| Glenn - USAEyes.org 2004-11-03, 7:12 pm |
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>I find it imperative to be "scared" enough on behalf of future
>patients to do my part to ensure they have a thorough understanding of
>the facts,
Absolutely! But the facts do not support WizKids' proclamations.
>particularly as relate to microkeratome inaccuracies, the
>uncertainties of the outcome with RST below 300um, and the
>difficulties of determining RST prior to a retreatment. I don't want
>to see potentially borderline patients subjected to elevated risks
>without their knowledge during the years-long processes of developing
>better keratomes, determining the finer points of topographical
>analysis and disseminating knowledge to other surgeons.
Then let's dessiminate a bit of relevant information right now.
Mechanical microkeratomes rarely provide a flap thickness as indicated
on the equipment. No competent surgeon expects a 160 micron head on a
mechanical microkeratome to actually provide a 160 micron thick flap,
but what the microkeratome does provide tends to be consistent from
patient to patient within a reasonably predictable range.
There are variables that need to be considered. A patient with a
thick cornea will often realize a flap that is a different thickness
than a patient with a thin cornea. Older patients who smoke will very
likely have a different flap thickness than a younger patient who does
not smoke. This is all with the same microkeratome. All these
seemingly irrelevant issues need to be considered by the surgeon as
s/he plans the surgery.
The flap created by a mechanical microkeratome is not of an even
thickness, but is meniscus in shape. The center is thinner than the
outer periphery, and the edges thin out to a point. Even if the flap
actually hit the 160 microns intended, it won't be 160 microns thick
throughout.
Because a flap created with a mechanical microkeratome has variability
of shape and thickness, the surgeon must plan for a reasonable margin
of error. A patient who has a low refractive error and thick flaps
may have more than enough margin for error, but a high myope with thin
corneas may not. Although on paper the numbers may come out okay, it
is important for the doctor to plan for the variables inherent to flap
creation.
Ectasia occurs when the amount of untouched cornea is too thin and the
cornea becomes unstable, bulging forward. If enough tissue remains
untouched and the cornea is healthy, ectasia will not occur. A
thicker flap will make for a thinner untouched cornea, but a flap too
thin can be problematic too.
The thinner center of the meniscus shaped flap created with a
mechanical microkeratome is why buttonhole flaps sometimes occur.
This is when the flap is too thin and the microkeratome breaks through
the surface of the cornea. A buttonhole or partial flap can also
occur if the suction that holds the microkeratome to the eye is broken
while the flap is being created. Normally, the partial or buttonhole
flap can be repositioned, allowed to heal for about three months, and
then the surgery can be repeated. Of course, the patient with a
buttonhole or partial flap must be monitored to be sure that
epithelial cells do not get under the flap and that the flap is
properly repositioned.
These issues are why preoperative measurement of corneal thickness is
so important. Additionally, many surgeons will take a thickness
measurement immediately after creating the flap to be certain that the
residual cornea is thick enough for the laser energy to remove the
appropriate amount of tissue. In this manner the doctor knows with
great certainty the actual thickness of the flap, how much underlying
tissue remains, and how much tissue will remain untouched after the
laser ablation.
The femtosecond laser microkeratome (Intralase) has resolved some of
these flap issues. The flap with a laser microkeratome is even in its
thickness from edge to center to edge. A buttonhole flap with
Intralase is just about impossible. The accuracy of an Intralase flap
is about 10 microns with a much smaller range of variance than a
mechanical microkeratome. If for any reason the procedure is aborted
and a partial flap is created, Intralase can simply start again where
it left off.
Of course, Intralase has its unique limitations too. A small
percentage of patients become very sensitive to light a few weeks to
months after Intralase. This malady responds well to steroids and
does resolve with continued healing, but is most certainly not
desired. Intralase creates a stromal surface that is slightly less
smooth than with the blade of a mechanical microkeratome, but actual
patient outcomes indicate that this makes no difference in visual
function at all.
These are just a few of the issues, concerns, and calculations that go
into providing LASIK or IntraLASIK. Should a patient know all these
details and more? Probably not, but every refractive surgeon should,
and every good refractive surgeon does. This is why selection of the
surgeon is so very, very important.
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.
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