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Contact Lens

Contact Lens

Background

The contact lens is a device worn in the eye to correct vision, although some people wear colored contact lens to enhance or change their eye color. The thin plastic lens floats on a film of tears directly over the cornea. For some forms of eye disease, contact lenses correct vision better than conventional spectacles. Many people prefer contact lenses over glasses for cosmetic reasons, and active sports enthusiasts prefer contact lens because of the freedom it provides them. There are basically three types of lenses: soft, hard, and gas-permeable. Soft contact lenses are usually more comfortable to wear, but they also tear more easily than hard contact lenses. Hard lenses also tend to "pop" out more frequently. Gas-permeable lenses are a compromise between the hard and soft, allowing greater comfort than hard lenses but less chance of tearing than soft lenses. Contacts are usually worn during the day and taken out every night for cleaning. Extended-wear lenses allow users to leave in their contacts for longer periods of time, even when they're sleeping. More recently, one-a-day contact lenses are gaining popularity among lens wearers. These contacts are worn for only one day and thrown away, eliminating the hassle of cleaning them every night.

History

The first contact lens was made by German physiologist Adolf Fick in 1887. Fick's lens was made of glass and was a so-called scleral lens because it covered the sclera, the white part of the eye. By 1912, another optician, Carl Zeiss, had developed a glass corneal lens, which fit over the cornea. Two scientists, Obrig and Muller, introduced a plastic scleral lens in 1938. It was made of the material commonly known as Plexiglas. Because it was lighter than glass, the Plexiglas lens was easier to wear. The first plastic corneal lens was made by Kevin Touhy in 1948.

To fit these early lenses, an impression was made of the patient's eyeball, and the lens was formed in the resulting mold. This procedure was doubtlessly uncomfortable, and the lenses themselves were often problematic to wear. Scleral lenses deprived the eye of oxygen, and many of these earlier lenses slipped out of place or popped out of the eye, and were often, oddly enough, difficult to remove. Touhy's first corneal lens had a diameter of 10.5 millimeters, and in 1954 Touhy reduced the diameter further to 9.5 millimeters, resulting in better wearability. Around this time the Bausch & Lomb company developed the keratometer, which measures the cornea, and eliminated the need for eyeball impressions.

The first successful soft contact lenses were developed by chemists in Czechoslovakia. In 1952, professors in the Department of Plastics at the Technical University in Prague set themselves a task of designing a new material that was optimally compatible with living tissue. They did not set out to create contact lenses, but by 1954 the team of Czech scientists had invented what is called a "hydrophilic" (for its affinity to water) gel, a polymer plastic that was suitable for eye implants. The scientists immediately recognized the new plastic's potential as a corrective lens, and they began experimenting on animals. These efforts were met with scorn by their colleagues in the optics field, but one of the scientists, Otto Wichterle, was undaunted and began perfecting soft contact lenses in his kitchen. Wichterle and his wife produced 5,500 pairs of contact lenses from their home for testing in 1961, and their success eventually got the attention of the wider scientific community. The American firm Bausch & Lomb licensed the technology and launched their Softlens in 1971. That first year alone, the firm sold about 100,000 pairs, and soft contact lenses have had great appeal with the public ever since.

Raw Materials

The raw material for contact lenses is a plastic polymer. (A polymer is a blend of materials created by linking the molecules of different chemical substances.) Hard contact lenses are made of some variant of polymethyl methacrylate (PMMA). Soft contact lenses are made of a polymer such as poly hydroxyethyl methacrylate (pHEMA) that has hydrophilic qualities, that is, it can soak up water and still retain its shape and optic functions. The science of lens material is always being updated by lens manufacturers, and the specific material of any contact lens may differ depending on the maker.

The Manufacturing
Process

Contact lenses may be produced by cutting a blank on a lathe, or by a molding process. The forming of the lens involves shaping the plastic into specified curvatures. The major curves of the lens are named the central anterior curve (CAC) and the central posterior curve (CPC). The CAC refers to the overall curve of the side of the lens that faces out. This outer contour produces the correct refractive change to fit the patient's visual needs. The CPC is the concave inner side of the lens. This conforms to the measurements of the patient's eye. Usually these two curves are formed first, and the lens is then called semi-finished. The lens is deemed finished when peripheral and intermediate curves are formed, and the edge is shaped.

Molding method

  • 1 Molding the lens can be carried out in several different ways. The lenses first developed in Prague were spin-cast. Three different fluids were poured into open rotating molds. The outside curvature of the lens was shaped by the mold, and the inside curvature was formed according to the speed of the rotation of the mold. The centrifugal force of the spinning mold led to the polymerization of the fluids so that the molecular chains linked to form the required hydrophilic plastic. A more reliable mass-production method is injection molding. In injection molding, the molten plastic is injected into the mold under pressure. Then the lens is removed from the mold and cooled. The lens is then finished on a lathe. It is also possible to produce lenses entirely through molding, that is, they need no lathe cutting. This is a recent development, made possible through highly automated, computer controlled mold production.

Lathe process

  • 2 The initial forming of the lens can also be done by cutting on a lathe. First a blank is made. The blank is a circle only slightly larger than the size of the finished lens. This can be cut from a plastic rod, or stamped from a plastic sheet. Next the blank is fastened to a steel button with a drop of molten wax. The button is then centered on a lathe, which begins to spin at high speed. A cutting tool, which may be a diamond or a laser, makes concave cuts in the blank to form the CPC. Indicators on the lathe measure the depth of the cuts to guide the lens operator.

    The button holding the blank is next moved to a lapping machine. The lapping machine holds the blank against a lapper, which is a revolving disk coated with an abrasive compound. The shape of the lapper matches the CPC of the lens. The lapping machine spins the blank in one direction, and the lapper in the other. It also moves the blank in a small figure eight motion. The abrasion polishes the lens surface.

    The polished lens is then mounted on a steel shaft called an arbor. The end of the arbor has been ground to match the CPC so the lens will fit on the shaft. The arbor is installed in a lathe, and the operator makes convex cuts in the lens to form the other major curve, the CAC. Now this side of the lens is polished, and the lapper is modified to fit the convex CAC. When this second side of the lens is polished, the lens is considered semi-finished.

Finishing

  • 3 The contact lens requires several more curves to be ground before the lens will fit exactly on the patient's eye. The final curves are the peripheral anterior and posterior curves and the intermediate anterior and posterior curves, which govern the shape of the lens nearest and next-nearest the edge. The lens is mounted on an arbor again by suction or with double-sided tape. The arbor is installed in the lathe or grinding machine. These shallower cuts may be ground with emery paper or cut with a razor blade. The diameter of the lens may also be trimmed at this time.

Quality control

  • 4 Quality control is very important for contact lenses, since they are medical devices and they must be custom fit. The lenses are inspected after each stage of the manufacturing process. The lenses are examined under magnification for anomalies. They are also measured by means of a shadow graph. A magnified shadow of the lens is cast on a screen imprinted with a graph for measuring diameter and curvature. Any errors in the lens shape show up in the shadow. This process may be automatically performed by computer.

Packaging

  • 5 After the lens has passed inspection, it is sterilized. Lens are boiled in a mixture of water and salt for several hours to soften the lens. Next, the lens are packaged. Standard packaging for lenses is a glass vial, filled with a saline solution and stoppered with rubber or metal. The hydrophilic material of soft contact lenses soaks up the saline solution, which is similar to human tears, and becomes soft and pliable. The lenses in this state are ready to wear.

The Future

The material for contact lenses is the subject of much research. Scientists are investigating different chemical recipes that may give plastic more desirable characteristics. One polymer currently being researched is a silicon-oxygen compound called siloxane. Siloxane forms a thin, flexible film and admits oxygen through to the eye 25 times better than current standard soft lenses. There are disadvantages to this compound, however: siloxane does not wet easily and it attracts lipids (fats) to its surface, causing it to cloud. Researchers have found a way to add flourine molecules to the siloxane compound, causing the material to resist lipids. Then they chemically attach a wetting agent, which changes its molecular shape when boiled in a saline solution, so that the material can soak up water like traditional soft lens. This material may ultimately lead to extended-wear contacts that can be worn for weeks at a time.

Researchers are also investigating new polymers that can be used for scleral lenses. For most people, comeal lenses are the norm, but the large scleral lenses are useful for patients with severely damaged corneas. Depending on the eye problem, some patients cannot regain their sight without a corneal transplant, but scleral lenses may help patients avoid eye surgery. Scleral lenses rest on the white part of the eye and form a vault over the cornea itself. This space over the cornea is filled with artificial tears, which serve to smooth out the cornea's damaged surface. In the past, scleral lenses have been uncomfortable because they do not allow enough oxygen to the eye, but investigations into new materials are focusing on more oxygen-permeable lenses.

Material for oxygen-permeable lenses has also been experimented on the space shuttle Endeavour. The designers of the experiment believe that microgravity conditions would promote a lens material that repels debris better and processes oxygen more effectively than polymers made in traditional labs. If commercially feasible, a new generation of contact lenses may be manufactured in space.

Where To Learn More

Book

Ruben, Montague, ed. Soft Contact Lenses: Clinical and Applied Technology. John Wiley & Sons, 1978.

Periodicals

"Making Eye Contact." Ad Astra, September-October 1993, p. 5.

"This Contact Lens Is A Sight For Sore Comeas." Business Week, April 20, 1992, p. 94.

"Extending Extended-Wear Contacts." Science News, September 5, 1992, p. 153.

Angela Woodward

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contact lenses

contact lenses The contact lens provides a good example of the lengths to which some humans will go to overcome deficiencies of the visual system. A contact lens is a foreign body and as such causes irritation and pain. In addition there is an excessive flow of tears to wash out the invading object and a strong urge to rub the eye, with the further risk of causing a corneal abrasion. The contact lens wearer has to be strongly motivated to learn to adapt to this irritant and develop the ocular tolerance necessary for comfortable lens wear. Vanity is a powerful driving force, immortalized by Dorothy Parker's malicious ‘Men seldom make passes at girls who wear glasses’. Assuredly there are other advantages with contact lenses in many types of sport, in the rain, and in working in confined and awkward positions, especially if protective goggles are needed as well. An early experiment by Thomas Young in 1801 to eliminate focusing by the cornea involved a convex lens fitted to a short glass socket filled with water which was applied to his eye. He immediately became hypermetropic (long sighted) in that eye, but using his optometer found ‘the same inequality in the horizontal and vertical refractions as without the water’. Interestingly, this showed that his astigmatism was not due to his cornea, which is certainly the commonest cause.

The first recorded use of a protective glass for an eye exposed after removal of the eyelid was in 1887. A blown glass contact lens made from a plaster mould of an eye with keratoconus (conical cornea) was used for optical treatment in 1888. A year later a ground glass contact lens was made to treat an ophthalmologist's own high myopia (short sight).

Scleral lenses covered more than the area of the eye seen between the eyelids, and ground and moulded glass lenses superseded blown glass until that, in turn, was followed by plastic lenses.

The author was present at a lecture by Wichterle in 1963 at the Institute of Ophthalmology, London when he introduced the soft hydrophilic plastic corneal lens. These cost 1 shilling (5p) each to produce, although Wichterle wryly commented that the cost to the patient would bear no comparison. In fact, having passed through expensive phases, daily wear, disposable soft lenses which are thrown away after use cost only £1 each, nearly 40 years later.

The search for new materials to make contact lenses continues, because a number of important constraints have to be met. The lens must allow oxygen and carbon dioxide easy passage between the air and the aqueous humour, the fluid behind the cornea. It must not interfere with the metabolic pump which maintains corneal transparency. The tear flow must be maintained, but not excessive. Adaptive changes occur in corneal sensitivity, so that the lens is better tolerated, but if sensitivity is blunted too far the risk of abrasions and infections increases. A soft lens is more comfortable because it conforms to the shape of the underlying cornea, but then it may not adequately correct any corneal astigmatism.

The most recent new soft contact lens — silicone–hydrogel — has such high oxygen permeability that up to 30 days of continuous (day and night) wear is advocated. Ophthalmologists always view such claims with scepticism because of the risks of low oxygen flow, drying out of the lens, accumulation of protein debris, and infection. If any contact lens is left in place overnight the further barrier to free oxygen exchange between the eye and the atmosphere imposed by the closed lids may be crucial. Daily wear, disposable soft lenses avoid these difficulties, and because each lens is from a sterile pack the armamentarium of small bottles of sterilizing fluids for reusable, hard lenses is banished.

Contact lens fitting has always been an art, necessitating careful consideration of many factors which allow accurate alignment of the optical centres of contact lens and cornea, with the proper thickness of tear film beneath the lens so that it may move slightly in relation to the globe but re-centres itself naturally. Any long-term effect that a contact lens may have on the underlying cornea shape, and thus its refractive power, must be monitored weekly by the patient checking that his spectacles continue to give optimal vision when worn.

Contact lenses can be used most advantageously to correct myopia, but can also be used for hypermetropia and corneal astigmatism. Any astigmatism not corrected by the contact lens (residual astigmatism) is due partly to the patient's own lens within the eye and partly to features of the contact lens itself. Making specially shaped contact lenses to reduce their rotation on the eye can help. Bifocal contact lenses to correct distance and reading vision, so that the obvious bifocal glasses with their ageing implications could be avoided, have not proved successful. Reading glasses for presbyopia still have to be worn over the contact lenses. Another way to overcome this problem is to fit a contact lens for distance viewing to one eye and one focused for reading to the other. There are significant implications for good quality fusion of the images from each eye, in that 3-dimensional vision and depth perception are compromised. This can be important when driving, and spectacles for optimal distance vision should be worn over such contact lenses under these circumstances.

Cosmetic contact lenses fulfil a variety of uses, ranging from different coloured lenses as matching accessories in the fashion industry to films where an opaque scleral lens could be used to simulate a blind or grossly damaged eye.

The new, soft disposable contact lenses can now be used for occasional or social wear, since the long period of adaptation and building up of wearing time with hard contact lenses is no longer necessary.

There are a number of clinical indications for contact lenses, and the irregularly-shaped ‘conical cornea’ is the principal one. Specially made lenses when held temporarily in contact with the eye assist the opthalmologist to obtain detailed, magnified views of otherwise inaccessible areas of the eye, for example in glaucoma patients. The original scleral contact lenses still have a role in treating severely damaged eyes, now that they are made of a highly permeable polymer allowing good gas exchange but giving essential protection.

Peter Fells


See also eyes; refractive errors; squint; vision.

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Contact Lens

Contact Lens

A contact lens is a small, shell-shaped piece of eyeware used to correct vision. It is placed directly over the cornea (the transparent, or clear, tissue over the pupil and the iris, or colored part of the eye). There are two types of contact lenses. One is the corneal contact lens which covers only the cornea, and the other is the scleral contact lens, which covers the cornea and a part of the sclera (the white part of the eye).

The contact lens is held in place by fluid attraction forces. These forces are easy to demonstrate. If two pieces of glass are placed together with water between them, it is easy to slide the pieces back and forth. But separating the two pieces of glass is very difficult. Today contact lenses are made from plastics and gels, but the original contact lenses were made of glass blown by glassblowers.

Contact Lenses in History

It is hard to say when the idea of a contact lens was born. Leonardo da Vinci (1452-1519; Italian painter, scientist and inventor) knew that glass and water affected vision. He sketched and described several types of optical devices demonstrating the principles of a contact lens. The first description of what was needed to make a contact lens was given by Sir John W. Herschel (1792-1871) in 1823. Three Europeans are generally credited with the invention of the contact lens: Adolf Eugene Fick, Eugene Kalt, and August Muller. In 1888 Fick described the first contact lens with refractive power (the ability to change vision by bending rays of light). Fick's "contact spectacle" was a thin, very small glass bowl. This bowl was placed on the eye, and the area between the bowl and the eyeball was filled with a liquid similar to tears.

On March 20,1888, a scientific paper was presented in Paris. Delivered by Kalt, the paper described a lens he made to treat keratoconus (a condition of the eye in which the cornea is cone-shaped; normally the cornea has a convex, or bulging outward, shape). Kalt's lens worked by pressing down on the cornea and making it flatter. This improved vision. When plastic contact lens were introduced, they were similar to Kalt's contact lenses. In 1889 Muller, a medical student at the University of Kiel in Germany, started his work on contact lenses. He was very myopic (near-sighted). His lenses were made to more closely match the curvature of the cornea. He knew they would be held in place by the tears between the lens and the cornea.

Between 1920 and 1940, the Zeiss Optical Works of Jena, Germany, and the Mueller Company of Wiesbaden, Germany, were making glass contact lenses. There were problems with eye irritation, however, and it was not possible to wear these lenses very long. Joseph Dallos understood that tears needed to pass between the lens and the cornea. Eventually it would be discovered that, because the cornea does not have its own blood supply, tears bring nutrients to it. Also, because it has no blood supply, the cornea must get its oxygen from the air.

Plastic Lenses

In 1936 William Feinbloom made the first contact lenses with plastic. These lenses were a hybrid (combination), however, of both plastic and glass. Feinbloom was also the first researcher to file a patent for bifocal (adjusted to two different focal lengths) and trifocal (adjusted to three different focal lengths) contact lenses. In 1936 the Rhom and Haas Company introduced a new, transparent plastic material called polymethylmethacrylate (PMMA). This material would be the main plastic used in contacts for the next forty years. In 1938, John Mullen and Theodore Obrig developed a technique for making scleral contact lenses with PMMA.

In 1948 Kevin Tuohy was the first person to make a corneal contact lens when his lab partner accidentally cut away the scleral part of a contact lens in production. Tuohy discovered that he could wear just the corneal part of the lens. There were still some problems, however, with making the contact lens fit properly. In 1950 George Butterfield changed the way contacts were made by creating a lens with different curves on the side of the lens next to the eye. This made the contact very similar to the cornea.

The Hydrogel Lens

The hydrogel lens is made of very pliable chemicals. It does not cause very much irritation to the eye. The idea for the hydrogel lens started with Professor Otto Wichterle, a polymer chemist from Czechloslovakia. Wichterle and one of his assistants created a material called hydroxyethylmethacrylate (HEMA). The researchers found that this material caused little irritation to the delicate tissues of the eye. In 1957 Wichterle tried to make contact lenses in a mold using HEMA. Unfortunately the lenses were too thick, and they often tore when anyone tried to get them out of the mold. Eventually, Wichterle was told that he could not get any more money for his research, so he continued his work at home. Over time, he was able to produce contact lenses that were thin and easy to make. In 1972 the Bausch and Lomb company introduced a hydrogel lens that has almost replaced the hard contact lens completely.

In addition to their cosmetic applications, contact lenses offer several medical advantages over eyeglasses. When cataracts (a disease of the lens causing it to get cloudy) are removed, contact lens are more effective than glasses in restoring sight. Unlike glasses, contact lenses can improve the vision of people with keratoconus, and they also allow people to play sports without having to woffy about breaking their glasses.

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contact lens

contact lens, thin plastic lens worn between the eye and eyelid that may be used instead of eyeglasses. Actors, models, and others wear them for appearance, and athletes use them for safety and convenience. Contact lenses may also be used to correct certain abnormalities of the eye that cannot be corrected by regular glasses. A. E. Fick, a Swiss physician, made the first contact lens in 1887. His heavy glass lenses exerted an uncomfortable pressure on the eyeball, covered the entire eye surface, and were difficult to fit. In 1938, the first plastic contact lens was made by Theodore E. Obrig, using a newly discovered methylmethacrylate plastic, known as Plexiglas or Lucite, that could be molded into the proper shape. The major drawback was that a solution placed between the lens and eye had to be changed every few hours, because the wearer's tears could not circulate beneath the lens. In 1950, the corneal contact lens was introduced. It covered only the cornea of the eye, floated on the tears of the wearer, and could be worn all day without difficulty. Recent improvements include flexible lenses that shorten the initial period of adjustment for the wearer and porous lenses that do not have to be removed each day. Today, contact lenses that "breathe" have become popular. They allow oxygen to get to the cornea, preventing blurred vision due to the corneal exhaustion syndrome.

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contact lenses

contact lenses pl. n. hard or soft lenses worn directly against the eye, separated from it only by a film of tear fluid. Contact lenses are used mainly in place of glasses to correct errors of refraction, but soft lenses (bandage lenses) may be used for protection to allow epithelial abrasions to heal.

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