The CK procedure was initially FDA approved for the correction of
hyperopia (farsighted, longsighted).  That means that it changes the
cornea to be more myopic (nearsighted, shortsighted).  After a short
time it was clear that CK was not the best technique for hyperopia
over about 1.00 diopter.  It just isn't quite up to the task of higher
hyperopia and LASIK or PRK seemed to do better.

CK regresses at a rate of about 1.00 diopter every 2-3 years, so that
means that it was suddenly really limited.  It could only correct a
bit more than what would regress in 2-3 years.  CK was looking at
becoming little more than a memory until some of the doctors thought
it might be a good technique to instill monovision in the plano
presbyope.

Plano means no refractive error.  Presbyopia is when the natural lens
of the eye is no longer able to change shape, thereby unable to change
focus from distance to near objects.

The reason doing a hyperopic correction on a plano presbyope makes
sense is because a hyperopic correction would make someone plano into
someone myopic.  There are two ways to describe myopia.  One is that
you cannot see things far away very well, and the other is that you
can see things close quite well.

Monovision is the technique of making the patient's non-dominant eye
myopic about 1-2 diopters and leave the dominant eye fully corrected.
After about 3-6 weeks the brain figures out to "look through" the
myopic eye at objects close and the plano eye at objects distant.
This can reduce the need for reading glasses as much as 85% of the
time.  For details on monovision, visit
http://www.usaeyes.org/faq/subjects/monovision.htm

So doctors started instilling monovision on plano presbyopes with CK
and something quite strange happened.  Patients were reporting being
able to see both distance and near in the CK eye.  This should not
have been the result.  The patient should not have been able to see
distance.

No one has an explanation for this phenomena, but they have a name for
it.  They call it "blended vision" rather than just monovision.  If
anything, this shows just how limited is current knowledge of how the
eye works.

Although several prominent CK doctors reported this blended vision
effect, it tended to be doctors who had a financial interest in the
company that makes the CK device.  I'm a cynic at heart.  While I do
not for a moment think any of these doctors would lie, I do believe
that their enthusiasm for CK may make their analysis of what is
happening just a bit biased.

CK for blended vision was an off-label use of the equipment.  That is
legal, but the company went through the extraordinary effort (and
expense) of getting FDA approval of CK for monovision.  No, they did
not call it blended vision and they did not submit data attempting to
affirm that patients would be able to see both distance and near in
the CK eye.

I wish I could point to a peer-reviewed study that shows blended
vision, but I cannot.  There have been numerous smaller studies
publicized that show blended vision to be a reality, but I do not
believe one has been peer-reviewed.  That is really the acid test.
That is, the acid test in addition to patient satisfaction.

Even if you did not get the blended vision effect that does seem to
occur at least some of the time with CK, you would get monovision.

If you decide to take this route, be absolutely certain that you
select a surgeon with significant practical experience with CK.  CK is
as much an art as science and not every surgeon is a good artist.

Anyone considering monovision with any kind of surgery absolutely
positively must try it first with contacts.  around 20-30% of people
simply can't stand it.  If you like monovision with contacts, you
would probably like monovision with CK.

Another thing, CK can have an unusual healing pattern.  You can get an
immediate effect, then vision quality goes downhill for about 30 days,
then levels out, then gets much better.  This process occurs in a
minority of patients.  Most patients have very little trouble if any
at all, but if you have CK and have problems, be patient.  Don't do
anything drastic for at least two months.

And lastly, this is surgery.  Real surgery.  Don't jump into CK
without doing your homework.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org
http://www.ComplicatedEyes.org

I am not a doctor.

Since this is sci.med.vision, let's talk about the science.

Enhanced reading ability following RK surgery was noted nearly two
decades ago.  This is due to the fact that the central cornea is flat
and the peripheral cornea is steep.  This is the same principle used in
concentric or aspheric bifocal contact lenses.

In the case of CK, the central cornea is steep compared to the
peripheral cornea.  The central cornea provides the reading power,
while the peripheral cornea provides the distance correction.

The same result can be achieved much more reliably and much more safely
with the Proclear Compatible system of bifocal contact lenses.  With
this system, the dominant eye has a distance center/near surround,
while the dominant eye is given a reading center/distance surround.  At
least the effect won't change over time, and you won't suffer the risk
of irregular astigmatism in the process.

DrG