In this retrospective study, 90 eyes with manifest myopia but without astigmatism that required refractive correction at Peking University Third Hospital from January 2018 to September 2022 were analyzed. All patients were sufficiently informed consent of the two algorithms, and the selection of the surgical approach was non-randomized based on preoperative examination and patient preference. Patients meeting the inclusion criteria were consecutively enrolled. The study received approval from the ethics committee of our institute(#KYC2023-363) 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.

The inclusion criteria were age between 18 and 49 years, medically suitable for LASIK, having zero manifest astigmatism in noncycloplegic refraction and only myopia with corrected distance visual acuity (CDVA) no worse than 20/25, 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 were: anisometropia greater than 1.50 D in the spherical equivalent, corneal thickness of less than 500 μm or predictive postoperative residual stromal bed less than 280 μm, topographic and/or tomographic evidence of corneal ectasia, previous ocular surgery, history of herpetic eye disease, corneal scarring, collagen vascular disease, pregnancy, and lactation.

Preoperative evaluation included UDVA and CDVA, manifest cycloplegic and noncycloplegic refraction, slit-lamp biomicroscopy, dilated fundus evaluation, corneal thickness, combined corneal topography and tomography (Sirius; CSO, Florence, Italy), and corneal topography (Fig. 1, Vario Topolyzer, WaveLight; Alcon Laboratories, Inc., Fort Worth, TX). The total corneal aberrations and optic quality in a 6-mm zone were obtained from combined corneal topography and tomography. Parameters of corneal aberrations and optical quality included optical path difference (OPD); root mean square (RMS) of higher order aberrations (HOAs), corneal astigmatism, spherical aberration (SA), coma and Strehl ratio (SR). Corneal cylinder data was obtained from the Vario Topolyzer Placido-based topography.

The anterior topographic data from the Placido Disc-based Topolyzer® Vario

All surgeries were performed by an experienced refractive surgeon (YC) under topical anesthesia. All flaps were created by the WaveLight FS200 femtosecond laser (Alcon Laboratories, Inc). The flap/canal/hinge parameters were: flap thickness of 110 μm; flap diameter of 8.5 to 9 mm; side-cut angle of 100°; hinge angle of 50°; and canal width of 1.5 mm. Following blunt dissection and flap lifting, the stromal bed was ablated with an excimer laser (EX500 WaveLight) using an optical zone of 6.5 mm with a 1.25-mm transition zone.

The software of PAE was offered by Dr. Mark Lobanoff. In the calculate table, after carefully checking and amending the transferred data, we input anterior and posterior corneal negative cylinder (power and axis) of 5 mm zone from Sirius combined topography and tomography [16]. Then, PAE uses geographic imaging software (GIS) to analyze the corneal topographic treatment image produced by the Contoura® Vision planning laptop. Each talus radius is measured, and its refractive effect is analyzed (Fig. 2). The refraction data (sphere, cylinder, and axis) used for all the PAE cases followed the calculated recommended results with PAE (Fig. 3).

The PAE uses geographic imaging software to analyze the corneal topographic treatment image. The talus from Fig. 2 is corresponding to the irregularities image from Fig. 1, and its refractive effect is analyzed

The PAE determines the lower-order astigmatic vector created by all sources (anterior cornea, posterior cornea and lenticular) and uses vector addition to calculate the final astigmatic vector. The figure (marked in red box) showing a manifest astigmatism is zero, but the anterior corneal cylinder and modified cylinder are not

The physiological Q-value provided preoperatively by the Topolyzer was taken as reference. The postoperative Q-target was programmed to be equal to the preoperative physiological Q. The ablation profile was centered on an estimated visual axis determined by the topographer (taking 75% of the pupil toward the corneal vertex [offset value]), which closely approximates the visual axis.

Infrared images of the iris were captured with the Topolyzer Vario topographer to control static and dynamic cyclotorsion during surgery. Patients were requested to focus on a pulsing green fixation light throughout the ablation. All treatments were centered on the pupil and all surgeries were programmed for emmetropia using the treatment nomogram provided by the manufacturer.

Postoperatively, all eyes received treatment with 0.1% fluorometholone (FML; Allergan pharmaceuticals, Dublin, Ireland) in tapering dose for 4 weeks, 0.5% levofloxacin (Cravit; Santen Pharmaceutical (China) Co. Ltd., Jiangsu, China) four times a day for 2 weeks, and lubricating drops four times a day for at least 4 weeks. The follow-up visits data had been obtained for stability at least 6 months. The follow-up examinations involved measurements of UDVA, slit-lamp examination, manifest refraction, CDVA, and combined corneal topography and tomography.

The Shapiro–Wilk test was used for confirming normality of data. Results were expressed as mean ± SD (range); normally distributed data were analyzed using t test between the two groups and the non-normally distributed data were analyzed using Wilcoxon signed-rank test preoperatively. The linear mixed model was applied considering the relevance between eyes in individual patients post operatively. The paired t-test was used to assess differences between preoperative and postoperative parameters under the assumption of normality, and Wilcoxon signed-rank test was used in case the data were not normally distributed. Comparison of the percentage of eyes between the preoperative and the postoperative used the chi-square test and Fisher exact test. Linear regression analyses were performed to compare achieved vs attempted outcomes. In corneal vector analysis, the keratometric refractive index (1.3375) was used. Data were analyzed using SPSS software (version 25.0; SPSS, Inc., Chicago, IL). A P value of less than 0.05 was considered statistically significant.

The results were based on the standard for reporting astigmatism outcomes of refractive surgery [17].