The pictures above illustrate the difference between radial keratotomy (RK) and photorefractive keratectomy (PRK).
Radial keratotomy (RK) is an invasive surgical procedure in which incisions usually cut through more than 90% of the thickness of the cornea. The incisions are intended to weaken the side structure of the eye so that internal pressures expand the sides and flatten the surface of the cornea to reduce nearsightedness.
Laser-PRK vision correction procedure does not affect the strength of the eye the way radial keratotomy (RK) does. In countries where PRK has been introduced, very few surgeons continue to perform RK.
Photorefractive keratectomy (PRK) is a process using the excimer laser that gently vaporizes very precise amounts of tissue from the surface of the eye. The process removes only 5-10% of the thickness of the cornea, or 25-50 microns, about the thickness of a human hair, in the center of the treatment zone which is approximately 6mm in diameter. Lesser amounts of tissue are removed toward the outer edges of the treated area.
LASIK, or laser in situ keratomileusis, is another invasive refractive surgery technique under investigation. With LASIK the surgeon uses a microkeratome, an automated knife-like device, that cuts a layer of tissue about 160 microns thick (one third of the cornea) across the surface of the eye. This flap of tissue, which remains attached on one side, is folded back and the excimer laser is used to remove the underlying stromal tissue. Following laser treatment, the flap is laid back in place.
LASIK is the newest laser surgery procedure, and shows great promise. This will likely be the way most refractive surgery is performed in the future, however some questions remain about the safety and efficacy of the LASIK process. Concerns about induced astigmatism if the flap is improperly repositioned and about adherence and healing of the flap are being evaluated.