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Eyes and Sight
A complete discussion of eyes and sight would have
to deal with dozens of variables, many of which are evaluated as part of
the screening for RK. For an introductory discussion, we can deal with
just three: the distance to an object being viewed, the shape of the cornea,
and the elasticity of the lens.
Normal Vision
 One
definition of “normal vision” is the ability to see clearly an object 20
feet away when the muscles of the eye are relaxed—commonly referred to
as 20/20 vision. For the relaxed, normal
eye, objects from as close as 20 feet to as far away as the moon are in
focus. There is no capability, indeed no need, to focus further away. To
see objects that are in the range from about 20 feet away to as close as
12 inches, the muscles of the eye contract to change the shape of the lens.
Near Sighted and RK
  In
a “near sighted eye” the cornea has too much curvature. The eye can only
focus on objects that are less than 20 feet away, in some cases only on
objects that are just inches away from the eye. RK causes the center of
the cornea to flatten so that it functions like a normal eye.
We only see through the central 30 to 40% of the cornea. RK does not
touch that central part. All of the surgery is done at the outer
edges, but it changes the shape of the entire cornea.
 The
cornea is soft enough that slight pressure will mark it, almost like childrens'
clay, except in this case the mark will disappear in a matter of minutes.
Using information from previous examinations, the surgeon uses specially
designed instruments to mark the incisions to be made. The number
of incisions—usually 4 to 8 but sometimes more—their depth and length determine
the amount of correction that will result. The incisions cause the outer
portion of the cornea to bow out slightly. This reduces the curvature,
i.e., flattens the center of the cornea.
A diamond bladed scalpel with a built in depth gauge is usually used
for the surgery.
Sometimes the correction occurs almost instantly. More commonly
it takes a period of time for vision to stabilize.
Astigmatism and RK
Astigmatism is an uneven curvature in the cornea or retina. If the
uneveness is in the cornea it can be corrected by RK. The most common method
is to make small incisions perpendicular to the radial ones.
Far Sighted
In a “far sighted” eye, the cornea is too flat. When the eye is relaxed
it can only focus on objects that are more than 20 feet away. The lens
of a normal eye only has to adjust over a fairly narrow range from 20 feet
away to as close as 12 inches. In a far sighted eye, the lens can
make some adjustments but is not capable of adjusting for the full range—it
cannot focus on nearby objects like books and menus. RK would make
the cornea even flatter so it won’t work for far sightedness. Instead,
a newer procedure to steepen the cornea is used.
Presbyopia
As we age, the lens looses its elasticity. The muscles are no longer
able to reshape the lens to focus on nearby objects. The symptoms are those
of far sightedness—problems with books and menus. The technical name is
presbyopia. Since the problem is in the lens rather than the cornea, RK
won’t help, except ...
RK Bifocals
One solution is to have one eye fully corrected—20/20—and have the other
under corrected or slightly nearsighted so that the lens in the under corrected
eye can focus to read. In a pair of bifocals the lower lens is a comparable
example of “under correction” for reading. Instead of arranging the correction
up and down as in a pair of glasses, it is arranged left and right. It
is an option but not a guaranteed solution. Some people find it difficult
to adjust to “monocular” vision, but others find it works very well.
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Updated March 23, 1998
URL http://www.lpf.com/source/rk/background.html
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