Thinner flaps to enhance safety, precision of corneal refractive surgery

Sub-Bowman’s keratomileusis and advances in femtosecond technology will help drive a rebirth of corneal refractive surgery.

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.

Daniel S. Durrie

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.

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.

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 or 60 µm of stroma, which cuts fewer of the nerves, and the recovery is faster.

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 Ziemer 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.

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.

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.

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.

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 industry is that patients continue to be afraid of the surgery. They are afraid of dry eyes. They are afraid of having pain after the 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.

For more information:

• Daniel S. Durrie, MD, can be reached at Durrie Vision, 5520 College Blvd., Suite 200, Overland Park, KS 66211; 913-491-3330; fax: 913-491-9650; e-mail: ddurrie@durrievision.com. Dr. Durrie has no direct financial interest in any of the products mentioned in this article. However, he is a paid consultant for AMO/IntraLase and Alcon.