This retrospective study comprised 21 consecutive patients (42 eyes) diagnosed with moderate to high myopia at the Department of Ophthalmology, Peking Union Medical College Hospital, Beijing, China, between February 2017 and October 2018. The study protocol followed the guidelines of the Declaration of Helsinki and the Institutional Review Board for Human Studies and was approved by the Peking Union Medical College Hospital Institutional Ethics Committee (S-K1768-1). Written informed consent was obtained from all the patients before the study was initiated.
Inclusion criteria for the study were as follows: borderline corneal topography (defined as 1 diopter [D] or greater inferior steepening in some areas but an inferior-superior value of less than 1.4 D or unevenly distributed corneal thickness) or high myopia with thin cornea , potential to improve postoperative refractive stability regarding myopic regression, and therefore lower risk of iatrogenic keratectasia. All patient were aged >18 years and had no other ocular pathologic signs (such as ocular surface infection or allergy). Exclusion criteria included history of ocular surgery or of conditions such as diabetes, autoimmune or endocrine pathologies, dry eye symptoms, insufficient follow-up, re-treatment of accelerated CXL, and pregnancy or lactation.
All procedures were performed by an experienced surgeon (Y.L.). After topical anesthesia with 0.5% proparacaine (Alcaine, Alcon-Couvreur; Puurs-Sint-Amands, Belgium, USP), a corneal flap was created using VisuMax (Carl Zeiss Meditec AG, Jena, Germany) femtosecond laser platforms. With a superior hinge, an intended flap diameter of 8.5 mm, a flap thickness of 90-95 µm, and corneal ablation was performed using the VISX S4IR excimer laser (Abbott Medical Optics, Santa Ana, CA, USA). Then, the corneal bed was saturated with a solution of 0.22% dextran-free riboflavin ophthalmic solution in normal saline (Vibex XtraTM, Avedro, Waltham, MA, USA), which was at the center of the cornea and allowed to soak for 90 s. After complete soaking of the riboflavin, the solution was rinsed from the eye with a 0.9% balanced saline solution. Next, UVA energy was applied at 2.7 J/cm2 with an irradiance of 30 mW/cm2 to the eye that underwent continuous (90 s) light illumination for LASIK Xtra (KXL system; Avedro, Inc., Waltham, MA, USA) (Table A). Finally, the eye was rinsed again with a 0.9% balanced saline solution, and a bandage contact lens (PureVision; Bausch & Lomb, Rochester, NY, USA) was applied to the cornea until complete re-epithelialization was achieved.
After surgery, all patients were prescribed topical 0.5% levofloxacin 4 times daily for 2 weeks, 0.5% loteprednol etabonate (Lotemax; Bausch & Lomb, Tampa, FL, USA), and preservative-free artificial tears (Hycosan; Hylo-Comod, Ursapharm Arzneimittel, Germany) 4 times daily for 1 month, and 0.2% carbomer eye gel (Liposic; Bausch & Lomb, Brunsbütteler Damm, Berlin, Germany) once daily at night for 1 month.
Examinations were performed preoperatively and postoperatively and at 1 and 3 days; 1 week; and 1, 3, 6, and 12 months postoperatively. Slit-lamp examination, best spectacle-corrected visual acuity with and without a pinhole, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA) using a logarithm of the minimum angle of resolution (logMAR) chart with tumbling E, corneal topography (TMS-4N; TOMEY, Erlangen, Germany), and dual Scheimpflug imaging (Gallilei; Ziemer Ophthalmology, Port, Switzerland), and ultrasonic pachymetry (TOMEY Ltd, Aichi, Japan) of the central cornea were performed. Goldmann applanation tonometry and an ocular response analyzer (Reichert Technologies; Depew, NY, USA) were used to measure intraocular pressure (IOP) and corneal biomechanical properties. The corneal demarcation line and the corneal epithelial and stromal thicknesses were measured and evaluated using anterior segment optical coherence tomography (AS-OCT) (Optovue RTVue XR, Optovue; Fremont, CA, USA). Corneal epithelial and stromal thickness profiles were obtained at the thinnest part of the central cornea using 16 peripheral measurements on the corneal vertex, and measurements and statistical analyses of the central 6-mm zone (inner areas, 3 mm zone of corneal vertex; outer areas, 6 mm zone of corneal vertex) were performed . For ultrasound pachymetry, the average measurements of the corneal thickness values were chosen (each single measurement represented the mean of 5 consecutive measurements). Postoperative evaluation included UDVA, corneal biomechanical parameters, corneal topography, and corneal thickness profiles (epithelia and stroma) using AS-OCT.
Data regarding all the evaluated parameters were recorded in a Microsoft Excel spreadsheet (Microsoft; Redmond, WA, USA), and statistical analysis was performed using SPSS for Mac (version 25.0; IBM Corp., Armonk, New York, USA). Data normality was tested using the Shapiro–Wilk test, and a paired t-test was performed to analyze changes between preoperative and postoperative data. If the data were not normally distributed, the Wilcoxon rank-sum test was performed to analyze changes between preoperative and postoperative data. Friedman analysis of variance with the Bonferroni correction was applied for repeated parameter measurements at the 12 months follow-up, in case data did not show a normal distribution. Statistical significant was set at P < 0.05.