Background
Transepithelial photorefractive keratectomy (TransPRK) is becoming increasingly popular in the treatment of myopia. TransPRK has a higher laser cutting frequency than traditional PRK. The unique feature of this technique is that it removes the corneal epithelium and stroma in a single step with one ablation profile. Its advantages include flap free, minimal trauma to the eye and without flap-related complications [
1]. Moreover, the corneal biomechanics are less affected than other refractive procedures, including Small Incision Lenticule Extraction (SMILE) [
2]. Also it allows reoperation. Previous studies have demonstrated that TransPRK is safe, predictable and effective in the correction of myopia and myopic astigmatism [
3‐
6]. A study showed that TransPRK and femtosecond-assisted laser in situ keratomileusis (LASIK) share similar refractive outcomes in myopia correction [
7]. Another study found that TransPRK using SmartPulse Technology (SPT) provides significant accelerated healing and visual rehabilitation than without SPT [
8]. However, there is a lack of comparative data on the safety, efficacy and refractive outcomes between low to moderate myopic eyes and high myopic eyes after TransPRK surgery.
This prospective clinical study evaluated the early visual acuity, refractive error and efficacy outcomes of TransPRK in different ranges of myopic eyes with low (< 2D) astigmatism.
Methods
Patient population and study design
This study enrolled patients consecutively between October 2016 and March 2017 at the Department of Ophthalmology at Peking University. Patients were divided into three groups: low myopia (≤ − 3.00D), moderate myopia (− 3.00D to − 6.00D) and high myopia (≥ − 6.00 D) [
9]. All the patients provided informed consent. The study followed the tenets of the Declaration of Helsinki and institutional review board.
Patient enrolment criteria
Inclusion criteria were as follows: age over 18 years with stable refraction for at least 12 months, corrected distance visual acuity (CDVA) of at least 20/25, cylinder refraction lower than 2.0 diopter (D), discontinued contact lens use for at least 1 month, free of ocular disease and estimated postoperative corneal stromal bed thickness of more than 350 μm.
Preoperative examination
Preoperative examination included slit-lamp examination, intraocular pressure measurement, corneal epithelium assessment by fluorescein staining, tear breakup time, Schirmer I test, UDVA and CDVA, corneal topography (Optikon SpA, Rome, ITALY), pentacam scheimpflug topography (Oculus, Wetzlar, Germany), manifest and cycloplegic refraction, ultrasound pachymetry and fundus examination.
Surgical technique
All surgeries were performed by a single surgeon using the SCHWIND Amaris 500E excimer laser platform (SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany). The ablation algorithm was calculated using ORK-CAM software. For each treatment, the epithelium thickness profile that 55 μm centrally and 65 μm peripherally based on the population statistic. The target refraction was emmetropia in all eyes. After surgery, the cornea was irrigated with a cool balanced salt solution and a soft bandage contact lens was applied for three to four days. Patients were instructed to use 0.5% levofloxacin (Cravit; Santen, Inc.) four times a day for one week and 0.1% fluorometholone (Allergan, Inc.) eye drops four times a day, then tapered progressively over the following four months.
Safety and efficacy
The safety index is defined as the ratio of postoperative CDVA/preoperative CDVA. The efficacy index is defined as the ratio of postoperative UDVA/preoperative CDVA.
Corneal wavefront aberration measurement
Corneal wavefront aberration were measured by a rotating Scheimpflug Camera (Pentacam; Oculus). The examinations were made in a dark room in the morning. Higher order aberrations (HOAs) of the cornea with a 6.0-mm analysis diameter were calculated separately from the total cornea preoperatively and 6 months postoperatively.
Discussion
This study demonstrated that one-step TransPRK could correct low to high myopia effectively. Six months after surgery, there was a significant improvement in UDVA, SE and astigmatism in the low, moderate and high myopia groups. More than 95% of the treated eyes were within ±1.00D of the intended target refraction. No eye lost two or more lines of CDVA.
Nearly 80% of the eyes in the low and moderate myopia groups and 65% of eyes in the high myopia group reached within ±0.50D of SE by six months after the operation. Previous clinical studies [
3,
4,
10‐
13] have reported acceptable visual and refractive outcomes after TransPRK. However, most of these studies concentrated on low and moderate myopia or high myopia only. In this study, we analyzed our results in different groups of myopia.
In our study, 100% of low and moderate myopia eyes achieved a UDVA of 20/20 or better six months after the operation, while 94% of the high myopia eyes achieved a UDVA of 20/20 or better. Our results are comparable to the previous studies of TransPRK [
3,
5,
7] and small-incision lenticule extraction. [
14,
15] We found a statistically significant difference in postoperative UDVA between the low and moderate myopia groups versus the high myopia group. The reason may be the increased HOAs of the cornea postoperatively or the changes of high myopia fundus preoperatively. However, there was no significant difference in the CDVA between the moderate myopia group and the high myopia group. This indicated that TransPRK for high myopia was safe.
In our high myopia group, 65.7% of eyes were within ±0.50D and 100% of eyes were within ±1.0D of the intended SE refraction six months postoperatively. Our results agreed to some extent with other studies. Antonios et al. [
3] found that 81.3% and 96.6% were within ±0.50D and ± 1.0D in high myopia patients 12 months postoperatively. Aslanides et al. [
13] reported 91.4% and 97.1% were within ±0.50D and ± 1.0D by using Mitomycin C (MMC) therapy for the prevention of haze. They got accurate results than us within ±0.50D. There were no significant differences in the SE within ±1.0D. We found a difference between the attempted and the achieved SE correction in the three groups, with a tendency of overcorrection. The overcorrection may be related to corneal dehydration during surgery. The longer time possibly increases dehydration of the corneal stroma [
1]. We suppose that the ablation of TransPRK should be modified in our future work.
In terms of safety, the mean safety index was greater than 1.0 in the three groups. The highest safety index was seen in the moderate myopia group in our study. In the low myopia group, 93.8% of eyes had no change or better CDVA postoperatively. In the moderate myopia group, 15.6% of eyes lost one line of CDVA and more than a half of eyes gained one or two lines of CDVA postoperatively. While in high myopia group, 22% of eyes lost one line CDVA. However, no statistically significant difference was found in the postoperative CDVA between the moderate and high myopia groups. The loss of the BCVA may be caused by the increase of the HOAs on the cornea postoperatively. Our results are more or less similar to other studies of refractive surgeries. Antonios R et al. [
3] reported that 81.3% of high myopia eyes were between±0.50D after the treatment of TransPRK. Serrao S et al. [
16] reported the safety index of the high myopia eyes treated by PRK was 0.81 one year postoperatively. Ikeda T et al. [
17] found 77% of high myopic eyes showed no change or gain in CDVA one year after LASIK. Torky MA et al. [
14] found that 88.2% of high myopic eyes got the SE within±0.50D by SMILE surgery six months postoperatively. Similarly, Jin HY et al. [
18] found that 87% of high myopic eyes got the SE within±0.50D by SMILE surgery .
Moreover, the efficacy and UDVA were improved significantly in each group. The highest efficacy index was seen in the moderate myopia group. No differences in efficacy were found between the high myopia group and the low myopia group. The study indicates that TransPRK is effective for moderate myopia, as well as mild and high myopia. The single-step ablation profile targets 55 μm centrally and 65 μm peripherally, using theoretical simulations for the scope of ablation optical zone (OZ). Different patients showed different corneal epithelial thicknesses. Mild myopia patients may be more influenced by the difference between the surgical setting of corneal thickness and actual corneal thickness.
Corneal HOA changes were evaluated in this study. We found a significant increase in total corneal HOAs after surgery. Previous studies had reported that HOAs were related to the shadows, halos and night vision glare [
19,
20]. The high myopia group showed significantly higher corneal HOAs than the low and moderate myopia groups. One study found that an RMS value of HOAs less than 1.0 had no noticeable effect on the clarity of retinal image, while blur could be seen with 1.0 to 1.5 μm of wavefront aberrations [
21]. This may cause the decreased CDVA and UDVA in high myopia group postoperatively.
In conclusion, our data shows that TransPRK is a safe and effective surgical option in mild to high myopia. A large sample size and long-term results are needed in furture studies.