Thin-flap LASIK with a High-frequency, Low-energy, Small Spot Femtosecond Laser – Effectiveness and Safety

Abstract

Objective: To evaluate clinical results of a high-frequency, low-energy, small spot femtosecond laser for the creation of thin corneal flaps in laser in situ keratomileusis (LASIK) used in a comparative case series at a private practice in Brussels, Belgium. Methods: A series of 75 patients selected for LASIK refractive surgery were enrolled for treatment with the Ziemer FEMTO LDV femtosecond laser and received a corneal flap of either 90 µm (59 patients, 103 eyes) or 80 µm (16 patients, 27 eyes) nominal thickness. Prospective evaluation included flap dimensions, intra-and post-operative complications and visual outcomes. 1-5 A thin flap may thus be desirable: it enables the treatment of higher corrections, permits larger ablation zones, induces fewer aberrations, has a lower enhancement rate and better functional results than a conventional >100 µm flap. Moreover, thin flaps help to maximise the RSB -staying further away from the critical 250 µm barrier -and preserve the biomechanical stability of the cornea, hence reducing the risk of ectasia.A critical prerequisite for thin flaps to be a practical alternative is to use a flap-making modality that creates flaps of uniform and predictable thickness. Femtosecond lasers have been shown to meet this condition better than mechanical microkeratomes. The practical limits of femtosecond lasers are determined by the mechanical stability and precision of the docking mechanism that applanates the cornea, by the pulse energy, by the capability of the laser optics to focus the laser beam in the cutting plane and finally by the quality of the achieved dissection.A thinner than conventional flap blends the advantages of lamellar and surface approaches: to preserve as much tissue as possible and at the same time retain an intact flap for fast recovery and protection. 1,6,7 Sub-Bowman keratomileusis (SBK) is a laser procedure that involves the use of a customised corneal flap between 90 and 110 µm with a diameter that is closely matched to the ablation zone of the excimer laser being used, typically ± 8.5 mm. 8One of the principal concerns in thin-flap LASIK is that very thin flaps induce the risk of intraoperative complications (pseudo-buttonhole): the thinner the flap, the closer you get to Bowman's layer. Ultra thin flaps are more difficult to handle and more easily displaced enhancing the risk of flap striae and irregular astigmatism. [9][10][11] Remaining tissue bridges can cause force to be required for separating the flap, which may cause a very thin flap to tear or to over-stretch.