American Academy of Ophthalmology answered:
Advanced surface ablation (ASA) is a refractive surgery technique used to treat nearsightedness (myopia), farsightedness (hyperopia) and astigmatism. With ASA, the outermost layer of the cornea, the epithelium, is removed or displaced to expose the stroma (the middle, thickest layer of tissue in the cornea). A computer-controlled excimer laser then reshapes the front surface of the corneal stroma. The epithelium is either replaced or assisted in healing back over the surface of the cornea underneath a bandage contact lens. ASA is usually recommended instead of LASIK for people with thin corneas. ASA procedures include photorefractive keratectomy (PRK) and laser epithelial keratomileusis (LASEK).
Photorefractive Keratectomy (PRK )
With PRK , the surgeon removes micro-thin layers of tissue from the outermost layer of the cornea, the epithelium. When treating myopia, or nearsightedness, the surgeon then uses the laser to remove central corneal tissue in a circular pattern, thereby flattening the cornea and weakening the focusing power of the eye. The tissue is removed in a controlled pattern programmed into the computer by the surgeon. When treating hyperopia, or farsightedness, the surgeon uses the laser to remove peripheral corneal tissue, thereby steepening the central cornea to increase the focusing power of the eye.
In comparison, when treating astigmatism, the laser is programmed to remove tissue in an elliptical pattern, selectively reshaping some portions of the cornea to form a smooth symmetrical surface. This procedure requires precise evaluation of the astigmatism so that the correct amounts of laser energy are delivered to the appropriate areas of the cornea. PRK is often a better option for people whose occupation makes it more dangerous to have a flap, as it could be dislodged accidentally.
Laser Epithelial Keratomileusis (LASEK)
With LASEK, your ophthalmologist (Eye M.D.) uses an alcohol solution to loosen and peel back the epithelium to expose the cornea. The excimer laser then resculpts the cornea, and the epithelium is placed back into position. After the procedure, a transparent bandage contact lens is placed on the cornea to promote healing.Advanced surface ablation (ASA) is a refractive surgery technique used to treat nearsightedness (myopia), farsightedness (hyperopia) and astigmatism. With ASA, the outermost layer of the cornea, the epithelium, is removed or displaced to expose the... More