LASIK

LASIK

Overview
LASIK or Lasik commonly referred to simply as laser eye surgery, is a type of refractive surgery
Refractive surgery
Refractive eye surgery is any eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea or cataract surgery. The most common methods today use excimer lasers to...

 for correcting myopia
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

, hyperopia
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

, and astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

. LASIK is performed by ophthalmologists using a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

.

LASIK is similar to other surgical corrective procedures such as photorefractive keratectomy
Photorefractive keratectomy
Photorefractive keratectomy and Laser-Assisted Sub-Epithelial Keratectomy are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University...

, PRK, (also called ASA, Advanced Surface Ablation) though it provides benefits such as faster patient recovery. Both LASIK and PRK represent advances over radial keratotomy
Radial keratotomy
Radial keratotomy is a refractive surgical procedure to correct myopia.- Discovery :The procedure was discovered by Svyatoslav Fyodorov who removed glass from the eye of one of his patients who had been in an accident. A boy, who wore eyeglasses, fell off his bicycle and his glasses shattered on...

 in the surgical treatment of vision problems, and are thus viable alternatives to wearing corrective eyeglasses or contact lenses for many patients.

The LASIK technique was first made possible by the Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...

-based Spanish
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 ophthalmologist Jose Barraquer
Jose Barraquer
José Ignacio Barraquer Moner was an ophthalmologist known to many as "the father of modern refractive surgery"...

, who, around 1950 in his clinic in Bogotá
Bogotá
Bogotá, Distrito Capital , from 1991 to 2000 called Santa Fé de Bogotá, is the capital, and largest city, of Colombia. It is also designated by the national constitution as the capital of the department of Cundinamarca, even though the city of Bogotá now comprises an independent Capital district...

, Colombia, developed the first microkeratome
Microkeratome
A microkeratome is a precision surgical instrument with an oscillating blade designed for creating the corneal flap in LASIK or ALK surgery. The normal human cornea varies from around 500 to 600 micrometres in thickness; and in the LASIK procedure, the microkeratome creates a 83 to 200 micrometre...

, and developed the technique used to cut thin flaps in the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and alter its shape, in a procedure he called keratomileusis
Keratomileusis
Keratomileusis is the surgical improvement of the refractive state of the cornea performed by lifting up the front surface of the eye by forming a thin hinged flap under which the shape of the cornea is changed by using an excimer laser or other surgical device, and was developed by José Ignacio...

.
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Encyclopedia
LASIK or Lasik commonly referred to simply as laser eye surgery, is a type of refractive surgery
Refractive surgery
Refractive eye surgery is any eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea or cataract surgery. The most common methods today use excimer lasers to...

 for correcting myopia
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

, hyperopia
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

, and astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

. LASIK is performed by ophthalmologists using a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

.

LASIK is similar to other surgical corrective procedures such as photorefractive keratectomy
Photorefractive keratectomy
Photorefractive keratectomy and Laser-Assisted Sub-Epithelial Keratectomy are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University...

, PRK, (also called ASA, Advanced Surface Ablation) though it provides benefits such as faster patient recovery. Both LASIK and PRK represent advances over radial keratotomy
Radial keratotomy
Radial keratotomy is a refractive surgical procedure to correct myopia.- Discovery :The procedure was discovered by Svyatoslav Fyodorov who removed glass from the eye of one of his patients who had been in an accident. A boy, who wore eyeglasses, fell off his bicycle and his glasses shattered on...

 in the surgical treatment of vision problems, and are thus viable alternatives to wearing corrective eyeglasses or contact lenses for many patients.

Technology


The LASIK technique was first made possible by the Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...

-based Spanish
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 ophthalmologist Jose Barraquer
Jose Barraquer
José Ignacio Barraquer Moner was an ophthalmologist known to many as "the father of modern refractive surgery"...

, who, around 1950 in his clinic in Bogotá
Bogotá
Bogotá, Distrito Capital , from 1991 to 2000 called Santa Fé de Bogotá, is the capital, and largest city, of Colombia. It is also designated by the national constitution as the capital of the department of Cundinamarca, even though the city of Bogotá now comprises an independent Capital district...

, Colombia, developed the first microkeratome
Microkeratome
A microkeratome is a precision surgical instrument with an oscillating blade designed for creating the corneal flap in LASIK or ALK surgery. The normal human cornea varies from around 500 to 600 micrometres in thickness; and in the LASIK procedure, the microkeratome creates a 83 to 200 micrometre...

, and developed the technique used to cut thin flaps in the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and alter its shape, in a procedure he called keratomileusis
Keratomileusis
Keratomileusis is the surgical improvement of the refractive state of the cornea performed by lifting up the front surface of the eye by forming a thin hinged flap under which the shape of the cornea is changed by using an excimer laser or other surgical device, and was developed by José Ignacio...

. Barraquer also researched the question of how much of the cornea had to be left unaltered to provide stable long-term results.

Later technical and procedural developments included RK (Radial keratotomy
Radial keratotomy
Radial keratotomy is a refractive surgical procedure to correct myopia.- Discovery :The procedure was discovered by Svyatoslav Fyodorov who removed glass from the eye of one of his patients who had been in an accident. A boy, who wore eyeglasses, fell off his bicycle and his glasses shattered on...

), developed in the USSR in the 1970s by Svyatoslav Fyodorov
Svyatoslav Fyodorov
Svyatoslav Nikolayevich Fyodorov was a Russian ophthalmologist, eye microsurgeon, professor, full member of the Russian Academy of Sciences and Russian Academy of Medical Sciences and politician...

, and PRK (photorefractive keratectomy), developed in 1983 at Columbia University by Dr. Steven Trokel, who in addition published an article in the American Journal of Ophthalmology in 1983 outlining the potential benefits of using the excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....

 patented in 1973 by Mani Lal Bhaumik
Mani Lal Bhaumik
Mani Lal Bhaumik is an Indian-born American physicist. He has been an author, lecturer, entrepreneur, and philanthropist.His early contributions to laser technology are exemplified by the development of the excimer laser at the Northrop Corporation Research and Technology Center in Los Angeles. As...

 in refractive surgeries. (RK is a procedure in which radial corneal cuts are made, typically using a micrometer diamond knife, and is completely different from LASIK).

In 1968 at the Northrop Corporation Research and Technology Center of the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...

, Mani Lal Bhaumik
Mani Lal Bhaumik
Mani Lal Bhaumik is an Indian-born American physicist. He has been an author, lecturer, entrepreneur, and philanthropist.His early contributions to laser technology are exemplified by the development of the excimer laser at the Northrop Corporation Research and Technology Center in Los Angeles. As...

 and a group of scientists were working on the development of a carbon-dioxide laser. Their work evolved into what would become the excimer laser. This type of laser would become the cornerstone for refractive eye surgery. Dr. Bhaumik announced his team's breakthrough in May 1973 at a meeting of the Denver Optical Society of America in Denver, Colorado
Denver, Colorado
The City and County of Denver is the capital and the most populous city of the U.S. state of Colorado. Denver is a consolidated city-county, located in the South Platte River Valley on the western edge of the High Plains just east of the Front Range of the Rocky Mountains...

. He would later patent his discovery.

The general term for changing a patient's optical measurements by means of an operation is Refractive Surgery. The introduction of lasers in refractive surgeries stemmed from Rangaswamy Srinivasan
Rangaswamy Srinivasan
Rangaswamy Srinivasan is an inventor at IBM Research. One of the famous inventions he has contributed to is LASIK.-Ablative Photodecomposition :...

's work. In 1980, Srinivasan, working at IBM Research
IBM Research
IBM Research, a division of IBM, is a research and advanced development organization and currently consists of eight locations throughout the world and hundreds of projects....

 Lab, discovered that an ultraviolet excimer laser could etch living tissue in a precise manner with no thermal damage to the surrounding area. He named the phenomenon Ablative Photodecomposition (APD). The use of the excimer laser to ablate corneal tissue for the correction of optical errors, such as myopia, hyperopia, and astigmatism, was first suggested by Stephen Trokel, MD, of the Edward S. Harkness Eye Institute, Columbia University, New York, NY. Dr. Trokel, along with Dr. Charles Munnerlyn
Charles Munnerlyn
Dr. Charles Munnerlyn is an American optical engineer. He constructed the first working excimer laser system for vision correction in 1985, and also developed the Munnerlyn Formula that dictates the amount of corneal tissue to be removed by the laser to correct vision disorders such as myopia and...

 and Terry Clapham, founded VISX, Incorporated. The first human eye was treated using a VISX laser system by Dr. Marguerite B. MacDonald, MD in 1989.

The first patent for LASIK was granted by the U.S. Patent Office to Dr. Gholam A. Peyman
Gholam A. Peyman
Gholam A. Peyman, MD is a Hall of Fame of Ophthalmology and retina surgeon who is also a prolific and successful inventor. Gholam Peyman has, thus far, been granted 124 US Patents covering a broad range of novel medical devices, intra-ocular drug delivery, surgical techniques, as well as new...

 on June 20, 1989, U.S. Patent #4,840,175, "method for modifying corneal curvature," encompassing the surgical procedure in which a flap is cut in the cornea and pulled back to expose the corneal bed. The exposed surface is then ablated to the desired shape with an excimer laser, after which the flap is replaced.

The LASIK technique was successfully applied in other countries before it arrived to the United States. The first U.S. Food and Drug Administration (FDA) trial of the excimer laser was started in 1989. The first use of the laser was to change the surface shape of the cornea, known as PRK. Summit Technology, under the direction of its founder and CEO, Dr. David Muller, was the first company to receive FDA approval for an excimer laser to perform PRK in the US. The LASIK concept was first introduced by Dr. Pallikaris
Ioannis Pallikaris
Ioannis G. Pallikaris is a Greek ophthalmologist who in 1989 performed the first LASIK procedure on a human eye. Pallikaris also developed Epi-LASIK....

 in 1992 to the group of ten surgeons who were selected by the FDA to test the Visx laser at 10 centers in the U.S. In 1998, the Kremer Excimer Laser serial #KEA 940202 received FDA approval for its singular use to perform LASIK in the US. Subsequently, Summit Technology was the first company in the US to receive FDA approval to mass manufacture and distribute excimer lasers to perform LASIK and Visx and several other manufactures then followed.

Dr. Pallikaris theorized the benefits of performing PRK after the surface was raised in a layer to be known as a flap performed by the Mikrokeratome developed by Barraquer in 1950. The blending of a flap and PRK became known as LASIK, which is an acronym. It quickly became very popular, since it provided immediate improvements in vision and involved much less pain and discomfort than PRK.

Today, faster lasers, larger spot areas, bladeless flap incisions, intraoperative pachymetry, and wavefront-optimized and -guided techniques have significantly improved the reliability of the procedure compared to that of 1991. Nonetheless, the fundamental limitations of excimer lasers and undesirable destruction of the eye's nerves have spawned research into many alternatives to "plain" LASIK, including LASEK
Lasek
Lasek may refer to:*LASEK or photorefractive keratectomy, a laser eye surgery technique*Lasek, Lower Silesian Voivodeship *Lasek, Kuyavian-Pomeranian Voivodeship *Lasek, Łódź Voivodeship...

, Epi-LASIK
EPI-LASIK
Epi-LASIK is a refractive surgery technique designed to reduce a person's dependency on eyeglasses and contact lenses. Invented by Dr. Ioannis Pallikaris , the technique is basically an automatic LASEK without alcohol:It can be better considered as superficial LASIK. The stromal bed is smoother...

, sub-Bowman’s Keratomileusis aka thin-flap LASIK, wavefront-guided PRK and modern intraocular lens
Intraocular lens
An intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...

es.

LASIK may one day be replaced by Femtosecond laser intrastromal vision correction via all-femtosecond correction (like Femtosecond Lenticule Extraction
Femtosecond Lenticule EXtraction
Femtosecond Lenticule EXtraction is a new form of refractive eye surgery similar to LASIK that creates a lenticule for removal and a corneal flap...

, FLIVC
FLIVC
FLIVC is a laser refractive eye surgery technology that is one of several possible alternatives to LASIK now under investigation...

, or IntraCOR), or other techniques that avoid permanently weakening the cornea with incisions and deliver less energy to surrounding tissues. One thermobiomechanical alternative, Keraflex, recently received the CE Mark
CE mark
CE marking is a mandatory conformity mark for products placed on the market in the European Economic Area . With the CE marking on a product the manufacturer ensures that the product conforms with the essential requirements of the applicable EC directives...

 for refractive correction, and is in European clinical trials for the correction of myopia and keratoconus. The 20/10 (now Technolas) FEMTEC laser has also recently been used for incisionless IntraCOR ablation on several hundred human eyes and achieved very successful results for presbyopia, with trials ongoing for myopia and other conditions.

Procedure



There are several necessary preparations in the preoperative period. The operation itself involves creating a thin flap on the eye, folding it to enable remodeling of the tissue beneath with a laser. The flap is repositioned and the eye is left to heal in the postoperative period.

Preoperative


Patients wearing soft contact lenses are usually instructed to stop wearing them 5 to 21 days before surgery. One industry body recommends that patients wearing hard contact lenses should stop wearing them for a minimum of six weeks plus another six weeks for every three years the hard contacts have been worn. Before the surgery, the patient's cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

s are examined with a pachymeter
Pachymeter
A pachymeter is a medical device used to measure the thickness of the eye's cornea. It is used to perform Corneal pachymetry prior to LASIK surgery, for Keratoconus screening, LRI surgery and is useful in screening for patients suspected of developing glaucoma among other uses.Modern devices use...

 to determine their thickness, and with a topographer to measure their surface contour. Using low-power laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

s, a topographer creates a topographic map
Topographic map
A topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features...

 of the cornea. This process also detects astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

 and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and the locations of corneal tissue to be removed during the operation. The patient typically is prescribed and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure.

Operation


The operation is performed with the patient awake and mobile; however, the patient is sometimes given a mild sedative
Sedative
A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....

 (such as Valium) and anesthetic eye drop
Eye drop
Eye drops are saline-containing drops used as a route to administer medication in the eye. Depending on the condition being treated, they may contain steroids, antihistamines, sympathomimetics, beta receptor blockers, parasympathomimetics, parasympatholytics, prostaglandins, non-steroidal...

s.

LASIK is performed in three steps. The first step is to create a flap of corneal tissue. The second step is remodeling of the cornea underneath the flap with the laser. Finally, the flap is repositioned.

Flap creation


A corneal suction ring is applied to the eye, holding the eye in place. This step in the procedure can sometimes cause small blood vessels to burst, resulting in bleeding or subconjunctival hemorrhage
Subconjunctival hemorrhage
thumb|right|250px| A stress induced subconjunctival hemorrhage in the left eye one week after hemorrhaging. Same hemorrhage four weeks after hemorrhaging. Some of the blood in the [[sclera]] has turned yellow, like a bruise....

 into the white (sclera
Sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

) of the eye, a harmless side effect that resolves within several weeks. Increased suction typically causes a transient dimming of vision in the treated eye. Once the eye is immobilized, the flap is created. This process is achieved with a mechanical microkeratome
Microkeratome
A microkeratome is a precision surgical instrument with an oscillating blade designed for creating the corneal flap in LASIK or ALK surgery. The normal human cornea varies from around 500 to 600 micrometres in thickness; and in the LASIK procedure, the microkeratome creates a 83 to 200 micrometre...

 using a metal blade, or a femtosecond laser microkeratome that creates a series of tiny closely arranged bubbles within the cornea. A hinge is left at one end of this flap. The flap is folded back, revealing the stroma, the middle section of the cornea. The process of lifting and folding back the flap can sometimes be uncomfortable.

Laser remodeling


The second step of the procedure is to use an excimer laser (193 nm) to remodel the corneal stroma. The laser vaporizes tissue in a finely controlled manner without damaging the adjacent stroma. No burning with heat or actual cutting is required to ablate the tissue. The layers of tissue removed are tens of micrometres thick. Performing the laser ablation in the deeper corneal stroma typically provides for more rapid visual recovery and less pain than the earlier technique, photorefractive keratectomy
Photorefractive keratectomy
Photorefractive keratectomy and Laser-Assisted Sub-Epithelial Keratectomy are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University...

 (PRK).

During the second step, the patient's vision will become very blurry once the flap is lifted. They will be able to see only white light surrounding the orange light of the laser, which can lead to mild disorientation.

Currently-manufactured excimer lasers use an eye tracking system that follows the patient's eye position up to 4,000 times per second, redirecting laser pulses for precise placement within the treatment zone. Typical pulses are around 1 millijoule (mJ) of pulse energy in 10 to 20 nanoseconds.

Repositioning of flap


After the laser has reshaped the stromal layer, the LASIK flap is carefully repositioned over the treatment area by the surgeon and checked for the presence of air bubbles, debris, and proper fit on the eye. The flap remains in position by natural adhesion until healing is completed.

Postoperative care


Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are continued in the weeks following surgery. Patients are usually told to sleep much more and are also given a darkened pair of shields to protect their eyes from bright lights and protective goggles to prevent rubbing of the eyes when asleep and to reduce dry eyes. They also are required to moisturize the eyes with preservative-free tears and follow directions for prescription drops. Patients should be adequately informed by their surgeons of the importance of proper post-operative care to minimize the risk of complications.

Higher-order aberrations


Higher-order aberrations are visual problems that cannot be diagnosed using a traditional eye exam, which tests only for acuteness of vision. Severe aberrations can cause significant vision impairment. These aberrations include starbursts, ghosting, halos, double vision, and a number of other post-operative complications.

There have always been concerns about LASIK because of its tendency to induce higher-order aberrations. The advancement of the LASIK technology has helped reduce the risk of clinically significant visual impairment after surgery. There is a correlation between pupil size and aberrations. Effectively, the larger the pupil size, the greater the risk of aberrations. This correlation is the result of the irregularity between the untouched part of the cornea and the reshaped part. Daytime post-lasik vision is optimal, since the pupil is smaller than the LASIK flap. But at night, the pupil may expand such that light passes through the edge of the LASIK flap into the pupil which gives rise to many aberrations, including the appearance of halos surrounding sources of light. There are other currently unknown factors in addition to pupil size that also may lead to higher order aberrations.

In extreme cases in which ideal procedures were not followed by ophthalmologists, and before key advances, some people could suffer debilitating symptoms such as serious loss of contrast sensitivity in poor lighting situations.

Over time, most attention has shifted from other aberrations and centered on spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

. LASIK and PRK tend to induce spherical aberration, because of the tendency of the laser to undercorrect as it moves outward from the center of the treatment zone. This is primarily an issue for major corrections. There are theories that posit that if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would occur. In eyes with few higher order aberrations, wavefront-optimized LASIK (rather than wavefront-guided LASIK) may well be the future.

Higher order aberrations are measured in micrometers (µm) on the wavescan taken during the pre-op examination, while the smallest beam size of lasers approved by the U.S. Food and Drug Administration is about 1000 times larger, at 0.65 mm. Thus imperfections are inherent in the procedure and a reason why patients experience halo, glare, and starburst, even with small naturally-dilated pupils in dim lighting.

Wavefront-guided LASIK


Wavefront-guided LASIK is a variation of LASIK surgery in which, rather than applying a simple correction of focusing power to the cornea (as in traditional LASIK), an ophthalmologist applies a spatially varying correction, guiding the computer-controlled excimer laser with measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the final result still depends on the physician's success at predicting changes that occur during healing. In older patients though, scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 from microscopic particles plays a major role and may outweigh any benefit from wavefront correction. Therefore, patients expecting so-called "super vision" from such procedures may be disappointed. Still, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos," the visual artifact caused by spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

 induced in the eye by earlier methods. Based on their experience, the United States Air Force has described WFG-Lasik as giving "superior vision results".

LASIK surgery results


The surveys determining patient satisfaction with LASIK have found most patients satisfied, with satisfaction range being 92–98 percent.

In March 2008, The American Society of Cataract and Refractive Surgery published a patient satisfaction meta-analysis of over 3,000 peer-reviewed articles from clinical journals around the world. Data from the prior 10 years revealed a 95.4 percent patient satisfaction rate among LASIK patients worldwide.

Safety and efficacy


The reported figures for safety and efficacy are open to interpretation. In 2003, the Medical Defence Union (MDU), the largest insurer for doctors in the United Kingdom, reported a 166 percent increase in claims involving laser eye surgery; however, the MDU averred that some of these claims resulted primarily from patients' unrealistic expectations of LASIK rather than faulty surgery. A 2003 study, reported in the medical journal Ophthalmology, found that nearly 18 percent of treated patients and 12 percent of treated eyes needed retreatment. The authors concluded that higher initial corrections, astigmatism, and older age are risk factors for LASIK retreatment.

In 2004, the British National Health Service
National Health Service
The National Health Service is the shared name of three of the four publicly funded healthcare systems in the United Kingdom. They provide a comprehensive range of health services, the vast majority of which are free at the point of use to residents of the United Kingdom...

's National Institute for Health and Clinical Excellence
National Institute for Health and Clinical Excellence
The National Institute for Health and Clinical Excellence is a special health authority of the English National Health Service , serving both English NHS and the Welsh NHS...

 (NICE) considered a systematic review
Systematic review
A systematic review is a literature review focused on a research question that tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question. Systematic reviews of high-quality randomized controlled trials are crucial to evidence-based medicine...

 of four randomized controlled trials before issuing guidance for the use of LASIK within the NHS. Regarding the procedure's efficacy, NICE reported, "Current evidence on LASIK for the treatment of refractive errors suggests that it is effective in selected patients with mild or moderate short-sightedness," but that "evidence is weaker for its effectiveness in severe short-sightedness and long-sightedness." Regarding the procedure's safety, NICE reported that "there are concerns about the procedure's safety in the long term and current evidence does not appear adequate to support its use within the NHS without special arrangements for consent and for audit or research."

Leading refractive surgeons in the United Kingdom and United States, including at least one author of a study cited in the report, believe NICE relied on information that is severely dated and weakly researched. Revised guidance (IPG164) was issued by NICE in March 2006 which states: "Current evidence suggests that photorefractive (laser) surgery for the correction of refractive errors is safe and efficacious for use in appropriately selected patients."

On October 10, 2006, WebMD
WebMD
WebMD is an American corporation which provides health information services. It was founded in 1996 by Jim Clark and Pavan Nigam as Healthscape, later Healtheon, and then acquired WebMD in 1999 to form Healtheon/WebMD...

 reported that statistical analysis revealed that contact lens wear infection risk is greater than the infection risk from LASIK. Daily contact lens wearers have a 1-in-100 chance of developing a serious, contact lens-related eye infection in 30 years of use, and a 1-in-2,000 chance of suffering significant vision loss as a result of infection. The researchers calculated the risk of significant vision loss consequence of LASIK surgery to be closer to 1-in-10,000 cases.

On February 25, 2010, Morris Waxler, former Food and Drug Administration (FDA) official in charge of approving laser vision correction (LASIK) devices, stated concerns about the risk of serious side-effects from LASIK and the original FDA approval process. His concerns about the safety of LASIK were discussed in an interview on Good Morning America
Good Morning America
Good Morning America is an American morning news and talk show that is broadcast on the ABC television network; it debuted on November 3, 1975. The weekday program airs for two hours; a third hour aired between 2007 and 2008 exclusively on ABC News Now...

.

On January 6, 2011, Waxler requested that "the Commissioner of Food and Drugs to withdraw FDA approval(PMA) for all LASIK devices and issue a Public Health Advisory with a voluntary recall of LASIK devices in an effort to stop the epidemic of permanent eye injury caused by lasers and microkeratomes used for LASIK eye surgery." Waxler alleged that "...the FDA was deprived of knowledge of the full extent of LASIK injuries prior to and during FDA reviews of documents submitted in support of the safety and effectiveness of LASIK devices under 21 CFR 812 and 21 CFR 814. In addition, LASIK manufacturers and their collaborators withheld safety and effectiveness information from their investigational device exemption (IDE) reports to the FDA. In addition, they hid LASIK injuries from FDA within the context of out-of-court settlement of innumerable lawsuits. Clinic-sponsored IDE studies cherry-picked, withheld, and hid data from FDA that clearly showed LASIK with excessive adverse event rates (greater than 1%). These activities were an industry-wide effort, organized wholly or in part by the manufacturers and their collaborators in order to circumvent FDA law and regulation. I will submit CONFIDENTIAL information on these matters separately to FDA’s Office of Criminal Investigation."

Patient dissatisfaction


Some patients with poor outcomes from LASIK surgical procedures report a significantly reduced quality of life because of vision problems or physical pain associated with the surgery. Most experienced and reputable clinics will do a full-dilation medical eye exam prior to surgery and give adequate post-operative patient education care to minimize the risk of a negative outcome. Patients who have suffered LASIK complications have created websites and discussion forums to educate the public about the risks, where prospective and past patients can discuss the surgery. In 1999, Surgical Eyes was founded in New York City by RK patient Ron Link as a resource for patients with complications of LASIK and other refractive surgeries. Surgical Eyes has now been superseded by the Vision Surgery Rehab Network (VSRN). As with Surgical Eyes before it, VSRN recognizes that the vast majority of patients achieve excellent outcomes. As such, neither VSRN nor Surgical Eyes was anti-refractive surgery. No patient advocacy organization has changed its official position in regard to refractive surgery, despite the allegations of Morris Waxler, PhD concerning criminal misconduct during the approvals process.

The FDA website on LASIK states: "Before undergoing a refractive procedure, you should carefully weigh the risks and benefits based on your own personal value system, and try to avoid being influenced by friends that have had the procedure or doctors encouraging you to do so." Consequently, prospective patients still need to fully understand all the potential issues and complications, as satisfaction is directly related to expectation.

The FDA received 140 "negative reports relating to LASIK" for the time period 1998–2006.

Potential complications


The most common complaint from patients due to refractive surgery is the incidence of "dry eyes." According to an American Journal of Ophthalmology study of March 2006, the incidence rate of dry eyes from LASIK after the six month post operative healing period was 36%. The FDA (Food and Drugs Administration) website states that "dry eyes" may be permanent.

The high incidence of dry eyes necessitates a proper preoperative and post operative evaluation and treatment for dry eyes. There are a number of successful treatments for dry eyes including artificial tears, prescription tears and punctal occlusion. Punctal occlusion is accomplished by placing a collagen plug in the natural drain of the eye. Dry eyes, if left untreated, can compromise the visual outcome and result in regression of the effect of LASIK or PRK, or in severe cases result in "chronic dry eye" where permanent chronic pain and visual impairment is a possible outcome. It also must be noted that some incidences of dry eye cannot be successfully mitigated using the above mentioned techniques, so a potential lasik patient must consider that dry eye can be a permanent outcome and untreatable.

The risk for a patient of suffering from disturbing visual side effects such as halos
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...

, double vision
Diplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...

 (ghosting), loss of contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 sensitivity (foggy vision) and glare
Glare (vision)
Glare is difficulty seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night. Because of this, some cars include mirrors with automatic anti-glare functions....

 after LASIK depends on the degree of ametropia before the laser eye surgery and other risk factors. For this reason, it is important to take into account the individual risk potential of a patient and not just the average probability for all patients. Aside from dry eye, the risk of flap dislocation, and other inherent risks, risk assessment involves a comparison between pre-operative and a prediction of post-operative optical aberration, which cannot be measured by a high contrast eye chart
Eye chart
An eye chart is a chart used to measure visual acuity. Types of eye charts include the logMAR chart, Snellen chart, Landolt C, and the Lea test.-Procedure:Charts usually display several rows of optotypes , each row in a different size...

, due to the physiology of visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....

. Post-operative optical aberration and light scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 are surgically created by both the corneal flap and laser ablation
Laser ablation
Laser ablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma...

 in the corneal stroma.

The following are some of the more frequently reported complications of LASIK:
  • Surgery induced dry eyes
  • Overcorrection or undercorrection
  • Vitamin D deficiency from Sun sensitivity
  • Visual acuity
    Visual acuity
    Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....

     fluctuation
  • Halos or starbursts around light sources at night
  • Light sensitivity
  • Ghost images or double vision
    Diplopia
    Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...

  • Wrinkles in flap (striae)
  • Decentered ablation
  • Debris or growth under flap
  • Thin or buttonhole flap
  • Induced astigmatism
    Astigmatism (eye)
    Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

  • Corneal Ectasia
  • Floaters
  • Epithelium erosion
  • Posterior vitreous detachment
    Posterior vitreous detachment
    A posterior vitreous detachment is a condition of the eye in which the vitreous humour separates from the retina.Broadly speaking, the condition is common for older adults and over 75% of those over the age of 65 develop it. Although less common among people in their 40s or 50s, the condition is...

  • Macular hole
    Macula
    The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...

    .


Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources:

Intraoperative complications

  • The incidence of flap complications has been estimated to be 0.244%. Flap complications (such as displaced flaps or folds in the flaps that necessitate repositioning, diffuse lamellar keratitis, and epithelial ingrowth) are common in lamellar corneal surgeries but rarely lead to permanent visual acuity loss; the incidence of these microkeratome-related complications decreases with increased physician experience. According to proponents of such techniques, this risk is further reduced by the use of IntraLasik
    IntraLASIK
    IntraLASIK, also known as Femto-LASIK or All-Laser LASIK, is a form of refractive eye surgery similar to LASIK that creates a corneal flap with a femtosecond laser microkeratome rather than with a mechanical microkeratome, which uses a steel blade. The only difference between LASIK and IntraLASIK...

     and other non-microkeratome related approaches, although this is not proven and carries its own set of risks of complications from the IntraLasik
    IntraLASIK
    IntraLASIK, also known as Femto-LASIK or All-Laser LASIK, is a form of refractive eye surgery similar to LASIK that creates a corneal flap with a femtosecond laser microkeratome rather than with a mechanical microkeratome, which uses a steel blade. The only difference between LASIK and IntraLASIK...

     procedure.
  • A slipped flap (a corneal flap that detaches from the rest of the cornea) is one of the most common complications. The chances of this are greatest immediately after surgery, so patients typically are advised to go home and sleep to let the flap heal. Patients are usually given sleep goggles or eye shields to wear for several nights to prevent them from dislodging the flap in their sleep. A faster operation may decrease the chance of this complication, as there is less time for the flap to dry.
  • Flap interface particles are another finding whose clinical significance is undetermined. A Finnish study found that particles of various sizes and reflectivity were clinically visible in 38.7% of eyes examined via slit lamp biomicroscopy
    Slit lamp
    The slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope...

    , but apparent in 100% of eyes using confocal microscopy
    Confocal microscopy
    Confocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...

    .

Early postoperative complications

  • The incidence of dry eye varies widely from research studies. A study by Hovanesian et al. reported that 48% of patients experienced dry eye symptoms at 6 months period post surgery.
  • The incidence of diffuse lamellar keratitis (DLK), also known as the Sands of Sahara syndrome, has been estimated at 2.3%. DLK is an inflammatory process that involves an accumulation of white blood cells at the interface between the LASIK flap and the underlying corneal stroma. It is most commonly treated with steroid eye drops, and sometimes it is necessary for the eye surgeon to lift the flap and manually remove the accumulated cells.
  • The incidence of infection responsive to treatment has been estimated at 0.4%. Infection
    Infection
    An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

     under the corneal flap is possible. It is also possible that a patient has the genetic condition keratoconus
    Keratoconus
    Keratoconus , is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve....

     that causes the cornea to thin after surgery. Although this condition is screened in the preoperative exam, it is possible in rare cases (about 1 in 5,000) for the condition to remain dormant until later in life (the mid-40s). If this occurs, the patient may need rigid gas permeable contact lenses, Intrastromal Corneal Ring Segments (Intacs), Corneal Collagen Crosslinking with Riboflavin or a corneal transplant.
  • The incidence of persistent dry eye has been estimated to be as high as 28% in Asian eyes and 5% in Caucasian eyes. Nerve fibers in the cornea are important for stimulating tear production. A year after LASIK, subbasal nerve fiber bundles remain reduced by more than half. Some patients experience reactive tearing, in part to compensate for chronic decreased basal wetting tear production.
  • The incidence of subconjunctival hemorrhage has been estimated at 10.5% (according to a study undertaken in China; thus results may not be generally applicable due to racial and geographic factors).

Late postoperative complications

  • The incidence of epithelial ingrowth has been estimated at 0.1%.
  • Glare is another commonly reported complication of those who have had LASIK.
  • Halos or starbursts around bright lights at night are caused by the irregularity between the lasered part and the untouched part. It is not practical to perform the surgery so that it covers the width of the pupil at full dilation at night, and the pupil may expand so that light passes through the edge of the flap into the pupil. In daytime, the pupil is smaller than the edge. Modern equipment is better suited to treat those with large pupils, and responsible physicians will check for them during examination.
  • Late traumatic flap dislocations have been reported 1–7 years post-LASIK.
  • Dry eye or in severe cases Chronic Dry eye. Due to nerves that are severed during the Lasik operation (around 70% of corneal nerves are severed), the lubrication system of the eye is affected and nerves may never recover to pre-operative condition. This may leave the patient with potential permanent dry eyes.

Other


LASIK and other forms of laser refractive surgery (i.e. PRK, LASEK and Epi-LASEK) change the dynamics of the cornea. These changes make it difficult for your optometrist and ophthalmologist to accurately measure your intraocular pressure, essential in glaucoma
Glaucoma
Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

 screening and treatment. The changes also affect the calculations used to select the correct intraocular lens implant when you have cataract surgery. This is known to ophthalmologists as "refractive surprise." The correct intraocular pressure and intraocular lens power can be calculated if you can provide your eye care professional with your preoperative, operative and postoperative eye measurements.

Although there have been improvements in LASIK technology, a large body of conclusive evidence on the chances of long-term complications is not yet established. Also, there is a small chance of complications, such as haziness, halo, or glare, some of which may be irreversible because the LASIK eye surgery procedure is irreversible.

The incidence of macular hole has been estimated at 0.2 percent to 0.3 percent. The incidence of retinal detachment
Retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blindness. It is a medical emergency.The retina is a...

 has been estimated at 0.36 percent. The incidence of choroidal neovascularization
Choroidal neovascularization
Choroidal neovascularization is the creation of new blood vessels in the choroid layer of the eye. This is a common symptom of the degenerative maculopathy wet AMD .-Causes:...

 has been estimated at 0.33 percent. The incidence of uveitis
Uveitis
Uveitis specifically refers to inflammation of the middle layer of the eye, termed the "uvea" but in common usage may refer to any inflammatory process involving the interior of the eye....

 has been estimated at 0.18 percent.

Although the cornea usually is thinner after LASIK, because of the removal of part of the stroma, refractive surgeons strive to maintain the maximum thickness to avoid structurally weakening the cornea. Decreased atmospheric pressure at higher altitudes has not been demonstrated as extremely dangerous to the eyes of LASIK patients. However, some mountain climbers have experienced a myopic shift at extreme altitudes.

In situ keratomileusis effected at a later age increases the incidence of corneal higher-order wavefront aberrations. Conventional eyeglasses do not correct higher order aberrations.

Microfolding has been reported as "an almost unavoidable complication of LASIK" whose "clinical significance appears negligible."

Blepharitis
Blepharitis
Blepharitis is an ocular condition characterized by chronic inflammation of the eyelid, the severity and time course of which can vary. Onset can be acute, resolving without treatment within 2–4 weeks , but more generally is a long standing inflammation varying in severity...

, or inflammation of the eyelids with crusting of the eyelashes, may increase the risk of infection or inflammation of the cornea after LASIK.

Myopic (nearsighted) people who are close to the age (mid- to late-forties) when they will require either reading glasses or bifocal eyeglasses may find that they still require reading glasses despite having undergone refractive LASIK surgery. Myopic people generally require reading glasses or bifocal eyeglasses at a later age than people who are emmetropic (those who see without eyeglasses), but this benefit may be lost if they undergo LASIK. This is not a complication but an expected result of the physical laws of optics. Although there is currently no method to completely eradicate the need for reading glasses in this group, it may be minimized by performing a variation of the LASIK procedure called "slight monovision." In this procedure, which is performed exactly like distance-vision-correction LASIK, the dominant eye is set for distance vision, while the non-dominant eye is set to the prescription of the patient's reading glasses. This allows the patient to achieve a similar effect as wearing bifocals. The majority of patients tolerate this procedure very well and do not notice any shift between near and distance viewing, although a small portion of the population has trouble adjusting to the monovision effect. This can be tested for several days prior to surgery by wearing contact lenses that mimic the monovision effect. Recently, a variation of the laser ablation pattern called PresbyLASIK
PresbyLASIK
PresbyLASIK is a surgical technique for presbyopic visual correction using Excimer LASER ablation.PresbyLASIK treatment uses the principles of LASIK surgery to create a multifocal corneal surface aimed at reducing near vision spectacle dependence in presbyopic patients...

, has been developed to reduce or eliminate dependence on reading glasses while retaining distance vision

There are reports of decrease in the number of corneal keratocyte
Corneal keratocyte
Corneal keratocytes are specialized fibroblasts residing in the stroma. This corneal layer, representing about 85-90% of corneal thickness, is built up from highly regular collagenous lamellae and extracellular matrix components. Keratocytes play the major role in keeping it transparent, healing...

s (fibroblast
Fibroblast
A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...

s) after LASIK.

Factors affecting surgery


Typically, the cornea is avascular because it must be transparent to function normally, and its cells absorb oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 from the tear film
Tears
Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of tears....

. Thus, low-oxygen-permeable contact lenses reduce the cornea's oxygen absorption, sometimes resulting in corneal neovascularization
Corneal neovascularization
Corneal neovascularization is the excessive ingrowth of blood vessels from the limbal vascular plexus into the cornea, caused by a low reception of oxygen, which is generally not received from the bloodstream, but through the air. One of the most common causes is contact lens wear, and to a...

—the growth of blood vessels into the cornea. This causes a slight lengthening of inflammation duration and healing time and some pain during surgery, because of greater bleeding.

Although some contact lenses (notably modern RGP and soft silicone hydrogel lenses) are made of materials with greater oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are warned to avoid over-wearing their contact lenses. Usually, it is recommended that they discontinue wearing contact lenses days or weeks before the LASIK eye surgery.

Age considerations


New advances in eyesight corrective surgery are providing consumers greater choices. Patients in their 40s or 50s who are considering LASIK surgery to improve their vision might want to consider to be evaluated for implantable lenses as well. "Early signs of a cataract might argue for surgery and implantation of multifocal lenses instead."

The FDA has approved LASIK for age 18 and over. More importantly the person's eye prescription needs to be stable for at least one year prior to surgery.

See also

  • Automated lamellar keratoplasty
    Automated lamellar keratoplasty
    Automated Lamellar Keratoplasty, commonly abbreviated to ALK uses a device called a microkeratome to separate a thin layer of the cornea and create a flap. The flap is then folded back, and the microkeratome removes a thin disc of corneal stroma below. The thickness and diameter of this disc...

  • Bates method
    Bates Method
    The Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...

  • Femtosecond laser intrastromal vision correction
  • Refractive error
    Refractive error
    A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...

  • Wavefront
    Wavefront
    In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...


External links