Eye Glasses and Contact Lenses

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Definition

Eyeglasses and contact lenses are devices that correct refractive errors in vision. Eyeglass lenses are mounted in frames worn on the face, sitting mostly on the ears and nose, so that the lenses are positioned in front of the eyes. Contact lenses appear to be worn in direct contact with the cornea, but they actually float on a layer of tears that separates them from the cornea.

Description

Eyes are examined by optometrists (O.D.) or by ophthalmologists (M.D. or D.O.—doctor of osteopathy). Prescriptions, if necessary, are then given to patients for glasses. The glasses are generally made by an optician. A separate contact lens-fitting exam is necessary if the patient wants contact lenses, because an eyeglass prescription is not the same as a contact lens prescription.

Over 140 million people in the United States wear eyeglasses. People whose eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. For people who are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina.

LENSES. Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of light that passes through it, and the greater the correction.

Lenses can be unifocal, with one correction for all distances, or they can be correct for more than one distance (multifocal). One type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects for distant objects (about 20 ft from the eyes). Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in). Conventional bifocals and trifocals have visible lines between the areas of different correction; however, lenses where the correction gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses are sometimes called progressives or no-line bifocals.

To be suitable for eyeglass lenses, a material must be transparent, without bubbles, and have a high index of refraction. The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact resistant than glass. The advantage of glass is that it is scratch resistant and provides the clearest possible vision.

Glass was the first material to be used for eyeglass lenses, and was used for several hundred years before plastic was introduced. The crown glass used for eyeglass lenses has an index of refraction of 1.52.

Optical-quality acrylic was introduced for eyeglass use in the early 1940s, but because it was easily scratched, brittle, and discolored rapidly, it did not supplant glass as the material of choice. Furthermore, it had a relatively low index of refraction, so it wasn't suitable for people with large refractive errors. A plastic called CR-39, introduced in the 1960s, was more suitable. Today, eyeglass wearers can also choose between poly-carbonate, which is the most impact-resistant material available for eyewear, and polyurethane, which has exceptional optical qualities and an index of refraction of up to 1.66, much higher than the conventional plastics used for lenses, and even higher than glass. Patients with high prescriptions should ask about high index material options for their lenses. Aspheric lenses are also useful for high prescriptions. They are flatter and lighter than conventional lenses.

There are many lenses and lens-coating options for individual needs, including coatings that block the ultra-violet (UV) light or UV and blue light which have been found to be harmful to the eyes. Such coatings are not needed on polycarbonate lenses, which already have UV protection. UV coatings are particularly important on sunglasses and ski goggles. Sunglasses, when nonprescription, should be labeled with an indication that they block out 99–100% of both UV-A and UV-B rays.

There are anti-scratch coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eliminate almost all glare and allow other people to see the eyes of the wearer. AR coatings may be particularly helpful to people who use computers or who drive at night. Mirror coatings that prevent other people from seeing the wearer's eyes are also available. There is a whole spectrum of tints, from light tints to darker tints, used in sunglasses. Tint, however, does not block-out UV rays, so a UV coating is needed. Polaroid lenses that block out much of the reflected light also allow better vision in sunny weather and are helpful for people who enjoy boating. Photosensitive (photochromatic) lenses that darken in the presence of bright light are handy for people who don't want to carry an extra set of glasses. Photochromatic lenses are available in glass and plastic.

FRAMES. Frames can be made from metal or plastic, and they can be rimless. There is an almost unlimited variety of shapes, colors, and sizes. The type and degree of refractive correction in the lens determine to some extent the type of frame most suitable. Some lenses are too thick to fit in metal rims, and some large-correction prescriptions are best suited to frames with small-area lenses.

Rimless frames are the least noticeable type, and they are lightweight because the nosepiece and temples are attached directly to the lenses, eliminating the weight of the rims. They tend to not be as sturdy as frames with rims, so they are not a good choice for people who frequently remove their glasses and put them on again. They are also not very suitable for lenses that correct a high degree of farsightedness, because such lenses are thin at the edges.

Metal frames are less noticeable than plastic, and they are lightweight. They are available in solid gold, gold-filled, anodized aluminum, nickel, silver, stainless steel, and now titanium and titanium alloy. Until the late 1980s, when titanium-nickel alloy and titanium frames were introduced, metal frames were, in general, more fragile than plastic frames. The titanium frames, however, are very strong and lightweight. An alloy of titanium and nickel, called Flexon, is not only strong and lightweight, but returns to its original shape after being twisted or dented. It is not perfect for everyone, though, because some people are sensitive to its nickel. Flexon frames are also relatively expensive.

Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are also offered in a variety of plastics (including acrylic, epoxy, cellulose acetate, cellulose propionate, polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. Epoxy frames are resilient and return to their original shape after being deformed, so they do not need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.

FIT. The patient should have the distance between the eyes (PD) measured, so that the optical centers of the lenses will be in front of the patient's pupils. Bifocal heights also have to be measured with the chosen frame in place and adjusted on the patient. Again, this is so the lenses will be positioned correctly. If not positioned correctly, the patient may experience eyestrain or other problems. This can occur with over-the-counter reading glasses. The distance between the lenses is for a "standard" person. Generally, this will not be a problem, but if a patient is sensitive or has more closely set eyes, for example, it may pose a problem. Persons buying readymade sunglasses or reading glasses should hold them up to see if they appear clear. They should also hold the lenses to see an object with straight lines reflected off of the lenses. If the lines don't appear straight, the lenses may be warped or inferior.

Patients may sometimes need a few days to adjust to a new prescription; however, problems should be reported, because the glasses may need to be rechecked.

Over 32 million people in the United States wear these small lenses that fit on top of the cornea. They provide a field of view unobstructed by eyeglass frames; they do not fog-up or get splattered, so it is possible to see well while walking in the rain; and they are less noticeable than any eyeglass style. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to the eye doctor; can lead to complications such as infections and corneal damage; and may not correct astigmatism as well as eyeglasses, especially if the astigmatism is severe.

Originally, hard contact lenses were made of a material called PMMA. Although still available, the more common types of contact lenses are listed below:

• Rigid gas-permeable (RGP) daily-wear lenses are made of plastic that does not absorb water but allows oxygen to get from the atmosphere to the cornea. (This is important because the cornea has no blood supply and needs to get its oxygen from the atmosphere through the film of tears that moves beneath the lens.) They must be removed and cleaned each night.
• Rigid gas-permeable (RGP) extended-wear lenses are made from plastic that also does not absorb water but is more permeable to oxygen than the plastic used for daily-wear lenses. They can be worn up to a week.
• Daily wear soft lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. Soft lenses are easier to get used to than rigid lenses, but are more prone to tears and do not last as long.
• Extended-wear soft lenses are highly permeable to oxygen, are flexible by virtue of their ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems. There is more of a risk of infection with extended-wear lenses than with daily-wear lenses.
• Extended-wear disposable lenses are soft lenses worn continually for up to six days and then discarded, with no need for cleaning.
• Planned-replacement soft lenses are daily wear lenses that are replaced on a regular schedule, which is usually every two weeks, monthly, or quarterly. They must also be cleaned.

Soft contact lenses come in a variety of materials. There are also different kinds of RGP and soft multifocal contact lenses available. Monovision, where one contact lens corrects for distance vision while the other corrects for near vision, may be an option for presbyopic patients. Monovision, however, may affect depth perception and may not be appropriate for everyone. Contact lenses also come in a variety of tints. Soft contacts are available that can change dark-colored eyes a different color. Even though such lenses have no prescription, they must still be fitted and checked to make sure that an eye infection does not occur. People should NEVER wear someone else's contact lenses. This can lead to infection or damage to the eye.

— Lorraine Lica, PhD

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.