Atomic force microscopy for the evaluation of corneal surface roughness after femtosecond laser flap creation and excimer ablation

Introduction It is well known that the femtosecond laser lamellar cut induces some degree of surface roughness. Nevertheless, as in femtosecond laser-assisted LASIK (FS-LASIK), an excimer LASIK ablation is performed, and the post-ablation stromal bed should show some degree of smoothening. We decided to compare, using atomic force microscopy (AFM), the roughness of the corneal stromal bed, after a femtosecond lasers device flap was created with or without an excimer myopic ablation. Methods Using 6 freshly enucleated porcine eyes, we created in every eye a flap using a femtosecond laser. Additionally, in 3 eyes, an excimer laser ablation to correct-3 diopters (D) was made. AFM imaging of the remaining corneal stroma was performed. Ten different square areas of 20 μm x 20 μm at the central area of the stroma of each corneal sample were studied. The roughness parameters used were the root-mean-square deviation from a perfectly flat surface. Results The RMS deviation was 360 ± 120 nm in femtosecond laser only, and 110 ± 20 nm in those cases where excimer is also involved ( p < 0.0001). Conclusions Our results show that the roughness of the surface treated with excimer is clearly lower than in the group with no excimer ablation; thus, the application of laser excimer after a flap created by femtosecond laser seems to soften the nano-irregularities created by this technique.