The anatomy of the cornea is discussed elsewhere on this website. The epithelium is highly regenerative, and can heal itself over the course of 36 – 72 hours. The remainder of the corneal layers cannot regenerate, and heal by the production of scar tissue.

Modifying the shape of the cornea implies that some of the corneal tissue need to be removed.  To treat myopia and astigmatism, the central cornea needs to be flattened.  Therefore, the central tissue is removed in a computer-controlled fashion with the excimer laser. In hyperopia, the central cornea is too flat, and needs to be steepened. This can be achieved by removing tissue from the periphery of the cornea, thereby steepening the center.

The PRK technique was introduced during the late 1980's. By utilizing this technique, the corneal shape is modified through the removal of Bowman's membrane, as well as superficial stroma. The epithelium is removed with a special brush at the beginning of the procedure, and the exposed Bowman's membrane as well as underlying stroma is removed with the excimer laser to flatten the area. Within 36 – 72 hours, the epithelium regenerates and closes the defect.  Over time, however, varying degrees of scarring may occur in the treated area, with the induction of side effects on the patient's vision. Today, however, due to modifications in technique, the application of special medication during the surgery, and improved lasers, the issue of scarring has become less prominent.

Advantages of this technique, include that it is easy to perform, while the cost thereof is cheaper. There are some disadvantages, however, including the following:

	pain and discomfort during the early post-operative period
	the risk of infection
	unstable vision in the early phases (up to 3 months post-operatively)
	limited scope of refractive correction
	the development of corneal scarring within the visual axis, with the consequent side effects on vision

However, if the laser treatment can be designed to only remove tissue to the level of the very superficial stroma, as is the case with low orders of corrections, it is possible to apply this treatment modality safely.