very thin lenses have disadvantages that include the requirement of disinfection between uses, incomplete astigmatism correction, low durability, and the possibility of chemical combination with some medications, which can damage the eyes. Therefore, much research is being carried out to improve them. For this reason, and because of the continued popularity of hard lenses, new kinds of soft and hard lenses are continually coming on the market.