Femtosecond intrastromal correction of myopia a viable option

Procedure requires no retractable flap, minimizes dry eye, enhances corneal structural stability and offers stable refractive outcomes.

Despite somewhat difficult dissection and limited potential for enhancement, small-incision femtosecond lenticule extraction yielded outcomes similar to those of LASIK, according to a study.

Small-incision femtosecond lenticule extraction (SMILE) is a variation of femtosecond lenticule extraction (FLEx), which involves the removal of stromal tissue in the treatment of myopia. FLEx was approved for clinical use by the European Union in 2008 but is not approved in the United States.

The procedure is performed with the VisuMax femtosecond laser (Carl Zeiss Meditec). More than 600 eyes have undergone FLEx, according to Walter Sekundo, MD.

“SMILE represents a step forward, with future potential to replace excimer laser in the medium and long-term … using femtosecond laser alone,” he said. “The overall satisfaction is high because the refractive outcomes are at the level of the excimer laser LASIK procedure.”

The 6-month feasibility study established the viability of intrastromal correction of myopia with the SMILE procedure.

Unlike FLEx, SMILE requires no retractable flap, eliminating the risk of flap dislocation and macrofolds, minimizing dry eye, enhancing corneal structural stability and offering more stable refractive outcomes, Dr. Sekundo said during a symposium sponsored by Carl Zeiss Meditec during the American Society of Cataract and Refractive Surgery meeting in San Francisco.

‘Flap’ and opposite paired incisions

The prospective study was conducted at two centers in Germany on 100 eyes of 50 patients. Patient inclusion and exclusion criteria were similar to those used in U.S. Food and Drug Administration studies related to LASIK. Mean manifest refraction spherical equivalent was –4.75 ± 1.56 D.

Dr. Sekundo and Marcus Blum, MD, performed all SMILE procedures between January 2007 and July 2008.

Study data at 6 months included 91 eyes of 48 patients with a mean age of 35.3 years. One-year follow-up data included 47 eyes that underwent voluntary examination.

Intraoperatively, mean lenticule diameter was 6.5 ± 0.3 mm, adapted to a scotopic pupil. Mean diameter of what was ostensibly termed a “flap” was 7.3 ± 0.2 mm. Mean flap thickness was 129.5 ± 7.6 µm. Mean stromal bed thickness was more than 300 µm.

The term “flap” is used loosely because the tissue that is lifted to allow access to the lenticule is technically not a flap, Dr. Sekundo said.

Drs. Sekundo and Blum created about 4.8-mm paired incisions placed 80° apart, superiorly or temporally. Paired incisions were chosen to prevent irregular astigmatism and enhance irrigation of the interface.

High refractive safety and stability

At 6 months postop, 53.3% of eyes gained one or two lines of best corrected visual acuity. One eye had a perforated flap and lost more than two lines of BCVA. However, BCVA in that eye subsequently recovered to loss of one line of BCVA at 1 year, Dr. Sekundo said.

“The refractive outcome was extremely satisfactory,” he said. At 6 months, 79% of eyes were corrected to within 0.5 D of targeted refraction; 85% of eyes were within 0.5 D of intended refraction at 1 year.

“The stability was just amazing,” he said. Manifest refraction spherical equivalent was –0.01 D at 1 month, –0.08 D at 3 months, –0.06 D at 6 months and +0.01 D at 1 year.

“I’ve been doing refractive surgery for the last 15 years, and I’ve never come across this stability in all of the surgeries I’ve ever done,” Dr. Sekundo said. “It’s something that is very particular for this procedure.”

At 6 months, 83% of eyes had uncorrected visual acuity of 20/20 or better, and at 1 year, 81% of eyes had UCVA of 20/20 or better.

Limited potential to correct astigmatism was attributed to the lack of a flap. This is possibly the downside of a flapless procedure because the flap creation by itself produces some biomechanical changes that might be necessary for astigmatism correction, he said. Thus, astigmatism more than 1.5 D of cylinder is best corrected by the standard, full-flap FLEx procedure.

“The downside of not weakening the cornea is that to correct high astigmatism, basically averaging above 1.5 D, you cannot succeed,” Dr. Sekundo said. “If I’m going to treat myopia with a reasonable, standard myopic astigmatism that’s 1 D, I would use SMILE. But if the standard is 1.5 D still, I would cut the full flap, like in the FLEx procedure.”

Results also showed total induced higher-order aberrations of 0.04 ± 0.07 µm, spherical aberrations of 0.01 ± 0.07 µm and coma (Zernike value 3) of –0.04 ± 0.12 µm.

“We basically have not induced any significant higher-order aberrations, neither for spherical aberrations nor for other things,” he said.

Mean subjective quality of vision, assessed on a questionnaire, was 92.3 on a scale of 0 to 100.

“And all patients said they would undergo this surgery again,” Dr. Sekundo said.

Complications included one case of nonprogressive epithelial ingrowth, three tears at the incision edge, three epithelial defects and one incomplete incision opening that was enlarged manually to extract the lenticule.

The SMILE procedure is more challenging than FLEx. However, the learning curve is acceptable for experienced surgeons, he said. – by Matt Hasson

• Walter Sekundo, MD, can be reached at Department of Ophthalmology, Philipps University of Marburg, Germany; e-mail: sekundo@med.uni-marburg.de. Dr. Sekundo is a consultant to Carl Zeiss Meditec.