Refractive surgery
Refractive eye surgery is any eye surgery used to improve the refractive state of the eye and decrease dependency on glasses or contact lenses. This can include various methods of surgical remodelling of the cornea or cataract surgery. The most common methods today use excimer lasers to reshape curvature of the cornea. Successful refractive eye surgery can help to reduce common vision disorders such as myopia, hyperopia and astigmatism.
According to surveys of members of the American Society of Cataract and Refractive Surgery, approximately 948,266 refractive surgery procedures were performed in the United States during 2004 and 928,737 in 2005.

# History
The first experimental studies about refractive surgery were developed by Lendeer Jans Lans, an ophthalmology teacher in Holland,  1896 where he published a theoretic work proposing penetrating corneal cuts to correct astigmatism.   In 1930 the Japanese ophthalmologist SatoTemplate:Specify made the first practical attempt to perform such surgery in military pilots.  He practiced radial cuts in the cornea to correct effects up to 6 diopters, but this procedure was soon rejected by the medical community because the high rate of corneal degeneration.   
In 1963, in the Barraquer ophthalmologic clinic (Bogota,Colombia)  Ignacio Barraquer developed the first proficient technique to refractive surgery, called keratomileusis (from the Greek Kerato: cornea and Mileusis: to sculpt) meaning corneal reshaping.  Keratomileusis allowed to correct not only myopia but also hyperopia.  The early surgeries were made removing a corneal layer, freezing it so it could be manually sculpted in the required shape and finally reimplant the layer (Keratomileusis with freezing).   In 1986 Dr Swinger improved the surgery (keratomileusis without freezing) but it was still a slightly imprecise technique.   
In 1958 Arthur Schawlow and Townes, from Bell laboratories published their theory of stimuled emission of shorter length waves, included light, which gave place to the development of  Laser.  In 1975 experiments with laser using a mix of argon and fluor ended with the invention of the Excimer.   This Laser was used with industrial purposes.  In 1980, R. Srinivasan, a scientist of  IBM who was using the Excimer to make microscopic circuits in microchips for informatic quipments, discovered that the Excimer could be used also  to cut organic tissues with high accuracy without significant thermal damage.  In 1983 Stephen Trokel, scientist of Columbia University in collaboration with Srinivasan performed the Photorefractive Keratectomy (PRK) or keratomileusis in situ (without separation of corneal layer) which was more technically exact, but the patients reported it to be very uncomfortable.  Also a delay in the healing was observed.  The first PRK was performed in Germany.  
In 1991 the Creta University and the Vardinoyannion Eye developed Lasik  that worked with the same principles, but removing first a lens shaped piece of the outer layer of the cornea, sculpting the inner layer and relocating the piece to cover, which allows a faster healing, without discomfort.  However, there exists debate over the stability of the healing with the corneal flap.  Current PRK  
procedures involve the removal of the corneal layer with an alcohol based solution for the corrective procedure, and then allowing the layer to regenerate.  However, this is somewhat painful for the patient following the procedure and takes longer for visual acuity to stabilize.

# Techniques

## Flap procedures
Excimer laser ablation is done under a partial-thickness lamellar corneal flap.
- Automated lamellar keratoplasty (ALK): The surgeon uses an instrument called a microkeratome to cut a thin flap of the corneal tissue.  The flap is lifted like a hinged door, targeted tissue is removed from the corneal stroma, again with the microkeratome, and then the flap is replaced.
- Laser Assisted In-Situ Keratomileusis (LASIK): The surgeon uses a microkeratome to cut a flap of the corneal tissue (usually with a thickness of 100-180 microns).  The flap is lifted like a hinged door, but in contrast to ALK, the targeted tissue is removed from the corneal stroma with an excimer laser.  The flap is subsequently replaced.  Another  method of creating this flap is by using a procedure called IntraLasik, in which a femtosecond laser is used to create the flap. Proponents of this method tout its superiority over "traditional" LASIK, but there are no conclusive independent studies to prove that this is a true statement.

## Surface procedures
The excimer laser is used to ablate the most anterior portion of the corneal stroma. These procedures do not require a partial thickness cut into the stroma. Surface ablation methods differ only in the way the epithelial layer is handled.
- Photorefractive keratectomy (PRK) is an outpatient procedure generally performed with local anesthetic eye drops (as with LASIK/LASEK) . It is a type of refractive surgery which reshapes the cornea by removing microscopic amounts of tissue from the corneal stroma, using a computer-controlled beam of light (excimer laser).  The difference from LASIK is that the top layer of the epithelium is removed (and a bandage contact lens is used), so no flap is created.  Recovery time is longer with PRK than with LASIK, though the final outcome (after 3 months) is about the same (very good). More recently, customized ablation has been performed with LASIK, LASEK, and PRK.
- Laser Assisted Sub-Epithelium Keratomileusis (LASEK) is a procedure that also changes the shape of the cornea using an excimer laser to ablate the tissue from the corneal stroma, under the corneal epithelium, which is kept mostly intact to act as a natural bandage.  The surgeon uses an alcohol solution to loosen then lift a thin layer of the epithelium with a trephine blade (usually with a thickness of 50 microns).  During the weeks following LASEK, the epithelium heals, leaving no permanent flap in the cornea. This healing process can involve discomfort comparable to that with PRK.
- EPI-LASIK is a new technique similar to LASEK that uses an epi-keratome (rather than a trephine blade and alcohol), to remove the top layer of the epithelium (usually with thickness of 50 microns), which is subsequently replaced. For some people it can provide better results than regular LASEK in that it avoids the possibility of negative effects from the alcohol, and recovery may involve less discomfort.

## Corneal incision procedures
- Radial keratotomy (RK) uses spoke-shaped incisions (usually made with a diamond knife) to alter the shape of the cornea and reduce myopia or astigmatism; this technique has now been largely replaced by the other methods (that use excimer laser).
- Arcuate keratotomy (AK) is similar to radial keratotomy, but the incisions on the cornea are done at the periphery of the cornea.  Arcuate keratotomy is used to correct astigmatism.  Although most incisional procedures are replaced nowadays by Lasik, AK is still used in some special cases (correction of residual astigmatism after a keratoplasty procedure or during cataract surgery).

## Other procedures
- 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 constriction. It can also be used to treat selected types of astigmatism.
- 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.
- Intra-Stromal corneal rings (Intacs) are approved by FDA for treatment of low degrees of myopia.
- Lens implantation inside the eye can also be used to change refractive errors.

# Expectations
The Council for Refractive Surgery Quality Assurance, an independent, nonprofit, patient/consumer health organization that provides information about refractive surgery and certifies LASIK surgeons, considers surgeons with results of 90% of patients achieving 20/40 or better and 65% achieving 20/20 or better with limited approximately 3% of refractive surgery patients experiencing a surgery induced complication at six months after surgery, with 0.5% being serious complications requiring extensive maintenance or invasive treatment as meeting the US national norms.
Many people with myopia are able to read comfortably without eyeglasses. Myopes considering refractive surgery are advised that this may be an advantage after the age of 40 when the eyes become presbyopic and lose their ability to accommodate or change focus.

# Risks
While refractive surgery is becoming more affordable and safe, it may not be recommended for everybody. Patients that have medical conditions such as glaucoma or diabetes, uncontrolled vascular disease, autoimmune disease, pregnant women or people with certain eye diseases involving the cornea or retina, are not good candidates for refractive surgery. Keratoconus, a progressive thinning of the cornea, is a common corneal disorder. It is believed that additional thinning of the cornea via refractive surgery may contribute to advancement of the disease 
, that may lead to the need for a corneal transplant. Therefore, keratoconus is a contraindication to refractive surgery. Corneal topography, pachymetry and, more recently, Pentacam exams are used to screen for abnormal corneas. Furthermore, some people's eye shape may not permit effective refractive surgery without removing excessive amounts of corneal tissue. Those considering laser eye surgery should have a full eye examination.
Although the risk of complications is decreasing compared to the early days of Refractive surgery,  there is still a small chance for problems sometimes serious. These include vision problems such as ghosting, halos, starbursts, double-vision, and dry-eye syndrome.   With procedures that create a permanent flap in the cornea (such as LASIK), there is also the possibility of accidental traumatic flap displacement years after the surgery,  with potentially disastrous results if not given prompt medical attention.