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Sub-Bowman's keratomileusis and advances in femtosecond technology will help
drive a rebirth of corneal refractive surgery.
by Daniel S. Durrie, MD
One of the topics that I think will be discussed
routinely in 2008 is the area of sub-Bowman's keratomileusis, or thin-flap
LASIK.
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"We
are now starting to
see it being led by science,
not just the vision results
that have been mainly driving
...development over the
years." |
The reason I think this is so important is that we are
now starting to see it being led by science, not just the vision results
that have been mainly driving the LASIK and PRK development over the years.
The science is coming from two areas: recent clinical data that shows that
thinner flaps have quicker visual recovery and better biomechanical
stability.
But it is also being driven by looking at the cornea in
more detail. John Marshall, PhD, of England, has presented data at several
meetings to show us that the fibers in the cornea do not have equal strength
throughout the whole area of the cornea. The stronger fibers are located in
the periphery of the cornea, outside of the central 6 mm. Also, looking at
the cornea from the front-to-back relationship, the stronger fibers are in
the anterior 160 μm of the cornea and the weaker fibers posteriorly.
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We have all known this for years. When we look at corneas
that have developed edema, the edema comes in the back layers of the cornea
where they are loosely attached together and mainly in the center.
What movement is happening with this new level of LASIK
surgery is to make the flap smaller so that we do not cut as many fibers out
in the periphery, and a lot of attention has been on whether we should make
the flap thinner.
What we have found clinically in comparison studies that
we have done in our center is that as we have moved from the 140-μm to
150-μm flaps that we used to make with both blades and with femtosecond
lasers into the area of approximately 100-μm flaps, we have seen fewer dry
eyes, faster visual recover, more accuracy as far as a lower enhancement
rate and, overall, better patient satisfaction.
Also, I think this is supported by the research of how
the cornea is structured because if we do a thinner, smaller flap, we cut
fewer fibers and nerves and, therefore, cause less trauma when we are doing
this elective surgery.
Back to the top.
Better Outcomes?
I think that the series of questions that will be out
there for the scientists and clinicians to look at, and also just for
discussion, will be about why we should go thin. As I said, cutting fewer
nerves and fibers makes sense, but does it show up in the clinical data and
the patient satisfaction?
One area that is being studied fairly intently is the
area of dry eyes. If we look at the early complaints of patients who had
LASIK 10 years ago, one of the big things that popped up was patients who
had dryness that lasted more than the first 3 weeks after surgery and some
for whom it continued to be a significant problem after 1 year.
What we have found as we have made the flaps thinner,
going from 180-μm flaps that we used to make in the old days to around the
140-μm range that was standard for a long period of time with blades, is
that now that we have these new tools such as femtosecond lasers, we can go
quite thin. Because the epithelium is approximately 50-μm thick, most of
these flaps that are being done have only involved 50 μm of 60 μm of stroma,
which cuts fewer of the nerves, and the recovery is faster.
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"Ophthalmologists
have a
tendency to think that
if thin is good, then thinner
is better. I think that we
ought to let the clinical
data lead us." |
We recently finished a study in which we followed
patients over 1 year, and their corneal sensation after thin-flap SBK
surgery has returned totally to normal sensation between the 6- and 12-month
period, whereas earlier published studies showed that there was still loss
of corneal sensation that lasted after a year.
The discussion that is going to take place now will get
into the question of whether or not this affects the overall risk of
keratoconus advancement or ectasia after surgery.
Certainly, it is too early to tell, but there is some
logic to say that if we cut fewer fibers, we would have less risk of
aggravating pre-existing keratoconus. The discussion about ectasia is a
worry that all doctors and patients have at the present time because we are
still trying to answer that question overall with better patient screening
and better surgery.
Steven G. Slade, MD, and I did a study in which we did
one eye with PRK and one eye with thin-flap SBK IntraLase (Advanced Medical
Optics).
We did all the tests that we could think of that were
clinically available for measuring biomechanics, including using a Reichert
Ocular Response Analyzer, a Zeimer Pascal Tonometer, doing sequential
analysis of topography and wavefront, which have all been reported as ways
to measure the biomechanics of the cornea.
We did not see any difference between the eyes that had
thin-flap SBK and PRK, so that is evidence that at least the previously
reported discussions that PRK had better biomechanics afterward than LASIK
does not seem to hold up with the data.
Back to the top.
How thin should we go?
The question that I hear all the time is, if thin is
better, why is it better, and how thin should we go? Ophthalmologists have a
tendency to think that if thin is good, then thinner is better. I think that
we ought to let the clinical data lead us.
We certainly know that flaps between 100 μm and 110 μm
appear to be safe and are giving good results.
I do not want to venture these types of philosophies like
we have seen in other areas of refractive surgery, where people want to
press the envelope. If flaps are too thin, they have a chance to fold and
break through the surface, even with femtosecond lasers.
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"My
personal bias
is that I think the
femtosecond lasers will
be safer because we
have computer-controlled
accuracy." |
There is another discussion that is going on regarding
whether this can also be done with manual microkeratomes. There have been a
couple of papers now reporting that with new microkeratomes, with better
blades, you can do flaps at 100 μm or less.
I think the jury is still out on whether this is just
going to be something that can be only done with a femtosecond laser.
My personal bias is that I think the femtosecond lasers
will be safer because we have computer-controlled accuracy, so my preference
is to use that laser to make the flap when we are going thinner.
Back to the top.
A changing paradigm
I think future studies need to be done. I think there is
reason to believe that people will be moving from the PRK camp that thought
PRK was the only way to go; they are starting to endorse the sub-Bowman's
space. We are also seeing a lot of people who have been doing LASIK for
awhile starting to think that thinner is better and moving into this area.
That brings up the other controversy, which I think is
more of just a discussion. Have we moved away from LASIK and PRK enough that
this deserves a new name?
I think it does. We started doing PRK in 1987. We started
doing LASIK in 1991. Those terms have served us well and explained the
procedure, but the procedure that we are doing today is dramatically
different from either one of those.
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"As
an industry, we need to
continue to make the procedure
safer and more accurate and to
continue to answer these
overall fears" |
It is more accurate, safer and has solved a lot of the
issues that we saw 10 years ago. I think that having ophthalmologists
discuss a new name, whether it is SBK or another variant, is an excellent
thing for our patients and also for the industry in general.
One of the problems that we are seeing with the growth in
the industry is that patients continue to be afraid of the surgery. They are
afraid of dry eyes. They are afraid of having pain after surgery. They are
afraid of bad flaps or developing haze. They are afraid of ectasia.
Overall, the thing that is holding back market growth is
this fear. As an industry, we need to continue to make the procedure safer
and more accurate and to continue to answer these overall fears.
My feeling is that this SBK discussion has been great in
leading us into better microkeratomes in general, both with femtosecond
laser and with blades, and also I think it has moved this level of safety up
much higher than just trying not to get a bad flap.
What we want to do is to have it be safe and cause the
least disruption in the cornea with the procedure. So 2008 will be the year
of the thin flap SBK discussion, and I think it will be great for the
industry.
Back to the top.
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