FEMTO FLAPS & BEYOND

Although similar to modern microkeratomes in terms of visual outcomes and complication rates, the femtosecond laser’s high precision and ease-of-use make it the preferred instrument for flap creation for most surgeons who have used them, according to Beatrice Cochener MD, CHU Brest, Brest, France. 
“There is still a learning curve and certainly you need to perform at least 10 procedures to master the technology. But now there is no need to adjust the parameters, since everything is already set with a very high degree of safety,” Prof Cochener said at the 18th ESCRS Winter Meeting in Ljubljana.
She noted that femtosecond laser systems have undergone a rapid and steady evolution since first introduced as an alternative to the microkeratome for creating LASIK flaps in the early years of this millennium. 
The improvements in the lasers used in refractive surgery include smaller, more tightly-packed cavitation bubbles that can produce an almost continuous cut. They are therefore free of the mechanically-resistant tissue bridges which could make flaps harder to lift.

MECAHNICAL LIBERATION 
“The femtosecond laser is in fact a reproducible laser scalpel that provides us with an easy flap removal thanks to the spontaneous mechanical liberation of the tissue bridges,” she said.
Other refinements include new algorithms for the creation of elliptical flaps or reversed edges designed for the mechanical reinforcement, and thinner flaps and interfaces with lower suction and vacuum pressure.
There are currently five femtosecond lasers for LASIK flap creation on the market. They are the latest version of the original femtosecond laser the Intralase® iFS (AMO), the LDV® (Ziemer), the Wavelight® FS 200, the Visumax® (Zeiss) and the Victus® (Technolas Perfect Vision).
One question that is still debated is whether the benefits of femtosecond lasers justify the higher cost. One of their main selling points is their higher predictability. Studies show that the thickness of LASIK flaps created with femtosecond lasers tends to vary from target values by only around 10 to 15 microns, compared to around 30 microns for the microkeratomes. “In terms of the predictability, the femto-cut appears to be more uniform, more precise and more reproducible than that achieved with a microkeratome,” Prof Cochener said.
However, the bulk of evidence from published studies to date suggests that the femtosecond laser’s improved accuracy in flap creation does not have a significant impact on visual outcomes, she noted. She cited two studies where there was no difference in mean refractive predictability and uncorrected visual acuity between those with flaps created with the latest generation of microkeratomes and those created with femtosecond lasers (Munoz et al, J Cat Refract Surg 2010; 36:934-44, Patel et al, Ophthalmology 2007; 114:1482-1490).
However, in another comparative study involving a total of 2,000 eyes, the femtosecond laser group had more rapid visual recovery and better uncorrected visual acuity than the microkeratome group (Tanna et al, J Refract Surg 2009 Jul; 25(7 Suppl):S668-71).
Studies also indicate that LASIK-treated eyes with femtosecond laser-created flaps have statistically similar results in terms of induced aberrations.
Complication rates with the two technologies are also very similar, although intraoperative complications are more common with microkeratomes and postoperative complications are more common with femtosecond lasers.
For example, in one comparative study the overall rate of flap complications was 14.2 per cent among eyes with microkeratome-created flaps and 15.2 per cent among eyes with femtosecond laser-created flaps (P = 0.5437). However, the rate of intraoperative complications was significantly higher in the microkeratome group, at 5.3 per cent, than in the femtosecond laser group at 2.9 per cent (P = .0111), and the postoperative flap complication rate was significantly lower in the microkeratome group, 8.9 per cent versus 12.3 per cent seen with femtosecond laser (P = .0201). The most common postoperative complication in both groups was diffuse lamellar keratitis (DLK), which occurred in 6.0 per cent of eyes in the microkeratome group, compared to 10.6 per cent of eyes in the femtosecond laser (P = .0002), (Moshirfar et al. J Cat Refract Surg 2010; 1925-1933). 

Femto refractive
Having proved their worth in LASIK-flap creation, femtosecond lasers now look set to enter a new era where they will be used as an alternative to excimer lasers in a potentially LASIK-beating corneal refractive surgery procedure called SMILE (small incision lenticule extraction). The technique involves the cutting of a lenticule within the stroma with the femtosecond laser and then removing it through a small incision.
The technique offers the potential to leave the anterior 40 per cent of the stroma intact. The anterior stroma has a resistance to deformation that is twice that of the posterior stroma. As a result, eyes undergoing the SMILE procedure should be less prone to regression and ectasia. 
“The potential advantages of the SMILE procedure suggest you may be able to safely correct higher amounts of refractive error than we can with the excimer laser, with less thermal effect and with better respect of corneal architecture and better postoperative comfort. In addition, you have one laser for doing everything,” Prof Cochener said.
However, she added some words of caution about the SMILE procedure. “All the assumed potential and promising advantages of this concept (specifically patented by Zeiss laboratory on its Visumax platform) need to be demonstrated by long follow-up studies of a larger series.”

Beatrice Cochener: beatrice.cochener@ophtalmologie-chu29.fr