I do not believe that safe and effective SBK can be performed with a microkeratome.
I introduced the IntraLase femtosecond laser (AMO) into our centre in Cologne in November 2005, having received some valuable advice from US surgeon Dr Peter Rapoza in Boston. More recently, we also added Ziemer's Femto LDV femtosecond laser to our practice.
Specifically, I began performing SBK (flap thickness: 90–100 μm) with the 60 kHz IntraLase femtosecond laser in February 2007. Now, approximately 90% of the LASIK cases I perform are Femto-LASIK procedures. My usual flap thickness is 110 μm, although we do create flaps of 100 μm or even 90 μm in select cases.
A microkeratome just isn't precise enough
Admittedly, a disadvantage of femtosecond laser SBK is the prolonged wound healing activity, probably caused as a consequence of heat and pressure released during the laser-tissue-interaction process. However, this disadvantage is outweighed by the improved flap adhesion in SBK where the defined side cut supports strong flap fixation.
Notwithstanding this, safety is our first and most important argument in favour of SBK. Thin flaps leave more residual bed tissue in the cornea, thus decreasing the risk of ectasia. Our SBK cases so far have not developed ectatic problems. We have also been able to increase the range of dioptric corrections that we perform, even in eyes with thin corneas. In our clinic, there have been no instances of buttonholes, false flap cuts or amputated flaps in the more than 3000 cases that we have performed in recent years.
Furthermore, flaps that are created with a femtosecond laser for SBK have a standardized overall geometry; the anatomy of the eye does not influence the outcome in any way because the cornea is applanated over the entire flap diameter. Moreover, there is no frictional deviation, as one sees with mechanical systems, which again is beneficial for the corneal surface.
Overall, there are three key reasons why I truly believe that SBK with a femtosecond laser is advantageous:
1. Precision: higher geometrical standards of created flaps are possible, regardless of the corneal shape.
2. Safety: no flap-related vision-threatening complications and cuts are reversible even in cases with suction loss.
3. Stability: decreased risk of ectasia, increased range of dioptric corrections, and improved flap-adhesion.
What does the future hold?
I think in the future, we will be cutting even thinner flaps and performing true SBK, i.e. immediately beneath Bowman's layer. Admittedly, this will not be achieved easily. To be less invasive, it will be necessary to decrease femtosecond pulse energy by further reducing pulse width while simultaneously increasing pulse frequency. Additional care must also be taken to prevent accumulation and breakthrough of gas-bubbles. Maybe combining the femtosecond laser with the OCT will be an elegant way to visualize and control the laser cutting process.1
We have also observed that it is more difficult to lift thinner flaps for re-treatments after SBK, which might be because of the slightly pronounced wound healing reaction in the anterior corneal lamellae. True SBK might therefore not be quite as widespread as some might believe.
SBK is a must-have!
Notwithstanding this, I think that, for doctors who have a high patient turnover, SBK is a must-have. For me, the conversion was similar to what we experienced in the 1980s with the introduction of the Nd:YAG laser. Laser capsulotomy simplified the treatment of secondary cataract and thereby helped pave the way for a revolution in IOL technology. Conversion to SBK, in my opinion, will follow a similar trend; eventually, everybody will be doing it.
To read Dr Massimo Camellin's counter-argument, please go to http://www.oteurope.com/2008debates/cutflap/no
1. O. Kermani, W. Fabian & H. Lubatschowski. Am. J. Ophthalmol. 2008;146(1):42-45.