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A Guide to Laser Vision Correction
By Ernest W. Kornmehl, Robert K. Maloney, Jonathan M. Davidorf
Addicus Books, Inc.Copyright © 2006 Ernest W. Kornmehl, Robert K. Maloney, and Jonathan M. Davidorf
All rights reserved.
The Human Eye and How Vision Works
Sight is our most precious sense. Our eyes enable us to take in the surrounding world. Without sight, the way we perceive the world would be forever changed. No wonder the eyes are often elevated in literature, art, religion, and philosophy to symbolize everything from the windows of the soul to supreme wisdom. Indeed, the eyes are a marvel of mechanics.
However, changes within the eyeball may occur, resulting in impaired vision. Objects that we once viewed with crystal clarity may become blurred or distorted. To better understand how vision may change, let's first examine the anatomy of the eye.
How the Eye Works
You may have heard the comparison between a camera and the human eye. Just as a camera takes in light and transforms it into an image on film, your eye does virtually the same thing, only the "film" is your retina and your brain "develops" the image. We see objects when light, which is reflected by the objects, passes through the eyeball lens and strikes the retina, at the back of the eye. Our brains then interpret the shapes, colors, and dimensions of the objects we see. A clearly focused object is the result of normal vision. However, just as an improper amount of light entering a camera lens will distort a photo, if light entering the eyeball does not strike the retina, the result may be distorted vision.
Anatomy of the Eye
Sclera and Cornea
The outer layer surrounding the eyeball is made up of two parts: the sclera and the cornea. The sclera — the white, opaque part of the eye — makes up the back five -sixths of the eye's outer layer and provides protection for the eyeball. The cornea, about the size of a dime and as thick as a credit card, makes up the remaining sixth of the eye's outer layer. It is the transparent dome, similar to the crystal of a wrist watch, at the front of the eyeball. The cornea provides most of the eye's focusing power, so small changes in its curvature can make an enormous difference in how clearly you see objects.
The cornea has three main layers. The epithelium is the thin outer protective layer of cells; it is made up of the same kind of tissue that covers most of your body, and is continually regenerating, or renewing itself. The stroma is the strong, fibrous layer that makes up 90 percent of the cornea's thickness and provides the cornea with its structure and shape. The endothelium is the single cell layer that lines the inside of the cornea and helps regulate the cornea's fluid content.
The iris, which determines one's eye color, is located behind the cornea. It is composed of connective tissue and smooth muscle fibers. The muscles of the iris control how much light passes through to the retina.
The pupil appears as a black circle in the middle of the iris. The pupil can be likened to the aperture, or shutter, of a camera. When it is very bright, as on a sunny day, the iris muscles make the pupil constrict, or become small, so only a small amount of light will pass into the eye. In darkness, the opposite happens, and the pupil dilates, or enlarges, to let in more light.
The lens is a circular structure located directly behind the pupil and held in place by slender, strong ligaments. Although most of the bending of light is accomplished by the cornea, the curved lens fine-tunes the angle of light passing through it, focusing the light onto the retina. When the ligaments tighten, the lens becomes flatter, or less convex, allowing you to see objects at a distance. When the ligaments relax, the elastic lens becomes rounder, or more convex, like a magnifying glass, so you can see objects that are close. This ability of the lens to refine the focus through flexing is called accommodation.
The vitreous humor is the jellylike substance, about 99 percent water, that fills the space between the lens and the retina on the inner back wall of the eye. Light passes through the vitreous humor before striking the retina.
The retina is a complex layer of nerve tissue that lines the inside back wall of the eyeball. Similar to film in a camera, the retina "captures" the image through an electrochemical reaction to light. Electrical impulses are then transmitted through the optic nerve to the brain, which interprets, or "develops," the image.
Common Vision Problems
Your eye doctor may refer to your vision problem as your refractive error, or focusing problem. How well you see is determined, for the most part, by how accurately your eyes are able to bend, or refract, light. In a normal eye, the focus comes to a point on the retina. But sometimes this does not occur. The result? Various forms of vision impairment, or aberrations. Vision problems fall into one of two basic groups: low-order aberrations and higher-order aberrations.
These aberrations include common refractive errors, such as nearsightedness, farsightedness, and astigmatism. The impairments can be corrected with eyeglasses, contact lenses, or LASIK, PRK, or other laser vision correction procedures.
Also known as nearsightedness, myopia is a condition in which you can see nearby objects well, but objects at a distance appear blurred. This happens when light bouncing off a faraway image enters the eye through the cornea and comes to a point of focus too soon, before it reaches the retina. Myopia may be due to a cornea that has too much curvature, which causes the light to "overbend" and focus in front of the retina. Myopia also occurs when the eyeball is too long — the retinal wall is too far back for the combined focusing power of the cornea and lens.
People with hyperopia, or farsightedness, see distant objects more clearly than nearby objects when they are young but may have difficulty with both as they get older. In hyperopia, the light rays coming into the cornea are not bent sharply enough and are focused behind, rather than on, the retina. The result is a blurred image. This usually happens in people whose eyeballs are too short from front to back or whose focusing muscles around the lens are too weak. Another cause of hyperopia, though rare, is a cornea that is not curved enough.
Because muscles are more elastic in youth, younger people who are mildly hyperopic can actually compensate for it by using the focusing muscles around the lens to fine-tune the focus by bending light more steeply. This action brings the point of focus forward toward the retina, allowing them to see more clearly. However, because the muscles weaken and the lens becomes less pliable as we age, these individuals eventually lose that ability and may no longer see well at a distance or close up. After age forty, they may be completely dependent on eyeglasses or contact lenses for both distant and close vision.
Many individuals with myopia or hyperopia also have some degree of astigmatism. People with significant astigmatism experience blurred or distorted vision with all objects, whether near or far. Astigmatism means that your cornea, instead of being spherical like the side of a basketball, is slightly oval, shaped more like the side of a football. Your cornea is more curved in one direction than the other. As a result, light rays entering the eye from different points on the cornea's surface are bent irregularly and are focused at several different points, rather than meeting at just one focal point. Almost everyone has a small degree of astigmatism.
Farsightedness is often confused with presbyopia, which literally means "old eyes." Presbyopia is the age-dependent need for reading glasses or bifocals. After age forty, and in most people by age forty-five, the ability to focus on an object close up, such as a restaurant menu, becomes more difficult. This happens to everyone. It is due to a loss of flexibility in the lens and a weakening in the muscles that enable the lens to flex and fine-tune the focus. Presbyopia typically continues to worsen until age sixty -five. When this occurs, people who already wear eyeglasses may need bifocals, and those who have never worn eyeglasses may require reading glasses.
Higher-order aberrations are focusing problems that are not correctable with glasses or contact lenses. Higher-order aberrations, which are a result of subtle irregularities in the focusing mechanism of the eye, cause a loss of crispness, clarity, and contrast. If you have significant higher-order aberrations, you may have trouble distinguishing between shades of gray. Higher -order aberrations may also affect one's night vision; people with these problems may see glare or halos around lights. Approximately 17 percent of visual errors are considered higher -order aberrations.
How Your Vision Is Measured
Most people who have had an eye exam that includes a test to measure visual acuity, or clarity or sharpness of vision, recognize the simple notation 20/20 as meaning "normal vision." What do those numbers mean? Let's say your vision is 20/40. That means you can see at twenty feet what a person with normal vision can see at forty feet. Your measure of visual acuity is determined by using the Snellen chart, that familiar eye chart with progressively smaller letters on each line. Although it is considered an accurate vision test, its results are sometimes affected by such variables as squinting, guessing at the letters, and room light.
So, numbers such as 20/20 or 20/40 describe your visual acuity but do not measure your refractive error — how accurately your eye bends, or refracts, light. When an eye doctor measures your refractive error, what you end up with is your eyeglass prescription. Finding an eye doctor whose measurements are impeccable is crucial, not just for your eyeglass prescription but also, as you will learn later, for laser vision correction.
Understanding Your Eyeglass Prescription
Your eyeglass prescription is written in numbers. The type and degree of refractive error is quantified in units of measure called diopters. If you have ever wondered what those numbers mean, here is how to read and understand your prescription.
To arrive at your prescription, your doctor takes three measurements during the eye exam: sphere, cylinder, and axis. Your prescription for glasses may look something like this:
OD – 1.25 — —
OS – 1.25 – .25 X 170
OD and OS refer to the right and left eyes, respectively. The first number next to OD or OS represents the sphere. The sphere measure tells the eye doctor where your eye focuses light: on the retina (normal vision), in front of the retina (myopia), or behind the retina (hyperopia). In other words, the sphere measure reveals whether you are nearsighted or farsighted. A negative diopter indicates myopia, or nearsightedness. A positive diopter indicates hyperopia, or farsightedness. The higher the number, the stronger the prescription. In the example above, the person has mild myopia (–1.25 diopters) in both eyes.
The number in the second column represents the cylinder. The cylinder measure indicates whether or not the patient has astigmatism. If the cylinder column is not blank, you have some degree of astigmatism. The larger the number, the more astigmatism you have. The example above reveals that this person has no astigmatism in the right eye, and a small amount (–.25 diopter) in the left eye.
If astigmatism is present, your eye doctor takes an axis measurement. The axis measure indicates where irregularity lies on the eyeball. In the prescription above, the astigmatism in the left eye is positioned at the 170-degree axis.
Nonsurgical Vision Correction Options
Eyeglasses have been around for hundreds of years. As early as the thirteenth century, inventors in China and Europe inserted magnifiers into frames, making the prototype for our modern-day eyeglasses. Like the early versions, today's eyeglasses work like magnifying glasses that enhance the eye's ability to focus sharply, whether near or far. The amount of curvature in the spectacle lens determines how light bends before it reaches your cornea. Vision is corrected, depending on the angle of refraction, to compensate for your focusing error.
Eyeglasses have a number of advantages. They are usually affordable, are easy to maintain, and can be adapted for a number of different uses, such as reading, active sports, and driving. They also have disadvantages. Eyeglasses may restrict peripheral vision, the outer part of your field of vision; may prove difficult in certain weather conditions, such as rain or snow; and may make images appear smaller or larger than they really are. They may cause a number of visual aberrations, including halos around lights, and the lenses usually need to be replaced as your vision changes. Eyeglasses may interfere with certain occupations and recreational activities — swimming, for example. And some people just don't like the way they look in eyeglasses.
Contact lenses offer another option for correcting vision. Like eyeglasses, they make up the difference between the amount of refraction your eye can accomplish on its own and what is needed for sharp focus. Because they are extremely thin and are custom -shaped for your cornea, contact lenses float on the surface of your eye; they are held in place by natural suction and are constantly lubricated by the eye's own moisture.
Contact lenses have some advantages over eyeglasses. For example, contacts enable the wearer to have more natural vision (including better peripheral vision), cause little noticeable change in cosmetic appearance, and allow more freedom in recreational activities. On the other hand, contacts may require maintenance — continuous, frequent cleaning. Users must buy cleaning and storage solutions. The lenses may tear easily. They may be inconvenient for traveling, and also are easily lost. Contacts may be uncomfortable for patients with dry eyes or for those who live and work in polluted city air. They may cause visual aberrations (including halos and uneven vision), and they always carry an increased risk of infection and possible corneal scarring. Individuals who live in higher altitudes may become intolerant of contact lenses over time because of the air's lower oxygen and humidity content.
The variety of contact lenses available today is dazzling. Costs for contacts vary widely, depending on the type you need.
This is a technique for treating myopia, or nearsightedness. Orthokeratology uses a series of rigid contact lenses that apply pressure to the cornea to flatten it. The effects are not permanent and require continued dependence on daily-wear maintenance lenses to retain the reshaping. Orthokeratology is generally only effective, even temporarily, for low levels of nearsightedness. The technique is expensive and high-maintenance and requires continuous follow-up visits. Long-term effects can include permanently warped corneas. The risk of infection may also be greater than that from normal contact lens wear.CHAPTER 2
Laser Vision Correction: An Overview
People have understood the mechanics of eyesight for thousands of years. Writings and drawings on this subject go back as far as 2000 B.C. And the quest to correct vision has never stopped. From the invention of eyeglasses hundreds of years ago to the fabrication of the first American contact lenses, the evolution of vision correction has indeed been astonishing.
Now zoom ahead a few decades to the development of laser surgery, today one of the most popular methods of vision correction. The advent of computers and laser technology has made it possible to perform laser eye surgery to correct the shape of the cornea.
Excerpted from LASIK by Ernest W. Kornmehl, Robert K. Maloney, Jonathan M. Davidorf. Copyright © 2006 Ernest W. Kornmehl, Robert K. Maloney, and Jonathan M. Davidorf. Excerpted by permission of Addicus Books, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
1 The Human Eye and How Vision Works,
2 Laser Vision Correction: An Overview,
3 Contemplating Laser Eye Surgery,
4 Your Consultation,
5 Wavefront Technology: How It Has Improved LASIK,
6 Undergoing LASIK,
7 After Your LASIK Procedure,
8 Risks and Complications,
10 LASIK and PRK Statistics: Your Chances for Success,
11 Other Vision Correction Procedures,
About the Authors,