Comparison Of Planned Versus Achieved Central Corneal Stromal Thickness Reduction In Smile Versus Fs-Lasik: A Retrospective Study

Many factors affect the accuracy of corneal thickness (CT) reduction in small incision lenticule extraction (SMILE)  and femtosecond laser-assisted in situ keratomileusis (FS-LASIK). Accuracy of planned corneal stromal thickness (CST) reduction is essential to the safety of laser vision correction. Overestimation of central CT reduction could increase the exclusion of eligible patients, while underestimation may increase the risk of iatrogenic corneal dilation and postoperative myopia progression. This study was to compare the accuracy of the planned central CST reduction in SMILE and FS-LASIK.

SMILE was performed by the VisuMax ® femtosecond laser system (Carl Zeiss, Jena, Germany). For FS-LASIK, all flaps were created by the WaveLight ® FS200 femtosecond laser (Alcon, Fort Worth, United States). The stromal bed was ablated with the WaveLight ® EX500 excimer laser (Custom-Q algorithm, Alcon, Fort Worth, United States). CT was measured by RTVue XR 100 Optical Coherence Tomography (Optovue Corporate, Fremont, United States) before and after the surgeries.

A total of 77 patients (43 for SMILE, 34 for FS-LASIK) were included in this retrospective study. According to preoperative manifest refraction spherical equivalent (MRSE), all enrolled patients were recorded into established subgroups, which included mild myopia subgroup (MRSE < − 3 D, 21 patients), moderate myopia subgroup (− 3 D ≤ MRSE < − 6 D, 26 patients), and high myopia subgroup (− 6 D ≤ MRSE < − 8 D, 30 patients). Key inclusion criteria: (1) age ≥ 18 years and ≤ 37 years; (2) preoperative best-corrected distance visual acuity (CDVA) ≥ 1.0; (3) sphere up to − 8 D; (4) cylinder < − 2 D. The planned central CST reduction was provided by surgery settings. the achieved central CST reduction was central CSTpre–central CSTpost.

There were no significant differences in patient demographics and other primary baseline data between SMILE and FS-LASIK groups. At postoperative 6-18 months, the central CST reduction was overestimated by 18.49 ± 6.42 μm in the SMILE group (P < 0.001) and underestimated by 2.56 ± 7.79 μm in the FS-LASIK group (P = 0.064). The planned-achieved difference (PAD) of central CST reduction was positively correlated with preoperative MRSE and with planned central CST reduction in both groups. When calculated by manifest refraction (MR) without nomogram adjustment, the central CST reduction was overestimated by 11.14 ± 6.53 μm in the SMILE group and underestimated by 2.83 ± 7.39 μm in the FS-LASIK group. 

In conclusion, SMILE and FS-LASIK provided similar visual outcomes during postoperative 6–18 months. The achieved central CST reduction was highly correlated with the planned central CST reduction for both SMILE and FS-LASIK, while FS-LASIK showed an even closer estimation than SMILE. When removing nomogram adjustment, the PAD of central CST reduction was significantly narrowed in SMILE and maintained in FS-LASIK, suggesting estimation using MR without nomogram adjustment may be feasible in practice. Furthermore, the full CT reduction can also be used for estimation.