Patients
Two hundred and eighty-three eyes of 283 patients who underwent bilateral myopia and myopic astigmatism correction with FS-LASIK or SMILE at Peking University Third Hospital from 2017 to 2019 were included in this prospective study.
Inclusion criteria were age ≥ 18 years, myopic sphere up to 8.00 D, cylinder up to 1.50D, with a documented refractive stability for a minimum period of 1 year and discontinuation of soft contact lenses for at least 2 weeks. Exclusion criteria included a residual stromal bed less than 280 μm, topographic evidence of corneal ectasia, noted complications during and after surgical procedure, previous ocular surgery, history of herpetic eye disease, collagen vascular disease, pregnancy and lactation.
In accordance with each patient's wish, SMILE was performed in 102 patients (SMILE group) and FS-LASIK was performed in 181 patients. Among FS-LASIK, 100 patients were randomly assigned to use WFO ablation profile (WFO group) and the other 81 patients were assigned to use TCAT profile (TCAT group). One eye of each patient was randomly chosen for analysis.
The study received approval from the Ethics Committee of our institute and was conducted in accordance with the tenets of the Declaration of Helsinki. A written informed consent was obtained from each patient prior to the surgical procedure.
Preoperative examinations
Preoperative evaluation included uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), manifest refraction, slit lamp bio-microscopy, dilated fundus evaluation, corneal thickness (A scan, Tomey Japan) , corneal tomography (Sirius,CSO,Italy) and corneal topography (Vario Topolyzer, WaveLight, Alcon Laboratories, Inc., Fort Worth, TX). The total corneal aberrations and optic quality in a 6-mm zone were obtained from corneal tomography. Parameters of corneal aberrations and optical quality included optical path difference (OPD), root mean square of higher order aberration (RMSh), RMS of corneal astigmatism, RMS of spherical aberration (SA), RMS of coma and strehl ratio (SR). Corneal topography data of kappa angle, static cyclotorsion compensation for excimer laser ablation and anterior corneal map for topography guided procedure were obtained from the Vario Topolyzer placido-based topography.
Surgical procedures
All surgeries were performed under topical anesthesia by an experienced refractive surgeon (YG Chen). For FS-LASIK, all flaps were created by the WaveLight FS200 femtosecond laser. The flap/canal/hinge parameters were as followed: flap thickness, 110 μm; flap diameter, 8.5 mm to 9.0 mm; side-cut angle, 90°; hinge angle, 50°; canal width, 1.5 mm. Following blunt dissection and flap lift, the stromal bed was ablated with excimer laser (EX500 WaveLight) using an optic zone of 6.5mm with a 1.25mm transition zone. The refraction data (sphere, cylinder and axis) used for the eyes in the WFO group was the subjective manifest refraction. The refraction data used for the eyes in the TCAT group was partially followed topography-modified refraction (TMR) scheme introduced by AJ Kanellopoulos8.
SMILE was performed with the VisuMax femtosecond laser system (Zeiss, Germany, Nomogram version 3.0). The energy setting was 140nJ, and the laser spot spacing was 4.5mm for the lenticule and cap interface, 2.0mm for the lenticule side cut and cap side cut. The spherical refraction data was 10% more on the basis of manifest refraction. The lenticule base thickness was 10~15 mm. The cap thickness was 120mm, and the cap diameter was 7.6mm with 2mm small incision width. The lenticule optical zone (OZ) was 6.5mm with 0.1mm transition zone for astigmatism.
Postoperative care and follow-up
Postoperatively, all the eyes received treatment with 0.1% fluorometholone (FML, Allergan, Inc.) in tapering dose for 4 weeks, 0.5% levofloxacin (Cravit, Santen, Inc.) four times a day for 2 weeks and lubricating drops four times a day for 4 weeks. Follow-up visits included postoperative day 1 and 7, month 1, 3 and 6. The follow-up examinations involved measurements of UDVA, slit-lamp examination, manifest refraction, CDVA and corneal tomography (Sirius, CSO, Italy).
Statistical analysis
Data were analyzed using SPSS software (version 21.0; SPSS, Inc., Chicago, IL). The Kolmogorov-Smirnov test was used for confirming normality of data. All the data were represented as mean ± standard deviation. The normally distributed data were compared among the three groups using One-way ANOVA. The non-normally distributed data were compared among the three groups using Kruskal-Wallis H test. Comparisons of the distribution of visual acuity and refraction among the three groups were analyzed by Pearson chi-square test. A P value of less than .05 was considered statistically significant.