for the Correction of Myopia and Astigmatism using the Argon-Fluoride Excimer Laser
An informational brochure developed by
Bobby Maddox, M.D.
©1990-2001 Robert M. Maddox, M.D.
Last Revision July 24, 2001
No materials herein may be reproduced in any manner without permission.
The Maddox Laser Eye Centers have two Excimer Laser systems -- one in El Paso, Texas and one in Juarez, Mexico -- allowing us to better serve refractive surgery patients on both sides of the border.
Our El Paso center is equipped with the Alcon Summit Autonomous LADARVision Excimer Laser System, "the future of laser vision correction," that combines the first patented eye tracker with unique, small-spot shaping to provide a smooth corneal surface and the potential for an early return to visual stability.
In Juarez, we use the LaserSight LSX Excimer Laser System.
Our two laser systems have enabled us to expand our treatment parameters to all modalities of refractive error, including myopia, hyperopia, astigmatism, and mixed astigmatism.
Dr. Bobby Maddox, an ophthalmologist practicing in El Paso, Texas, since 1975, has always been a pioneer in bringing new technology to benefit his patients. He routinely chose small incision phacoemulsification surgery for his cataract patients more than a decade before other surgeons in the area. In 1982, he established the first in-office outpatient surgery center in El Paso specifically designed for phacoemulsification of cataracts with intraocular lens implant, a procedure now widely used throughout the world as the procedure of choice for cataract surgery. He pioneered the Excimer Laser into Mexico in January, 1992, having brought the first Excimer Laser into the country of Mexico.
Dr. Maddox was born in Austin, Texas, and has spent most of his life in El Paso. As one of the great football legends of the El Paso High Tigers, he received the prestigious Ralph Marmolejo award from El Paso High for his outstanding performance at quarterback, punter, and safety. Along with his athletic prowess in football and track-and-field at El Paso High, Dr. Maddox competed in bodybuilding competitions as well, once capturing third place in the Mr. El Paso contest. Dr. Maddox has continued his lifelong interest in maintaining a strong physical fitness as well as a strong spiritual fitness. He and his family have dedicated themselves and their work to the honor and glory of God through His Son, Jesus Christ. His office is filled with reminders that God is there to carry us through our difficulties and to show us His love, if we would only ask Him, in Jesus' name.
Dr. Maddox received his medical training and his Doctor of Medicine degree from the University of Texas School of Medicine in Galveston, Texas. After a rotating internship in Austin, he completed a 3-year Residency program in Ophthalmology at Brown University and Rhode Island Hospital in Providence, Rhode Island. He was then awarded a Fellowship grant from the National Institutes of Health to participate in a study of laser surgery for the treatment of diabetic retinopathy, or retinal bleeding, which can lead to blindness. The year he spent treating and following the progress of diabetic patients at Massachusetts Eye and Ear Infirmary in Boston provided him with an invaluable experience in laser eye surgery that he continues to use for the benefit of his patients. His love for the border and the international culture of El Paso brought him back to El Paso, where he established his private practice in 1975. In 1981, Dr. Maddox was certified by the American Board of Ophthalmology, and is a Fellow of the American Academy of Ophthalmology.
Because of his commitment to use his profession to serve God, he has always found time to take care of the indigent citizens of El Paso and many communities in Mexico. His dedication to assist the poor has resulted in his receiving special commendation from several governmental authorities and media organizations in Mexico, and culminated in his being presented the Conquistador Award by the Mayor and City Council of El Paso -- the highest honor the city can bestow on an individual. His reputation for quality in the care of his patients resulted in his being invited to become a member of the Society for Excellence in Eyecare, a national organization of highly skilled eye surgeons, and was recently chosen to be a Fellow of the Society for Excellence in Eyecare as well as being invited by the International Institute for Advanced Laser Surgery to become a member of its five-member International Advisory Board for Excimer Laser Surgery. In 1999, Dr. Maddox was recognized as a "Pioneer in Refractive Surgery" by Summit Technology at the annual meeting of the International LASIK Society.
Dr. Maddox is also an internationally recognized educator, having addressed the 8th Congress of Ophthalmology in Lima, Peru, and the Third National Symposium of Optometry in Juarez, Mexico. In 1984, he participated in instructing other ophthalmologists, including some of his own former instructors, at an intensive course on Advanced Techniques in Cataract Surgery and Phacoemulsification, jointly sponsored in Philadelphia by the American Academy of Ophthalmology, Hahnemann University, Temple University School of Medicine, Scheie Eye Institute, and Wills Eye Hospital. His publications include articles detailing his technique for removing difficult cortical material during cataract surgery, published in Ocular Surgery News, as well as extensive patient educational materials. Dr. Maddox discovered a new and very important post-LASIK syndrome called Sands of the Sahara, or SOS. He has also served on physician educational panels, and he has contributed videos of his own surgical techniques to be used as instructional videos for the libraries of several major suppliers of ophthalmic surgery products. He has in the past conducted a nationwide series of physician certification seminars in postoperative care following Excimer Laser Photorefractive surgery.
His love of his profession and his dedication and commitment to the well-being of his patients has always led Dr. Maddox to be a pioneer in bringing his patients the best medical advances and medical technology science has to offer. He began offering Radial Keratotomy to his patients in 1982. In 1989, when he learned of the exciting results of early European and FDA studies of the Excimer Laser for correcting nearsightedness, he wanted to bring this revolutionary technology to his patients. All 46 clinical sites designated for the FDA studies in the U.S. were already assigned, and no new sites were being allowed. So Dr. Maddox sought to take advantage of the international character of El Paso and began an intensive 2-year process to receive a medical license in Mexico and to establish a clinical study for Mexico similar to the FDA study in the United States.
Dr. Maddox began working with the federal regulatory agencies in Mexico City in December, 1989, to establish an Excimer Laser Center in Mexico for the purpose of performing excimer laser surgery for the correction of myopia and corneal abnormalities. Dr. Maddox made a commitment to establish all aspects of the laser center in complete accordance and compliance with the laws of Mexico. In order to be totally within the regulations of the Ministry of Health and all other governmental agencies of Mexico, Dr. Maddox chose to apply for a license to practice medicine in Mexico. After fourteen months, he fulfilled all of the requirements for licensure and, in January, 1991, he was granted his licence to practice ophthalmology. The final step to establishment of the excimer laser center was completed in August, and the Excimer Laser Center began operations in January, 1992. After Dr. Maddox was awarded his license to practice ophthalmology in Mexico, he was also granted a permit by the Ministry of Health to establish an excimer laser site in Juarez, using the excimer laser system manufactured by Summit Technology, Inc., of Waltham, Massachusetts. Both the Ministry of Health and the Mexican Society of Ophthalmology in Mexico approved Dr. Maddox to perform excimer laser surgery and to conduct clinical studies for the Ministry of Health, as well as the protocol developed by Dr. Maddox for the studies to be conducted. These studies substantiated for Mexico the safety, predictability and efficacy of the 193-nanometer excimer laser in correcting refractive insufficiency of the cornea, as previously established in other worldwide studies. Millions of patients have undergone excimer laser photorefractive keratectomy surgery in countries throughout the world.
Corneal epithelium:(approximately 40-50 microns thick) The superficial collection of multiple layers of cells that line the corneal surface. These cells constantly regenerate as do the cells that cover the skin of the body.
Basement membrane: A small membrane laid down by the basal epithelial cells and upon which they rest. This membrane rests on Bowman's membrane.
Bowman's membrane:(approximately 15 microns thick) A membrane layer immediately below the epithelium and basement membrane which rests on the stroma.
Stroma: Makes up the bulk of the cornea; consists of multiple layers of collagen fibers, keratocytes, and water.
Stromal lamellae: Collagen fibers arranged in a transparent fashion within the stroma.
Keratocytes: The stromal cells.
Descemet's membrane: A membrane layer that is located beneath the stroma and above the endothelial cell layer.
Endothelial cells: A mosaic of cells lined up on Descemet's membrane. They cover the entire inner surface of the cornea and are responsible for keeping the cornea transparent.
Photorefractive keratectomy (also referred to as wide area ablation, corneal reprofiling, corneal sculpting, and laser keratomileusis) is the process of using the Excimer Laser to reshape the cornea in an effort to effect a change in the refractive characteristics of the eye and thereby correct or lessen myopia (nearsightedness), hyperopia (farsightedness), and/or astigmatism. Using the Argon-Fluoride Excimer Laser to accomplish photorefractive keratectomy is a dramatic departure from existing refractive procedures. Some of these existing procedures include radial keratotomy, in which multiple deep radial incisions or cuts are made into the cornea with a diamond blade; epikeratophakia for myopia and hyperopia, in which a donor corneal lenticle is reshaped with the corrective prescription and then sutured over the patient's own cornea; and myopic and hyperopic keratomileusis, in which the patient's own cornea or a donor cornea is shaved and reshaped like a contact lens with a corneal lathe and then resutured onto the eye. (Current technological offshoots of keratomileusis include myopic or hyperopic Lamellar Keratoplasty) and Excimer Laser PRK with the microkeratome, or LASIK.)
In photorefractive keratectomy for myopia, hyperopia, and astigmatism, the cornea is reshaped by the Excimer Laser without being cut or incised, normally effecting a change in the refractive properties of the cornea. The Excimer Laser uses photoablation, or high-powered, pulsed ultraviolet radiation (light energy or photons) to remove tissue with an extremely high degree of precision. The Excimer Laser is a unique computerized 193 NM Argon-Fluoride laser which can reshape the front surface of the eye (cornea), acting at the atomic and molecular level, in as little as 20 seconds, without creating significant thermal damage to surrounding tissue. This special characteristic allows the Excimer Laser to be referred to as a "cold" laser. ("Cold" is a relative term, in that other lasers produce larger amounts of heat and thermal damage than does the Excimer Laser.)
In an area of the central cornea, about the diameter of a drinking straw, 25 billion million photons (packets of light energy) per pulse shower down in a parallel fashion onto the cornea, photoablating, or removing, .25 microns of tissue with each laser pulse. [One micron equals one one-thousandth of a millimeter (1/1,000).] One cell (10 microns) has to be hit or pulsed 40 times in order to be completely photoablated at .25 microns, or 1/25000 of an inch, per pulse. The number of pulses needed to correct the myopia depends on the amount of myopia and the ablation zone size. Usually from 3% to 15% of the central corneal tissue is utilized for corneal reshaping for myopic corrections from -1.00 to -6.00.
[An explanation of the size of a micron: The average single human cell is approximately 10 microns in height. The diameter of a single human hair is approximately 50 microns, and the thickness of the central cornea (the front surface of the eye) is a little over 500 microns (half a millimeter). Therefore, only one-tenth of a cell is penetrated by the excimer photons when penetrating one micron.]
Since each photon has energy equivalent to 6.4 electron volts, and the energy required to hold the protein atoms together in corneal tissue is only 3 electron volts, these molecular bonds holding protein molecules together in the cornea are broken, and molecules and atoms of tissue fly away from the cornea, .25 micron layer by .25 micron layer, at supersonic speeds. (This effect is referred to as the "plume of photoablated tissue.") The laser's aperture (mechanical iris) simultaneously expands in a stepped fashion, until the desired optical zone and optical correction are achieved in myopia. Each pulse of 25 billion million photons acts only on those atoms of protein that are bonded together in an area of about 5 or 6 mm or larger, .25 microns in depth, or 1/40th of a single cell. (One cell is only 10 microns.) Each laser pulse lasts approximately 10 nanoseconds, which is 10 billionths of a second, at 10 pulses per second.
Refractive correction is achieved by stepped photoablation in myopia. The Excimer Laser system uses a mechanical iris, or aperture, to control the laser. After each set of laser pulses, the iris diameter widens and the laser ablates, or removes, a slightly larger ring of tissue to effectively flatten the corneal surface, thus creating a stepped curve. In essence, a prescription for glasses or contact lens becomes permanently microetched onto the front surface of the eye.
In most cases it is virtually impossible to detect any change in the cornea several months following surgery, even upon ophthalmic examination through a slit lamp biomicroscope by a well-trained physician. (The only way to detect the change that has occurred in the cornea is with a photokeratoscope.)
Before the Excimer Laser photoablation procedure can be performed, the surgeon must first remove the corneal epithelium (see illustration of cornea). After Excimer Laser photoablation of the cornea, the corneal epithelium will normally regenerate within a period of 48 to 72 hours. Immediately following Excimer Laser photoablation, a pseudomembrane forms, acting as a true osmotic type of barrier to impede water inflow. As soon as the epithelial cells regenerate and cover the area, the cells break down the pseudomembrane and begin to lay down elements of a new basement membrane. Hemidesmosomes anchor the basal epithelial cells down into the basement membrane, locking the newly formed epithelium straight down into the cornea onto the stromal lamellae. Within a period of four to six weeks, the new corneal epithelium stabilizes to the point that to mechanically push the epithelium off the cornea (for instance, by rubbing the eye) would be very difficult.
There is a distinct difference between RK (radial keratotomy) and Excimer Laser PRK (photorefractive keratectomy). RK reshapes the cornea by the surgeon's use of a diamond knife or scalpel to perform multiple deep and relatively wide incisions or cuts (90% corneal depth) into the periphery of the cornea, in a pattern resembling the radial spokes of a wheel, which weakens this area and allows the circumference of the cornea to increase, thus flattening the central cornea. Unfortunately, many people are under the false impression that radial keratotomy (RK) is indeed done with a laser beam. This is not true. Others also mistakenly believe that "laser surgery" reshapes the cornea by making deep incisions or cuts like RK, except that it is done with a laser beam instead of a surgical blade. This also is not true. Excimer Laser PRK and LASIK are considered to be the most technologically advanced methods in the world today for correcting myopia. This advanced technological breakthrough for the correction of myopia allows eye surgery to enter a totally new dimension - a world of its own - never before experienced by eye surgeons.
A summary of the highlights of the development of the Excimer Laser:
Although Excimer Lasers were first developed in 1975, the word EXCIMER, a contraction of the words "EXCIted" and "diMER," appeared in scientific literature as early as 1960. At first, the Excimer Laser was not developed for use in the realm of ophthalmology, but was initially used in 1975 in the plastics industry. The laser was developed for etching silicones and other polymers, and later with the hope of using this technology in manufacturing microcircuits and computer chips.
Excimer Laser surgery is a Western-developed surgical breakthrough. The other, more prevalent form of refractive surgery - Radial Keratotomy, or RK - was developed in the Eastern nations of Russia and Japan. Radial keratotomy, or RK, requires deep incisions or cuts into the cornea with a diamond blade, which changes its shape and structural integrity, in order to correct myopia. The Excimer Laser avoids having to make these deep incisions or cuts into the cornea, and therefore does not weaken the corneal structure. The Excimer Laser photoablates, or uses ultraviolet light energy at a specific wave length to break the cellular bonds of microscopic layers of the cornea in order to change the shape of the cornea to correct myopia or nearsightedness and other refractive errors. The technical term for the Excimer Laser in correcting myopia is Excimer Laser Photorefractive Keratectomy, most often referred to as Excimer Laser PRK.
In 1976, Dr. Dave Muller, Ph.D., formerly President of Summit Technology, Inc., built Cornell University's first Excimer Laser. In 1979 Dr. John Taboada, Ph.D. and colleagues initiated a study of the Excimer Laser on animal eyes at Brooks Air Force Base in San Antonio, Texas. A number of discoveries resulted from these studies, some of which were published in 1980 and 1981. The most intriguing was the observation of a smooth beam-shaped indentation on the cornea of rabbits with experimental Excimer treatment. They attributed the effect to a temperature jump in combination with a photochemical process. Subsequently in 1983, Dr. Taboada and Dr. Steve Trokel, M.D., an Ophthalmologist at Columbia University, met in New York City to complete a co-authored book on YAG Microsurgery. At that time, Dr. Taboada, who is now recognized as the originator of Excimer ablation for refractive surgery, apprised Dr. Trokel on the Excimer Laser process and its application to refractive surgery.
Dr. R. Srinivasan, Ph.D., an I.B.M. researcher in Yorktown, New York, demonstrated the precise photoablation capabilities of the Excimer Laser, which made the Excimer unique among lasers. In late September, 1982, and early 1983, Dr. Srinivasan was using the Excimer Laser for microetching microscopic circuit board technology in computer chips. He described the photoablated decomposition of plastic materials without thermal deformation, as well as decomposition of ultraviolet laser irradiation on biological tissues, such as aorta, bone, cartilage, and hair. He also showed that accurate and smooth microscopic grooves could be microetched on a single human hair with submicron precision without significant surrounding thermal damage to the hair. (A single human hair is approximately 50 microns in diameter.) Dr. Srinivasan microetched, or photoablated, about 30 microns of the human hair. He was impressed as to how sharply defined the edges were and how the microetched hair retained its cylindrical shape. This information was also published, and in 1983, Dr. Steve Trokel, M.D., saw the picture of the microetched hair and visited Dr. Srinivasan at his IBM laboratory in July, 1983. There Dr. Trokel did laboratory studies on rabbit eyes and bovine eyes, and confirmed a significant technological breakthrough. He is regarded as the first ophthalmologist to recognize the significance of the Excimer Laser in corneal refractive surgery.
In 1984, Dr. Olivia Serdarevic, M.D., while at Columbia University (Harkness Eye Institute) in New York, was working with laboratory animals. She was the first to apply the Argon-Fluoride Excimer Laser irradiation to create a therapeutic lamellar keratectomy. In the laboratory she was infecting animal corneas with fungal organisms and was applying the Excimer Laser to the surface of these corneas and was able to sterilize or eliminate the infecting organism. At the same time, she created a therapeutic lamellar keratectomy with a very smooth surface.
From that point on, much research and development began to spring up all over the world, especially in Western nations. The early pioneers include the following: Dr. Steve Trokel, M.D., USA; Dr. Francis L'Esperance, M.D., USA; Prof. John Marshall, Ph.D., England; Dr. Malcolm Ker-Muir, M.D., England; Dr. Theo Seiler, M.D., Ph.D., Germany; Dr. Olivia Serdarevic, M.D., USA; Dr. Carmen Puliafito, M.D., USA; Dr. Roger Steinert, M.D., USA; Dr. Marguerite MacDonald, M.D., USA; Dr. Charles Munnerlyn, USA; and others.
In 1983, Dr. Charles Munnerlyn of the United States started a project which resulted in construction of the first clinical prototype Excimer Laser for ophthalmology. He worked out mathematically the depth of ablation, diameter and edge angles. In 1984, Dr. Marguerite MacDonald from LSU started doing animal research with the Excimer Laser.
Dr. Theo Seiler of Germany ordered his first Excimer Laser in early 1984. In January, 1986, he was the first to create linear and arcuate keratectomies in sighted human eyes for the correction of astigmatism. He performed the first series of phototherapeutic keratectomies (PTK) in sighted human eyes in 1986 in cases of Salzmann's Nodular Degeneration, and for smoothing of the cornea after pterygium removal. On February 6, 1987, Dr. Francis L'Esperance, M.D. of Columbia University, New York, performed the first wide area Argon-Fluoride Excimer Laser superficial keratectomies (PRK) on a series of three human eyes. One of the patients had a malignant melanoma in his eye and was going to have the eye removed. The patient agreed to undergo Excimer Laser surgery prior to the removal of the eye for experimental purposes. This patient had 20/20 vision prior to the Excimer Laser; following the Excimer Laser, the eye was left farsighted, or hyperopic (+3.25 diopters), but was corrected to 20/20 with spectacle correction. In 1988, Professor John Marshall,Ph.D., of England, felt that he had sufficient data from laboratory studies to proceed with human exposure. In March, 1988, the first application of the Summit Excimed UV200 Excimer Laser PTK, or Phototherapeutic Keratectomy, in the U.K. was performed on a sighted human eye for "band keratopathy" (corneal opacity) in London at St. Thomas Hospital by Dr. Malcolm Ker-Muir with great success. In 1988, the United States Food and Drug Administration (FDA) recognized that the experimental research data on laboratory animals was sufficient and satisfactory; therefore, the FDA approved human clinical trials to be started at a number of investigative sites in the United States, approximately 46 sites. In July, 1988, Dr. Marguerite MacDonald, M.D., of LSU, performed Excimer Laser PRK on the first sighted eye with the longest follow-up in the world. In 1989, the first bilateral Excimer Laser PRK for myopia was done in Germany by Dr. Theo Seiler. In 1990, Dr. Howard Gimbel, of Calgary, Canada, began the first Canadian clinical trials for Excimer Laser PRK to correct myopia. On August 5, 1991, the Secretaría de Salud in Mexico City approved Excimer Laser PRK clinical trials to begin. The first Excimer Laser PRK for the correction of myopia in the country of Mexico was performed by Dr. Bobby Maddox, M.D., on January 8, 1992, in Juarez, Mexico. In the United States, the FDA study has completed well over 2,000 cases. The clinical trials that have been going on in Mexico since January, 1992, have been very successful thus far.
Excimer Laser technology has come a long way, and a continual refinement and enhancement of this technological breakthrough is expected as studies progress. The future of the Excimer Laser is going to be exciting to follow. The use of the Excimer Laser in ophthalmology may provide the greatest use of lasers in medicine during this decade and the decades to come. It is expected by most experts that the Excimer Laser will replace radial keratotomy for the most part.
The word EXCIMER is a contraction of the two words EXCIted and diMER. (EXCIted + diMER = EXCIMER). The word DIMER refers to the Argon-Fluoride molecules in the excited state. A dimer is basically a halogen combined with an inert or rare gas in an excited state. A dimer does not exist in the unexcited, or stable, state. The decay of the unstable molecules (Argon-Fluoride) to a stable state results in the emission of a highly energetic photon of ultraviolet light. The emission wavelength of Argon-Fluoride is 193 nanometers. The Excimer Laser is a unique laser in the ultraviolet 193-nanometer region of the electromagnetic spectrum. It differs from other, more commonly used lasers, such as YAG and Argon lasers in several ways.
The Excimer emissions occur in a train of individual pulses, typically 10 nanoseconds long. With a pulse repetition frequency of up to 50 Hertz, each pulse removes, or photoablates, as little as .25 microns of tissue. Remember that one cell is approximately 10 microns in greatest diameter, and one micron is one one-thousandth of a millimeter. Since the beam is unfocused, or parallel, each pulse showers down onto the central deepithelialized cornea about 25 billion million photons in a circular area of approximately 5 mm or more. Since the energy contained in each photon of the Excimer Laser UV light is about twice as strong as the energy holding the corneal protein molecules and atoms together, these molecular bonds are broken and the molecules and atoms of tissue fly away from the cornea, submicron layer by layer, at supersonic speeds. This is referred to as the plume of photoablated tissue. After each set of laser pulses, the mechanical iris, or aperture, in the laser delivery system slowly widens, or dilates, in a stepped fashion, toward the final goal of the selected optical zone size. Thus, the central cornea is flattened, leaving an exquisitely smooth refractive surface, with the microscopic appearance of a Fresnel lens. All of this takes place in about 20 seconds. The depth of the photoablation depends on the amount of myopia present and the selected optical zone size. A specific formula for the calculations was worked out by Dr. Charles Munnerlyn of the United States in the early 1980s.
Munnerlyn's PRK Formula:
Thickness of tissue removed [microns] = (Refractive charge [diopters] /3) x (Diameter of the ablated zone)^2
With a 6.00 mm optical zone and a -1.00 diopter correction, one will need to photoablate approximately 18.25 microns of corneal tissue, or about 3%. For a -6.00 diopter correction, one will need to photoablate 78 microns of corneal tissue. The central cornea, without the epithelium, is approximately 500 microns thick. Therefore, with a -6.00 myopic correction, we only have to photoablate about 15% of the entire central corneal thickness. Of course, if one chooses a larger optical zone size, then more photoablation would be necessary.
The precision and accuracy of excimer photoablation is expected to be far greater than any other method to date. The long-term goal of this procedure is to correct refractive disorders, including myopia, hyperopia and astigmatism without the aid of contact lenses or glasses. This new Excimer Laser surgery should give better results than radial keratotomy did and should be safer than RK. Therefore, most experts in the ophthalmic field feel that Excimer Laser photoablation surgery will replace and, for the most part, antiquate radial keratotomy surgery. However, combination of the use of Excimer Laser photorefractive keratectomy and radial keratotomy to effect a higher degree of refractive change may be possible. In some instances Excimer Laser PRK and AK (Astigmatic Keratotomy) can be combined to correct both myopia and astigmatism. (Excimer PRK may soon be able to correct the combination of myopia and astigmatism through the use of an ablatable erodible disk.)
[Excimer laser PRK (Photo Refractive Keratectomy) is not a form of RK (Radial Keratotomy) and should not be confused with RK. See Appendix for a comparison of PRK and RK.]
Patients with a high degree of myopia quite commonly can be temporarily overcorrected for the first few months following surgery; as the overcorrection regresses, the vision will become clearer. Overcorrection for the first few months following surgery is desirable, because any undercorrection in vision immediately following Excimer Laser surgery would continue to regress, making vision blurry for distance without correction. Therefore, the Excimer Laser surgery may have to be repeated or augmented by RK surgery in order to achieve best results. But if regression is minimal and the eye ends up slightly myopic, only part-time spectacle correction may be necessary.
The degree of preoperative myopia, which determines the amount of Excimer Laser surgery necessary may influence how soon best vision stabilizes. Best vision may stabilize in a few weeks or in a few months (3 to 6 months or longer), depending on the amount of laser photoablation necessary to correct the myopia.
The majority of patients who undergo photorefractive keratectomy for refractive purposes should be able to be free from or less dependent on contact lenses or glasses for distance vision. However, those who are at or above 40 years of age may have to use a correction for near vision for activities such as reading or needlework following excimer laser surgery due to the presbyopic phenomenon. Once a person reaches 40 years of age, the crystalline lens of the presbyopic eye loses its focusing ability. The patient who is presbyopic and myopic (nearsighted) can take off glasses or contact lenses and see up close without correction prior to refractive surgery. (The more myopic a person is, the closer the person must bring reading material in order to see the words clearly.) If vision is corrected for distance with the excimer laser, then reading glasses or other near vision glasses will be needed in order to see clearly up close. It is imperative for the patient to understand that there is a trade-off of vision without correction from near vision to distance vision. In other words, after excimer laser surgery or most other types of refractive surgery has been performed, chances are that distance vision will be much better without glasses or contacts than it was before surgery, but reading without glasses will be much more difficult than before. You feel as though your arm is too short, because you have to push your reading material further away to be able to read it. In some cases the patient will only be able to see up close with the aid of a spectacle correction. If the patient is over forty and would be uncomfortable with seeing blurry up close without glasses, the patient should not have the procedure. Excimer Laser PRK for myopia, hyperopia, or astigmatism alone generally is not able to give both near and distance vision without glasses to those patients in the over-40 age group, unless one is corrected for blended vision - i.e., one eye corrected for distance and the other eye corrected for near vision. Blended vision can be predetermined with contact lenses prior to Excimer Laser PRK, if necessary.
As with any surgical procedure, complications can occur; but serious complications are rare. Possible complications and risks of Excimer Laser PRK that are known at this time include the following:
1. Overcorrection or Hyperopia (farsightedness) following Excimer Laser PRK
a.Temporary Overcorrection
It is considered normal to be slightly temporarily overcorrected for the first few months following Excimer Laser PRK. Temporary overcorrection may cause difficulty for the treated eye in focusing near and far without correction, and may promote ocular imbalance (difficulty getting both eyes to work together), thus causing conditions such as double vision and eye strain. Generally, temporary overcorrection is minimal and is usually gone within three to six months. (In unusual circumstances, temporary overcorrection may last longer than the normal three to six month period.)
If a patient is in the presbyopic age range (usually 40 years of age and older), near vision will be weakened without spectacle correction and is temporarily weakened even more during this immediate three-month post-op period due to the presbyopic phenomenon or aging process. Individuals between 20 and 30 years of age are able to deal with temporary overcorrection better than those people in the presbyopic age group.
During the initial healing period, the patient may choose to patch the operated eye while reading or driving until the eye is stable and able to see well. However, this imbalance is usually eliminated by wearing a contact lens on the unoperated eye. During this healing period it may be necessary to augment the correction of your visual acuity with glasses as well. When the visual result is satisfactory and the eye is completely comfortable, the patient may consider having the second eye done in order to balance both eyes. This time period between treatment of eyes may be from as little as a few days to up to several months.
It is possible to have both eyes operated on the same day, but you will not be able to drive or read for at least 7 days following surgery. However, most patients who have had bilateral LASIK can return to work the next day.
b. Permanent Overcorrection
It is possible that, following Excimer Laser PRK, one could become hyperopic rather than myopic. The disadvantage of being hyperopic is that you may have to wear contact lenses or glasses following the procedure in order to correct the hyperopia. If you are between 20 and 30 years of age and have a mild overcorrection, it normally should not affect your vision until you are around 40 years old. However, you might need to wear reading glasses if you plan to read for an extended period of time. If you are between 20 and 30 years old and the hyperopia overcorrection is moderate, you may then need glasses for reading and, possibly, for distance vision. If you are 35 to 55 years old or older and you become hyperopic to a mild degree, you will have good distance vision. However, you will need reading glasses for close work, because at or near the age of 40 years, we all become presbyopic.
If you are above age 40, have had PRK, and have a moderate to marked overcorrection in the hyperopic range, you will not be able to read or see distance well without glasses or contact lenses. The hyperopia will then have to be corrected with a contact lens or glasses. Or refractive surgery can be performed after 6 to 9 months to reverse this. There are new surgical methods for correcting hyperopia:
These technologies are also in the investigative stages. Marked or moderate degrees of overcorrection are rare.
If you are 40 years of age or older, the healing process will be much slower than for a person who is 20 or 30 years of age. Therefore, if you are slightly overcorrected, which is normal initially, it will take longer for your eye to get rid of the hyperopia and stabilize. Current medical opinion is that even though the hyperopia does remain longer in patients who are over 40 years of age, it will eventually disappear in most cases.
2. Undercorrection (residual myopia following PRK)
Undercorrection may result in the need to wear glasses or contact lenses either full-time or part-time, especially when driving at night. Undercorrection may occur in a small percentage of patients following Excimer Laser PRK, and may require an enhancement or another treatment. Undercorrection is more commonly seen in higher degrees of myopia.
If you are just slightly undercorrected at the 3 month to 6 month period following PRK but are very satisfied with your vision, then no further surgery will be necessary. If you are moderately undercorrected, then we would prefer to wait approximately 6 months to 1 year and then decide whether to do a repeat PRK, or to add some radial keratotomy incisions to the peripheral cornea, in order to flatten the cornea more and obtain more correction. These are decisions that we will have to make if you desire further correction.
3. Regression of Effect and Corneal Haze
A normal amount of regression of effect and minimal corneal haze is expected with the normal healing response, going from mild hyperopia following the Excimer Laser PRK procedure to the targeted correction over the first few months post-op. By far and away, the majority of PRK's done show no scarring or opacification of the cornea and no significant damage to adjacent tissue after 1 year. As we said, generally there is expected to be a small degree of corneal haze present for a few months following the surgery, but this usually disappears. It is very unusual for this normal amount of corneal haze to cause any interference in visual acuity. Through the years, the minimal haze formation on the cornea has not proved to be clinically significant. There may be a transient temporary clinically significant haze and regression of the myopic effect that may develop in 2-3% of patients following PRK that is normally easily controlled and reversed by short-term potent corticosteroid drop treatment. It usually clears rapidly with reversal of the regression effect. If this occurs, it more commonly occurs in the higher degrees of myopia (i.e. -5.00, -6.00, or greater). In those rare cases where the central corneal haze becomes clinically significant (scar formation) and is not reversed by medical means (short-term corticosteroid drops), haze normally disappears or becomes clinically insignificant after 1-2 years. Out of those few patients who have had clinically significant corneal haze after 1 year (less than 1% in the -1.00 to -7.00 group), some have chosen to wait 6 months to 1 year longer and the haze has become insignificant. However, there have been some people who did not want to wait and wanted the haze to be removed by the Excimer Laser. This has been done in a few cases worldwide with clearing of the regression and haze in over 90% of cases following repeat Excimer Laser surgery that is directed toward removing this corneal haze. For the most part (over 99%), the corneal haze has been clinically insignificant thus far, especially in the -1.00 to 7.00 myopic group. However, the incidence of clinically significant haze increases in correction of higher degrees of myopia (-7.00 and beyond).
It is imperative that the patient faithfully instill eye medications for several months (approximately 5 months) following the Excimer Laser photoablation procedure. If the drops are stopped too soon, the patient's correction may significantly regress and the effect of the Excimer Laser treatment may be lost. Relatively long-term use of anti-inflammatory drops will help keep the cornea clear. Even though the eye may feel comfortable, a mild to minimal corneal haze will develop within 4 to 6 weeks postoperatively and peak at about 3 to 6 months postoperatively in most patients who have undergone Excimer Laser PRK. The anti-inflammatory drops work to clear this haze. This haze is normally clinically insignificant, and virtually disappears after 6 months to a year in most patients. In most cases, the haze is transitory and normally has no noticeable effect on the vision.
4. Loss of best corrected vision
Another complication that could occur in a very small percentage of patients is the loss of a few lines of best corrected vision. There have been hundreds of thousands of these surgical procedures done worldwide. There have been reports of some patients that have lost one, two, or three lines of best corrected visual acuity, and at 18 months following the PRK, most have regained their best corrected visual acuity. These were seen primarily in high myopes (>-7.00 diopters). Again, it is normally a loss of only one or two lines of vision, and in most cases the best corrected vision will return to normal after 6 months to 1 year or 1 1/2 years. It is possible that one could permanently lose one, or two, or even more lines of best corrected visual acuity, but this is not common.
When we refer to loss of 1 or 2 lines of best corrected vision, we are referring to the Snellen visual acuity chart at 20 feet away. For example, before surgery, suppose you were able to see the 20/15 line with your best correction in your glasses, but after surgery you were only able to see 20/20 or 20/25 with your best corrected vision. If this occurs, it usually occurs in the early post-op period following Excimer Laser PRK, and the eye normally regains the 20/15 best corrected vision several weeks to months later.
5. Excessive sunlight exposure following Excimer Laser PRK
There is some speculation that excessive sunlight exposure following PRK can interfere with the healing phase of the cornea and may cause some regression of the surgical correction. Therefore, we strongly recommend that you protect your eyes from intense sunlight by wearing dark glasses with ultraviolet protection for at least 6 months following the PRK surgery.
6. Glare or sensitivity to light
You should also be aware that, during the first 3 months following Excimer Laser PRK, you will usually experience somewhat more glare from car lights when driving at night than you will experience after 6 months to 1 year. Halos seen around lights at night are now rare with large ablation zone diameters of 6mm or greater.
7. Enhancements or re-operations (Higher Degrees of Myopia especially)
Remember that for those patients who have higher degrees of myopia (>-7.00 diopters), it may be necessary to combine repeat PRK or add radial keratotomy incisions after the initial PRK has stabilized, or do the LASIK procedure (Involves making a corneal flap with a microkeratome and lasering the bed instead of the surface of the cornea. The corneal flap is repositioned without sutures.) For extremely high degrees of myopia (>-10.00 diopters), other procedures may need to be used, such as the following:
At the present time, Dr. Maddox prefers to do LASIK in most cases of myopia, hyperopia, and astigmatism rather than surface PRK.
8. Contrast Sensitivity
Decrease in contrast sensitivity is present especially initially after the surgery has been done. Generally, contrast sensitivity levels return to normal after 6 months to 1 year.
9. Persistent corneal epithelial defect with foreign body sensation and a prolonged healing period and prolonged irritation
Normally, the epithelium covers over 3 to 4 days following Excimer Laser PRK. However, there are those rare cases who take as long as 5 to 7 days. If the epithelium is not completely covered by the third day post-op, the temporary soft bandage lens will be removed from the operated eye and the eye will be patched for 24 to 48 hours, and this treatment will usually allow it to heal over completely. For the first few weeks post-op, the eye may feel "dry" and you may use non-preserved artificial tear drops as frequently as needed. Even more rare is a persistent localized area of the cornea that causes a foreign body sensation; infrequently this has to be treated with the Excimer Laser or some other modality to allow it to clear.
10. Corneal ulcer or perforation from bacterial infection, with loss of eye (extremely rare)
No eyes have been lost to date as a direct result of Excimer Laser PRK. The highest risk for corneal infection is during the first few days following the Excimer Laser treatment, when the corneal epithelium has not covered over completely. This is the reason that the patient is to use antibiotic drops. Therefore, you will be asked to come in for a follow-up exam each day until the epithelium has healed over and the soft bandage contact lens has been removed. Your risk for corneal infection is increased until the epithelium has covered over and the soft bandage lens is removed.
11. Inability to wear contact lenses after Excimer Laser PRK
It would be rare not to be able to tolerate a contact lens after Excimer Laser PRK, especially if you could tolerate contact lenses prior to surgery.
12. Cataract formation
The Excimer Laser is an ultraviolet laser with a wave length of 193 nanometers. The maximal penetration is only 1 to 3 microns; hence, it is not believed to cause cataract formation. There have been no cataracts reported to date directly caused by the Excimer Laser. Worldwide, there have been a few reported that have been caused by long-term use of potent corticosteroid drops. Long term treatment with potent corticosteroid drops is rarely if ever recommended in current Excimer Laser treatment.
13. Temporary Glaucoma or increased intraocular pressure
The Excimer Laser does not cause glaucoma, but corticosteroid drops, especially the potent ones, could raise the intraocular pressure to such a level that it would be necessary to discontinue their use or add an antiglaucoma drop to lower the intraocular pressure to a tolerable level during the healing phase. Sometimes we use nonsteroidal drops or non-preserved artificial tears instead of the corticosteroid drops. For this reason, we do not routinely use potent corticosteroid drops following Excimer Laser PRK. We routinely use a very mild corticosteroid drop whose side effects are much less pronounced than the potent drops. Although the mild drops can occasionally give rise to a transient intraocular pressure elevation, it usually is only to a nonsignificant level of elevation. However, occasionally a mild corticosteroid drop can give rise to significant pressure spikes and this is the reason that your intraocular pressure will be checked periodically during your post-op visits. Therefore, if you have your treatment and fail to return for all follow-up visits and continue to use these drops, you could be risking an unchecked intraocular pressure spike for a prolonged period of time, with possible permanent optic nerve damage. This is also why we do not want you to refill your drops without consulting the doctor first.
14. Transient iritis (inflammation inside the eye)
Usually less than 1% of patients develop iritis during the epithelialization period following Excimer Laser PRK. The iritis normally clears with topical corticosteroid drops, or intramuscular injection of a systemic corticosteroid.
15. Temporary Fluctuation of Vision
This phenomenon commonly occurs during the first few weeks following Excimer Laser PRK surgery, but once the eye stabilizes, the fluctuation normally disappears. Permanent fluctuation of vision has not been a problem with Excimer Laser PRK. This type of fluctuation in vision is only transitory and is not like the fluctuation in vision seen from morning to night that can be seen with RK (Radial Keratotomy).
16. Irregular corneal curvature that may require further laser surgery
Rarely, several months following Excimer Laser PRK, one may end up with an irregular corneal surface that does not allow the patient to return to best corrected preoperative visual acuity. This condition can be corrected by enhancement or re-operation in over 90% of cases.
17. Ptosis or droopy eyelid (usually temporary)
It is felt that use of potent corticosteroids is the most common cause for ptosis, and it usually tends to be reversible. It could be caused from the lid speculum, or from post-op lid edema or swelling. It would be rare to have a permanently droopy eyelid following Excimer Laser PRK.
18. Dry Eye
There are a number of patients who have undergone Excimer Laser PRK who complain of a dry eye feeling for a few weeks to months following this refractive surgery. This is probably more related to corticosteroid use than anything; however, we do recommend that these patients use a non-preserved artificial tear drop as often as needed to relieve this sensation.
19. Decompensated Eye Muscle Imbalance (Rare)
If one has had a prior history of a crossed eye, but now is straight, this could recur after Excimer Laser treatment, especially if there is a substantial imbalance in the refractive error between the two eyes. Once both eyes are balanced by equal or similar refractive errors, they usually will straighten out. Rarely surgery has to be performed to correct this muscle imbalance.
20. Mutagenesis (Cancer)
There have been no reported cases of mutagenesis. Since the Excimer Laser 193 nanometer wave length penetrates only about 1 to 3 microns at the most, it is felt that it does not penetrate deep enough to affect the nucleus of the cell. Animal studies have not indicated any problem with mutagenesis as a result of Excimer Laser photoablation.
21. Retinal radiation effect from the Excimer Laser
Since the 193 nanometer wave length does not penetrate more than 1 to 3 microns, no damage to the retina or other intraocular structure has ever been reported. Some of the ultraviolet fluorescence, other than the 193 nanometer, in the 300 to 400 nanometer range is present to a certain degree; however the exposure during Excimer Laser PRK is no more than that received by the eye when one is walking outside for a few minutes on a bright, sunny day. The retina is the photoreceptor cell lining of the inside of the eye that receives and transmits light energy back to the visual cortex in the brain.
22. Corneal endothelial cell damage
Most studies indicate that there is no long-term or short-term significant endothelial cell loss following Excimer Laser PRK. Endothelial cells line the inside of the cornea and are responsible for its transparency.
23. Induced regular astigmatism
Significant amounts of astigmatism induced after Excimer Laser PRK are unusual, and it would be rare to be clinically significant. Regular astigmatism occurs when the cornea is shaped like an oval rather than a sphere and requires a special cylindrical lens or contact lens to correct. Significant amounts of induced regular astigmatism can be reduced or eliminated with a diamond blade or with the Excimer Laser.
24. Temporary Refractive Error Imbalance
Following Excimer Laser PRK on only one eye, the two eyes are left imbalanced unless a contact lens is used on the unoperated eye. Often, many patients who do not wear contact lenses believe that the two eyes will balance with use of their glasses. In most cases, one cannot comfortably balance both eyes together with glasses if one eye is nearsighted and the other eye has been corrected with Excimer Laser PRK. It is often difficult to convince the patient that he will not be able to wear glasses comfortably during this adjustment period, but should preferably wear a temporary soft lens over the unoperated eye. Those who insist on wearing their glasses after only one eye has been corrected with the Excimer Laser may do so, but may have to patch the operated eye while driving or reading until the operated eye is stable and able to see well. In addition, some individuals who choose this method of dealing with the imbalance prefer using no glasses at all after the operated eye recovers good vision if they are well under 40 years of age. If one is in the presbyopic age group (over 40) and chooses to wear a contact lens on the unoperated eye while the operated eye recovers, one will most likely need reading glasses or bifocals to be worn over the unoperated eye during the recovery phase with or without the contact lens. When the visual result is satisfactory and the eye is completely comfortable, the patient may consider having the second eye treated in order to balance both eyes. This time period between treatment of eyes may be as little as a few days to as long as several months.
It is possible to have both eyes operated on the same day, but you will not be able to drive or read for at least 7 days following surgery. However, most patients who have had bilateral LASIK can return to work the next day.
25. Pain
During the Excimer Laser PRK procedure, there is absolutely no pain. About 90% of patients have no significant discomfort or pain during the reepithelialization period. Often the only discomfort one has is the feeling of an eyelash in the eye. The light sometimes bothers the patients who are in the immediate post-op period. The 10% who experience pain say it can be from moderate to severe, and may last 2 to 3 days. We have found that with the use of oral pain medications, topical drops and a well fit soft bandage lens, the pain can be alleviated to a tolerable level for most patients. Some patients do not tolerate the contact lens very well, and it may be necessary to exchange this lens for another in order to achieve a tolerable comfort level. Occasionally, we have to eliminate the soft bandage lens entirely and patch the eye instead, especially if the epithelium is having problems covering the surface of the cornea. Sometimes the upper lid swells for a few days after surgery, and the nose runs. The unoperated eye tends to sympathize with the operated eye, and it will tear as well for a few days. If the soft bandage contact lens falls out during the night, you should leave it out and not try to put it back into the eye, so as to avoid contamination. Close the eye and sleep with your shield. It often is more painful without the soft bandage lens, so you may have to double up on your pain medications. Any patient that experiences this problem would be advised to return to the doctor's office the next morning for a replacement bandage lens. Once the epithelium has covered over completely (usually 3 to 4 days), the pain stops. The eye may feel dry or gritty for a few days following removal of the contact lens, and this would be the end of the "pain" experience. It is advised that all patients use non-preserved artificial tear drops every hour or two, especially for the first 3 to 4 days while the soft bandage lens is in place. These drops help to lubricate the eye and keep plenty of fluid in the eye so the soft bandage lens will be comfortable and not stick down to the surface of the eye. These are some of the reasons why we do not recommend you return to work for 3 - 4 days after surgery. Most patients will be able to work (if not outside work) during this period of time, but you may be one of the 10% who is not able to work and has moderate to severe pain during this period. Since we cannot predict which patients will fall into the 10% pain category, we recommend that all patients take it easy for 3 to 4 days following Excimer Laser PRK.
26. Stromal edema, or corneal swelling
Swelling of the treated area of the cornea is quite common during the initial healing period, but it would be rare for this condition to persist.
27. Long-term Effects
The long-term effects of this procedure are unknown. The first sighted eye with long term follow-up was done by Marguerite McDonald, M.D., at LSU in July, 1988. Most of the serious complications that have been thought possible have not been evident on patients who have undergone Excimer Laser surgery to date. New complications may be observed as the surgical procedure develops in the future. As with all types of surgery, there is a possibility of other complications due to anesthesia, drug reactions, or other factors that may involve other parts of the body, including death. (Extremely rare).
Presently, alternatives to Excimer Laser surgery for myopia, hyperopia, and astigmatism include the following:
The potential benefits which may possibly be derived from Excimer Laser surgery include the following:
Birth Control Pills
In general, if a patient is taking birth control pills, this would not be considered a contraindication to refractive surgery, especially if the patient has been taking the pills for a long period of time and does not plan to stop taking the birth control pills in the near future. It would be advisable for the individual who undergoes refractive surgery while on birth control pills probably to stay on the pills for at least 3 to 6 months during the healing phase, rather than stopping the birth control pills a few weeks or a month or so after the refractive surgery. This still may not cause significant problems, but there is an unknown factor involved that could affect the healing process in some way during the critical healing phase from 1 to 3 months. Also, there is a chance that if the individual went off the birth control pills immediately after surgery and became pregnant, this could in turn possibly affect the healing phase and outcome even more.
Hundreds of thousands of clinical procedures have been performed worldwide with the Excimer Laser System at laser centers located within the United States, Europe (including Germany, Sweden, Ireland, France, Italy and the United Kingdom), Canada, Japan, Mexico, and Australia. The Excimer Laser is presently approved for general use in over forty countries worldwide. In a news release dated October 23, 1995, Summit Technology, Inc. announced that it had received FDA approval to commercially market and sell its Excimer Laser for Photorefractive Keratectomy for laser correction of nearsightedness from -1.5 to -7 diopters using a 6mm ablation zone.
The Excimer Laser is a major advancement in eye care. Worldwide results to date in countries that have approved the Excimer Laser for general use, have been extremely encouraging. The procedure thus far has proven itself to be extremely accurate with no residual scarring of the cornea and with superior visual results in the majority of cases. Since the shape of the cornea is responsible for most of the refractive changes that take place in focusing images, such reshaping, or sculpting, of the cornea should result in visual correction never before possible.
Candidacy for the Excimer Laser surgery is determined by an evaluation exam. If you wear contact lenses, you must discontinue wearing the lenses for a few weeks before the exam (usually 3 weeks for soft daily wear lenses, 6 weeks for gas permeable and extended wear soft contact lenses, 8 weeks for [PMMA] hard plastic lenses) on at least one eye (preferably both) and wear glasses or one contact lens during this period of time. (The actual time required for removal of contact lenses may vary depending upon the doctor's recommendation for your particular case.) This is recommended in order to achieve the most accurate measurements of your natural corneal curvature. You may be required to have measurements taken every week for several weeks before Excimer Laser surgery to be sure the cornea is stable. If your cornea stabilizes sooner than expected after the removal of the contact lens, your surgery may be done sooner.
(Note to ladies preparing for Excimer Laser surgery: please discontinue application of eye makeup for at least one day prior to the date of surgery. The cornea usually reepithelializes after 3 to 4 days. Generally, you may resume eye lid makeup, preferably with new cosmetics, a few days after reepithelialization has taken place and the eye is comfortable. Be very gentle in applying your eye makeup and be sure not to bump your eye during the process. Use extreme caution with curling irons, makeup brushes, and hair or deodorant spray.)
If your evaluation exam proves that you qualify for surgery, we, or your referring eye doctor, will discuss surgical options with you. If you have the Excimer Laser surgery on one eye, you may wish to have the second eye done several days to months later. When the first operated eye is comfortable and sees well, and both doctor and patient are satisfied, then evaluation for Excimer Laser surgery on the second eye can be considered.
It is possible to have both eyes operated on the same day, but you will not be able to drive or read for at least 7 days following surgery. However, most patients who have had bilateral LASIK can return to work the next day.
However, if it is determined that you are not a good candidate for the Excimer Laser surgery, you will be given an explanation as to the reasons why you are not a good candidate. It may be recommended that you postpone your refractive surgery until further developments take place with the Excimer Laser. You may want to consider an alternative method of refractive surgery, or stay with your glasses or contact lenses for the time being.
1. Computerized Topographical Analysis (Video Keratography)
This is a very sophisticated, computerized, high-tech analysis machine that will record in detail the corneal topography (over approximately 6,000 points on the corneal surface), so that we may be able to see exactly what the corneal shape is prior to surgery and be able to follow that corneal shape after surgery to determine the impact of LASIK on the cornea.
2. Pupil Diameter
3. Pachymetry
Pachymetry will be measured to determine the thickness of the cornea. This measurement will also be done during surgery to determine the flap thickness and the thickness of the corneal bed.
4. Tonometry
Tonometry is taken to determine the intraocular pressure both preoperatively and postoperatively.
5. Endothelial Cell Count (ECC) (on selected patients)
This is a technique employed to determine the number of endothelial cells present on the back of the cornea, as well as the health of the endothelial cells. These measurements may be followed periodically after LASIK. Endothelial cells are responsible for corneal clarity and appropriate hydration of the cornea.
6. Contrast Sensitivity Analysis
This is a contrast sensitivity test that may be done prior to and following the Excimer Laser surgery. Contrast sensitivity measures the ability of the eye to distinguish images under varying degrees of lighting.
7. Horizontal Diameter of the Cornea
8. Eye Dominance
9.Refraction on the Automated Refractometer
10. Tear Test to Rule Out Dry Eye.
11. Complete Eye Exam
12. Others
The preoperative workup will be done in our El Paso office. Most LASIK patients will be treated in our El Paso office, but in special cases you may choose to have your surgery done in our Juarez office. If you are having your surgery done in Juarez, you will be transported in our van to our Juarez office in Mexico, which is approximately 15 minutes from our El Paso office. Patients are welcome to take one additional person with them to our Juarez office. Patients from the United States who are being done in Juarez should bring identification to prove U.S. citizenship with them, including birth certificate, Passport, voter registration card, or a notarized statement swearing U.S. citizenship. If the patient or visitor is from a country other than the U.S., that person should bring his birth certificate and Visa or Passport. Remember, most of our laser procedures are now being done in our El Paso office instead of our Juarez office.
Prior to surgery, you will be positioned under the microscope and you will be asked to fixate (concentrate) on a blinking red light. The unoperated eye will be taped shut and a shield placed over it. Make sure you do not squeeze your unoperated eye shut, because it will affect your ability to hold your operated eye steady. Just act as though the unoperated eye that is taped shut is open. Try to use both eyes together, and this will steady the operated eye.
Once you are positioned under the microscope, we will do a few test runs, so that you can get used to the procedure prior to the actual Excimer Laser surgery procedure itself. The eye will be anesthetized with topical anesthetic, and an eyelid speculum will be placed between the eyelids in order to hold the eyelids open during the procedure. You will be asked to fixate on the red fixation light under the microscope and we will carry out test pulses on the superficial skin-like layer on the surface of the eye (epithelium). Approximately 5 or 6 pulses will be placed on this superficial tissue. There will be absolutely no pain, but you will hear the snapping sound of the laser. You will also sense an odor as the ultraviolet Excimer Laser beam interacts and is absorbed within the superficial corneal epithelium.
Once you have passed the test pulse phase, we will be ready to go on to the Excimer Laser PRK treatment, which normally takes between 15 and 20 seconds. You will be asked to continue to look at the blinking red fixation light, and the optical center of the eye will be marked. Then the superficial, skin-like cells on the surface of the eye (epithelium) will be removed with a gentle removal technique, either mechanically or by laser. Once the epithelium has been removed, we have an extremely smooth surface, and the Excimer Laser treatment will begin. You will be asked to fixate on the green fixation light, and Dr. Maddox will be talking to you throughout the entire duration of the treatment. He will let you know when it begins; you will then hear the snapping sound of the laser; you must continue to look at the green light. When fixating on the green light inside of the red ring during the laser treatment, you may see the green light become somewhat blurry, and this would be normal. If during the Excimer Laser treatment, you inadvertently move your eye, the laser beam eye tracking system will take over and will follow small eye movements. It will be a much better treatment if you can hold your fixation for at least 15 to 20 seconds. Frequently, a multipass/multizone laser technique will be employed, in which case the full treatment is broken up into two or three "mini" treatments during the single treatment session. This technique takes slightly longer but is also quick and easy. The procedure is now over; medication will be placed in the eye, and a therapeutic soft bandage lens will be placed over the eye.
It is critically important that you remember during the operation to relax completely your shoulders, your neck, and your chin. Do not clench your hands or make your hands into fists and squeeze them. Doing so can detract from your steadiness. It is best to relax your hands, your legs, and your feet; relax your shoulders, your neck, and your chin. Do not chew gum during the procedure, and do not cross your legs. I will be reminding you of these things throughout the treatment session.
During the treatment, there is absolutely no pain whatsoever. However, several hours after the treatment, you may experience moderate to severe burning pain. The vast majority of our patients have no pain at all, but a few may have moderate to severe burning pain. You will be given a very strong analgesic (pain reliever) and muscle relaxant to help get you through the first couple of days, if necessary. Immediately after the surgery, you should go home or back to your hotel and try to rest. The unoperated eye is going to be affected in the sense that it is going to sympathize with the operated eye: the next morning it may be hard to open the unoperated eye; light may be very intense, and it may be very bothersome to both eyes for several days. You will be relying on the analgesic medication to allay the pain and to help relax you. Please remember that sometimes it takes 3 to 4 days for the epithelium to totally re-epithelialize, or cover over, and that you could have some uncomfortable pain for 3 to 4 days following the surgery. During this painful period, your upper eyelid may swell. Although the pain may be severe the first days following Excimer Laser PRK, all patients interviewed following PRK on one eye have enthusiastically stated that the pain would not keep them from having Excimer Laser PRK surgery on the other eye. The patients who have moderate to severe pain may have it for 2 to 3 days, but rarely longer.
Once the cornea is re-epithelialized, your vision is going to be blurry, and it will take approximately one week to 1 month before the vision starts to become clear. Your eyes will be imbalanced unless you have had both eyes operated on. You can wear a contact lens on the unoperated eye until the operated eye gets well in order to balance the eyes. Or, if you prefer, you can wear your glasses, but when you want to read or drive, you may have to patch the operated eye and use the unoperated eye until the operated eye is comfortable and can see well. Please realize that you will have to do this for several weeks. In most cases, one cannot comfortably balance both eyes together with glasses if one eye is nearsighted and the other eye has been corrected with the Excimer Laser PRK. In addition, some individuals who choose this method of dealing with the imbalance prefer using no glasses at all after the operated eye recovers good vision if they are well under 40 years of age. If one is in the presbyopic age group (over 40) and chooses to wear a contact lens on the unoperated eye while the operated eye recovers, one will most likely need reading glasses or bifocals to be worn over the unoperated eye during the recovery phase with or without the contact lens. Please be prepared to have some blurry vision out of the operated eye for a few months. It may take at least 3 to 6 months for the operated eye to stabilize and be able to achieve the desired excellent vision. It is perfectly normal for one to be hyperopic (farsighted) in the initial postoperative stages following Excimer Laser PRK; however, after about 3 to 6 months, all or most of the hyperopia should be gone. In some cases, it may take 12 months for the hyperopia to disappear, especially if you are over 40 years of age.
When the visual result is satisfactory and the eye is completely comfortable, one may consider having the second eye treated in order to balance both eyes. This time period between the treatments of each eye may be as little as a few days to as long as several months (unless bilateral surgery has been performed).
It is imperative that you see us or your own eye doctor, if you have been referred, for your recommended postoperative follow-up visits. You will be seen each day until the epithelium has completely covered the ablated area (usually takes 3 to 4 days). The soft bandage contact lens comes off as soon as total epithelialization takes place (usually 3 to 4 days). You will see us or your referring eye doctor at 1 month, 3 months, 6 months, 9 months, and 12 months. During these time periods, you will have some of the studies that will be necessary for follow-up for the clinical study. One of the most essential things to remember is that you will probably be using eyedrops for 4 to 5 months following your Excimer Laser PRK surgery. It is essential that you follow the exact regimen that is recommended. There have been patients who discontinued the drops too soon, and their operated eyes lost some of the correction that they had achieved. Therefore, it is critical that you remember to use the eyedrops for the prescribed period of time, even if you feel as though you already see well. Since the cornea does not have blood vessels in it, the healing period takes many months. Usually, most excimer PRK's stabilize between 3 and 6 months. Therefore, the drops are very necessary to help the cornea heal properly and to prevent the eye from overreacting to the Excimer Laser photoablation. Please note that the cost of eyedrops is not included in the surgery fee. You will be responsible for the cost of all postoperative medications.
We advise that you do not go back to work for at least 4 days following your Excimer Laser PRK, and do not drive until the eye is comfortable.
One of the reasons Dr. Maddox prefers LASIK in most cases is due to the more rapid rehabilitation. Most LASIK patients are able to return to work the following day. It is, however, generally recommended that you rest for a couple of days after your surgery.
Cornea - Transparent, dome-shaped front surface of the eye which overlies the iris. It is analogous to a clear watch crystal that overlies the face and dials of a watch.
Reported Precautions and Dangers of Contact Lens Wear
Contact lenses are not risk free. It has been shown that the risk in contact lens wearers of sight-threatening corneal infection (ulcer; microbial keratitis) is very real. It has been reported that 25% of patients who present with microbial (ulcerative) keratitis (which can lead to permanent loss of eyesight) are contact lens wearers.
Irreversible and permanent corneal warpage, corneal distortion and endothelial damage are real and may occur after years of long-term use of contact lenses.
The annual risk for microbial keratitis (corneal ulceration and infection) is as follows:
Pseudomonas Keratitis associated with contact lens wear is one of the most dangerous organisms that can threaten the integrity of the cornea. If not diagnosed immediately and treated aggressively, it can literally ulcerate and melt through the full thickness cornea in a matter of hours, and can potentially blind the eye or cause permanent loss of vision from corneal scar, which would necessitate corneal transplant. These organisms live very easily in a number of lens solutions, lens cases, oily places, etc.
Little corneal infiltrates with small overlying epithelial erosions in a contact lens wearer are potentially very serious and could lead to permanent vision loss if not treated promptly and aggressively. Therefore, it has to be assumed that the cornea is infected and treated as an ocular emergency until appropriate lab studies come back.
It is therefore obvious that one must use impeccable hygiene in the care and handling of contact lenses.
Pseudomonas Keratitis accounts for 1/3 to 3/4 of the organisms seen in contact lens-associated microbial keratitis. Approximately 25% are due to gram positive cocci such as staph., strep., etc.
1. GPC (Giant Papillary Conjunctivitis) from contact lens wear
One develops multiple tiny, inflammed bumps on the inner side of the upper lid, which can be very irritating to the eyes. This can be caused from protein buildup and accumulation on the surface of the contact lens and/or mechanical rubbing of the contact lens on the inner surface of the upper eyelids as one blinks.
Treatment:
Treatment:
The main concern with contact lens hypoxic keratopathy is that repeat bouts of persistent corneal hypoxia may eventually lead to permanent corneal damage with clinically significant endothelial cell dysfunction. The endothelium is a mosaic of cells that line the entire area of the inside of the cornea and are responsible for maintaining corneal transparency. If they are significantly compromised, then corneal decompensation can take place, the cornea can become opaque and one can experience significant reduction in vision, possibly on a permanent basis. This also predisposes the eye to a greater chance for microbial infection.
Treatment:
Exchange contact lenses for better fit and contacts that allow high Oxygen permeability. If this does not solve the problem, it is best to discontinue contact lens wear.
4. Dry Eye associated with contact lens wear
If one has a "dry eye", it is often difficult to wear contact lenses for long periods of time. Artificial tears without preservative are recommended. Some individuals who desire to wear contact lenses but have even a borderline dry eye situation have difficulty with comfort and clear vision with contact lenses.
5. Neovascularization of the Cornea in association with contact lenses
Superficial and sometimes deep blood vessels can abnormally grow onto the cornea from long-term use contact lenses. Only the examining doctor can see them under the microscope on examination. They grow because of mechanical rubbing and poor oxygenation by the contact lenses. It is undesirable for this to occur. If they proliferate to such a degree that they encroach on the visual axis area, they would cause a decrease in vision, glare, corneal distortion, corneal melt problems, etc.
6. Contact lens, lens case and contact lens solution contaminants
Discomfort and decreased wearing time can result from contact lens, lens solutions, and lens case contaminants.
Some of these contaminants are as follows:
Acanthomoeba is another extremely dangerous microorganism involved in contaminating contact lenses and cases, especially when using the "home made" saline and using tap water or well water to clean contact lenses. Acanthomoeba is very difficult to treat and cure. Some patients with this type of keratitis have to be treated for months to years with a poor prognosis for recovery of sight, in spite of corneal transplantation if it were to become necessary.
7. Contact lenses may cause permanent or irreverisble warpage of the cornea, which could induce permanent astigmatism.
Laser: an acronym for Light Amplification by Stimulated Emission of Radiation.
Photon: a packet of light energy, or quantum.
Excimer: a contraction of the two words Excited and Dimer. The dimer refers to the Fluoride-Argon molecules in the excited state. This dimer does not exist in nature in the unexcited state; in the unexcited state, the two atoms of Argon and Fluorine are not bound together. Fluorine gas is a halogen, and Argon gas is a rare or inert gas. Therefore, the excited dimer consists of atoms of Argon and Fluoride technologically bound together in a highly excited, temporary state to form a diatomic rare gas-halide. The decay of these unstable molecules (Argon-Fluoride) to a stable state results in the emission of a highly energetic photon of ultraviolet light. The emissions wavelength of the Argon-Fluoride Excited Dimer is 193 nanometers (193 nm). (One nanometer is one-billionth of a meter.) The excimer emissions are in a train of individual pulses, typically 10 nanoseconds (10 x 10-9 seconds) long, with a pulse repetition frequency of up to 50 Hertz (Hz), or 50 pulses per second. (Hertz is the international unit of frequency, equal to one cycle per second.) The typical pulse rate is 10 pulses per second, or 10 Hz coming out of the excimer laser.
Non-excimer ophthalmic lasers generally depend on one of two tissue/laser interactions:
1. Photocoagulation (typically achieved with Argon lasers):Photocoagulation occurs when laser light energy is absorbed by a target tissue and converted to heat. The thermal effects range from protein denaturation to vaporization and carbonization.
2. Photodisruption (typically achieved with Neodymium-YAG lasers): In photodisruption, a high-powered laser pulse causes optical breakdown and plasma formation with a resultant shockwave which mechanically disrupts the target tissue.
The excimer laser now offers a third tissue/laser interaction:
3. Photoablation: High powered, pulsed ultraviolet radiation removes tissue with an extremely high degree of precision (.25 microns per pulse, or 1/25000 of an inch per pulse). Initial studies of ultraviolet photoablation of organic polymers showed that such radiation could produce layer-by-layer removal of material and etch patterns with sub-micron precision. Moreover, ultraviolet photons are highly energetic and can directly break chemical bonds. Ultraviolet laser irradiation of biological tissue breaks these large molecules into diatomic and triatomic fragments and monomers which are ejected from the surface of the material being irradiated.
The Argon-Fluoride Excimer Laser is a revolutionary innovation and advanced treatment modality in an attempt to correct myopia, hyperopia and astigmatism, as well as superficial keratectomy to erase corneal scars and irregular corneal surfaces. When the Argon-Fluoride Excimer Laser is used in corneal reshaping to correct refractive errors, it breaks the carbon-to-carbon molecular bonds of the corneal tissue by the ultraviolet 193-nanometer wavelenghth of emission photochemical effect called photoablation. This photoablation effect is extremely superficial. Minimal thermal damage is created by the ultraviolet excimer laser, unlike traditional lasers in which the produced heat causes damaging effects to surrounding tissue. The pulsing excimer laser removes the tissue in microscopic layers, leaving virtually no underlying thermal trauma. The carbon-to-carbon bond holding most of the tissue together has an energy requirement of 3 electron volts. If an excimer laser photon is introduced, it can literally crack that bond. The photon-energy, or energy per photon, of the excimer photon is 6.4 electron volts, or 10-15 millijoules per photon. One laser pulse contains many photons. One excimer laser pulse contains 2.5 x 1016 photons. Therefore, the energy per pulse at the eye is equal to the 10-15 millijoules (single photon energy) times 2.5 x 1016 (number of photons in one pulse), which equals 25 millijoules (mj). (2.5 x 1016 = 25 billion million.) These excimer photons are like "photon scissors", breaking the carbon-to-carbon bonds of the corneal tissue. Hence, the excimer photon is incredibly energetic, having 3 times as much energy as the YAG laser photon and more than twice the energy as the Argon laser photon. The term that has been coined for the effect of the excimer laser on the tissue is photoablation. The key to the excimer laser is the short pulse duration (10 nanoseconds or 10 x 10-9 seconds) with high energy photons (energy per pulse is 25 millijoules at the eye) with the possibility of concentrating large numbers of these photons on tissue to crack the carbon-to-carbon bonding that holds tissue together.
For the first time, a no-touch system, or no-touch scalpel, with the ultimate resolution of a fraction of a micron, is available to surgeons. (One micron equals one one-thousandth of a millimeter.) So, without touching the eye, the excimer can change and sculpt the cornea (photon scissors) incredibly accurately with virtually no collateral damage conducted into the edges of the tissue affected. There is no significant mechanical effect to the surrounding tissues; and no crushing of tissue, as with a knife blade or scalpel.
PRK Changes anterior corneal surface by affecting the center of the cornea.
RK Changes anterior corneal surface by affecting periphery of the cornea.
PRK Photoablation of the central corneal surface. Involves a very small amount of central cornea with no incisions. PRK reshapes or sculpts the cornea by a shallow, rapid photoablation of microscopic amounts of tissue (often < 15% of total corneal depth).
RK Multiple deep peripheral corneal incisions. Involves peripheral cornea with multiple wound incisions (approx. 90% of total corneal depth).
PRK At this time, for the excimer correction of myopia, specific variables such as intraocular pressure, thickness of the cornea, keratometry readings (flat or steep), scleral rigidity, etc. do not indicate that they would directly affect the surgical outcome. There is some speculation that age may affect the corrective changes achieved following excimer laser PRK.
RK Many variables affect the outcome of RK surgery, such as age, intraocular pressure, thickness of the cornea, flat keratometry readings, scleral rigidity, etc. All these variables must be considered in addition to the degree of myopia present when performing RK surgery.
PRK Central corneal epithelium removed mechanically or with laser prior to photoablation; normally replaces itself in 3 days.
RK Peripheral epithelium incised or cut with surgical blade.
PRK Immediate postop comfort level: 90% have no significant postop pain. One can have severe, moderate, or little pain following PRK. With appropriate medications, the immediate postop pain can be markedly reduced. With the new postop medication regimen, postop pain is generally less severe than that following RK.
RK Immediate postop comfort level: One can have severe, moderate, or little pain following RK. Generally more painful than Excimer Laser PRK. 20-30% of RK patients have significant postop pain.
PRK A therapeutic soft bandage lens will be placed over the immediately postop eye until cornea re-epithelializes, normally in 3 days. It is recommended that the operated eye be shielded at night for 1 week postop with a plastic shield for protection during sleep.
RK Therapeutic bandage lens is not generally applied postop. The eye is not usually patched postop during waking hours, but it is recommended that the operated eye be shielded at night for 1 week postop with a plastic shield for protection during sleep.
PRK Potential for much more standardized results with computer-controlled process. For most corrections of -1.00 to -7.00 diopters, PRK can achieve maximum desired correction with only 1 application.
RK Surgeon-dependent procedure (less standardized results). For approximately 10% to 30% of those patients who undergo RK to correct myopia of -1.00 to -7.00 diopters, in order to achieve maximum desired correction it is necessary for the patient to undergo enhancement by a second and often a third RK procedure on that same eye.
PRK No significant structural weakening of cornea.
RK Structural corneal weakening common.
PRK Can be some regression of effect during the first year, usually stable at 3 to 6 months.
RK Can be regression of effect for months after RK surgery with unstable vision, especially with high number of incisions and redeepened incisions.
PRK Vision fluctuation during the day: So far, vision fluctuation during the day has not been a problem following excimer laser PRK.
RK Vision fluctuation during the day: Fluctuation of vision following RK can be a problem and is seen following RK in a limited number of patients. Because of the instability of the cornea, especially following maximum number of incisions and redeepening, the cornea weakens to such a degree that vision fluctuation may become a permanent situation in some cases.
PRK Microperforation eliminated
RK Microperforation possible since cuts are made to 90% of corneal depth.
PRK Less chance of damage to corneal endothelial cells.
RK Greater chance of endothelial cell damage.
PRK Less chance of glare problems at night from car lights. Glare problems with excimer or RK are not usually incapacitating at night and tend to be diminished to an insignificant level after 6 months to 1 year. Myopic people experience glare naturally prior to surgery and even more so with contact lenses. Glare following excimer is rarely worse than with contact lenses and usually less than that experienced with contact lenses. With the larger ablation zone, night glare is generally less than with RK.
RK Greater chance of glare problems at night from car lights. There is a greater chance of glare problems with starburst effect at night from car lights. However, this is usually not incapacitating, but potentially could be. This also depends on the optical zone size and the number of incisions. A patient with a large optical zone size and a few incisions may not have much glare or starburst, whereas a patient with a very small optical zone size and a large number of incisions with redeepening may experience this problem.
PRK Normally no scarring and no significant damage to adjacent tissue. By far and away, the majority of PRKs done show no scarring or opacification of the cornea after 1 year. Generally, there is expected to be a small degree of corneal haze present for a few months following the surgery, but this disappears. It is very unusual for this normal amount of corneal haze to cause any interference in visual acuity. Through the years, the minimal haze formation on the cornea has not proved to be clinically significant. There may be a transient temporary clinically significant haze and regression of the myopic effect that may develop in 2-3% of patients following PRK that is normally controlled and reversed by short-term potent corticosteroid drop treatment. It usually clears rapidly with reversal of the regression effect. If this occurs, it more commonly occurs in the higher degrees of myopia (i.e. -5.00, -6.00, or greater). In those rare cases where the central corneal haze becomes clinically significant and is not reversed by medical means (short-term corticosteroid drops), haze normally disappears or becomes clinically insignificant after 1-2 years. Out of those few patients who have had clinically significant corneal haze after 1 year (less than 1% in the -1.00 to -7.00 group), some have chosen to wait 6 months to 1 year longer and the haze has become insignificant. However, there have been some people who did not want to wait and wanted the haze to be removed by the excimer laser. This has been done in a few cases worldwide with clearing of the regression and haze in over 90% of cases following repeat excimer laser surgery that is directed toward removing this corneal haze. For the most part (over 99%), the corneal haze has been clinically insignificant thus far, especially in the -1.00 to -7.00 myopic group. However, the incidence of clinically significant haze increases in correction of higher degrees of myopia (-7.00 and beyond).
RK Scarring of corneal incisions present. Normally there is a mild or moderate amount of scarring that occurs with the RK incisions. Remember that these incisions are peripheral; but can contribute to some glare phenomenon, especially at night, which is normally inconsequential.
PRK Best corrected vision: Most of the time, best corrected vision is attained by 3 months, but it can take as little as 1 week or as long as 6 months or longer to attain best corrected vision.
RK Best corrected vision: It may take as little as 1 day or as long as 3 to 6 months to attain best corrected vision with radial keratotomy.
PRK Less chance for contact lens intolerance following PRK. It should be easier to fit contact lenses following excimer surgery vs RK surgery. However, one may or may not be able to wear contact lenses successfully after Excimer laser surgery. (The majority of patients would be able to wear contacts successfully if they wore contacts successfully prior to PRK.) One should wait 3 to 6 months before attempting contact lens wear.
RK Greater chance for contact lens intolerance following RK. If contact lenses are necessary to improve vision after RK, one should wait 3 to 6 months before fitting contacts. One may or may not be able to wear contact lenses successfully after RK surgery. (The majority of patients would be able to wear contacts successfully if they wore contacts successfully prior to RK.) However, because the deep RK incisions (90% deep) do not completely heal for 3 to 7 years or never (even though the surface cells heal over after a few days), it is felt that contact lens wear after RK versus after Excimer Laser PRK is more risky and it would be easier to develop a wound infection due to the chronic irritation of the contact lens over the radial incisions.
PRK Long term microbial keratitis and corneal stability: should not be a problem with PRK. One of the reasons why this is not a problem with PRK is that the corneal surface area of ablation generally is healed in 6 months to 1 year with complete stability in most cases. Since the ablation is only superficial, this area is generally considered healed 6 months to 1 year following excimer laser PRK.
RK Long term microbial keratitis (corneal infection). Occasional long-term incisional abscesses, particularly in patients who are eye-rubbers and/or have to go back to wearing their contact lenses. This can be a sight-threatening problem, especially if an incisional abscess occurs months to years following the initial RK surgery in association with contact lens wear.
PRK Generally slower visual recovery (sometimes only by a few days).
RK Generally quicker visual recovery.
PRK Phenomenon of progressive hyperopia thus far has not been seen with PRK.
RK Progressive hyperopia: A unique variable thus far seen with RK is referred to as progressive hyperopia, or consecutive hyperopia, or hyperopic "creep," which can be seen in a minority of cases. The eye continues to correct even years after the initial surgery has been done, and therefore the eye becomes progressively more and more farsighted. In the early RK years, there has been an incidence of 20-30% of patients who have exhibited this phenomenon. However, with modern-day surgery the incidence is less, about 5-20%. If this phenomenon is going to be seen, it is normally seen in high myopes who have had a maximum number of RK incisions with redeepening of the incisions. This apparently destabilizes the cornea to such a degree that allows progressive hyperopia. But, some cases of 4 and 8 cut RK have been reported. Therefore, in a certain percentage of RK patients, the eye never stabilizes after surgery.
PRK Shorter track record.
USA: 1988
RK Longer track record.
USA: 1978
Russia: 1974 - first human cases
(1972 research began)
PRK Number of cases done:
USA: thousands
Worldwide: hundreds of thousands
RK Number of cases done:
USA: over 1,000,000
Worldwide: several million
PRK Excimer Laser PRK may be used to correct undercorrected RK.
RK RK may be used to correct undercorrected Excimer Laser PRK.
PRK Excimer Laser PRK plus RK may correct higher degrees of myopia than either procedure alone.
RK RK plus Excimer Laser PRK may correct higher degrees of myopia than either procedure alone.
PRK Excimer Laser PRK more efficiently corrects higher degrees of myopia (-5.00 and greater) than does RK for patients who are in the 20's to early 30's.
RK RK is not as efficient in correcting higher degrees of myopia (-5.00 and greater) as is Excimer Laser PRK for patients in the 20's to early 30's.
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