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Refractive Eye Surgery is any eye surgery used to improve the refractive state of the eye and decrease the need for glasses. The most common methods today use lasers to reshape the cornea.

1 Techniques

• RK uses spoke-shaped incisions (usually made with a diamond knife) to alter the shape of the cornea and reduce myopia; this technique has now been largely superseded by other methods.

• PRK is an outpatient procedure generally performed with local anesthetic eye drops. It is a type of refractive surgery which reshapes the cornea by destroying microscopic amounts of tissue from the outer surface with a cool, computer-controlled ultraviolet beam of light (an excimer laser).

• LASIK is a more difficult procedure than PRK. It is performed for all degrees of nearsightedness. The surgeon uses a knife called a microkeratome to cut a flap of corneal tissue, opens the flap like a hinged door, removes the targeted tissue in the corneal stroma beneath it with the excimer laser, and then replaces the flap. Some variations don't use a microkeratome but cut the flap with a laser.

• LASEK is a procedure that permanently changes the shape of the cornea using an excimer laser to ablate a small amount of tissue from the front of the eye, just under the eye's skin or epithelium which is kept and replaced to act as a natural bandage.

• Thermal Keratoplasty is used to correct hyperopia by putting a ring of 8 or 16 small burns surrounding the pupil, and steepen the cornea with a ring of collagen contriction.

• Laser Thermal Keratoplasty (LTK) is a no-touch Thermal Keratoplasty performed with a Holmium Laser while Conductive Keratoplasty (CK) is Thermal Keratoplasty performed with a high-frequency electric probe. Thermal keratoplasty can also be used to improve presbyopia or reading vision after age 40.

• Lens implants can also be used inside the eye to change refractive error. Currently all refractive implants are under investigation by the Food and Drug Administration.

2 Risks

While refractive surgery is becoming more affordable and safe, it is not for everybody. People who are slow healers or who have ongoing medical conditions such as glaucoma or diabetes, uncontrolled vascular disease, autoimmune disease, pregnant women or people with certain eye diseases involving the cornea or retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu, are not good candidates for refractive surgery. Furthermore, some people's eye shape may not permit effective refractive surgery without removing dangerous amounts of corneal tissue. It is important that those considering laser eye surgery have a full examination. Unfortunately, since some surgeons eager to find business may accept patients unsuited to such surgery, prospective patients should choose their surgeon with care.

Other risks even for healthy people may be:

• InfectionInfection" is also the title of an episode of the television series Babylon 5; see Infection (Babylon 5). An infection is the detrimental colonization of a host organism by a foreign species. The colonizing organism interferes with the normal functioning and delayed healing: There is a less than 0.1 percent chance of the cornea becoming infected after PRK, and a somewhat smaller chance after LASIK. This is uncomfortable, but has no long-term effects after a period of four years.

• Undercorrection/Overcorrection: Predicting perfectly how your eye will respond to laser surgery is not yet possible. Therefore, you may still need corrective lenses after the procedure to obtain good vision. In some cases, a second procedure can be done to improve the result.

• Decrease in Best-Corrected Vision: After refractive surgery, a few patients find that their best obtainable vision with corrective lenses is worse than it was before the surgery. This may happen as a result of irregular tissue removal or the development of corneal haze.

• Excessive Corneal Haze: Corneal haze occurs as part of the normal healing process after PRK. In all but a few cases, it has no effect on the final vision and can only be seen by an eye doctor with a microscope. However, there are some cases of excessive haze that interferes with vision. As with undercorrections, this can often be dealt with by means of an additional laser treatment. The risk of significant haze is much less with LASIK than with PRK.

• Regression: In some patients the effect of refractive surgery is gradually lost over several months. This is like an undercorrection, and a re-treatment is often feasible. But, usually results are permanent.

• Halo Effect: The halo effect is an optical effect that is noticed in dim light. When the pupil enlarges to adapt to the dimmer light, a second faded image is produced by the untreated peripheral cornea. For some patients who have undergone PRK or LASIK, this can interfere with night driving.

• Flap Damage or Loss (LASIK only): Instead of creating a hinged flap of tissue on the central cornea, the entire flap could come loose. If this were to occur it could be replaced after the laser treatment. However, there is a risk that the flap could be damaged or lost.

• Distorted Flap (LASIK only): Irregular healing of the corneal flap could create a distorted corneal shape, which would decrease the best-corrected vision.

• Dry eye: Feeling of dryness, soreness, and discomfort in the eye.

• Altitude effects: Some refractive surgery patients have reported significant changes in vision with changes of altitude (perhaps because oxygen concentration can affect corneal swelling). A patient who achieves good vision at sea level may have poorer results in the mountains.

• Incomplete Procedure: Equipment malfunction may require the procedure to be stopped before completion. This is a more important factor in LASIK, due to its higher degree of complexity, than in PRK.

According to CRSQA (an industry body concerned with quality control of ocular surgery), a competent refractive surgeon will typically achieve results at the following levels:

• Around 90% of patients will receive 20/40 or better uncorrected visual acuityVisual acuity is the eye's ability to detect fine details and is the quantitative measure of the eye's ability to see an in-focus image at a certain distance. The standard definition of normal visual acuity (20/20 or 6/6 vision) is the ability to resolve (and, thus, 10% will not).
• Around 50% will achieve 20/20 or better (and 50% will not); patients with high myopiaNormal vision for a achromatopsic colour-blind person. Courtesy NIH National Eye Institute The same view when achromatopsic and myopic. Myopia also known as nearsightedness or short-sightedness is a refractive defect of the eye, where the person affected, hyperopia, or astigmatism, have poorer chances of achieving 20/20.
• Around 10% of patients will need retreatment
• "Less than 3%" of patients will have unresolved complications six months after surgery.