The present study involved SMILE treatment of moderate-to-high myopia in 447 eyes from 224 patients and included a 6 month follow up period. We controlled for surgical factors (e.g., laser energy setting and treatment zone) to precisely evaluate the predictability, efficacy, and stability of SMILE and to determine predictors that influence visual outcome.
The efficacy, predictability, and safety of SMILE in the present study were promising and comparable with other recently published studies [10–12]. Regarding efficacy, 84%, 73%, and 79% of patients were reported to have 20/25 or better UDVA in various previous studies [10–12]. According to the recent study of Sekundo et al. , which reported 1-year results, 98% of patients had 20/25 or better UDVA at 12 months, and 94% and 88% of patients had 20/20 or better at 6 and 12 months, respectively. In the present study, 97% and 79.8% of patients had 20/25 or better UDVA and 20/20 or better respectively at 6 months after surgery. The reason for the lower percentage of 20/20 or better in this study versus that of Sekundo et al.  is thought to be the larger number of eyes and higher average degree of myopic correction versus their study.
For predictability, 80.1%, 77%, 91% and 92% were within ±0.5 D and 94.2%, 95%, 98% and 100% were within ±1.0D of the attempted refraction in the various previous studies [10–13]. In the present study, 86.1% and 97.9% of patients were within ±0.5 D and ±1.0 D at the 6-month follow-up.
Regarding safety, Hjortdal et al.  reported that 2.4% of eyes lost two or more lines, 15% lost one line, and 83.1% were the same or improved. Anders et al.  reported that 4.1% of eyes lost one line, whereas 95.9% were the same or improved after 3 months. In the study by Shah et al. , 4% of eyes lost one line, and 96% were the same or improved at 6 months. Similarly, Sekundo et al.  reported that 11% of eyes lost one line, and 89% were the same or improved at 12 months.
In the present study, 0.3% of eyes lost two lines, 3% lost one line, and 96.7% were the same or improved at 6 months after surgery. Anders et al.  reported that there was a slight but significant regression from 1 week to 1 month after surgery. Sekundo et al.  also reported 0.08 D regression after 12 months. In the present study, no myopic regression was observed from 1 week to 6 months. It also needs to be confirmed whether regression will be evident after 12 months of observation.
According to Hjortdal et al. , age, corneal power, gender (female), and eye (left) were predictive factors that influenced SE correction errors in multiple regression analyses. These authors suggested that increasing age, corneal power, female gender, and right eye affliction influenced the refractive outcome using an undercorrection. However, in the present study, none of these factors were predictors that influenced errors in SE correction. For predictors that influenced UDVA, Hjortdal et al.  reported that increasing age and female gender were risk factors for worsening UDVA at 3 months after surgery. In the present study, however, age was the only risk factor for worsening UDVA at 6 months after surgery (decrease of 0.07 logMar per decade increase in age). However, the effect was so small that it appears clinically insignificant. The present study corroborated the previous predictor analyses of Hjortdal et al.  and suggested that SMILE is predictable regardless of diverse patient factors such as gender, corneal power, and amount of correction.
The slight delay in UDVA recovery in the early postoperative period is a unique characteristic of FLEx and SMILE [14–16]. Shah et al.  and Demirok et al.  reported that initial visual recovery after FLEx was slower than Femto LASIK despite successful refractive correction. Interface haze formation is one of the most common adverse events after this procedure that caused delayed visual recovery in the early postoperative period . However, the incidence of interface haze formation was decreased after using the higher frequency femtosecond laser and applying lower energy, which reduces irregularity of the interface surface [17, 18]. Others have reported that early visual recovery was affected by laser trajectory . A faster visual recovery was noted when the back of lenticule was scanned from the periphery to the center and the front of the lenticule scanned from the center to the periphery. Thus, this preferred method has been used in many studies, including that presented here.
We found that the initial visual recovery was faster compared with recent studies that also used the newer-generation femtosecond laser (500 kHz). Although 40% and 62.3% of patients had UDVA that was 20/25 or better on the first postoperative day in previous reports11–12, 81.2% of patients had similar UDVA values in the present study. In particular, 91.4% and 96.6% of patients who received 2.0 mm incision lengths (n = 58) had UDVA 20/25 or better on the first day and 1 week after surgery, respectively.
The faster initial visual recovery noted in the present study may be attributed to the use of different laser settings or to improved surgical factors, such as smaller incision lengths. Laser settings of higher pulse energy and larger spot spacing appear to aid faster initial visual recovery. The laser setting of 180-nJ pulse energy and 4.5-μm spot spacing in the present study showed faster initial visual recovery compared to the previously reported 120 to 150-nJ pulse energy and 2.5-μm spot spacing . Additionally, Hjortdal et al.  reported better visual acuity using 170-nJ pulse energy and 4.5-μm spot spacing compared to 125-nJ pulse energy and 2.5-μm spot spacing. Laser settings should be optimized in future studies to improve the initial visual recovery.
To our knowledge, no reported study has evaluated the effect of incision length on visual outcome after SMILE. The present study found that as incision length decreased, visual recovery on the first postoperative day was more rapid (P = 0.013). However, there were no significant differences among the various incision length groups after 1 week. The length of incision did not affect other refractive results, including error in SE and amount of astigmatism after surgery. However, there are several weaknesses in the comparisons among incision groups. Although all eyes were treated by one surgeon, there was no randomization of the incision groups and the numbers of eyes differed significantly by group. Moreover, this analysis of the effects of incision size was, in fact, retrospective, because we had not planned to analyze the effects of incision size on visual outcome when the study was designed. There is also a possibility that the surgeon may have become more skillful when the smaller incision group was treated compared with when the larger incision group was treated, in a relatively early period. In our experience, as the surgeon became adept at smaller incisions, the surgeon could achieve manipulations more readily, as with a larger incision, during the SMILE procedure. Thus, it becomes possible to benefit from the smaller incision, with increased corneal stability and rapid epithelial healing, which may aid in more rapid visual recovery. Presently, we cannot exclude the possibility of better initial visual outcomes because of the smaller incisions, not only greater surgeon experience. The relationship between incision length and HOA after surgery should be evaluated in future studies.
Hjortdal et al.  reported that the right eye showed better UDVA on the first postoperative day compared to the left eye. However, we did not observe any difference in UDVA in the left and right eyes 1 day after surgery.
One eye that had undercorrected SE of -1.63 D and 20/40 UDVA after 6 months was retreated in the present study. The eye had a refraction of emmetropia (-0.25 D) and 20/20 UDVA with no other complication 3 months after the surface ablation procedure (PRK).