Ординатура / Офтальмология / Английские материалы / The Eye Book A Complete Guide to Eye Disorders and Health_Cassel, Billig, Randall_2001
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Types of Lenses
Hard versus Soft
Contact lenses come in an impressive variety of materials, sizes, shapes, thicknesses, and colors. In general, they’re divided into two major categories: hard and soft.
Hard contacts have evolved significantly since their introduction in the 1940s. Initially their design improved as manufacturing techniques improved, but in the late 1970s came a major breakthrough: the development of hard contacts that “breathed” like soft lenses. We call these lenses rigid gas-permeable contacts, or RGPs.
The RGP lenses are more flexible and fit better than earlier hard lenses, and they last longer (with respect to “wear and tear”) and sometimes provide better vision than soft lenses. They’re manufactured by computer-con- trolled lathes that can create any kind of surface needed to correct someone’s vision. For example, if you have a high degree of astigmatism, an RGP lens can be ground with a curvature to match your cornea perfectly—pro- viding a healthier, more comfortable fit, and vision that’s usually superior to that offered by your glasses.
RGP lens materials also allow for bifocal segments to be added. In this sophisticated manufacturing process, a bifocal segment is fused into the lens; the result is similar to bifocal glasses that have a line (except this line is tiny, and it’s within a lens placed on your eye; for more on this, read on). Because RGP lenses move significantly on the eye with a blink, these lenses can be fit to trans-
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late or move the bifocal segment up so that it sits in front of the pupil while you’re looking down to read. (Because soft lenses don’t move quite as much, translating bifocal soft lenses don’t work very well.)
Another significant advantage to RGP lenses is that they can provide, in effect, a new cornea for people with a corneal problem that distorts vision. Because this lens maintains its shape on the eye—as opposed to molding itself to the eye, the way a soft lens does—it masks a corneal irregularity, helps correct the optical surface, and improves vision.
Soft contacts are referred to as hydrogels, because they’re made of a plastic material that holds water. By varying the amount of water that a lens holds, manufacturers can provide a broad assortment of materials for fitting. Water contents range from 38 percent to 79 percent. (There is no single material that’s absolutely best for everyone; by varying the water content, lenses can be custom-designed to fit specific needs.)
Soft contacts are available to correct myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and even presbyopia (that unfortunate loss of ability to focus at near distances that gets worse after our mid-thir- ties). For years patients with unusual prescriptions, astigmatism, or reading problems were told that they couldn’t wear soft contact lenses. But now innovations in lens manufacturing allow for almost any prescription to be made into a soft lens. Computerization and automation of lathing and molding soft contact lenses have made for extremely accurate lens prescriptions.
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Daily-Wear versus Extended-Wear
Daily-wear lenses are designed to be put in when you wake up and taken out before you go to sleep. This is very important. All daily-wear lenses are made to provide enough oxygen to the cornea for the eye to breathe while it’s open. No daily-wear lenses are appropriate for overnight wear.
Extended wear refers to any lens worn longer than twenty-four hours. (Note: Some people need extendedwear lenses for daily use, because their eyes require a higher level of oxygen.) These lenses are made to provide enough oxygen so that the eye can breathe even while you’re sleeping.
When they were first introduced in the late 1970s, ex- tended-wear soft contacts were advertised as “thirty-day lenses.” They’re not called that anymore. Research in the early 1980s quickly discovered that very few people could tolerate keeping lenses in their eyes for a whole month at a time, and those hardy souls who tried wearing them that long often ended up with big problems—corneal edema (excess fluid and swelling in the cornea), corneal abrasions, and infections. Subsequent research found that after even one week of continuous wear, there was a significant buildup of bacteria on and in the lenses— so much so, actually, that it’s been determined that contact lenses shouldn’t be worn for more than one week at a stretch. (However, it wasn’t until recently that the Food and Drug Administration forced contact lens manufac-
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turers to change their packaging for these lenses, limiting use to seven days.)
RGP extended wear didn’t become available until the mid-1980s. These lenses were approved for only sevenday wear, but they generally provided more oxygen than their soft-lens counterparts and were less prone to the nasty bacterial buildup.
More current thinking with regard to extended-wear lenses centers around the phrase flexible wear—which isn’t a particular type of contact lens but instead a concept of how these lenses should be worn. Flexible wear simply means that a lens material is fine for occasional overnight wear, and that you have the option of keeping your lenses in all night or taking them out before you go to sleep.
Disposable and Frequent-Replacement Lenses
Like everything else, as technology has improved, soft contacts have also gotten more complicated. Now, in addition to the original lens packaging—where you bought a single pair of lenses, built to last from about twelve to eighteen months—there are soft contacts that are packaged as disposable or frequent-replacement lenses.
Disposable extended-wear lenses, available since the late 1980s, are meant to be worn once—kept in your eyes for a week at a time—and then thrown away. Clinical studies in the 1980s found that many extended-wear lenses were more fragile than other kinds of contacts, more prone to wearing out or to being torn, which made
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this prospect too expensive for many people. In response to this, some lens manufacturers created entirely new production techniques and facilities to make lenses so reproducible, with so few defects, that large quantities of lenses could safely be dispensed at a very low cost per lens. Other manufacturers simply repackaged existing lens designs into lens “multipacks.” The safety and efficacy of this method can be called into question because some of the manufacturing techniques for these older designs don’t allow for the same lens reproducibility, and some of these older lenses provide less oxygen than the newer ones. Most recently, in 1995, came the first lens designed to be disposed of daily. If you have sensitivities to contact lens solutions or are prone to allergic reactions and infections due to contact lens coatings, or simply don’t have time to clean your contacts, then daily-wear disposables might be the healthiest choice for your eyes.
Frequent-replacement lenses are designed to be removed, cleaned, and then worn again—but replaced often, at two-week, one-month, or three-month intervals, depending on the brand and the individual wearer. These lenses are made for either daily use or extended wear. (However, any disposable, extended-wear lens can also be used as a daily-wear, frequent-replacement lens.) An important thing to remember is that although these lenses are replaced often, they must be cleaned and disinfected every time they are removed, if you intend to wear them again. Otherwise there is a significant risk of an eye infection.
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Getting Your Contact Lenses
Which Lenses Are Right for You?
Your first step in deciding which lenses are right for you is a comprehensive eye exam, to determine what your prescription is and also to make sure that your eyes are healthy.
One of the most important parts of this evaluation is your health history. (For more on eye exams, see chapter 3.) Have you had any eye disease—such as dryness, chronic infections, or corneal dystrophy—that may interfere with contact lens wear? Do you have any functional problems with your eyes—a muscle imbalance, perhaps, or any trouble focusing? Do you have any general health problems? (Even though the problem may not be in your eye itself, some conditions may make wearing contact lenses more difficult, so be sure to tell your eye doctor everything, even if it doesn’t seem important to you right now.) Are you taking any medications? (Some medications can dry out your eyes or otherwise interfere with contact lens wear.) Do you have any environmental allergies or sensitivities to preservatives? (These too may affect your comfort in contacts or your ability to use certain contact lens solutions.)
Another aspect of this evaluation is to determine whether your lifestyle—your work, habits, and hob- bies—lends itself to contact lens wear. For example, are you active in sports? Do you work in a dry or dusty environment? Do you stare at a computer for a large por-
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tion of your day? Are you routinely exposed to any chemical fumes that may be absorbed into a contact lens? While none of these might absolutely preclude your wearing contacts—many of us who stare at computers all day do just fine with our contacts, for instance—it’s certainly smart to consider everything before you make this investment, not after the fact.
Finally, it’s a good idea to clarify your goals and expectations with contact lenses. For example, if your goal is to wear contacts for competitive swimming, this may not be realistic, because of the chemicals and bacteria found in swimming pools. If your goal is to use ex- tended-wear soft contacts because you travel extensively on your job and don’t want the hassle of taking contact lens solutions with you, this may be a fine idea, so long as your eyes can tolerate this type of lens. (The dehydrating environment in airplanes may complicate your plans.)
Getting the Right Fit
Contacts that don’t fit right are about as useful as glasses that constantly slip down your nose; in other words, they’re not much help. Good fit is essential, because it lets the contacts do what they’re designed to do: give you the best vision possible.
To ensure good fit, your eye doctor will begin by measuring the corneal curvature and diameter for each eye. This curvature measurement, taken with an instrument called a keratometer, helps determine the right curvature of the back surface of a contact lens, so that it can be cus-
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tom-made to match your eye. The diameter of your cornea—important in determining what size lens to use—can be measured with a ruler, but often it’s best to try on a diagnostic contact lens (a nonprescription sample) of a known size; then the doctor can see how that lens fits by viewing it on your eye with something called a biomicroscope.
Your doctor will also take into account any surface irregularities on your eye that might hamper a good fit. Do you have any bumps or elevations on your conjunctiva (that clear layer over the “white” of your eye)? These may affect how your lenses move when you blink or may cause the edge of a lens to lift off a little. Is there any corneal disease? This too may affect how a contact lens would fit. And most important, might a contact lens aggravate any of these conditions?
When you blink, your contact lens should move up and down to circulate your tears on both sides of the lens. Good lubrication is very important; it helps keep your eyes and lenses from drying out. It also helps wash any debris, such as dust or even stray mascara, from your lenses and generally keeps the lens surface cleaner.
With this in mind, your eye doctor will also examine your eyelids. If you have a very large interpalpebral aperture (or “wide eyes,” a large distance between your upper and lower lids), it may be difficult to fit you with RGP lenses, and you may need soft lenses, which are larger. If your lashes tend to turn inward and rub on the lens, this can affect both your vision and comfort. Lids that don’t move smoothly across an eye can affect lens movement
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and lead to discomfort, dryness, and blurred vision. Similarly, eyelids that are very tight may move a lens too much, while lids that sag or are loose may not move a lens enough.
Next, your tears—their quantity and quality—will be inspected. Your “tear profile” also helps determine the kind of lens material you’ll need. Some people are more comfortable with soft contacts that hold high amounts of water; others don’t make enough tears to maintain a soft lens and would do better with an RGP lens that doesn’t absorb tears.
Next, the Test Drive
Once you’ve been examined for your prescription, measurements have been taken, and it’s been determined that your eyes are indeed healthy enough to tolerate contact lenses, your eye doctor will probably place “trial contact lenses” in your eyes—choosing lenses that best match your specific needs—to make sure all these measurements and assessments have been made correctly. What will they feel like? If you are trial-fit with a soft lens, you’ll probably have some awareness that there is a lens in your eye, but it should be fairly comfortable within a few minutes. RGP lenses at first usually feel much more like a foreign body in your eye, but they typically settle down within about fifteen to twenty minutes. Note: If either type of lens feels painful—like you have several eyelashes in your eye—then there’s something wrong, and the lens should be removed, rinsed, and tried again.
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Some lens designs take longer to settle on the eye than others. For instance, lenses that are designed to correct astigmatism and bifocal contact lenses usually need to sit on your eye for fifteen to twenty minutes before the fit can be assessed. Some soft-lens materials are more volatile than others; they dry more easily on the eye. These lenses should also be allowed a longer settling time, so that the doctor can see whether this material is likely to tighten on your eye as it dries.
Once the trial lens has had time to settle on your eye, the doctor can often double-check your prescription using hand-held lenses. This generally gives you a sense of how well you should see with your “real” contacts (actually, having your full prescription in a well-fitting contact lens should provide even better vision than what you’ll see with the trial contact demonstration).
Where to Purchase Your Lenses
Now it’s time to get your “real” contacts. After all the fitting determinations have been made, your new lenses will be ordered (or set aside, if the doctor already has them in stock). If you’re being fitted for contacts for the first time, or being refitted because of problems with a previous lens design, then you should buy your new lenses from your doctor or “lens fitter” (some states allow opticians to fit contact lenses).
Contact lens prescriptions are much more complex than glasses prescriptions. Glasses prescriptions just specify the power of the lens. Contact lens prescriptions specify a power, curvature, diameter, thickness, material,