Effect of corneal opacity on LASIK flap creation with the femtosecond laser.

Effect of Corneal Opacity on LASIK Flap Creation With the Femtosecond Laser To the Editor: I read the recent article by Tomita et al, which appeared in the January 2012 issue of the Journal of Refractive Surgery,1 and would like to offer some comments. Tomita et al, in their retrospective study, investigated the respective incidence of vertical gas breakthrough (VGB) for patients with corneal opacity undergoing Femto LDV (Ziemer Ophthalmic Systems AG, Port, Switzerland) and IntraLase FS 60 (Abbott Medical Optics Inc, Santa Ana, California) femtosecond laser fl ap creation for LASIK, and reached a conclusion that VGB with the IntraLase FS 60 is more common than that with the Femto LDV. Eyes included in the “IntraLase group” had corneal fl aps of 100to 130-μm thickness, whereas eyes in the “LDV group” had only 90-μm thickness corneal fl aps. Although the creation of a thinner corneal fl ap did not induce any cases of VGB in the LDV group thought to be potentially associated with the relatively thin depth setting, the study, not well-matched with corneal fl ap thickness, may have produced confounding outcomes. Also, the authors should have specifi ed where in the cornea the opacity was located and whether the femtosecond laser was focused on the scar plane to dissect the stroma for a fl ap creation, as these specifi cations would be helpful in interpreting the results presented. Occurrence of VGB during femtosecond laser fl ap creation is rare and considered to be causatively linked with a focal area of altered epithelium and thinned stroma occurring with corneal scars.2 If the femtosecond laser is directed to focus on the scar plane, some of the generated microbubbles are likely to coalesce into a bigger gas bubble and escape through the low resistance pathway to the surface, rather than progress with the advancing lamellar plane. With this understanding, the fl ap interface in this study may be either anterior or posterior to the corneal opacity (not documented in the article) to avoid the potentially imperfect photodisruption. As such, the difference in the incidence of VGB between the two groups could be attributed to the different photodisruption processes determined by the physical features of the two femtosecond laser systems. The cutting process of the IntraLase FS 60 femtosecond laser is driven by mechanical forces with a shockwave of approximately 4 μm,3 which is more likely to cause VGB as demonstrated in this study; the cutting process of the Femto LDV femtosecond laser is confi ned by the spot size 1 μm,4 and may explain why no VGB occurred in the LDV group even with a thinner corneal fl ap. However, there is a possibility that corneal opacity is adjacent to and well beneath the fl ap interface, which may predispose to VGB especially when the corneal stroma is dissected with a femtosecond laser–induced mechanical shockwave. I believe this study would be strengthened with a more defi nite specifi cation. Zhen-Yong Zhang, MD Shanghai, China