Abstract: Objective    To compare the effectiveness of LASIK flaps created with the Ziemer FEMTO LDV “Classic” femtosecond laser and Ziemer FEMTO LDV Crystal Line femtosecond laser. Methods    Fourier-domain optical coherence tomography (RTVue OCT) was used to measure the shapes of 120 LASIK flaps one week postoperatively in the patients created with the Ziemer “Classic” femtosecond laser (group A, 60 flaps) or the Ziemer Crystal Line femtosecond laser (group B, 60 flaps). The flap thickness was determined by 36 specified points on each flap. The regularity, uniform and accuracy of Ziemer “Classic” FS and Ziemer Crystal FS were compared. Nominal flap thickness was 110μm for two FS systems. Results    The mean flap thickness of group B was thinner than that of group A (103.09±2.38 v.s. 106.93±4.86μm; P<0.01, t-test for independent-samples). The flaps in group B were more regular than those in group A when measured from the center to the periphery. The maximum deviation from the pre-determined 110μm was 7μm in group A and  8μm in group B. Among the total 2160 measured points in 60 eyes, differences greater than 20μm were noted  in 0.13% of group A and 0.02% of group B. Conclusion    The flaps created with Ziemer Crystal Line femtosecond laser are more consistent and thinner than those created by Ziemer “Classic” femtosecond laser.

Key words:  Laser in situ keratomileusis, Fourier optical coherence tomography, Femtosecond laser, Corneal flap thickness