Refractive Surgery – Eye Care 1
How the Eye Works: The Short Version
Like a camera, the eye works by focusing light rays. Light entering the eye first passes through the transparent cornea, which is responsible for about 60 percent of the light-focusing ability of the eye. Light rays are then focused by the lens to make a clear image on the retina at the back of the eye. The image formed by the retina is transmitted to the brain as a series of electro-chemical impulses via the optic nerve.
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Anatomy of the Eye
To better understand refractive surgery, let’s briefly review the anatomy of the eye. Consider it Eye Anatomy 101.
Think of the eye as a camera. The lids are the lens cap, and the iris is the f-stop or diaphragm that controls the amount of light coming through to the lens. The lens then focuses that light onto the retina, which is the film in our camera. The retina is connected to the brain by a coaxial type of cable called the optic nerve. Keep this camera analogy in mind, as we’ll refer to it later. Cornea The cornea is the clear dome-shaped front window of the eye, which provides 60 percent of the eye’s focusing power. It is the only tissue in the human body that is transparent to light. In its normal state, the cornea has no blood vessels. Epithelial cells coat the outside of the cornea, providing both a smooth and a water-holding (tear) surface to allow clear vision. The lids not only act as windshield wipers but, by spreading the tears around, also serve to keep the eye from completely drying out. Sclera and Conjunctiva The sclera is the muscular outer coat of the eye that maintains the eye’s round shape. It is white and opaque to light. Covering the sclera and the inside of the eyelids is the conjunctiva, which is a clear, thin membrane containing mucous-producing cells to keep the ocular structures moist. Lens and Ciliary Body
Within the globe of the eye, situated just behind the cornea, is a (usually) clear, semi-solid structure called the crystalline lens. This lens is enclosed in a thin membrane (lens capsule), which is itself connected to the muscular part of the ciliary body by another membrane (zonule). Because it is the source of nutrient fluid, the ciliary body is extremely important to the health and function of the eye. The lens of the eye – composed of the capsule, cortex, and nucleus – is held under tension by the zonular membrane and thus is made thin.
In its relaxed state, the eye is able to discern distant objects clearly. As an object gets closer, the eye must change its light-bending power to accommodate the nearness of the object. How does this automatic focusing occur? The ciliary muscles are so arranged that when they contract, the zonule relaxes, allowing the lens to relax and become thicker in the center. This increased thickness bends the light more so that nearer objects are seen more clearly. The anatomical term for this act of focusing is called accommodation. It is the gradual loss of the property of accommodation that causes a loss of the ability to see near objects. This is called presbyopia, which occurs for most people in their mid-40s. When this crystalline lens becomes clouded from age or disease, it is called a cataract. Iris: Controlling the Light Entering the Eye Overlying the lens is the iris, the purpose of which is to control the amount of light entering the eye. It’s a structure made of thin elastic tissue with an opening in the center, surrounded by a circular muscle called a sphincter. The structure is the iris; the opening is the pupil. The backside of this tissue is composed of tissues containing brown pigment that acts to absorb light and prevent its scattering within the eye. Retina: “Film” Capturing the Image Coating the back of the eye and covering the sclera, is a thin layer of nervous tissue – an extension of the optic nerve – called the retina. The retina is analogous to the film of a camera. With its 10 layers, the retina is responsible for gathering light focused upon it by the cornea and lens. It converts the light to electrical signals that are amplified and sent to the brain for interpretation and construction of images. The brain actually performs the act of seeing, not the eye itself. back to top Refractive Errors: Don’t Take It Personally As a camera cannot produce a clear image if the incoming light is not focused on the film, the eye cannot produce a clear image if the incoming light is not focused precisely onto the retina. In a normal eye, the image is focused by the cornea and crystalline lens and falls on the retina. Why are some people nearsighted or farsighted? Why do some people have astigmatism? Blame it on heredity at least part of the time. The causes of refractive errors are not really known. Some researchers feel that reading at an early age may contribute to increasing nearsightedness. Some even suggest a link between increasing nearsightedness and intelligence. If you are nearsighted and reading this, you may be more intelligent than average. However, this does not mean that farsighted individuals are less intelligent! Myopia (Nearsightedness) An eye is nearsighted when the light rays are focused in front of the retina rather than on the retina. Nearsighted people see near objects more easily and better than those objects farther away. The condition occurs because the cornea and crystalline lens have excessive focusing power in relationship to the length of the eye. If the cornea and crystalline lens had less combined focusing power or if the eye were shorter, the light rays would focus on the retina. Hyperopia (Farsightedness) Farsighted people see faraway objects better than near objects. Farsightedness results when the cornea or crystalline lens have too little combined focusing power for the length of the eye. Light rays from a distant object are focused behind the retina rather than on it. If the eye were longer or if the crystalline lens had greater focusing power, the light rays would land on the retina. Astigmatism (Blurred Vision) If the focusing power of the eye is uneven, astigmatism occurs. The cornea should be round and spherical like a soccer ball; light rays then achieve a focal point. If the cornea is shaped more like a football or an egg, with two radii of curvature, there are two different focal points. This causes blurred vision. Astigmatism can occur with myopia or hyperopia, meaning the two focal points can be in front or behind the retina or can even straddle it. Presbyopia Presbyopia is the inability of the inner (crystalline) lens in the eye to focus images. Before the age of 40, when you want to see an object close to your eye, the inner lens changes in shape and focuses the light rays onto the retina. When you are in your 40s, a battle is continually being waged between focusing light by increasing the power of the inner lens and the gradual stiffening of the lens. It is a losing battle. Eventually, near objects such as print become increasingly difficult to see at normal distances, forcing you to hold these objects further away from your eye. At this point, reading glasses or bifocal lenses become necessary to keep the near objects in focus. back to top Refractive Surgery: Changing the Way Light Focuses inside the Eye Refractive surgery changes the way light focuses on the retina. There are a number of safe, effective refractive surgical procedures offered by Eye Care Associates, some of which use advanced laser technology to optimize comfort and precision. LASERS Ophthalmology was the first branch of medicine to utilize the laser in clinical practice. In the 1960s, early lasers produced heat energy and worked well for coagulating blood vessels, but they were not suitable for use on the cornea. Other lasers utilizing infrared light were used to cut membranes within the eye, especially after cataract surgery (YAG laser capsulotomy). Excimer Laser In the late 1970s, scientists at IBM developed an ultraviolet laser to etch microchips for the computer industry. The excimer laser was an ultraviolet laser that had the unique ability to strip away atoms and produce photo-decomposition of tissue. In 1983, a scientific paper detailing the first ophthalmic use of the excimer laser was published by Dr. Steven Trokel of New York. It described the unique properties of this laser to vaporize corneal tissue without producing heat or scarring. Clinical investigators conducted extensive research into and development of the excimer during the next 12 years, culminating with the approval of the first excimer laser for ophthalmic refractive use in the United States in 1996. Since that time, several companies have developed excimer lasers for treating refractive errors. The Machine: Elegant and Precise The excimer laser produces an invisible beam of ultraviolet light to remove microscopic amounts of corneal tissue – approximately one-third the thickness of a human hair – without causing damage to surrounding cells. The laser beam is directed onto the surface of the eye at a rate of six to 10 pulses per second. Each pulse removes a quarter of a micron, or 1/100,000th of an inch, of tissue from the eye. This precision assures total control over the correction process. Currently, our surgeons at Eye Care Associates use the VISX STAR™ S4 excimer laser, the most advanced excimer laser available in the United States. back to top Laser Vision Correction: LASIK LASIK stands for laser-assisted in-situ keratomileusis LASIK utilizes the excimer cool-beam laser to vaporize a small amount of corneal tissue beneath the surface of the cornea to reshape the cornea. LASIK is an acronym for Laser-Assisted in-Situ Keratomileusis, first described about 1993 by Greek eye surgeon Dr. Ionnis Pallikarus. Because of the successful surgical results, and the quick return of vision, LASIK has gained rapidly in popularity. Dr Sher went to Greece in 1996 to observe and master this new procedure from Dr. Pallikarus.
To perform LASIK, the surgeon uses an instrument called a microkeratome to create and fold back a thin layer of cornea. The creation of this flap takes about 30 seconds and the only sensation the patient feels is pressure and an occasional pinching of the eyelid. As with the PRK procedure, advanced computer technology, including Wavefront technology is used to adjust the laser for your particular prescription. You will be asked to look at a red blinking target light for a short time while the doctor watches your eye through a microscope The same excimer laser which is used with the PRK procedure, The VISX S4 laser, is equipped with a special tracking device that helps to center the laser on the proper position over your eye and follows your eye even if it moves.
The laser light pulses will then painlessly remove the tissue. You'll hear a steady clicking sound as the laser is operating. The higher your prescription, the more time the surgery will take. The surgeon has full control of the laser and can turn it off at any time.
The thin flap of corneal tissue is folded back into its original position where it bonds after several minutes of drying. There are no stitches required. The entire procedure takes 10 or 15 minutes and the only anesthetic required is a numbing anesthetic eye drop. Since only the edge around the thin layer of corneal tissue is needed to heal, the visual recovery is rapid and patients report very little post-operative discomfort or pain. This discomfort usually dissipates in a few hours. The return of vision is rapid, and 80 to 90% of the visual recovery may be present by the next day. Antibiotic and cortisone eye drops are used for about a week. Most patients return to work the next day. As with any surgical procedure, there are risks and complications. In LASIK, the most serious risks relate to the creation of the thin flap of cornea. Most of these problems can be managed but loss of significant vision is possible. While LASIK is associated with a number of intraoperative and postoperative complications it's important to note that, according to most studies, the incidence of severe, vision-threatening complications is less than 1%. Other less serious complications can occur in up to 5% of people who receive LASIK. Almost all LASIK complications, including the serious ones, can be treated and usually resolve within several months of surgery. Rarely, LASIK can cause permanent problems. In January 2002, the American Academy of Ophthalmology < the Eye M.D.
Association < reviewed the scientific literature and found LASIK safe and effective for correcting low-to-moderate nearsightedness and astigmatism, but less predictable for moderate-to-high nearsightedness. The assessment found that serious complications that result in permanent visual loss occur rarely with LASIK, but side effects such as dry eyes, nighttime starbursts and reduced contrast sensitivity occur more frequently.
A complete discussion of the benefits, risks, complications and alternatives are part of the initial consultation and subsequent visits. The US Food and Drug Administration (FDA) website is a good source of information: http://www.fda.gov/cdrh/LASIK/what.htm A copy of the booklet our office uses as a patient information booklet and informed consent document is viewable here. LASIK Retreatments
If a patient has persistent nearsightedness after undergoing LASIK to correct the visual error, additional treatment can be performed. The thin layer of cornea can usually be lifted again, especially within the first three or four months after the first surgery. Additionally, laser pulses are applied, and the flap is repositioned as before. Again, recovery and improvement of vision are quick. The need for additional treatment or enhancement is related to the severity of the preoperative nearsightedness. back to top Laser Vision Correction: Photorefractive Keratectomy (PRK) Photorefractive keratectomy (PRK) was the first laser vision correction procedure utilizing the excimer laser. Doctors at Eye Care Associates have been performing this procedure since 1989 as we participated in the earliest clinical trials of the excimer laser for laser vision correction. As with other technology, a number of improvements have been made over the last decade. It is currently being regularly performed with excellent visual results. The number of PRK procedures performed in the US and elsewhere diminished significantly after the introduction of the LASIK procedure in 1996. (More on LASIK below) In the last several years, due to significant advances in technology and medications which control wound healing, surface ablation techniques such as PRK are gaining in popularity. With over 14 years of experience with this procedure, its long term safety and stability have been proven. In 2004, approximately 70% of our laser vision correction procedures utilize LASIK and 30% utilize PRK The decision to offer PRK or LASIK to patients qualified for laser vision correction involves the consideration of numerous factors. These include anatomical considerations such as corneal thickness and curvatures as well as the patientıs occupational and recreational needs. For instance, the US Navy currently accepts PRK to meet vision requirements for all naval aviation including aircraft carriers. PRK or surface ablation uses the excimer laser to remove a precise amount of corneal tissue. The surface cells or corneal epithelium are first removed with the laser or gentle mechanical means. After the epithelial layer is removed over the central 6-8 mm, the excimer laser is used to vaporize a precise amount of the underlying corneal tissue which takes about 15 to 45 seconds, depending on the level of correction. Anesthetic ³numbing² eye drops are used during the procedure which is virtually painless. There is no cutting of the cornea or creation of a corneal flap. To treat nearsightedness, the laser reduces the curvature of the cornea. For farsightedness, it increases the slope of the curvature of the cornea. And for astigmatism, it makes the cornea more spherical or round. At the end of the surgery, a soft contact lens, which works as a bandage is placed on the eye and several different eye drops are administered. The corneal surface cells, the epithelium, regrow to resurface the cornea over the next 3 to 7 days. After these cells are back in place, the soft contact lens is removed and the vision improves very rapidly after that time. There is continued improvement over the next several months. During the first several days after the procedure, there is some irritation and discomfort in the operative eye with some patients describing this has a foreign body sensation or contact lens irritation. Several types of eye drops are given to reduce this discomfort. Most patients return to work the next day. The disadvantages of PRK, especially compared to LASIK, include more pain and discomfort than in LASIK. There is a slower healing time and the need for more postoperative visits and eye drops. Medications are commonly used for 2 to 4 months. The vision returns more gradually, and may take several weeks to return. Currently, there is an interval of approximately 2 weeks between performing PRK on the first and second eye. back to top Laser Vision Correction: LASEK LASEK is a poorly named procedure in that it is actually a variant of PRK, not LASIK. Diluted alcohol is applied to the center of the cornea for less than 30 seconds, and the epithelium is peeled back and left intact. The excimer laser is then applied to the surface of the cornea. At present, clinical studies are being done in an attempt to find alternatives to using alcohol in the procedure, which could make LASEK a more attractive choice. back to top Glasses and Contacts: Out of Your Life? Our goal is to reduce dependence on optical devices. Many of our patients function without glasses or contacts after a lifetime of being dependent on them. The reality is that some people will still need glasses or contact lenses for clearer vision while driving or at night. People who are in their 40s or older may still need corrective lenses for reading. However, the degree of correction is usually significantly less than before surgery. back to top Conductive Keratoplasty and NearVision™ CK Presbyopia and Baby Boomers
Presbyopia, Greek for “aging eye,” is a progressive condition that causes near vision to fade with age. Presbyopia is the most prevalent eye condition in the United States, affecting most people by the age of 40 and everyone by the age of 50. An estimated 90 million Americans – the “baby boomers” – either have presbyopia or will develop the condition within the next 10 years.
Symptoms of presbyopia include difficulty with all forms of near vision, such as reading, ordering from a menu, checking a price tag, dialing a cell phone, or even doing hobby work. People affected by presbyopia often own multiple pairs of reading glasses and must rely on them for even the most mundane activities. Farsightedness occurs when the patient’s cornea is too flat or the eye is slightly small in front-to-back diameter. Presbyopia is the gradual loss of near vision due to loss of flexibility of the crystalline lens of the eye. Conductive Keratoplasty (CK) CK does not utilize a laser or involve cutting the cornea. CK is performed using a probe thinner than a human hair that releases radiofrequency energy. Applied to the cornea in a circular pattern, the radio waves shrink small areas of collagen to create a constrictive band (like the tightening of a belt) that increases the curvature of the cornea, bringing near vision back into focus. NearVision™ CK refers to the CK procedure for presbyopia that is similar to CK for farsightedness. NearVision™ CK is used for the temporary improvement of near-vision in emmetropic presbyopes (those who require only reading glasses). CK is also performed for hyperopic presbyopes (those who require reading and distance glasses) but may not be as effective as LASIK or PRK for these individuals. NearVision™ CK is typically performed on just one eye, improving near-vision without significantly compromising the patient’s binocular distance vision. The distance vision will usually diminish by two to three lines on the eye chart due to induced nearsightedness. Usually, the non-dominant eye is chosen for near vision. CK is performed as an outpatient procedure, using only a drop of topical anesthetic. You will want to take the day off of work and not exert yourself too much for the rest of the day, but CK imposes no significant restrictions on your normal activities. Who is a candidate for CK or NearVision™ CK?
- You have had good distance vision without glasses or contact lenses throughout your life. This means that you do not have significant nearsightedness, farsightedness or astigmatism.
- You are over 45 and need reading glasses.
- You have difficulty reading menus, cell phones, labels, recipes, and the like.
- You experience eye fatigue when reading in poor lighting or at the end of the day.
- You constantly reposition reading material in an attempt to find the right focus.
What can I expect the day of the procedure? The procedure is done at the Phillips Eye Institute; you'll be there about two hours on the day of treatment, which includes the check in an hour before. The CK procedure itself takes approximately 10 minutes. You will feel some pressure on the eye and eyelids during the surgery, but the procedure does not hurt. Once finished, you don't have to wear a patch and can usually return to work the next day. In some patients, a soft contact lens is usually placed at the time of the procedure. The contact lens is removed in one to three days. Vision continues to improve during the first several weeks. There may be some irritation, tearing, and mild pain in the first day or two after surgery. Eye drops are given to reduce the pain and discomfort. On occasion, a pain pill is required, but this is unusual. Is Conductive Keratoplasty (CK) permanent? Unlike LASIK or PRK, it is not known how long the effects of CK will last. It is thought that there will be some loss of effect each year, and that the effect will last for about five years. Unlike myopia, which is stable in adult life, farsightedness progresses as we age, and it is difficult to differentiate between the effects of the progression and the regression of the surgical effect. Can CK be repeated? Yes. Additional applications of RF energy can be applied to enhance or restore the effect of the procedure. However, this can only be done once after the original procedure. Within the first year after surgery, this is done without fee. After one year, there is a reduced fee for the enhancement procedure. Is CK reversible? As with most vision correction procedures, CK is not reversible. Once the procedure has been performed, it is not possible to “remove" the effects of the procedure. This is an important factor that anyone thinking about surgery should carefully consider. Will my vision improve immediately after surgery? Patients usually notice an immediate improvement in their vision after the CK procedure. However, it normally takes several weeks for the eyes to reach the final level of correction. Will my vision fluctuate after the procedure is performed? Most patients will experience mild fluctuation in their vision after surgery, but many will never notice it. Any fluctuation will usually subside within a few weeks. Patients who undergo vision procedures for near vision usually require a longer stabilization period than those treated for distance vision. To find out more about NearVision™ CK and other refractive procedures, please contact Eye Care Associates today. back to top |
