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    Ultra thin-flap LASIK: how much better is it?

    Results of IntraLase (60kHz) Femto-LASIK with 90 micron flaps


    Although the incidence of post-LASIK ectasia is relatively low, the topic continues to be the focus of much debate.

    Omid Kermani, MD
    There are a number of risk factors known to increase the incidence of ectasia: an abnormal corneal shape, as seen in forme fruste keratoconus, is one major risk factor. The diagnosis of ectasia is based on computerized corneal topography and the condition is an undisputed exclusion criterion for LASIK.

    Regardless of the shape of the cornea, however, the residual thickness of the corneal bed after laser ablation is also believed to play a key role in the development of post-LASIK ectasia, as such, a residual bed thickness of 250 μm is an internationally accepted standard. Some surgeons even advocate using the preoperative corneal thickness as a determining factor in proceeding with LASIK. Certainly the residual bed should be thick enough to tolerate some degree of enhancement, if necessary.

    In the past few years, the ablation algorithms for modern refractive excimer laser systems have changed, with aspheric ablation zones of up to 9.0 mm total diameter now being more routinely used. This has led to an improvement in the optical quality of LASIK procedures because of the ability to reduce existing and induced spherical aberrations, helping to reduce related night vision disturbances. However, these algorithms have also increased the amount of tissue that is removed during the ablation procedure.

    In order for us to take advantage of these new treatment algorithms whilst, at the same time reducing the risk of ectasia, the indication range for LASIK has narrowed.

    It is believed surface ablation procedures such as PRK, LASEK or Epi-LASIK are superior with regards to their limited impact on corneal biomechanical stability. In my opinion, these procedures are, however, known to initiate a compound wound healing mechanism, thus necessitating pharmacological assistance of powerful topical agents such as Mitomycin C at the time of operation or prednisolone acetate for a prolonged period after the operation. Finally, even almost 20 years after the first PRK was performed on a human eye, the problem of pain control remains an issue.

    What are the real benefits of going thinner?

    The introduction of femtosecond laser technology to refractive corneal surgery has opened new perspectives in the preparation of the corneal flap. An ultra-thin flap, possibly designed as a Sub-Bowman's Keratomileusis (SBK), as proposed by Daniel Durry of Durrie Vision, Kansas, USA and Stephen Slade, MD of the Laser Center of Houston, USA, respectively could expand the indication of LASIK again and/or decrease the risk of post-LASIK ectasia.

    While the majority of mechanical microkeratomes are limited to flap thickness preparation of 120 to 130 μm, a femtosecond laser, theoretically, can safely produce any desired flap thickness. Further, the femtosecond laser is capable of creating a planar flap that has consistent thickness across the entire surface of the flap.

    Figure 1: Scanning electron microscope images of the stromal bed (2000 X) reveals the basic difference in the cutting processes. Mechanical dissection by the MK 2000 microkeratome with free fibril ends versus lamellar dissection due to gas bubble expansion with the IntraLase 60 kHz femtosecond laser (pressure causes "sealing" of the surface).
    Prior to initiating a clinical study in which we tested the 60 kHz IntraLase femtosecond laser versus a conventional microkeratome (MK 2000; Nidek), we performed experimental surgery on freshly enucleated pig eyes. The differences in the smoothness and evenness of ablation can be seen in Figure 1.

    Over the past 12 months we have treated 25 eyes (18 patients) with the IntraLase femtosecond laser, applying an intended flap thickness of 90 μm. At the same time 25 eyes (14 patients) received LASIK flap cuts with the MK 2000 microkeratome applying an intended flap thickness of 130 μm. All eyes were treated for myopia or myopic astigmatism with the Nidek EC 5000 CXIII refractive excimer laser.


    Dr Omid Kermani

    Omid Kermani, MD is clinical lead at ocumax® Eye Laser Center, PAN Klinik, Cologne, Germany. Dr Kermani may be reached by ...

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