PRK, Lasik, and LASEK refractive surgery techniques involve the process of removal of corneal tissue. If you are myopic, the removal is in the center of the cornea. If you are hyperopic, the removal is at the periphery of the cornea.

What is most important is that there remains a stable and untouched portion of the cornea to insure corneal integrity. Many surgeons feel that at least 250 microns of cornea must remain untouched.

Lasik requires that a thin layer of the cornea to be cut across the front of the visual axis and laid back like a hinged door. This is called a flap and the flap is normally around 180 microns thick, give or take. Laser energy is applied to remove tissue from the area under the flap, then the flap is laid back down over the reshaped area.

For Lasik to proceed correctly, the flap needs to be thick enough to remain stable and stay attached at the hinge. Also, there needs to be enough tissue under the flap to allow removal of tissue for the refractive change and keep the structural integrity of the cornea intact.

PRK and LASEK do not require this thick flap. With Lasik, the laser ablation starts at about 180 microns down into the cornea. With PRK and LASEK, the laser ablation starts virtually at the surface of the cornea.