In this study, we compared the subjective and objective visual quality before and after LASEK, FS-LASIK and SMILE for myopia. LASEK, FS-LASIK and SMILE all led to significant increase of subjective visual quality 6 months after surgery, but brought little impact on objective visual quality in which the difference was significant between patients with low myopia and high myopia. Besides, preoperatively and 6 months postoperatively, no statistical intergroup differences were detected in the comparisons of visual quality among three types of corneal refractive surgical techniques.
Interestingly, our results indicated that the subjective visual quality was improved after all the three surgical procedures with no statistical change detected in objective visual quality although values did vary among individuals. The improvement of subjective visual quality after three procedures has been supported by an abundant resource of evidence,(20-22) while the UDVA may be mainly improved by laser correction with modifying the cornea shape and adjusting the focus of light which enters through the central cornea,(20)together with the retinal or neural factors,(21) indirectly increasing the QIRC scores.(23) Although in our study, there was no significant change in objective visual quality parameters before and after LASEK, FS-LASIK and SMILE, it is worth noting that in all three procedures, objective visual quality decreased first and then showed an upward trend over time, with significant or inconspicuous differences, in agreement with the previous findings.(24-26)Considering that aberration and intraocular scattering are critical factors in determining the objective optical quality after refractive surgery,(27, 28) the immediate decline in objective visual quality may be the result of the increment of the ocular aberrations and forward intraocular scattering,(4, 27, 28)attributed to the following major factors: 1.surgical reasons: intraoperative flattening the cornea and damaging the tear film stability,(4) as well as 2.incision healing response: postoperative remodeling corneal shape(29) and rising the transient corneal haze.(30) Nevertheless, the limited 6 months follow-up and the trend of rising in objective visual quality at the end of the follow-up suggest that the conclusion is far from conclusive and the visual quality may be expected to continue increasing beyond 6 months. Further studies comparing the long-term visual quality are warranted.
We also found that despite not significantly different before surgery, the objective visual quality parameters, especially the MTF cutoff frequency, decreased as the degree of myopia increased after FS-LASIK and SMILE significantly. Similarly, Paquin et al indicated a quasi-linear correlation between aberration and SE.(31)Generally, the MTF cutoff frequency could reflect all the optical characteristics of the human eye, including the overall optical defects involved in retinal image degeneration, such as aberrations, scattering, and diffraction.(32) Hence, the explanation may be that compared with low myopes, high myopes might entail deeper ablation subsequently inducing greater inflammation under FS-LASIK,(33) and more Bowman's membrane microdisortions under SMILE,(34) together with longer tissue remodeling and wound healing processes.(35) Besides, previous studies have reported that there are less predictable outcomes and more risk of corneal ectasia in high myopic patients after laser refractive surgery.(36) Therefore, patients with high myopia may not be proper candidates for corneal refractive surgery. However, because of the greater functional impairment undergone by high myopes, the potential limitations of corneal refractive surgery might be still more acceptable.(37) On the other hand, substantial novel surgical techniques for high myopia correction, such as the implantation of intraocular lens, have advanced over the past few years,(38) and may provide greater reversibility or adjustability than corneal refractive procedures.(6) With these emerging trends, it is necessary for ophthalmologists to understand the pros and cons of each refractive surgery and weigh against optical corrective surgical options, with careful and appropriate patient selection.(37)
Despite that a couple of studies have been done on pairwise comparisons of visual quality between every two surgical procedures among LASEK, FS-LASIK and SMILE,(4, 13, 15) anecdotally, this study is the first to make comprehensive visual quality comparisons among all these three surgery techniques for different degrees of myopia, both objectively and subjectively. In our analysis, no significant intergroup differences were found in the comparisons of visual quality among LASEK, FS-LASIK and SMILE preoperatively and 6 months postoperatively, except in preoperative UDVA of high myopia, which has been supported by pairwise comparison results of previous studies.(6, 15) Therefore, LASEK, FS-LASIK and SMILE may be equally effective in visual quality improvement, and the selection of surgical procedures could rely on individual patient characteristics or surgeon preferences.
A major strength of our study is that we used OQAS system to estimate objective visual quality with good repeatability and reproducibility.(7)Although Hartmann–Shack wavefront sensors has been the most commonly used tool for visual outcome assessments, it is well known that the wavefront aberrometers overestimate visual quality in eyes where high-order aberrations and scatter are obvious due to limitation imposed by lens sampling.(8) Thus, in our study, for the objective visual quality estimation in eyes after corneal refractive surgeries where different levels of scattering exists because of differences in the degree of corneal haze,(39) the double-pass OQAS II may be a more appropriate option due to the property of the double-pass approach of capturing the comprehensive information on the retinal image which renders the technique extremely powerful in many of the conditions that specially influence scattering.
However, the findings of our study must be interpreted in the context of the following important limitations. First, the preoperative SE was statistically different between the SMILE and FS-LASIK groups for high myopic correction, as well as between three surgical procedures for moderate myopic correction, which might suggest a probable imbalance among the three surgical procedures because of different ablation depth, and thus affect outcomes. Secondly, this study did not collect sufficient long-term follow-up data. Although in our study, there was no significant change in objective visual quality preoperatively and 6 months postoperatively, there was a trend of rising at 6 months after surgery. Thus, our study may not be representative of the eventual postoperative visual quality level after surgery. Thirdly, there is absence of data in high myopia under LASEK, mainly due to the fact that clinically, high myopic patients generally choose intraocular surgery to correct myopia compared with LASEK, so it is difficult to collect data in high myopia under LASEK.