Copyright 2007 www.eyes-and-vision.com
What Is Lasik Eye Surgery?
You may
have heard many stories about lasik eye surgery, and that more and more
people have decided to "take the plunge" and undergo the procedure to
improve their quality of life. You know it has something to do
with lasers, obviously, but what exactly does this laser do?
To fully understand the function of this laser, we must first understand why the most common eye disorders happen. The most common eye disorders (nearsightedness, farsightedness, and astigmatism) are caused by "refractive errors". Refractive errors are caused by a misshapen cornea (the transparent outer surface of the eye through which light travels as it enters the eye).
In laser eye surgery, a computer controlled device called an eximer laser essentially "reshapes" your cornea.
In basic terms, the laser transfers your eyeglass or contact lens
prescription onto the surface of your eye. The reshaping is
accomplished through a process called photoablation, in which a
precisely controlled beam of ultraviolet light is used to stimulate the
molecules of your cornea to the point where certain targeted tissue
molecules break and vaporize.
These Are Not "Star Wars" Lasers
These Are Not "Star Wars" Lasers
Different Surgical Techniques For Different Eye Disorders
When treating nearsightedness
(where the cornea is too steep), the surgeon will remove tissue from
the center of the cornea to flatten the surface. When treating farsightedness
(where the cornea is too flat), tissue is removed from the outside
perimeter of the area to make the central portion of the cornea
effectively "steeper." For astigmatism, tissue is removed from the area of the cornea where it is unevenly shaped.
You can
compare the cornea to the glass that covers a camera lens. If
this "glass" is distorted in any way, it affects the "picture" that is
projected onto your retina, and passed onto your brain as an
image. Usually this problem is compensated for by corrective
eyewear or contact lenses. (For more details, see the eye anatomy page).
"Vaporizing tissue?"
This might sound quite extreme to you, especially when you consider how
sensitive our eyes are to pain. But these are not the destructive
laser beams you may have seen in science fiction movies. The eximer laser
is on the "cool" end of the light spectrum, and can only remove a
maximum of 0.25 microns of corneal tissue at a time. This is
approximately 1/500th the thickness of a human hair. Since a
human hair is about 125 microns thick, it would take 500 laser pulses
to break through a human hair. Your cornea is about 500 to 600
microns in thickness,
therefore
it would take 2,000 laser pulses or more to break through your
cornea! In most cases, the surgeon will remove between 10 and 160
microns of tissue to correct your vision problem, well within safety limits.
Much of the
computational work is actually done by the software that comes from the
manufacturer of the laser device. Your surgeon (or, in some
cases, a technician) enters your "refractive numbers" into the
software, which in turn calculates the exact number of pulses required
to achieve the desired level of correction. These calculations
can be adjusted by the surgeon depending on the specific needs of the
patient i.e. the thickness of his or her cornea, etc. These
adjustment factors are called nomograms. What the surgeon DOES
perform manually are the nomograms and the exact location on the cornea
where the laser is to be directed. It is very important that you
choose a surgeon that has performed hundreds, or even better, thousands
of laser eye procedures. This is covered in more detail on the
following pages:
What Does The Surgeon Actually Do?
