Refractive Surgery...
 

"Refraction" refers to the way the eye focuses light, the source of everything we see. There are three basic elements that determine the eye's ability to focus light: The shape of the cornea, the eye's transparent outer membrane. the power of the lens, which lies behind the pupil and completes the focusing process and the length of the eyeball.

Normal vision
In an eye with normal vision light rays are refracted in such a way that they meet at a light-sensitive membrane called the retina, which is located at the back of the eye. There, light images are changed into electrical impulses and sent by the optic nerve to the brain's vision-processing center.

Refractive errors occur when light rays entering the eye do not focus properly on the retina. Refractive errors are not diseases, but rather normal differences in visual ability. There are three basic types of refractive errors: myopia (nearsightedness), hyperopia (farsightedness), and astigmatism.

Myopia
In myopia, the most common type of refractive error, the cornea is too curved, the lens is too powerful, or the eyeball itself is too long. As a result, light rays are refracted too sharply and meet at a point in front of the retina. For this reason, faraway objects appear blurry.

Hyperopia
People with hyperopia, on the other hand, experience just the opposite problem. Because the cornea is too flat, the lens is too weak, or the eyeball is too short, light rays do not have enough space to come together before reaching the retina. As a result, hyperopic people see distant objects more clearly than nearby objects.

Astigmatism
Astigmatism, which often occurs with either myopia or hyperopia, is an irregular curvature of the cornea. It causes light rays to reach the retina in different locations and thus prevents the eye from focusing clearly at any distance.

Presbyopia
A fourth type of refractive error, presbyopia, occurs as the eye ages and the lens loses its ability to change focusing for close objects. Hence the need for bifocals or reading glasses.

Pre-Laser Eye Surgery...

The intent of laser eye (refractive)  surgery is to change the natural  curvature of the cornea  in order  to alter the eye's focusing power. There are presently two primary surgical techniques in the refractive surgeon's  arsenal to  accomplish this goal: Photo Refractive Keratectomy (PRK), and Laser Assisted In-situ Keratomileusis (LASIK). Before these techniques, and before the use of the Excimer laser, procedures, largely abandoned now, were used to change the curvature of the cornea.

Both of the techniques begin with applying topical anesthesia to the eyeball. These eye drops numb the cornea to any sensation. Once the cornea is sufficiently numb, the lids are then retracted and the cornea is marked with a special ink. This is to delineate the diameter of a clear zone directly in front of the pupil.

Radial Keratotomy (RK) was pioneered in the early 1970s by a Russian eye surgeon. By the end of the 70s, a number of U.S. eye surgeons had traveled to Russia to learn the procedure and import it back to their U.S. practices. It is the simplest to perform of the three techniques.

RK is performed solely on the outer part of the cornea. If incisions were made in the central cornea, the scar tissue resulting from healing would cause severe visual disturbances. Using an operating microscope, a diamond-edge scalpel is then used to cut a number of radial incisions (up to 90% of the corneal depth) in the periphery of the cornea, similar to cutting a pie. The incisions slightly weaken the peripheral cornea, causing it to bulge. This peripheral bulging, in turn, flattens the center of the cornea, weakening the focus power, and causing the focal point of light entering the eye to move backwards onto the desired retinal surface.

Automated Lamellar Keratoplasty (ALK) - ALK was used in the United States to treat relatively high degrees of myopia and some cases of hyperopia prior to the availability of the Excimer laser.

In the ALK procedure to correct nearsightedness the surgeon would employ a microkeratome to create a micro-thin, disk-shape flap from the top layer of the central corneal zone. This flap would be held back out of the way while the surgeon continues to use the microkeratome to flatten the underlying stromal bed in the central zone of the cornea. The flap was then replaced, without sutures, with a shield placed over the eye to protect it for 12 to 24 hours. 

The procedure was essentially the same for the correction of farsightedness, except that a deeper flap was created by the microkeratome. The central corneal zone was then pushed forward by the pressure inside the eye, resulting in a lessening of the farsighted condition.