Comparison of Visual Aberrations After SMILE and LASEK for Myopia.

I read with interest the article of Yu et al.1 about the “Comparison of Visual Quality After SMILE and LASEK for Mild to Moderate Myopia.” The authors compared the aberrations of the small incision lenticule extraction (SMILE) technique versus eyes treated with laser-assisted sub-epithelial keratectomy (LASEK), measuring the aberrations with a dilated pupil at 6 mm. After 3 months of the treatment, the aberrations were higher in the LASEK group. But if we look at the induced aberrations in the LASEK group, most of them were due to the spherical aberrations (SMILE: 0.26 μm, LASEK: 0.57 μm). The cause of this could be due to effective smaller optical zone. The authors used an optical zone from 6.5 to 6.6 mm in the SMILE group and 6.25 to 6.75 mm in the LASEK group. That means that the VisuMax (Carl Zeiss Meditec, Jena, Germany) has an effective larger zone than the MEL 80 (Carl Zeiss Meditec), but this is not due to the technique itself. In our opinion, the spherical aberrations can be reduced by targeting a larger optical zone or by optimizing the laser energy at the periphery.2 The spherical aberrations can be reduced if the excimer platform optimizes the laser energy at the periphery, considering the keratometry data. On the other hand, if we observe all other aberrations induced after the refractive surgery, the LASEK group had less vertical coma (0.4 μm) than the SMILE group (0.56 μm), in addition to less coma (0.81 and 0.69 μm, respectively) and vertical trefoil (-0.037 and -0.017 μm, respectively) or horizontal trefoil (-0.15 and -0.11 μm, respectively); these types of aberrations are due to centration. The authors do not mention where they centered the ablation; if this point is different from the center of measurement, there will be aberrations in the measurement. This problem is greater if we choose a small optical zone of 6.25 mm and measure the aberrations at 6 mm; if we have some shift of the treated versus centered point, this will again cause aberrations. The total ablation zone was larger than the flap created (9 mm vs 9.25 to 9.75 mm) in the LASEK group and the authors used a hinge so the total zone was definitively too small. LASEK or other surface techniques have the advantage of ablating less tissue than with SMILE. This allows us to choose larger optical zones and the ablation is still less deep than with the SMILE technique. I cannot understand why the total higher order aberrations are smaller than the spherical aberrations in Yu et al.’s study. It also would have been more interesting to compare the difference postoperatively versus preoperatively than preoperatively versus postoperatively.