Microkeratomes vs. the Femtosecond Laser
A discussion of the performance and economic issues.
BY CHRISTOPHER KENT, SENIOR ASSOCIATE EDITOR

For many years surgeons have used mechanical microkeratomes to create the flap during LASIK. Recently, however, the femtosecond laser has been making inroads into that market. Making the LASIK flap with the laser has the potential to resolve some microkeratome-related problems, while producing good outcomes and providing an incentive for blade-cautious patients to undergo the procedure.

At the same time, advances in technology are making mechanical microkeratomes more reliable, accurate and predictable than ever. Some new models boast failsafe devices, automatic self-testing, simplified assembly, a reduced learning curve, better visualization during surgery, real-time monitoring of functions and better manufacturer testing of blade quality. As a result, post-surgical complications such as DLK and epithelial defects are diminishing in frequency.

We asked a number of surgeons -- and the manufacturers of both mechanical and laser-based flap-making technologies -- to comment on the comparative advantages of mechanical microkeratomes and the femtosecond laser in several areas: predictability of flap thickness, complications during and after surgery, outcomes, practical considerations, patient appeal and financial issues.

Predicting Flap Thickness

One of the issues that has plagued mechanical microkeratomes is variability in the thickness of the LASIK flap, which can have serious consequences during and after the procedure.

Results of a study (reported at the ASCRS meeting earlier this year) by Kerry Solomon, M.D., director of the Magill Laser Center at the Medical University of South Carolina, confirmed that microkeratome flaps can be significantly thinner or thicker than the surgeon intends. Dr. Solomon and 18 other surgeons across the country studied 1,600 eyes and all of the major microkeratome brands. They found that a number of factors affect flap thickness, including mean pre-op pachymetry: Thicker corneas tended to end up with thicker flaps, and thinner corneas tended to end up with thinner flaps.

Because of this, the comparatively small variability in laser-produced flaps has become a selling point for the femtosecond laser. Perry Binder, M.D., associate clinical professor (non-salaried) at the Department of Ophthalmology at the University of California in San Diego, is co-medical monitor for clinical studies funded by IntraLase, manufacturer of the femtosecond laser. "Studies we've recently conducted, soon to be published, show a standard deviation of 12 microns for flaps made with the current laser. That means your chance of getting a 180-micron flap when you want a 130-micron flap is close to 1 in 1,000. If a microkeratome has a standard deviation of +/- 40 microns, the chance of having a flap exceeding 200 microns is 1 in 40. Having a standard deviation this low has a huge impact on the predictability of flap thickness."

James Salz, M.D., clinical professor of ophthalmology at the University of Southern California, agrees. "The femtosecond laser doesn't produce the high standard deviation or the wide range that you get with some mechanical keratomes. One microkeratome we tested had a standard deviation of about 25 microns, but more importantly, the range varied from 70 to 200 microns. Dr. Binder's studies have demonstrated not only a tighter standard deviation but a much narrower range. Knowing that the range and standard deviation are much smaller with the IntraLase is very comforting."

However, the variability between different mechanical microkeratomes is significant, which was borne out by Dr. Solomon's study. Two microkeratomes in his study produced standard deviations in flap thickness close to that found using the laser in Dr. Binder's study. AMO's Amadeus 140 produced a standard deviation of 15.5 microns, and Nidek's MK-2000 145 produced a standard deviation of 14.5. (Some studies of other microkeratomes, such as Moria's M2, have also found standard deviations in this range.)

It's worth noting that different studies of any mechanical microkeratome may show substantially better or worse results, making comparisons difficult.

Complications During Surgery

Possibly the strongest point in favor of laser-created flaps is the elimination of many types of intra-operative complications, a consequence of making the flap in a totally different way. To keep this in perspective, it's important to note that vision-threatening complications are very rare when mechanical microkeratomes are used. However, according to IntraLase, more than 69,000 surgeries have now been performed using the femtosecond laser without any severe complications being reported to the FDA.

"The advantage of the IntraLase is that it's very safe," agrees Lee Nordan, M.D., assistant clinical professor of ophthalmology at Jules Stein Eye Institute at the University of California at Los Angeles, and head of the Nordan Laser Eye Medical Group in Carlsbad, Calif. "If you start an IntraLASIK flap and a problem occurs -- for example, the patient moves -- the surgeon can go back a few minutes later and start over without any untoward problems.

"Part of the reason it's so safe is that the laser makes the dissection before making the vertical cut at the edge. If you stop before making the vertical cut, the gas the laser generates to separate the tissue dissipates and the tissue goes back to the way it was. With a microkeratome, you're making the edge cut first. You can't go back and change it."

When it comes to safety, Dr. Binder lists numerous benefits of the femtosecond laser. "With the IntraLase, you don't get decentered flaps, eccentric flaps, partial flaps, irregular flaps, buttonhole flaps, epithelial defects or ingrowth. Our recent studies have shown that the IntraLase is relatively insensitive to corneal thickness, which isn't true for microkeratomes.

"If you lose suction during a microkeratome pass, you can end up with a cap perforation. With the IntraLase you just reapply the suction ring and you can continue without a problem.

"With most mechanical microkeratomes it's difficult to accurately predict the length of your hinge. With the IntraLase you can dial in the hinge length based on an arc between 30° and 120°, and what you dial in is exactly what you get. This is also true for diameter; using a mechanical microkeratome, the steeper the cornea, the larger the diameter of your flap. IntraLase is insensitive to corneal curvature; you get predictable flap dimensions within a fraction of a millimeter.

"With a microkeratome, flap thickness is dependent on translation speed, and there's always a risk of blockage. That doesn't happen with the IntraLase because everything is computer controlled and standardized.

"And, of course, mechanical microkeratomes are sensitive to blade quality and manufacture."

Dr. Salz says he enjoys using the laser because it eliminates many of his concerns. "The rate of complications with almost any modern microkeratome is exceptionally low; maybe 1 out of 500. But when you encounter that problem, it's very disappointing.

"Many of the problems of traditional microkeratomes are really a matter of exposure, tight eyelids or deep-set eyes. With the IntraLase, these problems go away because you don't need a lid speculum. You just put this little suction device on, and it fits on almost anybody. You get the flap diameter you see on your applanation."

However, Dr. Nordan offers a note of caution. "Everybody wants to believe that LASIK can be done by technology, that the surgeon's not important. But making a flap by any means is still a surgical act. Even with the IntraLase, the flap is only as good as the surgeon -- and the outcome is only as good as the flap."

Complications After Surgery

So far, no one has invented a method of making the LASIK flap that eliminates every possible complication. Inflammation, dry eye, epithelial defects and other issues such as flap dislocation are still factors to be dealt with. But microkeratome manufacturers are constantly striving to minimize these problems, and their efforts are paying off. (For example, Moria now reports an epithelial defect rate of only 0.3%, and improvements in its M2 microkeratome have reduced DLK rates to near zero. Similarly, Nidek says there have been no reports of DLK in connection with the MK-2000.)

Dr. Binder notes that people have associated the femtosecond laser with an increased risk of DLK, or Sands of the Sahara Syndrome. He attributes this to less-than-ideal settings that were used in the early days of the instrument. "Initially, the energy level settings the company recommended were rather high, particularly on the side cut. This created a wider gap, allowing tears to enter the interface, causing peripheral interface inflammation in a doughnut shape.

"We've decreased the side cut energy from 6 microjoules down to between 3 and 4. Since then, in my experience, DLK is hardly a problem; in 700 eyes I've never had to lift the flap to irrigate with steroids. I've had to use hourly prednisone acetate in two cases, but that's about the same risk you might have doing a procedure with a microkeratome."

Dr. Binder says he believes the IntraLase does have an advantage in terms of flap displacement. "With a mechanical microkeratome, your side cut architecture is fixed; the angle is between 25 and 30°. With the laser you can vary the side cut angle from 30° to 90°. When you increase the angle, the force required to displace the flap increases dramatically."

Jeremy Meltzer, M.D., who practices in New York City and Bergenfield, N.J., discussed the rate of complications he's experienced with the IntraLase laser during an interview at ASCRS earlier this year. According to Dr. Meltzer, problems with DLK after use of the IntraLase have been overstated. "Out of 1,000 cases we've performed using the IntraLase, only 46 complications developed, including 15 cases of DLK, 11 flaps that needed repositioning, and 8 cases of abrasion. The laser causes inflammation a little more often, but when the instrument's settings are correct, it's a relatively minor problem." Dr. Meltzer also notes that the IntraLase flap has resulted in less post-op follow-up and handholding, and noticeably fewer complaints about dry eye after the surgery.

Recovery time after laser treatment has also been cited as a potential issue. "Recovery was a day or two slower in some cases of higher myopia," says Dr. Nordan, who pioneered use of the earlier models. "However, with the newer models of the IntraLase, the rate of recovery between IntraLASIK and LASIK with a microkeratome is clinically insignificant. The more rapid visual recovery is related to thinner (but not too thin) flaps."

According to a spokesperson for IntraLase, "A number of papers have now validated that 1-day post-op visual acuities with the IntraLase laser are consistent, and in many cases, better than results with a traditional microkeratome. The corneas are very quiet and clear, and users are reporting a faster recovery of corneal sensitivity, less dry eye and a significant reduction in retreatment rates."

Outcomes

In terms of visual results, no clear difference has yet been demonstrated between flap creation technologies; it appears that both technologies do remarkably well in this department. "In my hands, there's no difference in surgical outcomes between Intra-LASIK and the Amadeus microkeratome with a 140 micron thickness flap," says Dr. Nordan. "A good flap is a good flap. Visual outcome in either case still has a lot to do with the surgeon's skill in performing the excimer laser ablation and the replacement of the flap."

However, some surgeons who have used the IntraLase extensively believe their results may be a little better -- if only because of the elimination of some flap problems. And, as Dr. Meltzer points out, the shape of the flap is different, which may have an effect (although this hasn't been proven). "Keratome flaps tend to be meniscus shaped, whereas IntraLase flaps have a consistent depth across the entire flap," he explains. "This may be significant in terms of inducing fewer higher-order aberrations from flap creation."

Patient Appeal

There's no question that the femtosecond laser holds the best cards when it comes to patient perception of surgery. In fact, as nearly everyone we interviewed observed, the appeal of "avoiding the knife" has made the IntraLase laser a useful marketing tool for the surgeons who have purchased or leased one. Also, the idea of reducing some risks is something many patients are willing to pay for. Several doctors have noted that patients will travel significant distances to have the surgery done entirely by laser.

"Patient acceptance is very high," comments Dr. Salz. "Even if you can match the stats of the IntraLase using a mechanical microkeratome, patients won't see it as an equally good choice."

Dr. Meltzer also notes that because the laser is able to make thin flaps safely, he's been able to offer the procedure to more patients.

However, Ron Bache, director of global strategy and U.S. marketing/ refractive at AMO, expresses concern about the effect of appealing to patients by playing on the "no blade" point. "I think ads that insinuate that bladed LASIK isn't the standard of care anymore have a negative effect on the entire industry. Advertising that the IntraLase is safe is one thing. Implying that bladed LASIK is dangerous is quite another."

What Lies Ahead?

Whatever the pros and cons of both technologies, the IntraLase laser is slowly gaining market share. MarketScope, LLC, an industry research organization, estimates that the IntraLase laser was used in 7% of all U.S. vision correction surgeries in the second quarter of 2003, an increase from the 5% market share reported in the first quarter.

Although he believes the new technology to be valuable and expects its use to increase, Dr. Nordan doesn't believe that the IntraLase will ever totally replace the microkeratome. "The factors of surgical outcomes, relative safety, cost, efficiency and microkeratome skill must be evaluated by each surgeon. Each surgeon has to make an individualized decision."

Of course, mechanical microkeratomes continue to improve. As Donald Mikes, vice president of global marketing at Moria, put it, "Mechani-cal microkeratomes haven't reached their limits. We believe it's definitely possible to reduce flap thickness standard deviations, and our R&D efforts are focused on that objective."

Dr. Binder believes that the arguments in favor of the laser will eventually win out. "Mechanical microkera- tomes have limitations that can't compete with laser technology. For example, I don't believe it's possible to create a mechanical microkeratome that's insensitive to corneal thickness and curvature, unless the technology changes dramatically.

"Of course, I've used mechanical microkeratomes for years, and in most cases they work great. But using the IntraLase, you don't get decentered flaps, eccentric flaps, partial flaps, irregular flaps, buttonhole flaps, epithelial defects or ingrowth. And the studies I've participated in indicate that you can reduce the standard deviation for flap thickness by as much as 50% with the laser. It's hard to argue with a technological advantage like that."