Instead of the corneal flap cut out in the LASIK method in order to spare the outer layers of the cornea, only the epithelium is removed in the PRK/ LASEK / EPI-LASIK methods. It is the way in which it is removed that determines the name of the method.

PRK (Photorefractive Keratectomy)
PRK means literally the "removal of corneal tissue by light", to modify the refractive power of the cornea. This way of correcting visual defects has been used successfully since 1987. While in the beginning even strong defects where corrected in this manner, the method is used nowadays exclusively either when the thickness of the cornea is insufficient or when the visual defect is moderate, up to about 3.5 diopters.

In PRK the epithelium of the cornea is loosened up with an alcoholic solution and then removed with a spatula. The defect is then corrected with the Excimer laser working on the exposed layer (the stroma).

LASEK (Laser Epithelial Keratomileusis)
LASEK is a variation of the PRK technique. Like PRK, it is an alternative to LASIK when this method cannot be used because of the anatomical conditions (thin cornea, moderate visual defect).
 
In the LASEK method, the epithelium of the cornea is loosened up with an alcoholic solution and then carefully pushed aside with a spatula.

The visual defect is then corrected with the Excimer laser by ablation of the exposed internal layer called stroma. Finally, the epithelial layer is carefully pushed back over the layer that has been treated.

EPI-LASIK (epithelial LASIK)
EPI-LASIK is another variation of the PRK/LASEK technique. As these methods, EPI-LASIK also represents an alternative to the LASIK method, for cases where the anatomical conditions do not allow the use of LASIK (thin cornea, moderate visual defect).

In EPI-LASIK, the outermost layer of the cornea, the epithelium, is separated from the underlying stroma using a microkeratome with a blunt blade, and pushed aside. The visual defect is then corrected with the Excimer laser working on the exposed stroma. Finally the epithelial layer is pushed back over the treated surface.

The difference to PRK/LASEK is that with the EPI-LASIK method the epithelial cells are not treated with an alcoholic solution. In certain cases, using this "non-alcoholized" epithelial carpet to close the wound after treatment with the Excimer may influence the wound-healing process positively. On the other hand, scraping off the epithelium with the microkeratome may represent a stress factor for the tissue, canceling the advantage of the alcohol-free treatment.

Properties and advantages of these methods

• Results can be predicted relatively precisely
• Some pain during the wound-healing phase, lasting a few days
• Recovery period of about 4 to 6 weeks (with fluctuations in visual acuity)
• Appropriate when anatomical/medical conditions do not permit the use of LASIK
• Later corrections are more complicated than with LASIK

PRK is also used to treat superficial scars or pathological modifications due to diseases of the cornea. It is then called PTK (Phototherapeutic Keratectomy). In PRK / LASEK / EPI-LASIK, the first step consists in the removal of the epithelium of the cornea. The correction of the visual defect by Excimer laser is performed as in the LASIK / Femto-LASIK methods.

The epithelium of the cornea regenerates completely within 3 days, independently of exterior Influences. The stroma of the cornea underlying the epithelium, a compact collagen layer, can be modeled and does not regenerate itself. Whatever the treatment, PRK or LASIK, the correction by Excimer laser is always performed on the exposed corneal stroma. In PRK / LASEK / EPI-LASIK, it is the upper stroma layer which is modified. In LASIK, the excision of the flap permits to model the tissue layers under the stroma surface with the laser (the surface of the stroma remains intact).