Comparison of corneal ablation depth predictability between small incision lenticule extraction and femtosecond laser-assisted in situ keratomileusis for myopia

Background

Recently, small incision lenticule extraction (SMILE) procedure is used to correct myopia.The clinical safety and efficiency of SMILE have been approved, but its predictability to corneal ablation depth is brought into focus.

Objective

This study was to compare the predictability of ablation depth in central cornea between SMILE and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for myopia.

Methods

A non-randomized controlled clinical study was performed.Two hundred and seventy eyes of 135 myopic patients who were going to receive corneal refractive surgery were included in Beijing Tongren Hospital from October 2015 to May 2016.SMILE and FS-LASIK were performed on 138 eyes of 69 patients and 132 eyes of 66 patients matched in demography respectively under the informed consent.Central corneal thickness was measured by RTVue FD-OCT before and 1 week after surgery.The refractive power, actual ablation depth (difference of central corneal thickness before and after surgery) and central corneal cutting error (difference between theoretically expected ablation depth and real ablation depth) were intergrouply compared, and the correlation of real ablation depth with theoretically expected ablation depth was assessed.

Results

No significant difference was found in spherical power, astigmatic power and spherical equivalent after surgery between SMILE group and FS-LASIK group (t=-1.826, -1.405, -1.420, all at P>0.05). The actual ablation depth was (76.96±15.27)μm in the SMILE group, which was significant lower than (96.76±16.52)μm of theoretically expected ablation depth (t=-23.016, P<0.01); however, there was no significant difference in the FS-LASIK group between actual and expected ablation depth([77.92±18.69] μm versus [77.42±15.60] μm) (t=-0.604, P=0.547). The central corneal cutting error was (20.55±8.51)μm in the SMILE group and (7.17±5.97)μm in the FS-LASIK group, showing a significant difference between them (t=14.950, P<0.01). The positive linear correlations were seen between actual and expected ablation depth in both SMILE group and FS-LASIK group (r=0.799, 0.867, both at P<0.01). The actual ablation depth was increased over expected ablation depth, with the regression equations of Y=3.892+ 0.749X in the SMILE group and Y=3.443+ 0.957X in the FS-LASIK group.

Conclusions

The actual corneal ablation depth is less than expected corneal ablation depth in SMILE procedure, while in FS-LASIK procedure, the actual corneal ablation depth appears to be consistent with the expected one, inferring a good predictability in corneal ablation depth in FS-LASIK surgery.