Pre-Laser Eye Surgery...
The intent of laser eye
(refractive) surgery is to change the
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.