LASIK is the acronym for Laser-Assisted in Situ Keratomileusis, a type of refractive laser eye surgery performed by ophthalmologists for correcting myopia, hyperopia, and astigmatism. The procedure is generally preferred to photorefractive keratectomy, PRK, (also called ASA, Advanced Surface Ablation) because it requires less time for the patient's recovery, and the patient feels less pain, overall; however, there are instances where PRK/ASA is medically indicated as a better alternative to LASIK. Many patients choose LASIK as an alternative to wearing corrective eyeglasses or contact lenses.
The LASIK technique was made possible by the Colombian-based Spanish ophthalmologist Jose Barraquer, who, around 1950 in his clinic in Bogotá, Colombia, developed the first microkeratome, used to cut thin flaps in the cornea and alter its shape, in a procedure called keratomileusis. He also provided the knowledge about how much of the cornea had to be left unaltered to provide a stable long-term result.
Later technical and procedural developments included the RK (radial keratotomy) started in the 70’s in Russia by Svyatoslav Fyodorov and the development of PRK (photorefractive keratectomy) in the 80’s in Germany by Theo Seiler.
In 1968, at the Northrup Corporation Research and Technology Center of the University of California, Mani Lal Bhaumik and a group of other scientists, while working on the development of a carbon-dioxide laser, would develop the Excimer laser, where molecules that do not normally exist come into being when xenon, argon or krypton gases are excited. This would form the cornerstone for LASIK eye surgery. Dr. Bhaumik announced his discovery in May of 1973 at a meeting of the Denver Optical Society of America in Denver, Colorado. He would later patent it. 
The introduction of Laser in this refractive procedure started with the developments in Laser technology by Rangaswamy Srinivasan. In 1980, Srinivasan, working at IBM Research 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).. Dr. Stephen Trokel published a paper in the American Journal of Ophthalmology in 1983, outlining the potential of using the excimer laser in refractive surgeries.
Using these advances in laser technology and the technical and theoretical developments in refractive surgery made since the 50's, LASIK surgery was developed in 1990 by Lucio Buratto (Italy) and Ioannis Pallikaris (Greece) as a melding of two prior techniques, keratomileusis and photorefractive keratectomy. It quickly became popular because of its greater precision and lower frequency of complications in comparison with these former two techniques. Today, faster lasers, larger spot areas, bladeless flap incision, and wavefront-optimized and -guided techniques have significantly improved the reliability of the procedure as 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 all-femtosecond correction (Femtosecond Lenticule EXtraction, FLIVC), LASEK, Epi-LASIK, sub-Bowman’s Keratomileusis aka thin-flap LASIK, wavefront-guided PRK, and modern intraocular lenses.
There are several necessary preparations in the preoperative period. The operation itself is made by creating a thin flap on the eye, folding it to enable remodeling of the tissue underneath with laser. The flap is repositioned and the eye is left to heal in the postoperative period.
Patients wearing soft contact lenses typically are instructed to stop wearing them approximately 5 to 7 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 had been worn.  Before the surgery, the patient's corneas are examined with a pachymeter to determine their thickness, and with a topographer to measure their surface contour. Using low-power lasers, a topographer creates a topographic map of the cornea. This process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and locations of corneal tissue to be removed during the operation. The patient typically is prescribed an antibiotic to start taking beforehand, to minimize the risk of infection after the procedure.
The operation is performed with the patient awake and mobile; however, the patient typically is given a mild sedative (such as Valium) and anesthetic eye drops.
LASIK is performed in two steps. The first step is to create a flap of corneal tissue. The second step is remodeling of the cornea underneath the flap with laser. Finally, the flap is repositioned.
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 into the white (sclera) 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 using a metal blade, or a femtosecond laser microkeratome (procedure known as IntraLASIK) 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 be uncomfortable.
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 adjacent stroma. No burning with heat or actual cutting is required to ablate the tissue. The layers of tissue removed are tens of micrometers 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 (PRK).
During the second step, the patient's vision will become very blurry once the flap is lifted. He/she will be able to see only white light surrounding the orange light of the laser. This can be disorienting.
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. The average power for each pulse is usually in the milliwatt range  Typically, each pulse is on the order of 10–20 nanoseconds.
Reposition 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.
Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are discontinued in the weeks following surgery. Patients are also given a darkened pair of goggles to protect their eyes from bright lights and protective shields to prevent rubbing of the eyes when asleep. Patients should be adequately informed by their surgeons of the importance of proper post-operative care to minimize the risk of post-surgical complications.
Higher-order aberrations are visual problems not captured in a traditional eye exam which tests only for acuteness of vision. Severe aberrations can effectively cause significant vision impairment. These aberrations include starbursts, ghosting, halos, double vision, and a number of other post-operative complications listed below.
Concern has long plagued the tendency of refractive surgeries to induce higher-order aberration not correctable by traditional contacts or glasses. The advancement of LASIK technique and technologies has helped reduce the risk of clinically significant visual impairment after the surgery. One of the major discoveries was the 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. There are other currently unknown factors in addition to pupil size that also affect higher order aberrations.
In extreme cases, where ideal technique was not followed and before key advances, some people could suffer rather debilitating symptoms including serious loss of contrast sensitivity in poor lighting situations.
Over time, most of the attention has been focused on spherical aberration. 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 really a significant issue for only large corrections. There is some thought if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would be induced. Hence, in eyes with little existing higher order aberrations, "wavefront optimized" lasik rather than wavefront guided LASIK may well be the future.
In any case, higher order aberrations are measured in µm (micrometers) on the wavescan taken during the pre-op examination, while the smallest beam size of FDA approved lasers 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 a small naturally dilated pupils in dim lighting.
Wavefront-guided LASIK is a variation of LASIK surgery where, rather than apply 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 which occur during healing. In older patients though, scattering from microscopic particles plays a major role and may exceed any benefit from wavefront correction. Hence, patients expecting so-called "super vision" from such procedures may be disappointed. However, while unproven, 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 induced in the eye by earlier methods.
The incidence of refractive surgery patients having unresolved complications six months after surgery has been estimated from 3% to 6%. The risk for a patient of suffering from disturbing visual side effects like halos, double vision (ghosting), loss of contrast sensitivity (foggy vision) and glare 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. The following are some of the more frequently reported complications of LASIK:
- Surgery induced dry eyes
- Overcorrection or undercorrection
- Visual acuity fluctuation
- Halos or starbursts around light sources at night
- Light sensitivity
- Ghost images or double vision
- Wrinkles in flap (striae)
- Decentered ablation
- Debris or growth under flap
- Thin or buttonhole flap 
- Induced astigmatism
- Corneal Ectasia
- Epithelium erosion
- Posterior vitreous detachment
- Macular hole
Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources:
- 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 and other non-microkeratome related approaches, although this is not proven and carries its own set of risks of complications from the 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. "Traditional" LASIK is the fastest form of the procedure, as IntraLasik requires the flap to be left flipped over for up to forty minutes to allow the surgeon to clear the haze over the cornea created by the laser microkeratome during the procedure.
- 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, but apparent in 100% of eyes using confocal microscopy.
Early postoperative complications
- The incidence of diffuse lamellar keratitis (DLK), also known as the Sands of Sahara syndrome, has been estimated at 2.3%. When diagnosed and appropriately treated, DLK resolves with no lasting vision limitation.
- The incidence of infection responsive to treatment has been estimated at 0.4%. Infection under the corneal flap is possible. It is also possible that a patient has the genetic condition keratoconus 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.
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 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 a "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 per cent to 0.3 per cent. The incidence of retinal detachment has been estimated at 0.36 per cent. The incidence of choroidal neovascularization has been estimated at 0.33 per cent. The incidence of uveitis has been estimated at 0.18 per cent
Although the cornea usually is thinner after LASIK, because of the removal of part of the stroma, refractive surgeons strive to maintain a minimum 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. There are no published reports documenting scuba diving-related complications after LASIK.
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".
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 is 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.
Factors affecting surgery
Typically, the cornea is avascular, because it must be transparent to function normally, its cells absorbing oxygen from the tear film. Thus, low oxygen-permeable contact lenses reduce the cornea's oxygen absorption, sometimes resulting in corneal neovascularization—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.
A 2004 Wake Forest University study established that heat and humidity affect LASIK surgery results, both during the procedure and in the two weeks before the surgery.
The surveys determining patient satisfaction with LASIK, have found most patients satisfied, with satisfaction range being 92–98 percent.
Some patients with poor outcomes from LASIK surgical procedures report a significantly reduced quality of life because of vision problems. Patients who have suffered LASIK complications have published websites to educate the public about the risks, and discussion forums, where prospective and past patients can discuss the surgery. Frequently, negative outcomes are a result of the patient being a poor LASIK candidate, either from an underlying medical condition or from non-compliancy of care following the procedure. An experienced and reputable clinic will do a full dilated medical eye exam prior to surgery and give adequate patient education about post-operative care to minimize the risk of a negative outcome.
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 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's National Institute for Health and Clinical Excellence (NICE) considered a systematic review 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.
On October 10, 2006, 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 21, 2007, the Food and Drug Administration (FDA) issued a Class I recall of the LADAR-6000 surgical laser, manufactured by Alcon.  The recall was because the algorithm used to calculate the laser treatment left some patients with inaccurate surgical outcomes that could not be re-treated with additional surgery.