Patients and sampling
This double-masked, randomized contralateral clinical trial was conducted on myopic and myopic astigmatism patients scheduled for PRK at Noor Eye Hospital, Tehran, Iran in 2020 based on good clinical practice principles. Inclusion criteria were age 18–50 years, myopia of −2.00 to −8.00 diopters (D), astigmatism of −4.00 D or less, a stable refraction for at least one year, and contact lens wear abstinence for three weeks. The exclusion criteria were any ocular pathology, corneal dystrophy, keratoconus or keratoconus suspect, glaucoma or glaucoma suspect, diabetes, autoimmune diseases, and history of ocular surgery.
This study adheres to CONSORT 2010 guidelines. The tenets of the Helsinki Declaration were observed in all stages of the study. The protocol of this study was approved by the Ethics Committee of Tehran University of Medical Sciences; it is registered and publicly accessible at: https://www.irct.ir/search/result?query=IRCT20200317046804N1.
Written informed consent was obtained from all participants prior to enrollment.
Randomization, concealment, and masking
Randomization was done in two stages. First, each patient was randomly assigned to one of the two study arms (A and B). In the second stage, randomization was done to allocate fellow eyes of each patient to a type of treatment. In arm A, the randomly selected eye received tPRK and the fellow eye received mPRK. In arm B, the randomly selected eye received tPRK and the contralateral eye received aaPRK. Balanced block randomization with a block size of four was done in both stages to allocate the patients to arms or eyes to treatment. The random sequence of blocks was obtained from the random number table. To maintain concealment, randomization was done by a person other than the surgeon using sequentially numbered, sealed, opaque envelopes. The patients and assessing technicians were unaware of the allocation of eyes until the end of the study, and the surgeon was blind to allocation until surgery.
All procedures were done using the Schwind Amaris excimer laser (SCHWIND eye-tech-solutions GmbH & Co KG) by the same surgeon (H.H.). The target refraction was emmetropia for all eyes. After controlling the fixation and head position by the surgeon and the Schwind system, first the left eye and then the right eye received the allocated treatment.
In mPRK, the cornea was marked with an 8.5 mm marker (B.S.A.Co.) and the epithelium was removed using a hockey-stick spatula. In aaPRK, a 8.5 mm well was placed on the central cornea and filled with ethanol 20% (Sina Daru). After 30 s, the alcohol was absorbed using a small dry polyvinyl alcohol sponge (Merocel, Medtronic, Inc.) and the loose epithelium was removed from the cornea. In both conventional methods, the stroma was ablated with laser immediately after epithelial debridement.
For tPRK, ORK-CAM software module (SCHWIND eye-tech- solutions GmbH & Co KG) was used for automatic measurement of the amount of corneal ablation. In single-step tPRK, first, the cornea is ablated for refractive correction, then, additional ablation is done to compensate for the epithelial thickness (software default is 55 μm).
The optical zone (OZ) was set to 6.5 mm for all eyes. Laser was applied at a frequency of 1050 Hz and wavelength of 193 nm. To reduce patients’ anxiety, the horizontal diameter of the cornea (white to white) was measured using a Castroviejo caliper (B#8232, B.S.A.Co.) after the ablation was over. Mitomycin C (MMC) 0.02% was mixed with fluorescein and applied to the stromal surface for 5 s per one diopter correction using a sponge. The image of the ablated cornea was captured under the cobalt blue light using the camera mounted to the excimer laser machine. Then, the ocular surface was washed with copious amounts of balanced salt solution (BSS). After instilling one drop of chloramphenicol (Sina Daru), a bandage contact lens (Ciba vision) was placed on the cornea. The epithelial removal time in mPRK and aaPRK was measured with a stopwatch. For all cases of tPRK, the epithelial removal time was 13 s as indicated by Schwind’s software.
The postoperative regimen for all patients included betamethasone 0.1% (Sina Daru) and chloramphenicol 0.1% (Sina Daru) eye drops every 6 h and preservative-free artificial tears (Artelac Advanced, Bausch & Lomb) every three hours. Chloramphenicol was discontinued two days after bandage contact lens removal. Betamethasone was continued for two weeks after the operation; thereafter, fluorometholone 0.1% (Sina Daru) was administered every 6 h and tapered over the next three months considering the amount of stromal ablation.
Pre and postoperative assessment
Before the operation, uncorrected and corrected distance visual acuity (UDVA and CDVA) were measured using the Snellen SC-2000 chart (Nidek Inc.) and manifest and cycloplegic refraction were measured using retinoscopy (ParaStop HEINE BETA 200; HEINE Optotechnik) for all patients. They also received a complete ophthalmic examination using slit lamp (Haag-Streit), the Goldmann applanation tonometer mounted on the slit lamp, and the Pentacam HR (Oculus Optikgeräte GmbH).
For epithelial healing assessment, the patients were examined daily by an ophthalmologist until complete healing was achieved. In each visit, slit lamp biomicroscopy was done to assess the CED, corneal transparency, presence of filamentary keratitis, and other complications. For accurate measurement of the CED area, the bandage lens was removed and the cornea was photographed using the Photo-Slit Lamp BX 900 (Haag-Streit) after dying the cornea with fluorescein. Then, a new bandage lens was fitted. After complete re-epithelialization, the bandage lens was removed.
The CED area was calculated using the ImageJ software. Each image was imported into the software and the white-to-white corneal diameter was set on the image as the reference distance. The CED border was marked using a cursor, and its area was calculated automatically by the software.
Pain intensity and other symptoms were recorded at each visit by an interviewer blinded to treatment allocation. An 11-point numerical scale was applied to rate the severity of pain, foreign body sensation, burning, photophobia, dry eye, and tearing with 0 indicating no complaints and 10 indicating maximum complaints. The patient was asked about the presence of each symptom in either eye. If the answer was positive, further questions were asked to determine the eye with more severe symptoms. Then, the patient was requested to rate the severity of the symptom in each eye. The 11-point numerical scale was again administered on the 7th postoperative day through a telephone interview.
The patients were evaluated for visual and refraction outcomes and haze development three months after the operation. Corneal haze was assessed using slit lamp biomicroscopy and graded subjectively according to the method suggested by Fantes et al.  as follows: 0 = clear cornea; 0.5 = trace haze seen only by oblique illumination; 1 = minimal cloudiness barely seen with direct and diffuse illumination; 2 = mild easily visible opacity; 3 = moderate dense opacity that partially interferes with iris details; 4 = opaque cornea.
The sample size was determined based on the mean and standard deviation of the CED size in tPRK and mPRK groups in the third postoperative day in a contralateral eye study . Using 0.56 ± 0.99 and 2.54 ± 1.01 mm for the tPRK and mPRK groups, respectively, and a type I error of 5% (two sided), 99% power, and tPRK-to-mPRK groups ratio of 1:1, a sample size of 10 in each group was calculated. Given that the main purpose of this study was to compare CED size in the first three postoperative days, and since the difference between the two groups in the first and second days after surgery may not be significant, a sample size of 40 in each group was considered. The Stata statistical software: Release 14 (Stata, Corp LP, College Station) and R package version 3.5.2 were used for data analysis. The Q-Q plot was applied to assess the distribution of variables. In each arm, paired t test was used to compare pre- and postoperative visual acuity and refraction, epithelial removal time, subjective symptoms, and corneal haze between tPRK and conventional methods (mPRK and aaPRK).
Qualitative variables were compared using chi square and Fisher’s exact tests. Safety was assessed based on CDVA changes after surgery compared to baseline, and the safety index was calculated as the mean postoperative CDVA divided by the mean preoperative CDVA. The efficacy index was calculated as the mean postoperative UDVA divided by the mean preoperative CDVA. Generalized estimating equations (GEE) were used to compare safety and efficacy indexes between the study groups while controlling the correlation between fellow eyes (unstructured correlation matrix). Accuracy was assessed by comparing the three-month spherical equivalent refractive errors with the intended target refraction. A linear random mixed-effect model was applied to compare the trend of changes in indexes between tPRK and conventional PRK methods, and an unstructured correlation matrix was used to evaluate the correlation between the fellow eyes and follow-up times. An intention-to-treat approach was adopted for analysis. P values less than 0.05 were considered significant.