The cornea is the clear covering of the front of the eye which bends (or refracts) light rays as they enter the eye. For clear vision to occur, the cornea must have the correct shape and power to focus incoming light rays precisely on the retina at the back of the eye. If the cornea is to steep, too flat or irregular in shape, it cannot bend light at the angle needed to focus on the retina. As a result, eyeglasses or contact lenses may be needed to refract the incoming light rays at the angle needed for clear vision. In a nearsighted eye, for example, the cornea's shape causes incoming light rays to focus in front of the retina.

All corneal modification procedures are intended to alter the curvature of the cornea so that incoming light is refracted at an angle that allows it to focus precisely on the retina. This may eliminate the need for eyeglasses or contact lenses, or reduce the power of prescription lenses needed.

Those interested in any of the corneal modifications should first have a comprehensive eye examination to evaluate their eye health and determine their vision needs.

PRK
In photorefractive keratectomy, the doctor uses an excimer laser to remove a thin layer of tissue from the center of the cornea. The laser produces a uniform, concentrated beam of high energy ultraviolet light that is precisely focused onto the cornea. During the procedure, the laser's light energy is converted into chemical energy which evaporates molecules of corneal tissue in very precise amounts. By altering the curvature of the cornea in this way, PRK helps reduce or eliminate moderate nearsightedness and astigmatism. Candidates for PRK should be at least 18 years old, have stable vision and no abnormalities of the cornea or external eye.

RK
In RK, the doctor surgically flattens the cornea by making a series of small, radial (like the spokes of a wheel) incisions in the periphery of the cornea. This flattens the cornea and changes its focusing power so that light rays fall precisely on the retina. Radial keratotomy is used to treat nearsightedness and some forms of astigmatism. Candidates for RK should be at least 18 years old, have stable vision and no abnormalities of the cornea or external eye.

LTK
In LTK, the doctor uses a laser to heat a series of small, dot-shaped areas of the cornea. This causes the corneal tissue to expand and the curvature of the cornea steepens. LTK is used to treat people who are farsighted. Candidates for LTK should be at least 40 years old, have stable vision and no abnormalities of the cornea or external eye.

Lasik
The Lasik procedure uses both surgery and a laser to treat nearsightedness, farsightedness and astigmatism. In Lasik, the doctor surgically creates a flap of tissue on the front surface of the cornea. The flap is folded back and a laser is used to reshape the layer of tissue underneath (the middle layer of the cornea). Once this is completed, the flap is repositioned and allowed to heal. Candidates for Lasik should be at least 18 years old, have stable vision and no abnormalities of the cornea or external eye.

ALK
In ALK, a thin layer of tissue is removed from the front surface of the cornea. Then, to reshape the cornea, the doctor removes tissue from the middle layer of the cornea using an instrument called a microkeratome, which functions similarly to a carpenter's plane. Finally, the top layer of tissue is replaced. ALK is usually reserved for patients with severe nearsightedness or low degrees of farsightedness.

Ortho-K
Ortho-K is a non-surgical procedure that involves the wearing of a series of specially designed rigid contact lenses to progressively reshape the curvature of the cornea over time. The results of the painless procedure are not permanent, thus retainer contact lenses must be worn periodically to maintain improvements made in vision. Ortho-K is used to treat low to moderate nearsightedness and low degrees of astigmatism.