April 2007

 

REFRACTIVE SURGERY

 

The evolution of laser refractive surgery: Are surgeons ready for the next chapter?


by Kristine A. Morrill EyeWorld Contributing Editor

   

Consistent visual results are the ultimate goal

In the 15+ years of laser refractive surgery, surgeons and the companies that make the technology have been on a quest to develop a procedure that delivers the Holy Grail so longed for, but so difficult to achieve—consistent and excellent visual results for all patients. From PRK to ALK to LASIK and back to PRK again, the refractive pioneers have studied and pondered how the cornea responds to excimer laser treatments. Along the way, customized treatments have come along, faded out and bounced back again. “People were led to believe that 20/12 vision was achievable for everybody with Custom ablation,” said

Ronald Krueger, M.D., Cleveland. “It didn’t work perfectly that way, and part of the reason was because of the poorly understood confounding factors of the treatment, such as biomechanics and wound healing.”

The result has been the development of two distinct camps in laser refractive surgery—the PRK proponents (concerned with biomechanics of LASIK) and the advocates of LASIK (concerned with the wound healing of PRK).

The PRK proponents maintain that the procedure is safer, reducing the risk for corneal ectasia as well as avoiding the biomechanical effects caused by flap creation. LASIK advocates point to a relatively painless procedure with rapid visual recovery and better visual outcomes. Recently, however, a third option has arrived on the refractive scene. Sub-Bowman’s keratomileusis, or thin-flap LASIK, may well be able to bridge these two procedures and at the same time deliver the visual results refractive surgeons have long sought.

Here, some of the leading refractive surgeons discuss PRK, LASIK, and the path that is leading them to the next step in laser refractive surgery.

In the beginning…

Italian surgeon

Lucio Buratto, M.D., Milan, was among the first surgeons in the world to perform PRK, and he remembers well the challenges of the early version of this technique. “At the time, we had very small optical zone because that’s all the laser could do and we were very concerned about thinning the cornea or damaging the cornea during ablation,” recalled Dr. Buratto. “The broadbeam laser created a shockwave on the cornea and the impact on Bowman’s and the stroma induced many complications and haze.”

It quickly became obvious that this early form of PRK would not succeed because of the pain, haze, and slow visual recovery patients experienced. The glowing predictions of consumers throwing away their glasses and contact lenses simply did not come to fruition.

Dr. Buratto said instincts told him and other surgeons that it would be better to work inside the cornea, particularly when dealing with high levels of refractive errors. “One of the main concepts of PRK is that if we stay on the surface of the cornea, we can correct more, but we are also removing the sub-Bowman’s layer with PRK, so we’re removing one of the two basic membranes,” he said.

Was LASIK the answer?

Richard L. Lindstrom, M.D., clinical professor of ophthalmology, University of Minnesota, Minneapolis, also started out with PRK, but quite happily switched to LASIK when mechanical microkeratomes were introduced.

“We went rapidly from 5% to 95%,” said Dr. Lindstrom. But, in 10 years of performing LASIK, Dr. Lindstrom and other surgeons in Minneapolis, encountered complications due to the microkeratomes used. They encountered “slipped flaps, striae, DLK, and epithelial ingrowth. In addition, as we’ve watched our patients for a period of time, we have seen that some of our patients have developed corneal ectasia,” he said. Dr. Lindstrom added that with mechanical microkeratomes, particularly the first- and second-generation tomes, the flaps were often too thick, with poor predictability.

“With the earlier generations of mechanical microkeratomes, there was a lot of variability,” said Long Island, N.Y., refractive surgeon Eric D. Donnenfeld, M.D. “You could have flaps that were too thick or too thin, so we chose a more moderate flap thickness of 120 to 140 microns because we wanted to avoid buttonholes.”

These flap-related complications, coupled with less than stellar results when combined with customized ablations, made a number of refractive surgeons question the safety and effectiveness of LASIK.

Back to the surface

Around the time that people began to question the effectiveness of customized treatments,

Marguerite McDonald, M.D, Tulane University, New Orleans, did a comparison study between customized PRK and customized LASIK using the LadarVision system (Alcon, Fort Worth, Texas). The PRK results were better. Dr. Donnenfeld does point out that VISX (Advanced Medical Optics, AMO, Santa Ana, Calif.) did a similar customized comparison with its wavefront-guided system and found no difference between PRK and LASIK outcomes. But, the study results did trigger a renewed interest in PRK.

“There were a lot of things that suggested that PRK would ultimately be better because of reduced biomechanics and other variables caused by the creation of the flap,” recalls Dr. Krueger. “We now know that this was mostly due to the variability in flap thickness that we see with mechanical microkeratomes.”

But, the number one factor driving the move back to surface ablation was safety. As Dr. Lindstrom noted previously, longer-term follow-up of LASIK patients began to uncover cases of cornea ectasia, as well as bad flaps and buttonholes in the short-term. Dr. Buratto has seen just three cases of ectasia in 15 years but calls these cases a “very, very bad experience.”

“We know that ectasia is dramatically reduced with surface ablation and the recommendations of the ectasia committee was that if a patient has even mildly irregular corneal topographies, you should do surface ablation,” said Dr. Donnenfeld. Long a proponent of PRK because of higher degrees of safety, Prof. John Marshall, Ph.D., St. Thomas Hospital, London, remembers urging caution when people began to jump on the LASIK bandwagon: “I was the lone voice in the wilderness because I was saying we need to look at this a bit more carefully.” Prof. Marshall was not surprised when people began to reconsider PRK. “If you work on the surface, you have two days of nastiness, and you have at least three months of vision that isn’t great,” said Prof. Marshall. “But, if you look at the one-year data, certainly in our hands, you see slightly higher visual acuity performance values.”

Still, there is that “nastiness” that makes it a difficult sell for patients interested in laser refractive surgery. “We can never make PRK work as well as LASIK in terms of patient satisfaction,” according to Dr. Lindstrom. “I don’t think there is anything we can do to make them 20/15 on the first day and allow them to go back to work and drive.”

Dr. McDonald doesn’t view it quite so negatively.

“I haven’t done LASIK in three years now. I went back to surface ablation because the results with wavefront laser surgery are even better than with LASIK,” she said. “We can now effectively deal with the previous drawbacks of surface ablation…the gap between the two procedures has closed a lot. Virtually all of my surface ablation patients are able to drive legally without glasses on the first day post-op.” Notwithstanding the position take by Dr. McDonald and a number of other refractive surgeons, according to Dr. Lindstrom, Prof. Marshall and others, it is time to think thin.

The Next K in refractive surgery

“We’ve made a lot of progress on the LASIK side with better flaps and more computer control of the procedure,” said

Daniel S. Durrie, M.D., clinical professor of ophthalmology, University of Kansas, Overland Park. “But in the public’s eye, it’s a good news, bad news scenario with quick visual recovery but problems with dry eye and keratoectasia.”

The answer, according to Dr. Durrie, is a new “K”—sub-Bowman’s Keratomileusis (SBK). With this technique, a 90- to 100-micron flap is created and lifted followed by excimer laser ablation. The thin flap preserves Bowman’s that, according to Prof. Marshall, is integral to the structural integrity of the cornea.

Stephen Slade, M.D., Houston, first floated the concept of SBK beginning in 2003 and 2004, in part to avoid ectasia without the negatives of surface ablation. In the beginning, there were a fair number of skeptics of this approach, but now, greater numbers of surgeons are warming to the concept.

“The neat thing about thin flaps is that you are actually preserving a ‘crust’ of Bowman’s even though it’s cut around the edge,” said Prof. Marshall. “This could actually be better biomechanically than the PRK procedure because you’re putting back the tough anterior stromal material together with this nice crust of Bowman’s.”

A contralateral, prospective clinical study on 50 patients that Dr. Durrie and Dr. Slade did earlier this year seems to indicate that SBK does overcome the lackluster visual performance of PRK during the first three months following surgery. In the study, patients underwent PRK on one eye and SBK, or thin-flap LASIK, on their fellow eye. Flaps were created with the IntraLase FS Femtosecond Laser (IntraLase, now AMO), and all eyes were treated with the Alcon LadarVision Custom Cornea system.

One and three-month visual results showed a statistically significant difference in uncorrected visual acuity between the two groups, with the SBK group outperforming the PRK group (89% versus 49% were 20/20 or better at one month). Forty-two percent of the femtosecond group gained lines of vision compared to 19% at one month. Subjective patient evaluation found that at one month, 40% of the PRK group had fluctuating vision compared with 29% of the SBK group.

The key to the success of SBK is the ability to make a planar flap of consistent thickness across the entire surface of the flap, not just in the center. “The type of flaps that we were making with the mechanical microkeratomes were thinner in the center and thicker in the periphery,” said Dr. Lindstrom. “If you really pushed it with a classic mechanical keratome, you would end up with a flap that was only epithelium in the center or worse, a buttonhole, both of which are very difficult to manage.”

Currently, the most consistent way to make a thin, planar flap is with a femtosecond laser. “With the thinner flaps created by the femtosecond laser, we can lift the Bowman’s layer and work on just the first few layers of the stroma,” said Dr. Buratto. “This means we can have all of the advantages of a very superficial ablation without having to remove Bowman’s and without having to touch the epithelium and damage it.”

These thinner flaps minimize the biomechanics seen with thicker LASIK flaps, leading to better clinical results. “By capturing they eye’s optics with wavefront, we discovered that there were optical changes that we were creating with the surgery that were undesireable. We were creating aberrations,” said Dr. Krueger. “Biomechanical experts are now really understanding what is going on and know that if we cut less, there are fewer problems,” explains Dr. Durrie.

In addition to the reduction in the biomechanical effects of the flap in SBK, the safety profile of this procedure is similar to what is seen with PRK. “Clearly, when you go to a thinner flap with a mechanical microkeratome, even the modern ones, there is a risk of flap complications,” said Dr. Donnenfeld. “That risk with the femtosecond laser has almost disappeared. As long as the surgeon handles the thin flaps delicately, I think it’s the safest way to make flaps today.”

Although little clinical data exists aside from the study of SBK from Drs. Durrie and Slade, a recently published study from Greece suggests that the concept may be valid. Ophthalmologists from the University of Crete published in the August issue of the Journal of Cataract & Refractive Surgery long-term follow-up on eyes that LASIK with flaps of less than 80 microns. Although the flaps were not intended to be that thin, the study found all the patients had rapid visual recovery with good visual outcomes and no long-term flap-related complications. While urging additional studies, the Greek ophthalmologists conclude that what they called “superficial” LASIK could be a new approach that combines surface and LASIK procedures.

Drawbacks to thin flaps

Although the proponents of SBK are growing, there does exist some skepticism about the ease of handling thinner flaps. Belgium refractive surgeon

Erik L. Mertens, M.D., who prefers Epi-LASIK to PRK and LASIK, wonders if there will be increased occurrences of buttonholes.

“In theory, it’s a very good idea, but when you get thinner and thinner, the risk of buttonholes will increase; even with the femtosecond, there is still a risk,” said Dr. Mertens. “I don’t know whether it’s a good idea to preserve the cornea, but create a higher rate of complications.”

“It’s been our experience that thinner flaps are not more prone to complications,” said Dr. Lindstrom. “Some outcomes are suggesting that you get better results because of less retraction of the corneal tissue because you’re cutting fewer fibers.” In his experience, Dr. Lindstrom has found no difficulty in handling the thinner flaps and has found the visual results are better when compared to standard LASIK.

Adds Dr. Durrie: “In our SBK study, we use the 60 kHz laser with 100-micron flaps and found the flaps very easy to handle. I don’t think this will be a problem for surgeons.”

Potential controversy

The proponents of SBK acknowledge that their push to move the laser refractive surgery market to a place that sits between PRK and LASIK will not be without controversy. “We have powerful industry forces at work where you have one group trying to defend the microkeratome and say it’s okay to use thicker flaps,” said Dr. Lindstrom. “Then you have the femtosecond laser which can make safer, thinner, more predictable flaps. Finally you have a group pushing surface ablation.”

Still, said Dr. Lindstrom, controversy is not such a bad thing. “That’s the fun part and in the end the truth wins out. The patients will have a big vote.”

Dr. Slade thinks that while controversial, SBK will bring the different sides together. “I think the PRK people will accept this because they’ve always said we need to be closer to the surface. Bascially, SBK is PRK with a little bit of Bowman’s—it’s a wonderful meeting point.”

Dr. McDonald said that while she is open to the concept of thin-flap LASIK, she will stay on the sidelines of SBK until there are published studies that show superiority of results. In the meantime, she prefers advanced surface ablation. “The studies comparing the two are just wrapping up. True sub-Bowman’s surgery was not even technically possible until recent refinements in the femtosecond laser,” said Dr. McDonald. “Now that it is technically possible, we have to see which approach provides the best clinical results and safety.” What remains to be seen is if mechanical microkeratome manufacturers can find a way to match the flaps created by the femtosecond laser. On this subject, at least Drs. Durrie and Slade remain divided.

“Metal microkeratomes cannot follow into this, so while they’re trying to battle it out with all these weird, negative ads, basically they are being left in the dust,” said Dr. Slade. On the other hand, Dr. Durrie believes that mechanical microkeratome manufacturers will be obligated to try to match femtosecond laser flaps. “The femtosecond is setting a new criteria for making flaps. It might be easier to control with a computer, but someone will try with a mechanical microkeratome,” he said. “I am worried that this might raise the complications rate, which is not something the industry needs at this time. The lasers give us safety and control. “

Final thoughts

From the viewpoints of Dr. Lindstrom and Prof. Marshall, the most important factor in all of this is what is best for the patient. “For patient comfort, you need to be within the stroma,” said Prof. Marshall. “But, from a biomechanical standpoint, you need to be as superficial as you can get. Finally, to eliminate wound-healing problems, you must avoid any damage to the epithelium. That’s why, to my mind, the femtosecond laser offers us the best of both worlds, as of now.”

Dr. Donnenfeld echoed this position: “The headline for me is that thinner flaps offer us more room for doing wider and deeper ablations. They reduce the risk of ectasia and may also lie a bit more regularly and smoothly.” Consider that one year ago, Dr. Donnenfeld used a mechanical microkeratome 100% of the time, today he now uses the femtosecond laser almost 100% of the time. “There’s one reason only—it’s better for my patients.”

Although there are many assertions about the potential benefits and safety of SBK, as the “new K in refractive surgery,” the clinical data will speak for itself, say Dr. Krueger. In this regard, ASA and SBK will be featured at a subspecialty day at the annual ARVO convention, May 2007. Many of these aforementioned discussions will be supported by clinical data.

Editors’ note: Dr. Donnenfeld has financial affiliations with AMO (Santa Ana, Calif.), Alcon (Fort Worth, Tex.), Allergan (Irvine, Calif.), and Bausch & Lomb (Rochester, N.Y.). Dr. Durrie is a consultant for Alcon, Nidek (Gamagori, Japan), WaveLight (Erlangen, Germany), and other ophthalmic companies. Dr. Krueger is a consultant for Alcon. Dr. Lindstrom is a consultant for Acufocus (Irvine, Calif.), AMO, Bausch & Lomb, and IntraLase (now, AMO, Santa Ana, Calif.). Dr. Marshall is a consultant for IntraLase. Dr. McDonald is a consultant for AcuFocus, Allergan, Norwood Abbey (Melbourne, Austrailia), and other ophthalmic companies. Dr. Slade is a consultant for Alcon and Bausch & Lomb. Dr. Merten has financial affiliations with Alcon, Bausch & Lomb, and STAAR Surgical (Monrovia, Calif.).

Contact Information

Donnenfeld: 516-766-2519, eddoph@aol.com

Durrie: 913-491-3737, Ddurrie@Durrievision.com

Krueger: 216-444-8159, Krueger@ccf.org

Lindstrom: 612-813-3633, rllindstrom@mneye.com

Marshall: june.spacey@kcl.ac.uk

McDonald: margueritemcmd@aol.com

Mertens: +32 3 8282949, e.mertens@zien.be

Slade: 713-626-5544, sgs@visiontexas.com

       
The evolution of laser refractive surgery: Are surgeons ready for the next chapter? The evolution of laser refractive surgery: Are surgeons ready for the next chapter?
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