November 2007

 

REFRACTIVE SURGERY

 

Femto laser for intrastromal ablation


by Matt Young EyeWorld Contributing Editor

   
Excimer laser ablation

Excimer laser ablation, pictured here, could find itself going head to head with intrastromal ablation in years to come Source: Mackool Eye Institute

If successful, the femtosecond laser could perform both flap cutting and lenticule extraction

Femtosecond lasers in ophthalmology are known for their flap-cutting proficiency. While their indications are growing, one in particular stands out: intrastromal ablation. In fact, those familiar with femtosecond technology know that the idea of using it for said purpose has been around the block. Steven C. Schallhorn, M.D., former director of cornea and refractive surgery, Naval Medical Center, San Diego, noted that the picosecond laser, an earlier, slower version of the femtosecond, had been considered for this purpose. Now, Carl Zeiss Meditec (Jena, Germany/Dublin, Calif.) is aggressively researching its VisuMax femtosecond laser system precisely for this, and those who have performed the studies say the indication looks very promising. VisuMax is approved by the Food and Drug Administration for flap cutting but is not indicated for intrastromal ablation, which still is experimental. Intrastromal ablation via femtosecond technology is especially intriguing because if it works, the procedure would be wholly different from LASIK, and yet a direct competitor. Essentially the technology allows a femtosecond laser to perform both flap cutting and lenticule extraction. Dr. Schallhorn, for one, thinks the technology has some appeal but said excimer laser technology is currently much more precise than femtosecond for ablation purposes. However, Walter Sekundo, M.D., Eye Clinic of the Gutenberg University, Mainz, Germany, a primary researcher of this intrastromal technique, which Zeiss is calling Femtosecond Lenticule Extraction (FLEX), said early tests show the procedure holds great promise.

A new ablation

The way FLEX works is via femtosecond pulse to perform laser processing of the lenticule back side, lenticule front side, the flap side cut. and lenticule removal after flap opening. So far, Dr. Sekundo has provided results for 23 eyes that have undergone FLEX. Those results show that the procedure can work but still carries risks at its current developmental stage. Dr. Sekundo appears to believe that when the technology is refined further, it could be an alternative refractive procedure used in clinic day-to-day. In the 23 eyes studied, the lenticule diameter was 6.00 to 7.3 mm adapted to mesopic pupil size. The remaining stromal bed was greater than 300 microns. Patient myopia was less than 8 D. Mesopic astigmatism was less than 0.75 D. Target refraction was –0.5 to –0.75 D. Pre-op, average patient manifest refraction spherical equivalent was –3.88 D. At one month, it was 0.40 D, at three months it was 0.30 D, and at six months (16 eyes by that point had been followed) it was 0.25 D.

“This is remarkable refractive stability after six months,” Dr. Sekundo said.

FLEX’s effect on best spectacle-corrected visual acuity (BSCVA), however, looks like it could use some adjustment. At one month, 30% of eyes lost one line of BSCVA, although that figure narrowed to 17% by three months. Thirty-nine percent were unchanged at one month, and 30% gained one line of BSCVA. At three months, 30% of eyes were unchanged and 52% gained one line. No eye lost two or more lines, or gained two or more lines, of BSCVA. Still, Dr. Sekundo said, “We were very much surprised how well it actually worked.” Certainly, 71% were within +/–0.5 D of attempted manifest refraction spherical equivalent at three months. Further, analysis of the first 10 cases with FLEX found that the procedure did not induce higher-order aberrations (HOAs). That’s interesting, because essentially the procedure—without any need for wavefront—is capable of being an HOA-free procedure. Further, there is no haze or DLK that develops afterward, results showed. Of course the former has traditionally been a problem with surface ablation and the latter an issue with LASIK. Some eyes did, however, develop microstriae. Once nomograms are adjusted, however, results should also get better, Dr. Sekundo said. “I truly believe that FLEX shows extremely high potential to achieve aberrationally neutral myopic treatments,” Dr. Sekundo said. “It’s not as good as the MEL 80 [a Zeiss excimer laser] but it’s new.”

Another view

Dr. Schallhorn, meanwhile, is a bit more skeptical about the precision of intrastromal ablation with femtosecond technology but believes it can be refined.

“With current technology, I don’t believe it will be as precise as the excimer laser,” Dr. Schallhorn said. “It requires more development.”

Today, Dr. Schallhorn said, one pulse of a femtosecond laser is less accurate at removing tissue than one pulse of an excimer laser. However, it’s possible that this could change with further research, he said. “Technology only moves in one direction,” Dr. Schallhorn said. “It’s conceivable that with further advances intrastromal ablation could be viable with future technology.”

It’s also interesting to note that while revolutionary in some aspects, intrastromal ablation is essentially aiming to do the same thing that an excimer laser does.

“To correct myopia, intrastromally you have to remove a lenticule of tissue; a positive lens power because the myopic eye has too much positive power,” Dr. Schallhorn said. “The excimer laser does the same thing with LASIK or PRK. It removes a positive lens power.”

Nonetheless, if FLEX can remove that positive lens power with safety and efficacy comparable to an excimer laser, refractive surgery could find a strong new technological contender within its industry.

Editors’ note: Dr. Schallhorn has a financial interest with Advanced Medical Optics (Santa Ana, Calif.). Dr. Sekundo has spoken on behalf of FLEX for Carl Zeiss Meditec.

Contact Information

Schallhorn: 619-920-9031, scschallhorn@yahoo.com

Sekundo: +49 (0) 61 31/17-5445, sekundo@augen.klinik.uni-mainz.de

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