Generality/Definition
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Surgical procedures aimed at improving the focusing power of the eye are called refractive surgery. There are three primary types of refractive errors: myopia, hyperopia and astigmatism. Persons with myopia, or nearsightedness, have more difficulty seeing distant objects as clearly as near objects. Persons with hyperopia, or farsightedness, have more difficulty seeing near objects as clearly as distant objects. Astigmatism is a distortion of the image on the retina caused by irregularities in the cornea or lens of the eye. Combinations of myopia and astigmatism or hyperopia and astigmatism are common.
Source: LASIK (webeyemd.com)
Treatment
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The lasik is present since 10 years in the refractive surgery and it didn't have any changes in the principles, but great changes in the cutting quality and in the efficiency. The lasik technique has the advantage to be able to correct stronger shortsightedness than the surface PRK (Surface Excimer laser). It has a shorter visual recovery delay . The lasik principle ( Laser Assisted In-situ Keratomileusis ). The lasik intervention allows to cut a cornea level ( 160 ), in order to operate directly in the stroma. The operation 1. A succion ring is placed on the eye. It is useful for two reasons: To maintain the eye in a proper position during the operation, to support and to guide the microkeratome in its cutting advancement. 2. The surgeon is placing the microkeratome on the succion ring. 3. The microkeratome advancement is regular and programmed, a corneal flap with a thickness representing 20 or 25% of the cornea ( 160 ) but it will stand together with the cornea with an hinge. 4. Once the cutting is done, the microkeratome comes back to its starting position and the surgeon is able to remove the microkeratome and the succion ring. 5. The surgeon lifts the cornea cover with a surgical spatule. 6. The laser process can begin. It's strictly similar to the surface PRK . 7. At the end of the laser intervention, the surgeon is replacing the cornea cover without any suture and the natural cicatrization is playing its full role in 2 or 3 minutes. 8. A protection shell is placed on the eye treated, because the link cornea flap \ cornea is still fragile for a moment.
Source: Excimer laser correction- LASIK (myopes.com)
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The excimer laser ( excited and dimer ) is using a fluorine and rare gas ( argon ) mixing. Its use allows not to have contacts between the instrument and the cornea ( avoids an infection ). Sending a ray in the UV spectrum has the consequence to pulling off the molecules. It sprays a cornea film at each impact ( around 0.25 m ). The fineness of the cornea films vaporized is the reason of the correction's accuracy of the excimer laser. The operation The intervention takes place on local anaesthesia, with using just a collyrium a few minutres before the operation. It will allow to anaesthetize the cornea. The patient is installed on a surgical armchair to ensure a perfect position under the laser. Using a retractor allows to maintain the eye in the opened position without pain or any risk. The surgeon must take off the epithelium to do this intervention. It's possible then to ablate directly in the corneal stroma. There are many existing techniques: the first one consists in using a surgical spatule and to scratch the eye's surface. Anyway, the most used technique is the use of a brush, which takes off a cylindrical surface of epithelium, superior than the zone to treat. The practicians prefer yet to use again a surgical spatule on the surface. To obtain correct results, it is necessary to remove epithelium of the zone to treat. There is an other technique mainly used, which consists to expose the epithelium with an alcoholized solution, in order to peel it and to remove it with a surgical spatule. The practician draws now the central aim with a corneal marker and he asks the patient to watch a light situated in the center of the ray. It's possible to begin the treatment. The operation is very short and allows then to avoid a decentralized treatment. When the eye is moving a lot, the treatment must be immediately stopped. When the eye becomes stable again, you can continue to operate. At the end of the intervention, some healing drops can be added on the eye and some analgesics are also prescribed.
Source: Surface Excimer laser (myopes.com)
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Radial Keratotomy (RK) and Photorefractive Keratectomy (PRK) are other refractive surgeries used to reshape the cornea. In RK, a very sharp knife is used to cut slits in the cornea changing its shape. PRK was the first surgical procedure developed to reshape the cornea, by sculpting, using a laser. Later, LASIK was developed. The same type of laser is used for LASIK and PRK. Often the exact same laser is used for the two types of surgery. The major difference between the two surgeries is the way that the stroma, the middle layer of the cornea, is exposed before it is vaporized with the laser. In PRK, the top layer of the cornea, called the epithelium, is scraped away to expose the stromal layer underneath. In LASIK, a flap is cut in the stromal layer and the flap is folded back. Another type of refractive surgery is thermokeratoplasty in which heat is used to reshape the cornea. The source of the heat can be a laser, but it is a different kind of laser than is used for LASIK and PRK. Other refractive devices include corneal ring segments that are inserted into the stroma and special contact lenses that temporarily reshape the cornea (orthokeratology).
Source: LASIK (webeyemd.com)
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