There has been much discussion about LASIK flap thickness. Specifically, there is a trend toward making thinner flaps, but how thin is too thin?
Originally, microkeratomes were set to cut flaps of 160 to 200 microns. Such flaps are easily manipulated and may be less prone to striae than thin flaps. As surgeons pushed the limits of corrections and attempted to treat higher levels of myopia, the depth of available stroma quickly diminished. Furthermore, with newer generation lasers, larger optical zones, and wavefront procedures, more tissue per pulse is removed. Ectasia is a major concern if the cornea is weakened too much, and now we recommend that the residual stromal bed be a minimum of 250 microns. More flexibility and control in flap dimensions is desirable, so microkeratomes were designed to allow surgeons to make larger diameter as well as thinner flaps. Initially, some surgeons reported an increased risk of flap striae with thin flaps. This has not proven to be an issue with flaps greater than 120 microns, but how about with thinner flaps?
In addition to leaving a thicker, stronger stromal bed, thin flaps may also diminish the risk of post-LASIK dry eye by causing less damage to the corneal nerves. As a result, there has been a shift toward thinner flaps. Some surgeons routinely make flaps of 100 microns or less, and a new name has even been given to this procedure: SBK or sub-Bowman’s keratomileusis. Advocates of SBK explain that a thin flap combines the advantages of LASIK and PRK without the disadvantages. Preserving the flap results in faster visual recovery, less discomfort, and minimal risk of haze, and the thinness of the flap preserves the biomechanical properties of the cornea, which are weakened with traditional LASIK flaps. Recent studies have also shown that corneal sensitivity after SBK is similar to that after PRK.
The more precise femtosecond laser keratome (Intralase) enables flaps of this depth to be accurately and reproducibly created. However, ultrathin flaps are more fragile and difficult to manipulate. Care must be taken in handling such thin flaps to prevent tearing, buttonholes, and unwanted striae.
What is the optimal flap thickness? What are the real advantages and disadvantage to a 80 micron versus 90 micron versus 100 micron flap? Some surgeons believe 100 microns is the ideal depth, but others are advocating flaps as thin as 70 microns. Such thin flaps, however, also risk disrupting Bowman’s membrane. If this occurs, then haze, pseudobuttonholes, and irregular astigmatism may result. I believe flaps should not be less than 100 microns for exactly these reasons. Otherwise it’s safer to do surface ablation, especially since newer techniques and postop regimens allow faster and more comfortable recovery. There is certainly an optimal flap thickness that will preserve the advantages of LASIK, not weaken the cornea, and not increase the risk of flap complications. Hopefully, future studies will provide the answers.
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