lasik lasik lasik or lasik (laser-assisted in situ keratomileusis), commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and an actual cure for astigmatism, since it is in the cornea. lasik surgery is performed by an ophthalmologist who uses a laser or microkeratome to reshape the eye's cornea in order to improve visual acuity. for most people, lasik provides a long-lasting alternative to eyeglasses or contact lenses. for the drug used for hypertension, see lasix. for the slovakian footballer, see richard lásik. lasiklasik surgery using an excimer laser at us national naval medical center bethesdaspecialtyophthalmology, optometryicd-9-cm11.71meshd020731medlineplus007018 lasik is very similar to another surgical corrective procedure, photorefractive keratectomy (prk), and lasek. all represent advances over radial keratotomy in the surgical treatment of refractive errors of vision. for patients with moderate to high myopia or thin corneas which cannot be treated with lasik and prk, the phakic intraocular lens is an alternative. as of 2018, roughly 9.5 million americans have had lasik and, globally, between 1991 and 2016, more than 40 million procedures were performed. however, the procedure seemed to be a declining option as of 2015. process the planning and analysis of corneal reshaping techniques such as lasik have been standardized by the american national standards institute, an approach based on the alpins method of astigmatism analysis. 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." the procedure involves creating a thin flap on the eye, folding it to enable remodeling of the tissue beneath with a laser and repositioning the flap. contact lenses patients wearing soft contact lenses are 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. the cornea is avascular because it must be transparent to function normally. its cells absorb 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. pre-operative examination and education in the united states, the fda has approved lasik for age 18 or 22 and over because the vision has to stabilize. more importantly the patient's eye prescription should be stable for at least one year prior to surgery. the patient may be examined with pupillary dilation and education given prior to the procedure. before the surgery, the patient's corneas are examined with a pachymeter to determine their thickness, and with a topographer, or corneal topography machine, to measure their surface contour. using low-power lasers, a topographer creates a topographic map of the cornea. the procedure is contraindicated if the topographer finds difficulties such as keratoconus the preparatory process also detects astigmatism and other irregularities in the shape of the cornea. using this information, the surgeon calculates the amount and the location of corneal tissue to be removed. the patient is prescribed and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure and is sometimes offered a short acting oral sedative medication as a pre-medication. prior to the procedure, anaesthetic eye drops are instilled. factors that may rule out lasik for some patients include large pupils, thin corneas and extremely dry eyes. flap creation flap creation with femtosecond laser a soft 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 causes a transient dimming of vision in the treated eye. once the eye is immobilized, a flap is created by cutting through the corneal epithelium and bowman's layer. this process is achieved with a mechanical microkeratome using a metal blade, or a femtosecond laser 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 uses an excimer laser (193 nm) to remodel the corneal stroma. the laser vaporizes the 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 micrometers thick. performing the laser ablation in the deeper corneal stroma provides for more rapid visual recovery and less pain than the earlier technique, photorefractive keratectomy (prk). during the second step, the patient's vision becomes 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. the excimer laser uses 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 the 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 told to rest and are given dark eyeglasses to protect their eyes from bright lights and occasionally 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. occasionally after the procedure a bandage contact lens is placed to aid the healing, and typically removed after 3–4 days. patients should be adequately informed by their surgeons of the importance of proper post-operative care to minimize the risk of complications. wavefront-guided wavefront-guided lasik is a variation of lasik surgery in which, rather than applying a simple correction of only long/short-sightedness and astigmatism (only lower order aberrations 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 result still depends on the physician's success at predicting changes that occur during healing and other factors that may have to do with the regularity/irregularity of the cornea and the axis of any residual astigmatism. another important factor is whether the excimer laser can correctly register eye position in 3 dimensions, and to track the eye in all the possible directions of eye movement. if a wavefront guided treatment is performed with less than perfect registration and tracking, pre-existing aberrations can be worsened. in older patients, scattering from microscopic particles (cataract or incipient cataract) may play a role that outweighs any benefit from wavefront correction. when treating a patient with preexisting astigmatism, most wavefront-guided lasik lasers are designed to treat regular astigmatism as determined externally by corneal topography. in patients who have an element of internally induced astigmatism, therefore, the wavefront-guided astigmatism correction may leave regular astigmatism behind (a cross-cylinder effect). if the patient has preexisting irregular astigmatism, wavefront-guided approaches may leave both regular and irregular astigmatism behind. this can result in less-than-optimal visual acuity compared with a wavefront-guided approach combined with vector planning, as shown in a 2008 study. thus, vector-planning offers a better alignment between corneal astigmatism and laser treatment, and leaves less regular astigmatism behind on the cornea, which is advantageous whether irregular astigmatism coexists or not. the "leftover" astigmatism after a purely surface-guided laser correction can be calculated beforehand, and is called ocular residual astigmatism (ora). ora is a calculation of astigmatism due to the noncorneal surface (internal) optics. the purely refraction-based approach represented by wavefront analysis actually conflicts with corneal surgical experience developed over many years. the pathway to "super vision" thus may require a more customized approach to corneal astigmatism than is usually attempted, and any remaining astigmatism ought to be regular (as opposed to irregular), which are both fundamental principles of vector planning overlooked by a purely wavefront-guided treatment plan. this was confirmed by the 2008 study mentioned above, which found a greater reduction in corneal astigmatism and better visual outcomes under mesopic conditions using wavefront technology combined with vector analysis than using wavefront technology alone, and also found equivalent higher-order aberrations (see below). vector planning also proved advantageous in patients with keratoconus. no good data can be found that compare the percentage of lasik procedures that employ wavefront guidance versus the percentage that do not, nor the percentage of refractive surgeons who have a preference one way or the other. wavefront technology continues to be positioned as an "advance" in lasik with putative advantages; however, it is clear that not all lasik procedures are performed with wavefront guidance. 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 induced in the eye by earlier methods. a meta-analysis of eight trials showed a lower incidence of these higher order aberrations in patients who had wavefront-guided lasik compared to non-wavefront-guided lasik. based on their experience, the united states air force has described wfg-lasik as giving "superior vision results". topography-assisted topography-assisted lasik is intended to be an advancement in precision and reduce night-vision side effects. the first topography-assisted device received fda approval september 13, 2013. history barraquer's early work in the 1950s, the microkeratome and keratomileusis technique were developed in bogotá, colombia, by the spanish ophthalmologist jose barraquer. in his clinic, he would cut thin (one hundredth of a mm thick) flaps in the cornea to alter its shape. barraquer also investigated how much of the cornea had to be left unaltered in order to provide stable long-term results. this work was followed by that of the russian scientist, svyatoslav fyodorov, who developed radial keratotomy (rk) in the 1970s and designed the first posterior chamber implantable contact lenses (phakic intraocular lens) in the 1980s. laser refractive surgery in 1980, rangaswamy srinivasan, samuel e. blum and james j. wynne at the ibm research laboratory, discovered that an ultraviolet excimer laser could etch living tissue, with precision and with no thermal damage to the surrounding area. the phenomenon was termed "ablative photo-decomposition" (apd). five years later, in 1985, steven trokel at the edward s. harkness eye institute, columbia university in new york city, published his work using the excimer laser in radial keratotomy. he wrote, "the central corneal flattening obtained by radial diamond knife incisions has been duplicated by radial laser incisions in 18 enucleated human eyes. the incisions, made by 193 nm far-ultraviolet light radiation emitted by the excimer laser, produced corneal flattening ranging from 0.12 to 5.35 diopters. both the depth of the corneal incisions and the degree of central corneal flattening correlated with the laser energy applied. histopathology revealed the remarkably smooth edges of the laser incisions." together with his colleagues, charles munnerlyn and terry clapham, trokel founded visx usa inc. marguerite b. macdonald md performed the first human visx refractive laser eye surgery in 1989. patent a number of patents have been issued for several techniques related to lasik. rangaswamy srinivasan and james wynne filed a patent application on the ultraviolet excimer laser, in 1986, issued in