Comparison of Refractive and Visual Outcomes after Transepithelial Photorefractive Keratectomy (TransPRK) in Low versus Moderate Myopia

Abstract

Is it possible to obtain good results in myopia of 2 or fewer diopters (D) with transepithelial photorefractive keratectomy (TransPRK) changing the optical zone and epithelium thickness? We retrospectively analyzed two groups of 296 eyes with a minimum follow-up of 4 months. Group A had 2 or less D, treated with an optical zone (OZ) 0.2 mm bigger than recommended, and a central epithelium thickness of 60 microns, and group B had 2 D to 5 D, with the recommended optical zone, and a 55-micron epithelium ablation at the center. The outcomes were not different between the two myopic ranges; the postop uncorrected distance visual acuity was 20/20 ± 4 in both groups (p = 0.2), which was −0.3 ± 0.8 lines worse than the preoperative corrected distance visual acuity in both groups (p = 0.5). The safety of the treatments resulted in a change of 0.0 ± 0.7 lines in the low myopia group, versus a gain of +0.1 ± 0.8 lines in the moderate myopia group (p = 0.1). The deviation from the intended target was −0.04 ± 0.33 D in the low myopia group and +0.07 ± 0.32 D in the moderate myopia group (p < 0.0001); the postoperative spherical equivalent was 0.00 ± 0.33 D in the low myopia group and +0.10 ± 0.31 D in the moderate myopia group (p < 0.0001). The postop refractive astigmatism was 0.32 ± 0.16 D in both groups (p = 0.5). In conclusion, the refractive and visual outcomes after TransPRK are comparable in low myopia changing the optical zone and epithelium thickness versus moderate myopia with standard optical zone and epithelium thickness.

Customized versus Standard Epithelium Profiles in Transepithelial Photorefractive Keratectomy

“…The difficulty lies in performing corrections with TransPRK for low-myopia corrections, when the epithelial thickness cannot be measured, because if the epithelium is thicker than the standard given by the software (55 microns) at the center, the OZ may be smaller than the precalculated value, which can result in undercorrection [3]. Therefore, in a previous paper, we proposed [2] that, in cases in which the epithelial thickness cannot be measured, the OZ should be enlarged by at least 0.2 mm, and the central epithelium thickness input should be 60 microns. These adjustments affect more tissue, as the laser penetrates deeper than necessary, but they can be used to safely correct low myopia with TransPRK.…”

“…As previously described [2], we used surface ablation TransPRK with an aberrationfree ablation pattern. The treatment was planned with the ORK-CAM planning module.…”

“…In Group A, with the standardized optimized epithelial thickness profile, we input the central epithelium thickness at 60 and 70 microns at a 4 mm radial distance, as described previously [2]. In Group B, with the customized epithelium thickness profile, the input was the mean epithelium thickness, as measured with the AS-OCT, and the 4 mm input was automatically increased, by the software, to 10 microns.…”

“…The laser ablation algorithm has been described previously [2]. In brief, a flying-spot delivery system that operates at 1050 Hz with a super-Gaussian beam profile of 0.54 mm full width-half maximum [11] is used.…”

“…We recently published the refractive and visual outcomes after TransPRK treatment for low-myopia (2 diopters (D) or less) [2] corrections, using a larger optical zone and a thicker epithelium than those proposed by the standard software. One of the main difficulties with low-myopia corrections with TransPRK is that, when the epithelium is thicker than the precalculated value, the ablation zone becomes smaller and, sometimes, no correction occurs [3].…”