Skepticism About LASIK Flap Dogma
In the contest between microkeratomes and femtosecond lasers, some surgeons question whether femto flaps are really “all that.”
By René Luthe, Senior Associate Editor
Femtosecond lasers have been hailed as the new standard of care in flap creation, touted as making flaps superior to those created by microkeratomes. While it's undeniably true that the laser's precision allows for certain characteristics of flap architecture that aren't possible with a mechanical device, those benefits are being questioned in some quarters. Some recent research has fomented a bit of agnosticism about the advantages femto flaps can deliver. Here, proponents and skeptics weigh in on the evidence.
On Second Thought …
Recent research by veteran refractive surgeons has led them to question the superiority of femtosecond lasers for flap creation. A study appearing in the May issue of the American Journal of Ophthalmology reported that the planar configuration of the femtosecond laser flap did not offer any advantage in visual outcomes. Corneal higher-order aberrations and visual acuity remained stable three years after LASIK.1 The findings would seem to throw a monkey wrench into “The Great Femto Advance” of recent years, in which 69.5% of the flaps made in LASIK surgery have been made with femtosecond lasers, according to Market Scope's second-quarter survey for this year.
The study's principal investigator, Sanjay V. Patel, MD, says that he and his fellow investigators went into their randomized investigation of the fellow eyes of 21 patients with myopia or myopic astigmatism with open minds. Bladeless flaps were made with the IntraLase FS (AMO), while the Hansatome (Bausch+Lomb) was used to create the mechanical microkeratome flaps. Yet the investigators did not see the advantages that femtosecond-made flaps are reputed to deliver. The study showed no differences in corneal total high-order aberrations, spherical aberration, coma or trefoil between methods of flap creation at any examination (corneal topography and visual acuity were measured before surgery and at one, three, six, 12 and 36 months postop). Additionally, uncorrected and best-corrected VA did not differ between the two methods of flap creation at any examination, and they remained stable postoperatively through 36 months.
And while corneal total high-order aberrations over a 4-mm optical zone remained slightly elevated in the microkeratome group at three years, Dr. Patel says that there was a similar trend in the femtosecond group, but that the study lacked sufficient statistical power, because of a small sample size, to show the difference. Both microkeratome and femtosecond groups had elevated corneal total high-order aberrations over a 6-mm optical zone at three years.
As for femtosecond's presumed advantage of fewer complications, Dr. Patel has yet to see definitive evidence. While the precision of the femtosecond laser theoretically should improve flap creation safety, he points out that problems such as buttonholes and free flaps are very uncommon with the current generation of microkeratomes.
“You would need a very large randomized, controlled study population to determine if femtosecond lasers really create fewer such complications,” he says. “The safety advantage has not been clearly established, and will likely take a meta-analysis to answer that question.”
Dr. Patel attributes the decrease in complications caused by the mechanical microkeratome partly to advances in technology and partly to greater experience among surgeons.
“Surgeons are better informed about the factors associated with flap creation complications, and they recognized when they might have difficulty with a microkeratome,” he says.
Figure 1. Left: Anterior segment OCT shows a 62-micron stromal gap between the IntraLase flap edge and the surrounding peripheral corneal bed. Right: A smaller stromal gap or “femtosecond furrow” in this immediate postop case.
Yet whatever the reason, Dr. Patel reiterates that his group found the same quality of vision in patients with microkeratome-created flaps as in the bladeless group. Dr. Patel says that the team will follow up the eyes again at five years.
James S. Lewis, MD, of Philadelphia shares with the study's authors a less-than-full embrace of femtosecond laser-created LASIK flaps. His attitude can be summed up by the title of a presentation he gave at the 2009 American Academy of Ophthalmology Meeting — “Keep Your Mechanical Microkeratome for LASIK Surgery (Or, Don't Dress Like the Emperor).”
He began taking patients immediately from their LASIK surgery to an anterior segment OCT device (RTVue, Optovue) to image their flaps; some made with a mechanical microkeratome, some made with the IntraLase. “We wanted to see the geometry, the anatomy of the flap edge,” Dr. Lewis explains. “I don't believe anyone has examined the flap architecture minutes after surgery. Most studies use images from one week to three months after LASIK.” He found that the mechanical microkeratome (he used a Moria SBK) produced a semi-planar shaped flap “pretty close to planar, but not planar, whereas the IntraLase flaps were perfectly planar. I am not sure this has any clinical significance.”
Dr. Lewis explains that he had been skeptical of the claims of femtosecond proponents that the reverse bevel possible with the latest generation femtosecond lasers create greater stability, minimizes striae and prevents epithelial ingrowth.
“While these claims sound nice, there are a couple of things that didn't make sense to me as a corneal surgeon. You can't really expect a 60-micron thick ledge (a 110-micron flap equals 50 microns of epithelium plus 60 microns of stroma), something that's 110 microns thick to stabilize a 9000-micron flap. So I decided to image the flap edge and surrounding corneal bed.”
The Moria SBK produced an acute angle approximately 30 degrees at Bowman' membrane. “I fully expected the femtosecond laser to produce more of a 90- or even 120-degree angle at Bowman's membrane,” Dr. Lewis says.
When he examined the edge of the laser-made flap, he discovered something he did not expect: a break between the flap and the untouched corneal stroma peripherally. These “little gutters” he saw in the femto cases ranged from 20 to 80 microns wide. They were present in every meridian except the hinge region (Figure 1).
“That was unexpected. I was expecting the angle to be well defined, as these arguments have been made that it's so much better and so much more stable,” Dr. Lewis says. “In actuality, I found discontinuity, a total absence of stroma and possibly stromal tissue loss. A week later you don't see the gap in the OCT. What you see is an epithelial plug filling the furrow with a slightly different tissue density.”
He says he saw these gaps in every one of 30 Intralase cases performed by different surgeons. “Nothing suggestive of tissue loss was found in the SBK cases.”
In the femto cases, Dr. Lewis continues, “We do not see the well-defined edges that are theorized to make enhancement safer and epithelial ingrowth a thing of the past. In fact, what we see may be the first sign that the femtosecond laser is not a panacea.”
He suspects the possible loss of stroma may be related to the side cut the femtosecond lasers use to complete the flap. Side cuts require more energy from the laser than the lamellar portion of the cut, Dr. Lewis notes; this may be how tissue is destroyed.
Further, he agrees with Dr. Patel that femto-made flaps do not deliver superior optics for the patient. “This higher-order aberration argument is beyond silly,” he says. “It uses Zernicke instead of Wavelets, it ignores tissue registration, tissue curvature, tissue healing and reduces everything to a single value, the root mean square. Don't get me started, this is another subject for another day.”
Dr. Lewis notes that using femtosecond lasers adds an economic burden to the practice as well. The fee charged for utilization of the femtosecond laser adds urgency to increasing, or at least maintaining, LASIK volumes — no mean feat during an infamous slump in LASIK procedures and in the midst of a recession. Additionally, charging patients extra money for “alllaser LASIK” over conventional LASIK is problematic for some surgeons.
“How in good conscience can you believe and advertise that all-laser LASIK is superior and then, when someone can't come up with the extra cash, go back to the microkeratome?” he asks. As a result, Dr. Lewis says, some surgeons solve this ethical dilemma by simply absorbing the cost of the femtosecond laser for those patients unable to pay the additional fee.
Exceptions to Every Rule
Yet even if Drs. Patel and Lewis don't find femto-made flaps all they are cracked up to be, both agree that there are situations in which the femtosecond laser does offer advantages. For instance, Dr. Lewis points out that new surgeons who have already started with femtosecond lasers, or those who don't perform many LASIK procedures, should probably stick with the femtosecond lasers or perform PRK. “You can't dabble in mechanical microkeratomes,” he says. “If you have a decent volume and you have the skill set, that's terrific. But many are not even learning mechanical microkeratomes today.”
Dr. Patel agrees with Dr. Lewis and also bases his exceptions to mechanical microkeratomes on the individual patient's anatomy and corneal parameters. Features such as too-small eyelid fissures or too-prominent brows can prevent the surgeon from obtaining good suction with a mechanical microkeratome, and in these cases, femtosecond lasers could be advantageous, in lieu of a sufficiently compact microkeratome. Dr. Patel says he does not use corneal thickness as a determinant of when to use the femtosecond laser. “Although the femtosecond laser should cut a thinner and more precise flap, which is important for maintaining a safe residual bed, I take the more conservative route and assume that everyone will end up with a thick flap, regardless of the method of flap creation.”
In contrast, Dr. Lewis has studied more than 500 Moria SBK flaps and found the central corneal flap thickness to be 99 microns with less than a 10-micron standard deviation. He feels he leaves even more residual stroma with a mechanical microkeratome than is possible with any femtosecond laser (Figure 2).
Figure 2. Flap thickness profile with Moria One-Plus SBK.
Standing Their Ground
While questions may be growing about the superiority of femto-made flaps, many surgeons affirm their advantages (Figure 3). Mitchell Jackson, MD, of Lake Villa, Ill., for one, remains unconvinced by the AJO study. He reports that he uses the IntraLase in 90% of his refractive surgery cases for three important reasons:
1. Increased quality of vision. This includes improved contrast sensitivity, as well as aberration and acuity advantges.
2. Increased predictability. This, Dr. Jackson says, relates to increased flap thickness consistency based on standard deviation data compared to mechanicals. “That would result in more likelihood of being able to do an enhancement. Additionally, research shows there is less potential ectasia risk and less dry eye.”2,3
3. Increased safety for the patient, thanks to the lower risk of ectasia and dry eye. “Theoretically, thinner flaps eventually result in less dry eye, compared to a mechanical microkeratome where there's much more variability in flap thickness,” Dr. Jackson explains.
Eric D. Donnenfeld, MD, of Long Island, concurs. “The advantage of the ability to make femtosecond flaps is the ability to customize the hinge, the flap characteristics and size,” he says. “Because of this, we can reduce the incidence of dry eye, which is one of the most common concerns of LASIK today.”
“That ability to reduce dry eye is significant, considering that the FDA task force made a big deal about dry eye,” Dr. Jackson says. “You want things that will help guard against that postop.”
Additional reasons for Dr. Jackson's skepticism derive from the AJO study itself. He notes that the femtosecond laser used was a first-generation IntraLase (15 kHz) rather than the current model (150 kHz). “And minimal preop data was obtained, especially whole-eye aberrometry data. Their sample size overall was very low to really be that powerful as well.”
Karl G. Stonecipher, MD, of Greensboro, NC, also has some issues with the study. “The authors acknowledge that total higher-order aberrations increased by one month after LASIK, and although they remained statistically elevated through three years in the microkeratome group only, the small sample size resulted in insufficient statistical power to detect small changes within the bladeless group.”
Safety and Predictability are Paramount
For his part, Dr. Donnenfeld feels that whether the planar configuration of femto-made flaps offers an advantage for higher-order aberrations is rather beside the point. “That's a minor aspect of the advantage of doing a femtosecond LASIK flap.” He too appreciates femtosecond lasers for the enhanced safety they bring to the LASIK procedure.
“I consider a femtosecond flap the most important safety addition to LASIK,” Dr. Donnenfeld explains. “Those safety aspects are that femtosecond flaps have essentially eliminated buttonholes, free flaps and irregular flaps.” These complications may be relatively rare even with mechanical microkeratomes these days, but when they do occur, “they can cause significant vision loss,” he says. “So the worst complications of flap surgery have been removed” by femto-created flaps.
Figure 3. Inverse flap made with an IntraLase FS laser.
Dr. Stonecipher is another fan of the femtosecond's safety and predictability. “The newer femtosecond lasers are adding a whole other layer of safety, as well as flap quality and flap predictability,” he says.
And while both surgeons say that the technology of the mechanical microkeratomes is improving, they contend that mechanical techniques still have important disadvantages. “We're still left with most studies showing that femtosecond lasers come out ahead, albeit at still minute levels in the big picture,” Dr. Stonecipher points out. “In my series of between 28,000 to 29,000 eyes, there's still a fourfold difference in favor of the femtosecond laser in comparison to the microkeratome. In terms of complications, which mainly fall into the category of epithelial-related concerns, you end up with a whole other onslaught of complications related to epithelial issues.” A better flap means the surgeon is less likely to induce higher-order aberrations and other healing-related issues, he adds, which would transmit to visual quality.
Dr. Donnenfeld also notes that the reverse side-cut that can be made with the femto increases flap adherence. “A study by Michael Knorz showed it created three times more flap adhesion and flap stability, and we've presented data that the reverse side-cut also reduces the risk of dry eye after LASIK.”
Further, mechanical microkeratomes cannot make reproducible flaps as thin as the femto can, nor can they customize the size of the flap to the desired 10th of a millimeter, Dr. Donnenfeld says. And while current generation mechanical microkeratomes are safer than previous models, they will always carry with them the risk of buttonholes and free flaps because of the keratome's built-in possibility of a loss of suction. It's a possibility that doesn't exist, Dr. Donnenfeld points out, with certain femtosecond lasers.
As for the “gutters” that Dr. Lewis reports, Dr. Donnenfeld responds that multiple peer-reviewed studies have documented that the stromal bed and hinge edges are more regular with the IntraLase femtosecond laser than with a microkeratome. The next-generation 150 kHz femtosecond laser, he says, creates an even smoother flap.
“And the reverse side-cut of the 150 kHz laser allows the flap to be embedded into the stromal bed, essentially eliminating the gutter seen with traditional microkeratomes. Finally, the typical 30-degree side-cut of most keratomes allows for flap slippage and increases the risk of epithelial in growth compared to an IntraLase femto second flap.”
As to the issue of investing in access to a femtosecond laser at a time when refractive procedures are down and the economy is in a slump generally, some surgeons are convinced that its advantages are well worth the price.
“I don't think you can put a price on safety,” Dr. Donnenfeld says. “As far as I'm concerned, the moderate increase in expense in making a flap with a femtosecond laser is far outweighed by the security that you know you're going to have a perfect flap every time, and it's going to reduce the risk of complications to levels that are much smaller than could possibly be achieved with a keratome.”
The decreased need for enhancement also makes the femto invaluable to refractive surgeons, its proponents say. Dr. Stonecipher reports that use of the femtosecond laser has reduced his enhancement rate by fourfold. He believes the femto has improved the surgeon's ability to “hit the target” the first time because the femto-made flap results in a much drier bed than that made with a microkeratome.
“The technological aspects of the mechanical microkeratome can be nomogrammed to some extent. However, if I'm lasering a little more moist bed than I am with a femtosecond-made flap, that's going to show up in my predictability and final outcomes,” he explains. “But that's the nice thing with the femtosecond laser — when you lift the flap, the bed is pristine and dry, as opposed to the mechanical keratome where it tends to have more moisture.”
Additionally, Dr. Stonecipher has more faith in the science behind femto's advantages than do Drs. Patel and Lewis. “I have a series of 28,000+ eyes that shows reduced enhancement rates, improved outcomes and reduced complications,” he says. “That is why I can say I am glad I made the switch to femotsecond lasers.”
He points out that research over the past eight years conducted by researchers such as Kim, Knorz and Durrie and Kezirian and others appearing in multiple publications bears out these advantages.4,5,6
Maintaining Healthy Skepticism
Essentially, Dr. Lewis simply urges surgeons to exercise a little caution with the Latest Big Thing in refractive surgery.
“There's such an obsession with the femtosecond laser that I don't know if we're ascribing to it features of superiority that it doesn't have,” he says. “That doesn't mean it's not a great machine and won't be important in our ophthalmic future. I just think that a well-maintained microkeratome in the hands of someone who is experienced can reliably achieve an excellent flap that is more physiologic and more cost effective.”
He points out that surgeons have hundreds of years of history with metal-blade corneal incisions. “Basically no tissue loss, separation of tissue with a nonreactive blade; we know we're not singeing the edges of collagen fibers, we know we're not inducing any chemical or energy-related change in tissue. Basically, we have a clean, steel incision, and the body is not in any way mangled. Using a femtosecond laser to create a flap involves processes and tissue responses that are new.” He concedes, however, that this could be an unsubstantiated concern.
It is possible, Dr. Lewis says, that long-term follow up may not be kind to femotsecond lasers. “Fifteen or 20 years ago, who would have believed that a perfectly executed and successful RK would leave such a large number of progressive hyperopes? “Consider that there may be unanticipated findings. Consider dressing differently than the emperor!” OM
1. Calvo R, McLaren JW, Hodge DO, Bourne WM, Patel SV. Corneal Aberrations and Visual Acuity after Laser in Situ Keratomileusis: Femtosecond Laser versus Mechanical Microkeratome. American Journal of Ophthalmology. May 2010. 149:785-793.\
2. Reinstein DZ, Srivannaboon S, Archer TJ, Silverman RH, Sutton H, Coleman DJ. Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part II: quantifying population risk. J Refract Surg. 2006 Nov;22:861-870.
3. Slade SG, Durrie DS, Binder PS. A prospective, contralateral eye study comparing thin-flap LASIK (sub-Bowman keratomileusis) with photorefractive keratec-tomy. Ophthalmology. 2009 Jun;116:1075-1082.
4. Kim J Y, Kim MJ, Kim TI, Choi HJ, Pak JH, Tchah H. A femtosecond laser creates a stronger flap than a mechanical microkeratome. Invest Ophthalmol Vis Sci. 2006 Feb;47:599-604.
5. Knorz MC. Comparison of Flap Adhesion with a Femtosecond Laser and mechanical microkeratome in rabbit eyes. Preseteted at: 2008 Meeting of the American Society of Cataract and Refractive Surgery, Chcago. April 5, 2008
6. Durrie DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis: prospective contralateral eye study. J Cataract Refract Surg. 2005 Jan;31:120-126.