Myopia is the most common ocular disorder that causes visual dysfunction. Myopia was reported to affect approximately 1406 million people worldwide in 2000, among whom163 million exhibited high myopia (2.7% of the world population)[1]. As the prevalence of myopia is still increasing yearly, some scholars predict that, 49.8% and 9.8% of the world’s population will have myopia and high myopia, respectively, by the year 2050[1]. Uncorrected refractive error has brought and will bring an increasing financial and social burden per annum[2].

Therefore, additional techniques have been introduced to correct refractive errors. In 1998, laser-assisted subepithelial keratomileusis (LASEK) became a priority surgical choice to correct refractive errors. With rapid development, small incision lenticule extraction (SMILE) emerged as the latest generation technology in 2008. Several studies have proved that LASEK and SMILE are both safe, effective and predictable[3,4].

With the improvement in refractive surgery, more concern has transferred from safety and efficacy to postoperative visual quality. However, all corneal refractive surgeries will inevitably elevate the risk of postoperative glare, haloes and night vision defects because they change the natural path of light to the retina. These changes are even more pronounced in patients with high myopia due to the thicker ablation depth/lenticule thickness.

The postoperative higher-order aberrations (HOAs) of LASEK are lower than those of laser in situ keratomileusis (LASIK), which requires a flap, whereas LASEK does not[5]. Also, thanks to the flapless and other minimally invasive features, SMILE, an all-femtosecond laser refractive procedure, had been proven to yield better visual quality than LASIK[6]. However, to our knowledge, studies comparing the long-term visual quality between LASEK and SMILE with respect to correcting high myopia are still needed. Therefore, the aim of the present investigation was to compare the 1-year outcome of visual quality in patients undergoing refractive surgery using SMILE or LASEK to correct high myopia, including wavefront aberrations, contrast sensitivity (CS), objective scatter index (OSI) and modulation transfer function (MTF) cut-off frequency.

Forty-six eyes of 24 patients underwent LASEK, 50 eyes of 26 patients underwent SMILE, and these patients completed all of the pre- and postoperative examinations. There was no significant difference in the average age between the two groups (t = 1.284, P = 0.205), and the results of the preoperative examinations between the two groups were comparable (Table 1).

Kulkarni et al. reported 1-year visual and refractive outcomes of LASEK[8]. It was found that 84% of the treated eyes in the high myopia group attained a UCVA of 20/20 or better and that 60% achieved a CDVA of 20/15. The mean postoperative SE was below -0.5 D postoperatively, and no serious complications occurred. Kim et al. reported that[9], one year after SMILE, 78.4% of the treated eyes in the high myopia group attained a UCVA of 20/20 or better, 43.2% had an unchanged CDVA, 47.2% gained one line, and 6.4% gained two lines. The mean postoperative SE was -0.25 ± 0.35D postoperatively and no visually threatening complications occurred.

In our study, both LASEK and SMILE were proved effective, with no significant difference observed between the CDVA before the operations and the UCVA achieved after. In addition, all of the patients attained a UCVA of 10/20 or better postoperatively, with 79.2% and 76.9% of the treated eyes in the LASEK and SMILE groups, achieving a UCVA of 20/20 or better, respectively. Furthermore, both the LASEK and the SMILE groups gained a better postoperative CDVA compared with the preoperative CDVA, with safety indices of 1.17 ± 0.17 and 1.19 ± 0.14 in the LASEK and SMILE groups, respectively. Consistent with previous studies[4,8,9,10], our findings showed that both LASEK and SMILE are safe and effective.

Yu et al. reported that there are no significant differences in UCVA between SMILE and LASEK in patients with mild to moderate myopia 3 months postoperatively[11]. The current study, which showed that there were no significant differences in the postoperative UCVA, efficacy index, safety index and SE between LASEK and SMILE 1 year postoperatively, suggests that LASEK is equally effective and safe as SMILE for correcting high myopia.

The differences in the ablation depth/lenticule thickness and postoperative CCT of the present study between the two groups showed that the lenticule thickness of SMILE was thicker than the ablation depth of LASEK. This phenomenon may due to the distinction between the laser types. The VisuMax femtosecond laser needs to remove a greater amount of corneal tissue than does the MEL-80 excimer laser for correction per 1.0D of myopia[11,12,13]. Nevertheless, there was no significant difference in the SE between the two groups.

With the development of refractive surgery, researchers have shifted more attention from efficacy and safety to visual quality postoperatively. It is well known that visual quality of human eye can be affected by several factors, such as tear film stability, corneal shape, wavefront aberration, pupil size, lens density, etc. Until now, no matter what kind of refractive surgery, have the potential to change the shape of cornea, raise the wavefront aberration and damage the tear film stability, Thereby, causing a potentially deleterious effect on the postoperative visual quality[14,15], and resulting in symptoms such as glare and haloes. However, there was still no reported clinical study comparing the visual quality after SMILE and LASEK for correcting high myopia, let alone the long-term difference. Therefore, we measured visual quality parameters including wavefront aberration, CS, OSI and MTF cut-off frequency 1 year after SMILE and LASEK to determine which surgery is the better choice for the long-term visual quality when correcting high myopia.

In a comparative study, Ganesh et al. found that postoperative HOAs were significantly fewer in the SMILE group than in the FS-LASIK group, whereas both operations caused an increase in HOAs[16]. McAlinden et al. attributed postoperative aberrations induced by LASIK rather than LASEK to the flap creation[17]. Benefiting from the flapless characteristic, both LASEK and SMILE induced fewer HOAs than LASIK[5,17].

Our results of wavefront aberration showed that HOAs, especially SA, increased more significantly 1 year after LASEK than after SMILE, while HOAs, including SA and coma aberrations, increased significantly in both the LASEK and the SMILE groups postoperatively, which was also observed by Yu et al. in their 3-month study[11].

HOAs are the crucial factors in determining the visual quality after refractive surgery. Among them, SA plays a more important role in visual quality than do coma-like aberrations, particularly under a large pupil diameter[18]. Therefore, increased HOAs, especially SA after surgery, may elevate the potential risk of glare and haloes in dark environments. Regardless, LASEK or SMILE both flatten the cornea, thus raising aberrations. Furthermore, postoperative refractive reduction and corneal remodeling associated with the incision healing process, may increase HOAs[19]. As it induces a lower incision-healing response and is an all-in-one femtosecond laser procedure, SMILE can minimize changes in the corneal shape[20], thus resulting in fewer HOAs. Our results demonstrated that SMILE, through a lower wavefront aberration induction, may achieve a better visual quality than LASEK for the correction of high myopia.

With regard to CS, our results revealed no significant difference before and after surgery in both LASEK and SMILE at all spatial frequencies. Moreover, there was no significant difference between the two groups postoperatively across all spatial frequencies.

Yu et al. reported that both SMILE and LASEK cause a slight reduction in high spatial frequency CS 1 month after surgery, which is recovered 3 months postoperatively[11]. Townley et al. found no significant CS changes occurred in the LASEK group under photopic or scotopic conditions 1 year postoperatively[21]. A study by Tan et al. looking at the outcomes following SMILE showed that both mesopic and photopic CS decreased significantly 3 months after surgery at higher frequencies but was recovered by 1 year[22].

Contrast sensitivity, defined as the ability to detect the minimal disparity in luminance between two objects, was considered a better index of visual quality than visual acuity. A few studies have shown that CS declines after corneal refractive surgery but recovers to preoperative levels within 3 months to 1 year after surgery[11,22]. LASEK and SMILE are both prone to raised wavefront aberrations and instability of the tear film during the early postoperative stage[11,23,24]. Over time, owing to restability of the tear film and a gradually restored elevation of wavefront aberration, long-term CS will rise again. Liou et al. found that, compared with contact lens correction, spectacle lens reduced the CS when correcting high or severe myopia, and they attributed this finding to the image shrink caused by the vertex distance effect[25]. The statistically unchanged CS of the present study occurred because of either the postoperative recovery effect within 1 year, or the lower preoperative CS caused by the examined spectacle lens correction, or even both. Based on this result, regardless of the reason, patients with high myopia were unaffected in CS after both LASEK and SMILE.

Intraocular scattering also plays an important role in visual quality and might reduce the contrast of the retinal image, particularly in patients with refractive medium opacity and after refractive surgeries[26,27]. Lee et al. reported that intraocular scattering is a reliable predictor for retinal image quality after LASEK and LASIK[28]. Based on a double-pass system technique, the OQASII system is the objective, quantitative and repeatable instrument for image quality measurements of the retina[29] and has been used in optical quality evaluation after several corneal refractive surgeries[28,30]. Therefore, it was applied to compare the visual quality postoperatively.

Our results showed that there were no significant differences between the two groups regarding both OSI and MTF cut-off frequency, although these values were both better in SMILE than in LASEK.

The OSI is an index reflecting the intraocular scattered state of light[31]. The MTF cut-off frequency represents the highest spatial frequency at the lowest contrast (1% of contrast) while the MTF reaches a value of 0.01[31]. Lee et al. reported that the OSI and MTF cut-off frequency were not significant correlated with ocular aberrations[28]. Ondategui et al. compared photorefractive keratectomy (PRK) and LASIK and concluded that corneal refractive surgery may deteriorate these values postoperatively[15]. They also found a significant correlation between OSI and achieved refractive correction and thus attributed the increment of ocular scattering to the ablation procedure rather than to flap creation after refractive surgery. Miao et al. reported that SMILE caused an increment in the OSI 20 days after surgery, which was gradually recovered 3 months postoperatively[30]. The refractive corrections of the present study were comparable between the two groups. Therefore, after a 1-year postoperative recovery, the OSI and MTF cut-off frequency after LASEK were not significantly worse than those after SMILE when correcting high myopia.

One limitation of the present study is that we did not measure the preoperative baseline of the OSI and MTF cut-off frequency. However, there were no significant differences in the preoperative CDVA, achieved refractive correction and other important refractive values between the two groups. Furthermore, the surgical patients had no serious ocular disease, such as cataract, which could affect the refractive media. Considering that the OSI and MTF cut-off frequency were associated with these factors[15,29], these values were likely to be comparable between the two groups preoperatively.

In conclusion, the current 1-year study indicates that both LASEK and SMILE are excellent surgical options for correcting high myopia. When considering all the visual quality measurements, SMILE may be a superior option over LASEK in providing a painless, high-refractive-accuracy procedure that subsequently yields better visual quality. A larger sample size and a visual-quality questionnaire will be employed in further research.