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REVIEW ARTICLe Visual Quality After Femtosecond Laser Small Incision Lenticule Extraction Huamao Miao, MD, PhD, Tian Han, MD, Mi Tian, MD, Xiaoying Wang, MD, PhD, and Xingtao Zhou, MD, PhD Abstract: Femtosecond laser small incision lenticule extraction (SMILE) is a newly developed form of “flapless” corneal refractive surgery with all-in-one technology. Femtosecond laser SMILE is increasingly attractive for both doctors and patients because it is minimally invasive and does not require a flap to be lifted during surgery. It exhibits many advantages in terms of morphology, biomechanical effects, corneal wound healing, and nerve rehabilitation. However, visual quality assessment after refractive surgeries is just as important as these advantages and correlates with patient satisfaction. Evaluation indexes for visual quality include visual acuity, contrast sensitivity, aberration, intraocular scattering, and so on. This paper reviewed visual quality and patient satisfaction after SMILE for myopia correction. Key Words: femtosecond laser, small incision, lenticule extraction, myopia, visual quality, aberration, scattering, contrast sensitivity (Asia-Pac J Ophthalmol 2017;0:0–0) C orneal refractive surgeries for myopia correction have consistently and rapidly improved; these surgeries have shifted from using excimer lasers to using femtosecond lasers, from laser ablation to lenticule extraction, and from large corneal incisions to small 2-mm incision lenticule extraction. Femtosecond laser small incision lenticule extraction (SMILE) is a newly developed form of “flapless” corneal refractive surgery employing all-in-one technology. To correct myopia and astigmatism, 2 steps are performed in SMILE. First, a convex-shaped lenticule and a 2-mm upper corneal incision are produced using the femtosecond laser; after this, the lenticule is dissected and extracted through the 2-mm incision.1 Compared with laser in situ keratomileusis (LASIK), the small incision necessary for SMILE eliminates postoperative cap-related complications; moreover, instead of the 2 laser platforms needed in LASIK, SMILE can be completed using only 1 platform, which is more efficient. When compared with From the Key Laboratory of Myopia, Ministry of Health, Department of Ophthalmology, Eye and ENT Hospital of Fudan University, Shanghai, China. Received for publication September 17, 2016; accepted January 19, 2017. H. M. and T. H. contributed equally to this study and are considered co-first authors. Supported in part by the National Natural Science Foundation of China (Grant No. 81570879) and the Outstanding Academic Leaders Program of Shanghai Health System (Grant No. XBR2013098). The authors have no other conflicts of interest to declare. Reprints: Xingtao Zhou, MD, PhD, Department of Ophthalmology and Vision Science, Eye and ENT Hospital of Fudan University, No.19 Baoqing Road, Xuhui District, Shanghai, China. Email: [email protected]. Copyright © 2017 by Asia Pacific Academy of Ophthalmology ISSN: 2162-0989 DOI: 10.22608/APO.2016171 laser-assisted subepithelial keratomileusis (LASEK), SMILE preserves the epithelium layer and the Bowman membrane, causes less postoperative corneal irritation, and reduces the risk of haze development. All of these advantages make SMILE a popular corneal refractive technique for myopia correction. Clinical and experimental studies have revealed that SMILE has less of an effect on the corneal nerve and corneal sensitivity than other corneal refractive surgeries2‒7; furthermore, it potentially reduces corneal biomechanical stability to a lesser extent than other techniques.8,9 It is of great importance to assess visual performance after refractive surgery. In this review, the main outcomes of recent studies about objective optical quality and subjective visual quality after SMILE are summarized. Visual and Refractive Outcomes Since 2011, many prospective clinical studies have demonstrated the safety, efficacy, predictability, and stability of SMILE for myopia and astigmatism correction1,10‒20; the results are summarized below. The visual and refractive results may have differed slightly among studies due to the different inclusion criteria employed [eg, different ages, preoperative refraction values, and corrected distant visual acuity (CDVA)], attempted refraction related to the surgery, and country/area in which the surgeries were performed. Researchers have reported that in patients with a preoperative CDVA of 20/25 or better, 85% to 100% could achieve a postoperative CDVA of 20/20 or better at 3 or 6 months after SMILE. Ten to 30% of those patients gained 1 line of CDVA and 4‒10% gained 2 or more lines, though in 50‒80% of patients, the CDVA remained unchanged. A few patients exhibited CDVA loss in the earliest period after SMILE; in general, less than 10% of the patients lost 1 line, and extremely few (less than 3%) lost 2 or more lines of CDVA. Even so, some of the CDVA loss was recovered in the long run (6 months or more). In Vestergaard et al’s study,10 they observed that decentered optical zone, dryness, interface scattering, and irregular topography were several reasons for the loss of CDVA. Donate et al21 found that an energy level close to the plasma threshold [a cut energy index of 20 (100 nJ)] during SMILE provides a faster and better visual recovery.21 Ultimately, more than 90% of the patients had a postoperative uncorrected distance visual acuity (UDVA) reaching 20/40 or better and more than 60% reaching 20/20 or better at 3 or 6 months after SMILE.1,10‒17 By the 3-month or 6-month follow-up in previous clinical studies,1,10‒18 90% or more eyes were within ±0.5 diopters (D) and 95% were within ±1.00 D of the target refraction. A strong consistency was found between the achieved correction and the attempted refraction, with a correlation coefficient of 0.9 or higher. Visual acuity and refraction usually began to achieve stability from 1 week or 1 month postoperatively. Hjortdal et al11 found that Asia-Pacific Journal of Ophthalmology • Volume 0, Number 0, Month 2017 Copyright © 2017 Asia-Pacific Academy of Ophthalmology. Unauthorized reproduction of this article is prohibited. www.apjo.org | Miao et al Asia-Pacific Journal of Ophthalmology • Volume 0, Number 0, Month 2017 a small undercorrection was associated with older age (0.10 D per decade) and steeper corneal curvature (0.04 D per D). However, this finding could partly be affected by the operative design where a small residual myopic correction was performed in patients older than 40 years. Some recent studies reported longer-term results after SMILE. Miao et al22 found consistent UDVA and CDVA values at 6 and 18 months after SMILE for moderate to high myopia correction. At 18 months postoperatively, 67% of the eyes gained 1 or more lines of CDVA, and none lost 2 or more lines. In addition, 91% and 100% of the eyes were within ±0.5 D and ±1.0 D of the intended refraction at 18 months, respectively. The mean spherical equivalent (SE) was −0.19 D at 6 months and regressed slightly to −0.23 D at 18 months, but the cylindrical diopters remained unchanged. Han et al19 found no significant regression between 4-year and 6-month outcomes. Pederson et al23 compared the outcomes 3 months and 3 years after SMILE and found no difference in UDVA, though CDVA increased by −0.03 logMAR; in this study 78% and 90% of the eyes were within ±0.5 D and ±1.0 D of the target refraction at 3 years, respectively. No difference in SE was observed between the 2 follow-up times (−0.31 D to −0.39 D). Although a minor regression (0.36 D) of total corneal refractive power was detected, no significant change in SE was observed. A 5-year observation of the first group of patients treated with SMILE worldwide was reported by Blum et al24; by 5 years, 58% of the 56 eyes gained 1 to 2 lines of CDVA, and 48.2% and 78.6% of the eyes were within ±0.5 D and ±1.0 D of the intended refraction, respectively. A mean regression of 0.48 D was observed, although this was mainly due to 7 eyes; 6 of these eyes exhibited an axial length (AL) elongation rather than a steepening of the cornea, and the other eye showed cap perforation resulting in corneal scarring. Overall, refractive regression tends to increase with time, and it seems to be mostly caused by AL elongation; however, the SMILE regression data are no worse than the data from other keratorefractive procedures. Contrast Sensitivity Visual acuity is an overall parameter to measure the eye’s ability to recognize objects of different sizes at the 100% contrast level. However, contrast sensitivity testing provides a more comprehensive description of visual function than the standard visual acuity chart testing; it is a measure of the ability to discern objects of different sizes and contrasts. Contrast sensitivity was also investigated after SMILE. Sekundo et al12 used the Vector Vision CSV 1000E Test to measure contrast sensitivity at 3, 6, 12, and 18 cycles per degree (cpd) 1 year after SMILE. No significant postoperative contrast sensitivity change was found in either the photopic or scotopic environment, and the contrast sensitivity at different partial frequencies showed stability at different time points. Similarly, in Vestergaard et al’s study,13 no significant contrast sensitivity change was found 6 months after SMILE. These results suggested that SMILE had little influence on the formsense function of the visual system; they also showed that postoperative contrast sensitivity was stable. Higher-Order Aberrations Higher-order aberrations (HOAs) can increase after corneal refractive surgeries. Consistent results concerning HOA changes | www.apjo.org after SMILE have been reported. Shah et al14 reported that total HOAs, coma, spherical aberration, and residual fourth-order astigmatism increased significantly at 6 months postoperatively. Moreover, Sekundo et al12 found that HOAs increased from 0.17 ± 0.08 μm to 0.27 ± 0.10 μm, coma from 0.28 ± 0.19 μm to 0.55 ± 0.30 μm, and spherical aberration from −0.01 ± 0.23 μm to −0.12 ± 0.20 μm at 1 year postoperatively. Different components of HOAs changed differently. Previously, Miao et al22 analyzed aberration changes 18 months after SMILE; we found that the total HOAs, spherical aberration, and coma increased significantly, but no significant change in trefoil or tetrafoil was observed. Among these aberrations, coma was induced most, and the changes in HOAs were consistent with the changes in coma; thus, we suggested that the postoperative total HOA induction was mainly due to the coma increase. Postoperative induced coma may be associated with a mild decentration. Li et al15 revealed that vertical coma showed the greatest increase in magnitude; this was found to be associated with the magnitude of horizontal decentration. Moreover, according to Wu et al’s study,25 SE together with corneal resistance factor (CRF) showed a negative liner relationship with changes of third- to sixth-order HOAs and spherical aberration but a positive linear relationship with changes of vertical coma. Long-term changes of spherical aberrations were also observed. Spherical aberrations increased at 6 months after SMILE in our previous study; however, the difference no longer existed at 18 months.22 Pedersen et al23 found that corneal spherical aberration and total corneal HOAs decreased from 3 months to 3 years while coma remained stable. Han et al19 found that no significant changes in whole-eye aberrations were detected among the 1month, 6-month, or 4-year follow-ups postoperatively. When compared with other corneal refractive surgeries, SMILE induces fewer postoperative HOAs. For example, total HOAs and spherical aberration were induced less after SMILE than after LASEK, whereas coma was induced at a similar rate.26 When compared with femtosecond laser‒assisted LASIK (FS-LASIK), more coma was induced after SMILE. Yao et al16 observed aberrations more than 3 months after SMILE and FS-LASIK; they found that HOAs and coma increased in both groups, and spherical aberration increased in the FS-LASIK group. The increase of coma was greater in the SMILE group, whereas the increase in spherical aberration was greater in the FS-LASIK group. Lin et al17 also found that total HOAs, spherical aberration, and coma increased after both surgeries, whereas total HOAs and spherical aberration were induced less after SMILE, and coma was induced less after FS-LASIK at 3 months. Two studies compared SMILE and femtosecond lenticule extraction (FLEx) groups and found similar aberration changes.13,20 Intraocular Scattering Due to the microcosmic nonhomogeneity characteristics and different refractive indexes of ocular refractive media, light scatters as it follows its pathway and forms intraocular scatter light. An excessive amount of intraocular scattering projecting to the retina could reduce the contrast sensitivity and cause unsatisfactory visual performance. Intraocular scatter light could increase after refractive surgery.27,28 When evaluating the retinal image quality after SMILE, it is essential to take intraocular scattering changes into consideration. Our study used a double-pass system © 2017 Asia-Pacific Academy of Ophthalmology Copyright © 2017 Asia-Pacific Academy of Ophthalmology. Unauthorized reproduction of this article is prohibited. Asia-Pacific Journal of Ophthalmology • Volume 0, Number 0, Month 2017 to evaluate objective forward scatter light after SMILE and found that postoperative intraocular scattering had a tendency to increase temporarily and then decrease to preoperative baseline in 3 months; however, the increase was within an acceptable range. Moreover, lower postoperative intraocular scattering was associated with younger age, lower preoperative refractive degree, and lower preoperative intraocular scattering.29 Our further study suggested that stable postoperative intraocular scattering may occur for a longer period; we found that intraocular scattering levels at 6 and 18 months after SMILE were not significantly different from the preoperative level.22 Agca et al30 used in vivo confocal microscopy to evaluate corneal backscattering after SMILE; they found that the backscattered light increased after SMILE in the early period and then decreased to the preoperative level at 6 months. Xu et al31 used a subjective method (C-quant) to measure the postoperative scatter light and found a 58% increase at 1 month after SMILE and a 51% increase at 1 year. However, the increases did not exhibit statistical significance. In addition, they found that SMILE induced less postoperative scatter light when compared with LASIK and epipolis LASIK (Epi-LASIK). Haze and anterior keratocyte loss were considered to relate to the scatter light increase after excimer laser surgeries, and this could be associated with corneal wound healing.32‒35 However, further study is needed to investigate the mechanism of a temporary intraocular scattering increase after SMILE. Patient Satisfaction Patient satisfaction comprises the patient’s overall feeling about the surgery and is part of a comprehensive assessment of visual quality. Sekundo et al1 carried out a questionnaire survey administered to 48 patients 6 months after SMILE; the mean quality of vision was graded at 92.3 on a scale of 0 (very poor) to 100 (best ever known). All eyes exhibited improved vision; it was found that 28.4% were markedly improved, 68.2% were extremely improved, and 3.4% were somewhat improved. Furthermore, 93.3% of the patients indicated that they would undergo the procedure again. The remaining 6.7% were not sure if they would do so again because they had some trouble with night vision or driving at night. None of the respondents complained of glare, whereas 7.7% reported dryness compared with 2.2% before the surgery. Shah et al14 conducted a survey at 3 months postoperatively in 41 patients; the most common complaint was mild dryness, but no patients complained about the increase of halos, double vision, or night-driving vision deterioration. In addition, on a scale from 0 (very dissatisfied) to 10 (very satisfied), all patients reported being very satisfied with the treatment. The mean satisfaction was 9.3 ± 1.1 on a scale of 0‒10 at 3 months postoperatively in Vestergaard et al’s study.10 Here, 95% of patients were willing to recommend the treatment to others. Marked to extreme improvement of overall quality of vision was reported in 95% of the patients, 4% perceived that it was moderately improved, and 1% perceived that it was not or only slightly improved. A comparative study included 34 patients who received SMILE in 1 eye and FLEx in the other; the mean satisfaction score was 9.3 ± 1.1 on a scale of 0‒10, with no difference between the 2 treatments.13 Investigations using larger sample sizes and longer-term follow-up studies also showed high patient satisfaction after SMILE. Ivarsen et al18 reported that the mean 3-month satisfaction score in 1036 eyes of 808 patients was 9.34, whereas 6 patients (9 eyes) Visual Quality After SMILE reported a score of 5 or less. The unsatisfied patients complained about monocular ghost images, blurred vision due to haze, unstable vision due to dryness, or residual cylinder due to preoperative high cylindrical diopters beyond the treatment range. After treatment, some of the symptoms were ameliorated, and only 2 patients were still dissatisfied at the last visit (more than 1 year later) due to blurred vision or residual astigmatism. Our study reported that the mean 18-month satisfaction score for SMILE was 9.31 ± 0.64; this study found that 47.83% of the patients were very satisfied and 52.17% were satisfied, whereas no patient was unsatisfied.22 Conclusions In conclusion, SMILE is a safe, effective, and highly satisfactory procedure for myopia correction. The visual and refractive outcomes exhibit stability soon after the treatment, and contract sensitivity at different spatial frequencies is barely influenced. However, younger patients should be reminded of possible refractive regression due to their continued AL elongation. Finally, we suggest that future studies on the following topics may help to elucidate visual quality after SMILE: (1) coma was induced most after SMILE; therefore, optimization of the surgical technique to reduce coma is needed in the future; (2) long-term corneal remodeling might lead to the decrease of certain components of HOAs after SMILE and this regularity could be further observed; and (3) a temporary increase of intraocular scattering after SMILE was detected; the reason for this needs to be further discussed, and this may be helpful in improving optical quality in the early period and patient satisfaction. References 1. Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95:335‒339. 2. Dong Z, Zhou X, Wu J, et al. Small incision lenticule extraction (SMILE) and femtosecond laser LASIK: comparison of corneal wound healing and inflammation. Br J Ophthalmol. 2014;98:263‒269. 3. Vestergaard AH, Gronbech KT, Grauslund J, et al. Subbasal nerve morphology, corneal sensation, and tear film evaluation after refractive femtosecond laser lenticule extraction. Graefes Arch Clin Exp Ophthalmol. 2013;251:2591‒2600. 4. Li M, Niu L, Qin B, et al. Confocal comparison of corneal reinnervation after small incision lenticule extraction (SMILE) and femtosecond laser in situ keratomileusis (FS-LASIK). PLoS One. 2013;8:e81435. 5. Denoyer A, Landman E, Trinh L, et al. Dry eye disease after refractive surgery: comparative outcomes of small incision lenticule extraction versus LASIK. Ophthalmology. 2015;122:669‒676. 6. Wei S, Wang Y. Comparison of corneal sensitivity between FS-LASIK and femtosecond lenticule extraction (ReLEx flex) or small-incision lenticule extraction (ReLEx smile) for myopic eyes. 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J Fr Ophtalmol. 2008;31:489‒493. | www.apjo.org © 2017 Asia-Pacific Academy of Ophthalmology Copyright © 2017 Asia-Pacific Academy of Ophthalmology. Unauthorized reproduction of this article is prohibited.