Cornea
PK and beyond: the potential of the femtosecond laser

At a glance: Femto advantages
• Femto laser delivers more rapid recovery of BSCVA & less astigmatism • Femto laser in PK improves presicion of donor-host alignment, resulting in faster recovery & improved visual acuity • Predominant disadvantage to femto for PK: location of laser vs. operating room

EyeWorld explores the applications of the laser technology in corneal surgery

Almost seven years after it was first customized to create corneal flaps, the femtosecond laser has become a well-established tool in LASIK procedures. Its foray into corneal surgery, however, is continuing to show great potential.
Corneal specialists first tested the laser’s potential in PK. In a January/February 2007 article published by Ophthalmology Times, Roger Steinert, M.D., professor of ophthalmology, University of California, Irvine, Calif., wrote, “By harnessing the precision and flexibility of the femtosecond laser, [corneal surgeons] are able to create almost an infinite variety of cuts that can enable us to improve the precision and success of corneal surgery. These innovations will allow corneal surgeons to speed recovery, make a secure incision and provide a better quality of vision to our patients.”
More recently, Dr. Steinert co-authored a study demonstrating these possibilities. The research team compared the performance of the femtosecond laser zig-zag incision and conventional blade trephination for PK and concluded that the laser technology delivered more rapid recovery of BSCVA and induced less astigmatism than the latter.
Published in the September 2009 issue of the journal Ophthalmology, the research included 49 eyes of 43 patients who underwent femtosecond laser zig-zag incision pattern PK and 17 eyes of 14 patients who underwent conventional Barron suction trephination PK performed contemporaneously. Lead researcher Marjan Farid, M.D., assistant professor of ophthalmology, University of California, Irvine, and colleagues wrote that all PKs were closed with an identical, 24-bite running nylon suture technique.
Topographically determined astigmatism, best spectacle-corrected visual acuity (BSCVA), and recovery of full visual potential were the main outcome measures in the study. Ranging from 1 to 12 months, the post-op follow-up showed a significant difference in average astigmatism between the groups at post-op month 1 (P = 0.013) and 3 (P = 0.018). By the third month, the study authors wrote, the average astigmatism was 3 diopters (D) in the zig-zag group and 4.46 D in the conventional group. Patients with normal macular and optic nerve function (nZZ = 32; ncon = 14) were found to have a significant difference in BSCVA at month 1 (P = 0.0003) and month 3 (P = 0.006) with 81% of the zig-zag group versus 45% of the conventional group achieving BSCVA of ≥20/40 by month 3 (P = 0.03).
In the discussion section, Dr. Steinert and colleagues wrote, “The ability of the femtosecond laser to produce PK incisions improves the precision of donor–host alignment, which translates into faster recovery of higher levels of visual acuity.”
“The use of appropriate adhesives, corneal “welding” techniques with lasers, or alternative minimal suturing styles has the potential to advance the optical performance of PK,” they wrote.

Beyond PK

Even whilst the safety and efficacy of the femtosecond laser’s use in full thickness corneal transplantations are starting to be determined, experts have already gone on to explore the technology’s application in partial thickness corneal transplants as well.
These procedures include Deep Anterior Lamellar Keratoplasty (DALK) and Descemet’s Stripping Endothelial Keratoplasty (DSEK), and early results are promising, Dr. Farid said. Previously very difficult manual dissections are now being replaced by a laser, so instead of using a blade to try to manually do them, now the laser does it effortlessly, she explained.
In the aforementioned study’s discussion, Dr. Farid and colleagues wrote that they had also been using the zig-zag–shaped incision combined with the deep anterior lamellar keratoplasty big-bubble technique described by Anwar and Teichmann for keratoconus patients. “In this group of young patients in which risk of endothelial rejection is higher than in older patients, a zig-zag deep anterior lamellar keratoplasty would theoretically eliminate the risk of immune endothelial rejection and still provide the additional benefits of wound stability and low astigmatism from the femtosecond zig-zag incision,” they postulated.
Precision cuts with the femtosecond laser potentially provide a perfect fit in partial thickness transplants for the donor cornea and the patient’s cornea, said Christopher J. Rapuano, M.D., professor of ophthalmology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, and co-director, Cornea Service, Wills Eye Institute, Philadelphia. However, he said, “The femtosecond laser has a little harder time cutting through dense scars, so it will do it but it’s not as easy as it is cutting through relatively normal cornea.” Surgeons who are already performing these procedures say there is a learning curve, Dr. Rapuano said.

Logistical obstacles

Despite these exciting possibilities, there are some downsides to using the technology as opposed to manual dissection. For example, Dr. Rapuano said, “Most people don’t have a laser in their operating room.”
“Most people who have this laser have it in their laser center because it’s primarily used for LASIK, so it’s not particularly convenient to use the laser in the laser center and then transport the patient back to your operating room,” he explained.
In addition, once the patient’s cornea has been cut, it is significantly weakened and during that transport time, if they bump their eye it could theoretically open up their eye in a somewhat uncontrolled fashion, Dr. Rapuano said.
However, the above study’s authors wrote, “Potential risks with greater donor manipulation and risks of perforation with hypotony and choroidal hemorrhage during transport from laser suite to the operating room remain theoretical at this point. In our series of patients who have undergone this procedure, no additional risks have been seen compared with traditional blade trephination.”
Dr. Rapuano also said that it’s also expensive to buy the equipment and there’s an added cost each time you make a cut. Using a suction ring on the patient is sometimes not ideal as well, he said.
Finally, with the laser, it takes a reasonably larger amount of time to cut the cornea, Dr. Rapuano said. With just a metal blade it takes one or two minutes each for the donor and recipient, whereas with the laser, it will take upwards of half an hour.
However, Dr. Rapuano noted, “I think [the femtosecond laser] has the potential to really change the way we do corneal transplants and it’s basically still in its infancy.”
Dr. Farid said that right now, researchers will look at continued ways of cutting the cornea in order to create the optimal results for corneal transplantation.

Editors’ note: Drs. Farid and Rapuano have no financial interests related to their comments.

Contact information:

Farid: mfarid@uci.edu
Rapuano: 215-928-3180, cjrapuano@willseye.org