Femtosecond laser-assisted corneal flap cuts: morphology, accuracy and histopathology. Invest Ophthalmol Vis Sci.

PURPOSE. Precision in corneal flap cutting is essential in LASIK surgery. Current mechanical microkeratomes have a very good performance record; however, in a few cases, complications can occur during the microkeratome pass and flap cut. Femtosecond lasers offer an alternative to the mechanical cut and can provide additional features regarding the flap morphology. In this study, we analyzed femtosecond laser flaps regarding their morphology, cut accuracy, and histopathology. METHODS. Forty-five fresh porcine cadaveric eyes were prepared for femtosecond laser flap cutting with the Femtec femtosecond laser system (20/10 Perfect Vision, Heidelberg, Germany). The eyes were assigned to three different thickness groups, with 120-, 140-, or 180-m cut depth, respectively. In addition, different flap diameters ranging from 8.0 to 9.5 mm and rim edge angulations between 60°and 90°were performed. After the cut, the eyes were examined under a microscope regarding accuracy and potential defects, and flap thickness and diameter were measured. In addition, flaps were prepared for further histopathologic examination. RESULTS. All flap cuts were easily performed without any intraoperative complications. Flap thickness measurements revealed a median (in micrometers) of 110.5 (intended thickness 120), 142.5 (intended 140), and 180.0 (intended 180), respectively. The flap diameter for an intended size between 8.0 and 9.5 mm was within a range of Ϯ0.4 mm, the median at the maximum was 0.3 mm off. Histopathology revealed very low to almost no changes in the stromal structure of the cornea and correct hinge angulations. CONCLUSIONS. LASIK flap cuts were easily performed without any complications. The accuracy and morphology were very precise and consistent. Histopathology revealed a smooth cut with hinge angulations, as expected. (Invest Ophthalmol Vis Sci. 2006;47:2828 -2831) DOI:10.1167/iovs.05-1123 P recisely cut corneal flaps are essential for successful laser in situ keratomileusis (LASIK) treatments and calculation of ablation profiles. Even with modern mechanical microkeratomes, variations in flap thickness and morphology can be found. Differences in flap thickness can be related to several factors, including the microkeratome model used. 1 The manufacturers have made extensive improvements in their microkeratomes over the past years, to create more precise flaps. However, many complications of LASIK surgery are still related to the flap-cutting process. 2 An alternative way of producing LASIK flaps involves modern lasers that are based on the femtosecond laser technology. At the moment, two femtosecond laser systems are commercially available on the market and are U.S. Food and Drug Administration (FDA) approved for the creation of LASIK flaps. One of these systems is the Femtec femtosecond laser system (20/10 Perfect Vision, Heidelberg, Germany). The wavelength of this laser is 1 m infrared, and the spot size adjustable to several micrometers. By photodisruption of the corneal tissue-a nonthermal ablation processthe stromal layers can be divided and a LASIK flap created. The laser-induced vaporized tissue forms a cavitation gas bubble that mainly consists of CO 2 , N 2 , and H 2 O and diffuses out of the cornea through normal mechanisms. 3,4 The advantage in comparison to conventional mechanical microkeratomes is that common complications of LASIK surgery like button holes, free caps, or high variations in flap thickness can be avoided, and the shape of the flap can be individualized to the needs of the patient and surgeon. The purpose of this study was to investigate the performance of the Femtec femtosecond laser system in regard to quality of flap cutting as well as accuracy, reliability, and histopathologic characteristics of corneal flaps. MATERIALS AND METHODS In an experimental laboratory setup, 45 fresh porcine eyes were used for cutting corneal flaps. Before the procedure the corneal epithelium was removed with a corneal scraper in all eyes, to have consistent smooth corneal surfaces, which is important to measure an exact thickness of the created flap. Afterward, the eyes were fixed in an interface, positioned, and attached, by applying suction energy, to the Femtec laser system (20/10 Perfect Vision). The laser spots were applied in a single-pass spiral pattern disc move from the periphery to the center of the cornea, followed by an anterior movement of the spots to cut through the anterior corneal surface. The laser settings were as follows: stromal bed energy of 3 J; bed spacing 8/10 (8-m line separation and 10-m spot separation); rim energy of 4.4 J; rim spacing 3/6 (3-m line separation and 6-m spot separation); edge angulation of 60°, 75°, or 90°; and flap diameter of 8.0, 8.5, 9.0, or 9.5 mm. The intended flap thickness was set at 120, 140, or 180 m. After completion of the laser cut, each eye was observed under the microscope for any damage or complications due to the laser cut. A flap spatula (15470 spatula; Geuder, Heidelberg, Germany) was introduced into the interface, and, by slight movements to both sides, the flap was separated from the underlying stroma. The intent was to cut the flap completely free, to facilitate the investigation of the shape and thickness of the flap. After the flap was removed from the eye, in a subgroup of 12 eyes, the diameter was measured and recorded with a millimeter scale. Thirty eyes were used for thickness measurements, with 10 being in each thickness group. The thickness measurements were performed with a micrometer (Digimatic; Mitutoyo Inc., Kangawa, Japan) measuring instrument (accuracy, 1 m) by positioning the flap immediately after completion of the laser cut without any further manipulation, between two microscope coverslips. The coverslips were compressed with a locking screw with the lowest pressure possible until the first stop of the screw, and the thickness was noted