Journal of Refractive Surgery

Letters to the Editor Free

Effect of Corneal Opacity on LASIK Flap Creation With the Femtosecond Laser

Zhen-Yong Zhang, MD

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 flap 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 “Intra-Lase group” had corneal flaps of 100- to 130-μm thickness, whereas eyes in the “LDV group” had only 90-μm thickness corneal flaps. Although the creation of a thinner corneal flap 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 flap thickness, may have produced confounding outcomes. Also, the authors should have specified 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 flap creation, as these specifications would be helpful in interpreting the results presented.

Occurrence of VGB during femtosecond laser flap 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 flap 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 confined by the spot size <1 μm,4 and may explain why no VGB occurred in the LDV group even with a thinner corneal flap. However, there is a possibility that corneal opacity is adjacent to and well beneath the flap 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 definite specification.

Zhen-Yong Zhang, MD

Shanghai, China

The author has no financial or proprietary interest in the materials presented herein.

References

  1. Tomita M, Chiba A, Matsuda J, Nawa Y. Evaluation of LASIK treatment with the Femto LDV in patients with corneal opacity. J Refract Surg. 2012;28(1):25–30. doi:10.3928/1081597X-20111213-01 [CrossRef]
  2. Seider MI, Ide T, Kymionis GD, Culbertson WW, O’Brien TP, Yoo SH. Epithelial breakthrough during IntraLase flap creation for laser in situ keratomileusis. J Cataract Refract Surg. 2008;34(5):859–863. doi:10.1016/j.jcrs.2007.12.043 [CrossRef]
  3. Zhang ZY, Chu RY, Zhou XT, et al. Morphologic and histopathologic changes in the rabbit cornea produced by femtosecond laser-assisted multilayer intrastromal ablation. Invest Ophthalmol Vis Sci. 2009;50(5):2147–2153. doi:10.1167/iovs.08-2400 [CrossRef]
  4. Lubatschowski H. Overview of commercially available femtosecond lasers in refractive surgery. J Refract Surg. 2008;24(1):S102–S107.

Reply:

We thank Dr Zhang for his comments regarding our article.1 When we created 90-μm LASIK flaps using the IntraLase FS 60 (Abbott Medical Optics Inc, Santa Ana, California) in patients with corneal opacity, we often experienced vertical gas breakthrough (VGB). In our study, we explained the characteristics of the IntraLase FS 60 and Femto LDV (Ziemer Opthalmic Systems AG, Port, Switzerland). When the IntraLase FS 60 is used for flap creation, the flap is created by dissecting a flap interface followed by a side cut. If defects are present in the Bowman layer due to corneal opacity, the gas produced when cutting a flap interface may break through the damaged part of the Bowman layer and escape underneath the epithelium. The damaged part of the cornea may heal, but the wound does not return to its original structure.2 Because there is no side cut at this stage, the produced gas does not have a way to escape. In contrast, the Femto LDV dissects the flap interface and the side cut at the same time. Therefore, any gas produced can escape from the side cut and does not cause VGB even when a 90-μm flap is created.

Dr Zhang suspected that the flap interface in our study may have been either anterior or posterior to the corneal opacity. Corneal opacity observed in LASIK patients is generally caused by previous infections triggered by contact lens wear. In these cases, the opacity developed from underneath the epithelium into the stroma without a break.3 Our observations of the depth of the opacity by slit-lamp in this study are shown in the Table. In most cases, if a 90-μm flap is created, the laser would dissect through the corneal opacity. To ensure an adequate distance between the dissecting surface and the epithelium, flaps created with the IntraLase FS 60 were on average 19-μm thicker to avoid VGB in patients with corneal opacity. However, 27 cases of VGB occurred. If we had created 90-μm flaps using the IntraLase FS 60 for these corneal opacity patients, the incidence of VGB may have been significantly higher than reported in our study.

Flap creation time was approximately the same for both femtosecond lasers. As Dr Zhang mentioned, the cutting process of the two lasers is different. The amount of gas produced by one laser pulse and its shockwave is larger when using the IntraLase FS 60.4 However, because the spot size and subsequent gas production from photodisruption is small with the Femto LDV, flap creation needs approximately 10 times more laser pulses. Therefore, we hypothesized that the total gas produced using the Femto LDV should be approximately the same as that of the IntraLase FS 60.

Most of the corneal opacity observed in LASIK candidates was caused by infections triggered by contact lens wear. The opacity would have developed from just underneath the epithelium to the stroma and the Femto LDV could not have avoided corneal opacity when 90-μm flaps were created. We believe the location of corneal opacity in the cornea does not affect the discussion of our study.

Minoru Tomita, MD, PhD

Tokyo, Japan

The author has no financial or proprietary interest in the materials presented herein.

References

  1. Tomita M, Chiba A, Matsuda J, Nawa Y. Evaluation of LASIK treatment with the Femto LDV in patients with corneal opacity. J Refract Surg. 2012;28(1):25–30. doi:10.3928/1081597X-20111213-01 [CrossRef]
  2. Cintron C, Szamier RB, Hassinger LC, Kublin CL. Scanning electron microscopy of rabbit corneal scars. Invest Ophthalmol Vis Sci. 1982;23(1):50–63.
  3. Khurana RN, Li Y, Tang M, Lai MM, Huang D. High-speed optical coherence tomography of corneal opacities. Ophthalmology. 2007;114(1):1278–1285.
  4. Lubatschowski H. Overview of commercially available femtosecond lasers in refractive surgery. J Refract Surg. 2008;24(1):S102–S107.

Opacity Levels

Opacity Level* Percentage of Eyes
Femto LDV Group (n=205) IntraLase Group (n=200)
Light (up to 110 μm) 33.2 20.0
Medium (up to 120 μm) 49.7 72.5
Heavy (up to 130 μm) 17.1 7.5
Authors

The author has no financial or proprietary interest in the materials presented herein.

10.3928/1081597X-20120701-08

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