This prospective cohort study was performed on myopic patients undergoing refractive surgery at Noor Eye Hospital in Tehran, Iran in 2020. Myopia was defined based on manifest refractive spherical equivalent less or equal − 0.5 diopter (D). The moderate myopia was defined as spherical equivalent (SE) 3.00 to 6.00D and high myopia SE > 6.00D. Patients were recruited from myopic cases undergoing refractive surgery. Eligibility criteria for surgery were based on age (≥ 20 years), no sign of ectasia, stable refraction in the past 12 months (a change of ± 0.50 D or less), and residual stromal bed (RSB) thickness RSB + cap > 400µm for SMILE, not including the epithelium RSB > 300µm for femto-LASIK, and ≥ 350 µm for PRK). All patients had been advised to stop wearing contact lenses for at least 4 weeks prior to surgery. Inclusion criteria of this study were myopia of 3.0 D or more, refractive astigmatism of 2.0 D or less, and no corneal surgical history. Patients undergoing SMILE were enrolled consecutively. For each case in the SMILE group, one case each was matched from those undergoing femto-LASIK and PRK.
Matching
Only one eye per individual was enrolled. Patients in each myopia groups (3.00 to 6.00 D and > 6.00 D) were matched in terms of age, gender, and corneal thickness. Matching was based on a range of ± 3.0 years for age and ± 5.0 µm for corneal thickness.
Ethical considerations
This protocol of this study was reviewed and approved by the Ethics Committee of Tehran University of Medical Sciences (ID: IR.TUMS.MEDICINE.REC.1399.193). Written informed consent was obtained from patients to participate in the study. The study adhered to the tenets of the Helsinki Declaration at all stages.
Surgical techniques
SMILE
SMILE was performed using the VisuMax laser platform (Carl Zeiss Meditec AG, Jena, Germany). After topical anesthesia, patients were asked to fix their gaze on an internal light source. First, the posterior surface of the lenticule was cut from the periphery to the center, and then the anterior surface was cut from the center to the periphery. The parameters for lenticule creation were: cap thickness = 120 µm, cap diameter = 7.7 mm, incision angle = 52◦, incision width = 3.0 mm, optical zone = 6.5 mm, and transition zone = 0.1 mm for cases of moderate myopia, and cap thickness = 120 µm, cap diameter = 7.2 mm, incision angle = 52◦, incision width = 3.0 mm, optical zone = 6.0 mm, transition zone = 0.1 mm for high myopia. The postoperative treatment regimen included chloramphenicol eye drop 0.5% (Sina Darou, Tehran, Iran) every 6 hours for 3 days, betamethasone eye drop 0.1% (Sina Darou, Tehran, Iran) every 6 Hours for 1 week, and preservative free artificial tears (Hypromellose) every 6 hours for 1 month.
Femto-LASIK
For Femto-LASIK, after inducing topical anesthesia, first a 110 µm thick flap was created using Femto LDV (Ziemer Ophthalmic Systems AG, Port, Switzerland). Then the flap was lifted, and wave-front optimized ablation was performed using WaveLight Allegretto EX500 (Alcon, Fort Worth, TX, US) in the 6.50mm optical zone for moderate myopia and 6.00mm for high myopia patients with a blend zone of 1.25mm. The postoperative treatment regimen included chloramphenicol 0.5% every 6 hours for 3 days and betamethasone 0.1% every 6 hours for 7 days.
PRK
For PRK, first the corneal epithelium was mechanically scraped without alcohol. Then the WaveLight Allegretto EX500 (Alcon, TX, US) excimer laser was used to free-aberration ablate the 6.50mm optical zone for moderate myopia and the 6.00mm optical zone for high myopia with a 1.25mm blend zone. After laser treatment, a sponge soaked in 0.02% Mitomycin-C was applied to the ablated stroma for 10 seconds per corrected diopter. After rinsing with 30cc sterile balanced salt solution, a bandage contact lens (Ciba vision, Duluth, GA) was applied. The postoperative treatment regimen included betamethasone 0.1% four times a day, levofloxacin eye drop 5mg / ml four times a day, and artificial tears as needed. Daily examinations continued until observation of complete epithelial healing. The bandage contact lens was removed upon reepithelialization, and levofloxacin was discontinued; betamethasone and artificial tears were continued for another 2 weeks, after which fluorometholone 0.1% drops (Sina Darou, Tehran, Iran) was prescribed to be tapered over a course of 3 months.
Pre- and post-operative examinations
Total C-HOA, coma, spherical aberration (SA), and trefoil (3rd to 7th order) were exported in 3 and 6 mm zones (to simulate photopic and mesopic conditions, respectively) using Sirius (Costruzione Strumenti Oftalmici, Florence, Italy) by a single technician before and 3 and 12 months after surgery. The patient was seated in a dark room for 10–20 minutes to dilate the pupil at least 6 mm; the pupil size was measured using a pupilometer (Colvard; Oasis Medical, London, UK).
In addition to C-HOA, uncorrected and corrected distance visual acuity (UDVA and CDVA) were measured using the Snellen SC-2000 system (Nidek Inc., Tokyo, Japan), and refraction was determined using retinoscopy (ParaStop HEINE BETA 200; HEINE Optotechnik, Herrsching, Germany).
Statistical analysis
Analyses were performed using SPSS version 21 (IBM Corp., Armonk, NY, USA). Multiple generalized estimating equations (GEE) were used to examine and compare the 12-month changes in C-HOA indices between the 3 studied groups. Given the individual matched design, unstructured correlation matrix was used in correlation analysis between groups. The significant level was 0.05. Refractive surgery safety index was calculated as postoperative CDVA / preoperative CDVA and efficacy was calculated as postoperative UDVA / preoperative CDVA.