Flap Issues Snapshot, 2009

Improved technology makes complications less frequent and less significant.

BY RENÉ LUTHE, SENIOR ASSOCIATE EDITOR

While flap complications in LASIK surgery haven't exactly gone the way of the buggy whip yet, the growing use of femtosecond (FS) laser keratomes and improvements in mechanical microkeratomes are making them less frequent, according to leading refractive surgeons in the United States. Here are the insights of researchers who have presented findings at recent ophthalmic meetings.

Femtosecond Continues to Gain Ground

While the majority of refractive surgeons continue to use mechanical microkeratomes, data show that femtosecond laser microkeratomes keep gaining converts. According to the "U.S. Trends in Refractive Surgery: 2008 ASCRS Survey" by Richard J. Duffey, M.D. and David Leaming, M.D., 33% of surgeons reported using the IntraLase (from Abbott Medical Optics) and 1% use Ziemer's Femto LDV. While this means that mechanical microkeratomes still rule, the figures do represent an 11 point increase over the number of users from only the year before.

Similarly, Karl G. Stonecipher, M.D., medical director of TLC in Greensboro, N.C., found in his survey of 101 refractive surgeons at the ASCRS meeting that 40% use an FS laser (Figure 1).¹ The reason they are increasingly chosen by surgeons, he says, is that they show significantly fewer, and less severe, complications than those associated with mechanical microkeratomes.

Figure 1. From "Controversies in Refractive Surgery," Karl G. Stonecipher, M.D.

"In the old days, we were definitely concerned each time we made a flap," says Dr. Stonecipher. "If you ask any surgeon using a microkeratome from the early ’80s through the mid ’90s, they just held their breath when they put the suction ring on and made that pass, concerned they were going to get an epithelial defect or abrasion, or a loose epithelium or, heaven forbid, an irregular flap. As blade technology evolved, that got less stressful. But the real complications of flap making didn't really start to come to what I consider a nominal number until the femtosecond laser."

The buttonholes, incomplete flaps and free caps were not common with mechanical microkeratomes, points out William B. Trattler, M.D., of the Center for Excellence in Eye Care in Miami and a volunteer assistant professor of ophthalmology at the Bascom Palmer Eye Institute, but they were challenging when they did occur (Figure 2). According to Dr. Stonecipher's research, all three are significantly less frequent with the FS laser.

Figure 2. Incomplete flap, with scarring at the flap edge, which is within the visual axis.

While a review of the literature showed that mechanical microkeratomes had a 0.3% rate for buttonholes and Dr. Stonecipher's prospective data from 13,721 cases treated with a mechanical microkeratome showed only 0.015%, his data from 9,061 cases with FS showed 0.0% for buttonholes. For incomplete flaps, the rate of complications according to a review of the literature for mechanicals showed 1.2%, and his prospective data for mechanicals showed 0.19% — while his FS group showed only 0.05%. For free caps, the figures were 0.1%, 0.01% and 0.0%, respectively.

Subtle microstriae are a more common complication of mechanical microkeratomes, Dr. Trattler says, and while they may not be visible under the slit lamp, they can degrade the performance of the patient's visual system.

Similarly, a study by Myung and colleagues compared 1,000 eyes that underwent LASIK using an FS laser keratome with 1,000 eyes that underwent the procedure with a mechanical microkeratome.² Their findings: The FS laser keratome "appears to be superior in reducing postoperative complications," including DLK, flap complications, flap striae and epithelial downgrowth.

"Another common complication that occurs with metal microkeratomes, and to a lesser extent with planar femtosecond flaps, is postoperative dry eye," Dr. Trattler says. "Following the creation of a flap, there is reduced corneal sensitivity that can take many months to more than a year to recover. A study by Dan Durrie showed that metal microkeratome flaps tended to have a larger reduction in corneal sensitivity than planar femtosecond flaps with the same flap thickness. The reduced corneal sensitivity is a major part of the development of postoperative dry eye."

What's the Hold Up?

With such persuasive data for FS laser microkeratomes, the figures of 33% and 40% use for them among surgeons may seem puzzling. Cost issues, particularly during the current economic downturn, are the answer, says W. Bruce Jackson, M.D., director general of the University of Ottawa Eye Institute.

"There's cost of the equipment, there's cost per case, and there's the size of the laser, fitting it into a surgical suite," he explains. "It also takes longer. So there are a number of factors that you would have to look at seriously, unless you've got the patient volume, especially in these times when volumes are down, as to whether to incorporate femtosecond."

Continued innovation, though, may make FS even harder to resist in the future. The new iFS Advanced Femtosecond Laser from AMO, Dr. Jackson points out, allows the surgeon to create a more custom-shaped flap.

Mechanicals Have Their Advantages

Yet with approximately 60% of LASIK surgeons still using mechanical microkeratomes, the modality obviously still has its attractions.

David Huang, M.D., Ph.D., medical director of the Doheny Laser Vision Center and Charles C. Manger III M.D. chair of corneal laser surgery, says he bases his choice of keratome primarily on flap thickness and the likeliness of an enhancement in the patient's future.

"If I need to make a very thin flap, for example, in someone with high myopia and a thin cornea, I tend to use the IntraLase and set it at 100 μm," Dr. Huang explains. "Because the variation is small between flap and also within the flap, it's a relatively planar profile. I know I can get a relatively thin flap, so I'm confident there will be more stromal bed to ablate."

In patients who ar older and hyperopic, he opts for the Zyoptix XP (Bausch & Lomb), especially given the likelihood of more regression occurring with hyperopic ablation and additional hyperopia to come.

"And I tend to use a mechanical microkeratome because I might have to relift a flap, say, 2 years later, which would be difficult if it were an IntraLase flap. I think the XP flap is also very reproducible in flap thickness. And it's easier to relift because I think the surface is probably smoother on a microscopic level."

To avoid complications, he advocates regularly measuring the flap thickness with an OCT system such as the RTVue with a corneal adapter module. In a poster presented by Dr. Huang and colleagues at ARVO that evaluated the repeatability of LASIK flap thickness measurement using Fourier-domain optical coherence tomography (OCT), the repeatability of the flap measurement was 2.25 to 3.8 μm inside the 2 mm central corneal diameter.³

"A surgeon would have more confidence with the residual stromal bed thickness calculation, for both primary LASIK and enhancement, once he has confidence in an accurate measurement of flap thickness," says Camila Saroli, M.D., a coauthor on the poster.

Dr. Trattler agrees with the importance of intraoperative flap thickness measurements. "We know that if you get a thicker-than-expected flap, that potentially can lead to excessive thinning of the residual stromal bed, and result in ectasia. So intraoperative measurements are an important safety step," he says. "It may not be necessary for every case," he says. Before creating the flap, calculate the residual stromal bed thickness. If the procedure will end up close to the lower limit of the bed that you feel comfortable with (250 to 300 μm), intraoperative measurements are important to make sure that you did not get an unexpectedly thick flap, which would lead to excessive thinning of the residual stromal bed. "If the lower limit is 250 μm and you calculate that you'll be at 270 μm after subtracting the estimated flap depth and ablation depth from the preoperative corneal thickness, then intraoperative pachymetry can help make sure that a thicker-than-expected flap is not accidentally created." If this ends up being the case, he says, put the flap back down and do a PRK procedure rather than proceeding with LASIK.

Still, Dr. Huang says he has not seen a flap complication "in years," which he attributes to ever-improving blade technology. Dr. Stonecipher agrees. The majority of refractive surgeons, he points out, still opt to use mechanical microkeratomes.

"What have we done there that's gotten better with the technology? Obviously, the consistency of the flap has gotten better," Dr. Stonecipher says. "One of the things that we always were concerned about during the evolution of this technology was the thickness of the flap and the standard deviation of that thickness. That has gotten remarkably tighter as the keratome technology has improved." A planar flap is now more possible with a keratome, he says. "I think that some of the newer keratomes are allowing you to get more of a femtosecond laser-like flap," he adds, suggesting that the biggest technological advance is the blade itself. "With more precise technology, the blades are allowing for overall better-made flaps with lower complication rates."

FS Issues

Femtosecond lasers, as impressive as surgeons find them, however, still have issues to resolve — or, put more positively, new frontiers to conquer. The primary concern for Dr. Jackson is the patient interface and docking for comfort and centration. Flap size creation and placement also must be addressed in the future, he says.

"I think being able to align where the treatment is being placed on the stromal bed and the flap creation to match that in size and shape will be the way to go in the future," he says, "so that we're not creating large flaps for ablations to try to fit into, but we can tailor the flap size to match the ablation and the position on the cornea."

Dr. Stonecipher regards vertical gas breakthrough as the most significant current challenge with FS lasers. While he says it is not a common problem, it tends to occur when surgeons try to make the flap too thin. To avoid vertical gas breakthrough, he recommends not making the flap thinner than 100-110 μm.

He lists opaque bubble layer as another problem that, while not significant, does require the surgeon to delay the surgery until it resolves, which slows the procedure time. And while there are few flap-related issues associated with the FS, Dr. Stonecipher cautions that there is a learning curve with these devices, mainly with centration of the flap. "People have aborted cases because they haven't well centered the actual flap over the center of the pupil."

Renée Solomon, M.D., in private practice in New York, N.Y., presented research at this year's ASCRS and ARVO meetings on the creation of flaps. One of the issues that remains unresolved, she says, is the flap's smoothness after it is created.

"The reason this is important is because irregular corneal flaps can create an irregular refracting surface," Dr. Solomon explains. "And that irregular refracting surface can result in loss of best-corrected visual acuity, decreased contrast sensitivity, glare and halos. There have been some hypotheses that the smoother the surface, the better the final optical quality and visual outcomes."

She presented data comparing the smoothness of flap beds via scanning electron microscopy with the iFS Advanced Femotsecond Laser, the IntraLase (60 kHz), the Zeimer Femto LDV, and the Zyoptix XP mechanical microkeratome on 16 human eyebank eyes.⁴ The iFS, Dr. Solomon found, created the smoothest stromal surface, which she speculates may be due to its ability to scan the tissue with closer spot and line separation, lower energy and significantly faster flap creation.

"What needs to be evaluated is the impact of this stromal bed regularity on quality of vision issues and on strength of adherence of the flap," she says. "Those are areas for further study." OM

Editor's note: Dr. Trattler has financial interests with Abbott Medical Optics, Allergan and Inspire Pharmaceuticals, among other companies. Dr. Stonecipher has financial interests with Alcon, Allergan, AMO and Inspire, among others. Dr. Jackson has financial interests with AMO and Allergan. Dr. Huang has a financial interest in OptoVue.

References

1. Stonecipher KG. Controversies in Refractive Surgery. Presented at American Society of Cataract and Refractive Surgery Meeting 2009; San Francisco: April 6, 2009.
2. Myung K, Kanellopoulos AJ. Comparison of Femtosecond LASIK to Mechanical Microkeratome LASIK in 1,000 Consecutive Cases. Poster presented at Association for Research in Vision and Ophthalmology 2009 Meeting; Fort Lauderdale, Fla: May 3, 2009.
3. Salaroli CR, Li Y, Ramos JLB, Huang D. Repeatability of LASIK Flap Management with Fourier-Domain Optical Coherence Tomography. Poster presented at Association for Research in Vision and Ophthalmology 2009 Meeting; Fort Lauderdale, Fla: May 3, 2009.
4. Solomon R. Scanning Electron Microscopy Ultrastructural Comparison of 150 kHz Femtosecond Laser Vs. Microkeratome Lamellar LASIK Flap Creation. Presented at American Society of Cataract and Refractive Surgery Meeting 2009; San Francisco: April 6, 2009.