In this retrospective study, we compare data from 154 eyes of 86 patients, treated at the same private practices. This study was approved by the local research ethics committee (No. 2022–1980) and complies with the Declaration of Helsinki. Our study is registered in the German Clinical Trials Register, DRKS (DRKS−ID: DRKS00030977). All patients gave informed consent for the use of their routinely collected data for research purposes. In addition, all patients were older than 18 years and none had any ocular disease, previous ocular surgery or trauma, or general disorders affecting the eye. Patients with any systemic diseases that might affect the eye were excluded from surgery. We included only eyes with a spherical equivalent (SE) of −1 to −8 diopters (D).

For comparison, we formed two groups. Group A patients received conventional tPRK treatment with Amaris750. Group B patients underwent refractive surgery using the combined PTK−PRK method using the MEL90 laser. To avoid sampling bias, we randomly select the 154 eyes (86 patients) from a larger cohort (approximately 197 eyes) using the random number function of Excel (Microsoft Excel 2017, Microsoft®). To reduce any bias caused by different preoperative refractive errors and thus different ablation depths, all groups were matched regarding the preoperative SE.

All patients were examined before surgery according to a standard protocol. Visual acuity in terms of subjective refraction, including uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), was collected within 2 weeks before and 6 months after surgery. We also obtained topographic data using Schwind Anterior Segment Analyzers (Peramis and Sirius, Schwind eye−tech solutions) for tPRK patients and the TMS−5 Scheimpflug tomograph (Tomey) and WASCA aberrometer (Carl Zeiss) for PTK−PRK patients all at an optical zone (OZ) of 6.5 mm. For treatment planning, we used the K values of the respective topography device as well as those of an auto-refractometer/keratometer (Nidek).

All operations were performed by the same surgeon using two different laser platforms with a repetition rate of 500-Hz for the MEL90 and 750-Hz for the Amaris 750. For all treatments, topical anesthesia (Oxybuprocaine, Conjuncain EDO® 0.4 mg/mL, Bausch & Lomb) was used. Patients were asked to focus their gaze on the fixation light to center the ablation zone. Group A patients underwent a single stage tPRK procedure in which the excimer laser simultaneously reshapes the corneal epithelium and stroma to correct the refractive error. Patients in group B first received PTK treatment for epithelial ablation, with a depth of 50 μm and diameter of 8mm. Following this, refractive ablation of the stroma was performed using the PRK mode, without any further modifications. Our own ablation nomogram was utilized for this step, and no wavefront-guided mode was employed. It is important to acknowledge that the SCHWIND Amaris750 incorporates an aspherical epithelium ablation, taking into consideration the impact of keratometry and the associated higher energy loss in the periphery. In contrast, the MEL−90 does not account for peripheral energy loss. Regardless of the method employed, all refractive ablations were consistently performed with an OZ of 6.5 mm and a transition zone of 1.5 mm. Mitomycin C (MMC, 0.02%) was applied for 15–30 s (sec), depending on ablation depth. For corrections up to −4 D, MMC was applied for 20 s, while durations of 45 s were employed for corrections exceeding −4 D.

At the end of the procedure, all eyes got a therapeutic contact lens for 5 days, as well as preservative-free eyedrops of ofloxacin (Floxal EDO®, Bausch & Lomb) and dexamethasone (Dexa EDO®, Bausch & Lomb). Postoperative care included the application of Nepafenac eyedrops (0.1%, Nevanac®, Novartis) for 5 days, as well as hyaluronan (Hylo−Comod®, Ursapharm) eyedrops and dexamethasone (Dexa EDO®, Bausch & Lomb) eyedrops for 6 weeks.

Statistical analysis was performed using R Core Team software (R Foundation for Statistical Computing 2021). Refractive results are represented as standard graphs for reporting outcomes in refractive surgery [12], which show the efficacy, safety, predictability, and accuracy of each treatment. The efficacy index (EI) describes the ratio between postoperative UDVA and preoperative CDVA, whereas the safety index (SI) describes the ratio between postoperative CDVA and preoperative CDVA. Predictability is evaluated as the proportion of eyes achieving a postoperative SE within ± 0.50 D of targeted visual acuity and was analyzed using the least squares method. The differences in percent (%) of eyes within 0.50 D between the groups were tested with Fisher's exact test.

The differences in pre- and postoperative parameters were tested using either the independent t−test or Mann–Whitney test, based on whether the assumptions of parametric test were satisfied. Normality was tested with Shapiro–Wilk test, homogeneity of variances with Levene test, and outliers, if any, were identified using the box plot method. The changes within the groups were tested with either Wilcoxon signed rank test or paired t test. And the differences in changes were tested with either Mann–Whitney test or independent t test.