Abstract
Purpose:
To analyze the spherical aberration induced by myopic small incision lenticule extraction (SMILE) and compare with a matched LASIK population.
Methods:
Retrospective analysis of consecutive SMILE treated eyes using a standard spherical lenticule profile (6, 6.5 or 7mm optical zones) to matched LASIK eyes using wavefront guided-optimised profile on the Carl Zeiss MEL80 excimer platform (6mm zone). Ablation depth vs myopia treated for each of the employed profiles were plotted for comparison. For each eye corneal spherical aberration change (6mm) and total postoperative relative tensile strength [based on a previously published model (J Refract Surg. 2013 Jul;29(7):454-60] were calculated and plotted against myopia corrected.
Results:
Mean SEQ was -7.99±2.53D (-2.51 to -13.31) in the 6mm SMILE group (n=72), -5.77±1.83D (-2.63 to -11.19) in the 6.5mm SMILE group (n=69) and -3.95±1.34D (-1.65 to -8.31) in the 7mm SMILE group (n=170). Respectively for SMILE vs LASIK, mean optical zone was 6.70mm vs 6.08mm, mean tissue removal was 107μm vs 81μm, mean induced spherical aberration was 0.11μm vs 0.31μm and mean postop tensile strength was 73% vs 57%. The induced spherical aberration decreased as expected for larger SMILE optical zones; the regression line slope was 0.081 for 6mm, 0.059 for 6.5mm, and 0.030 for 7mm. Comparing LASIK and SMILE 6mm zone treated groups, the induced spherical aberration was similar (slope 0.074), but ablation depth was higher for LASIK (equivalent to a 6.25mm SMILE). SMILE with a 7mm optical zone induced approximately 1/3 of the spherical aberration of the LASIK group (eg for -10 corrections 0.1982 vs 0.5804-μm Z4,0) but leaving significantly greater relative postoperative tensile strength (eg 55% more for a -10 correction) despite consuming more tissue (27% for a -10 correction).
Conclusions:
Despite SMILE lenticule profiles being spherical, induction of spherical aberration in SMILE was lower than aspheric LASIK for equivalent tissue removal. In preserving stronger anterior stromal lamellae, SMILE optical zones can be safely increased to improve spherical aberration control while still leaving postoperative relative corneal tensile strength higher than for an equivalent modern aspheric LASIK procedure.
Keywords: 679 refractive surgery: comparative studies