SMILE is a safe and efficient myopia correction method. SMILE has gained wide acceptance and developed rapidly in recent years. For example, Zhou et al.1 improved the lenticule separation and extraction procedures and developed the SMILE with continuous curvilinear lenticulerhexis (SMILE-CCL) technique. As described previously, the SMILE-CCL technique exhibits outcomes similar to or better than those of traditional SMILE. SMILE-CCL is more efficient and offers good-quality lenticule extraction and satifactory refractive outcomes.1,2
Bowman's layer microdistortions have been observed after both traditional SMILE and SMILE-CCL.1,3 These microdistortions can be measured and quantified through the use of high-resolution Fourier-domain optical coherence tomography (FDOCT). Yao et al.3 found that microdistortions are associated with refractive lenticule thickness and surgical experience in traditional SMILE, and they tend to be induced in the central rather than the peripheral area. The question was raised as to whether the twisted segments of the Bowman's layer would lead to reduced optical quality. Following SMILE, microdistortions were found to barely affect postoperative visual acuity and aberrations.3 Besides such aberrations, intraocular scattering is another important indicator by which to assess optical quality following refractive surgery. Beerthuizen et al.4 observed microstriae in the flaps of two patients after LASIK, and one experienced increased straylight; thus, they suggested that microstriae might affect visual quality. A temporary increase in intraocular scattering was detected after traditional SMILE, but the mechanisms underlying that scattering have not yet been established.5 An increase in ocular scatterlight could be due to heterogeneous changes in any part of the ocular media. The current study is the first to evaluate early changes in intraocular scattering and the relationship between scattering and Bowman's layer microdistortions following SMILE-CCL.
Patients and Methods
All of the SMILE-CCL procedures were performed by the same surgeon (XZ) at the Eye and ENT Hospital of Fudan University using the VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Berlin, Germany) as the operation platform. The inclusion criteria were: age 18 to 40 years, no more than a −0.50 diopter (D) annual refraction increase in the past 2 years, no wearing of contact lenses in the previous 2 weeks, spherical diopters no more than −10.00 D, cylindrical diopters no more than −3.00 D, and corrected distance visual acuity (CDVA) of 20/20 or better. Exclusion criteria were: keratoconus or suspicion of keratoconus, severe dry eye, corneal scar, and a history of herpetic keratitis, cataract, glaucoma, retinal pathology, or ocular surgery.
Ninety-three eyes of 93 myopic patients (44 women, 49 men) who met the criteria for SMILE-CCL were consecutively enrolled. Mean age was 26.09 ± 4.39 years (range: 18 to 38 years); the mean spherical equivalent (SE) was −6.31 ± 2.16 D (range: −10.25 to −2.25 D). The eyes were divided into two groups according to the SE: a high myopia group (57 eyes) with SE of less than −6.00 D and a low to moderate myopia group (36 eyes) with SE of −6.00 D or greater. The preoperative refraction, preoperative corneal thickness, and lenticule thickness of the two groups are summarized in Table 1. The SE was higher, and lenticule thickness was thicker, in the high myopia group (P < .05); there were no significant differences between the two groups with regard to age, cylindrical diopters, or corneal thickness (P > .05).
This study adhered to the tenets of the Declaration of Helsinki and its protocols were approved by the ethical committee of the Eye and ENT Hospital, Fudan University. Each patient gave written informed consent after being told the benefits and possible risks of the study.
As described in previous studies,1,2 the parameter settings were as follows: 500-kHz repetition rate, 130-nJ pulse energy, 110-μm cap thickness, 7.5-mm cap diameter, lenticule diameter of 6.3 to 6.9 mm, and a 90° angled side cut of 2 mm in the 12-o'clock position. After the femtosecond laser scanning was finished, the CCL technique1 was performed as follows: (1) a spatula was used to dissect the cap-lenticule interface, (2) approximately 0.3 mm of the inferior edge of the lenticule was dissected from the stromal bed, and (3) microforceps were used to grasp the edge of the lenticule and then, in a clockwise motion, the lenticule was completely separated and removed. All surgeries were uneventful, and routine postoperative medications were administered. The patients were followed up at 20 days and 3 months postoperatively.
Intraocular scattering was measured using a double-pass optical quality analysis system (OQAS II; Visiometrics, Terrassa, Spain). The Objective Scatter Index (OSI) parameter was used to estimate intraocular scattering. During the measurements, images of a light source (780-nm laser diode) reflected on the retina were acquired, and the OSI was calculated as the ratio of the amount of light in the peripheral zone (an annular area of 12 and 20 minutes) to the central zone (central peak of 1 minute of arc) of the retinal image. This method has been used in previous studies to evaluate intraocular scattering following refractive surgeries.5–7 To obtain a clear retinal image, both the spherical and cylindrical errors of −0.50 D or more were corrected before taking measurements. A spherical refractive error exceeding −8.00 D or a cylindrical diopter of −0.50 D or higher was corrected with an external lens; any spherical diopter less than −8.00 D was automatically corrected by the double-pass system. All measurements were conducted in mesopic conditions with a 4-mm artificial pupil.
Bowman's layer microdistortions were measured using an anterior segment FD-OCT (RTVue, software version 6.2; Optovue, Inc., Fremont, CA). We obtained line scans of four meridians (0°, 45°, 90°, and 135°). The convex component of each microdistortion above the baseline level of the Bowman's layer was defined as a peak, and the width of each peak in the central 4-mm zone of the Bowman's layer was measured (Figure 1). The mean number of microdistortions (Mcount) for each meridian was calculated as the total number of all peaks in the four line scans, divided by 4; the mean width of the microdistortions (Mwidth) of each line scan was calculated as the total width of all peaks of the four line scans, divided by 4. Two examiners (HM, XL) performed the measurements, and one masked investigator (MT) analyzed the images.
A 32-year-old woman with preoperative spherical equivalent of −6.25 diopters (D). The Bowman's layer microdistortions in the central 4-mm optical zone were measured using Fourier-domain optical coherence tomography. Two microdistortions existed in the horizontal line scan (0° meridian); the width decreased from (A) 768 μm at 20 days to (B) 593 μm at 3 months postoperatively. The Objective Scatter Index values were (C) 0.4 preoperatively, (D) 1.4 at 20 days postoperatively, and (E) 0.8 at 3 months postoperatively.
Statistical analysis was performed using SAS software (version 9.3; Cary, NC). A linear mixed model was used to detect the changes in repeated measurements of OSI before and after the operation. The Bonferroni correction was used for multiple comparisons between time points. To compare Mcount and Mwidth between 20 days and 3 months postoperatively, a paired t test or signed-rank test was chosen by the results of a skewness–kurtosis test for normality. A t test or ranked-sum test was used to compare parameters between the high myopia and low to moderate myopia groups and between patients with and without microdistortions, according to the normality test. A multiple linear regression model was used to explore the potential impact factors of the postoperative OSI values, whereas a Poisson regression model was used for a similar purpose on the number of micro-distortions. A P value less than .05 was considered statistically significant.
At 3 months postoperatively, the efficacy index was 1.09 ± 0.17 (range: 0.8 to 1.3) and the safety index was 1.09 ± 0.16 (range: 0.8 to 1.5). The CDVA remained unchanged in 49.5% of all eyes, 38.7% improved one line, 4.3% improved two lines, and 7.5% lost one line. None of the eyes lost two or more lines of CDVA. The mean SE was 0.08 ± 0.18 D; 87.1% of the eyes were within ±0.25 D and 100% were within ±0.50 D of the target refraction.
The mean OSI value was 0.67 ± 0.44 before surgery; it increased to 1.09 ± 0.62 at 20 days and then declined to 0.84 ± 0.44 at 3 months (P < .05) (Table 2, Figure 1). The same trend existed in both groups. The OSI value was higher in the high myopia group than in the low to moderate myopia group before and after the operation (P < .05).
Changes of Objective Scatter Index After SMILE-CCL
Bowman's layer microdistortions existed in 58 eyes (62.37%) at 20 days and in 45 eyes (48.39%) at 3 months postoperatively (P < .05). At 3 months, microdistortions existed in 68.42% of the eyes in the high myopia group and 16.67% of the eyes in the low to moderate myopia group (P < .05). Table 3 shows the microdistortion changes between 20 days and 3 months for those eyes that had microdistortions at 20 days. The width of the microdistortions per meridian was 283.18 ± 197.19 μm at 20 days; this decreased to 156.00 ± 159.86 μm at 3 months (P < .05) (Figure 1). The number of microdistortions per meridian was 0.93 ± 0.62 at 20 days, and this decreased to 0.55 ± 0.51 at 3 months (P < .05). Both groups showed similar micro-distortion changes.
Mcount and Mwidth Changes in Patients With Corneal Bowman's Layer Microdistortions at 20 Days After SMILE-CCL
Table 4 shows that the preoperative SE was higher and corneal thickness was thicker in eyes with microdistortions than in those without at 3 months (P < .05), whereas preoperative cylindrical diopters and postoperative OSI values were similar between the two groups (P > .05).
Preoperative Parameters in Eyes With and Without Microdistortions at 3 Months Postoperatively
In Tables 5–6, the results of regression analyses show that the OSI values at 3 months increased as age (b = 0.02, P = .03) increased and preoperative SE (b = − 0.09, P < .05) decreased; meanwhile, no significant association was found between postoperative OSI and gender or microdistortion parameters (P > .05). The number of microdistortions increased as preoperative SE (b = −0.28, P = .01) and preoperative corneal thickness (b = −0.02, P = .01) decreased.
Multiple Linear Regression Analysis of Factors That Influence the OSI at 3 Months Postoperatively (N = 93)
Poisson Regression Analysis of Factors That Influence the Mcount at 3 Months Postoperatively (N = 93)
Increased scatterlight following SMILE has been observed through the use of various measurement tools. A temporary increase in backscatter from the corneal anterior stroma has been detected soon after SMILE, using confocal microscopy8 and the Pentacam (Oculus Optikgeräte, Wetzlar, Germany).9 A slight but insignificant increase in forward straylight has been observed at 1 month after SMILE using a subjective method (C-quant).10 The double-pass technique is an objective way of measuring intraocular scatterlight because it is accurate and offers good repeatability.11 In the current study, we found that OSI values increased soon after SMILE-CCL, but then decreased with time. In addition, higher preoperative SE was associated with higher postoperative OSI. These results align with the findings of traditional SMILE.5 Although in the current study we observed short-term OSI changes, our previous study regarding optical quality evaluation at 18 months after traditional SMILE proved that the OSI values remain stable over longer periods.12
FD-OCT can be used to observe micro-level changes in the cornea that cannot not be easily detected with the naked eye. Microdistortions were found after both traditional SMILE and SMILE-CCL.1,3,13,14 In our study, the wave-like microdistortions were less obvious in the low to moderate myopia group than in the high myopia group, suggesting that after the refractive lenticule was extracted, the larger posterior surface of the flap and flatter residual stroma bed matched better in eyes with lower myopia. Another finding is that a thicker preoperative cornea yielded fewer corneal microdistortions; this may be because more residual stroma bed would be left in eyes with a thicker cornea, which could serve as better conditions for maintaining corneal shape.15 Moreover, the number and width of the microdistortions decreased with time in both groups, indicating that a better matching between the two layers was gradually achieved not long after the operation.
Scattering occurs when light penetrates through each component of the ocular refractive media. An increase in scatterlight could increase the risk of glare vision in the early period after SMILE; this might be induced by some irregular corneal changes in either the interfaces or the stroma. The current study found that postoperative OSI and microdistortions showed similar changes. First, they were both more significant in eyes with higher myopia; second, they both became less obvious with time. However, regression analysis found no significant relationship between these two findings. It has also been reported that microdistortions after traditional SMILE are not associated long-term with either visual outcomes or wavefront aberrations3; therefore, we can conclude that the twisted segments of the Bowman's layer barely affect the optical quality.
Nonetheless, it is uncertain why the scattering increased following corneal intervention. Increased straylight has been found in eyes with microstriae (in a LASIK treatment group) and haze (in a photorefractive keratectomy treatment group).4 Neither visible microstriae nor haze was observed after SMILE-CCL in the current study. Zhao et al.2 used an electron microscope to evaluate the surface quality of the lenticules extracted using SMILE-CCL or traditional SMILE; both groups scored high points, indicating good regularities in the interfaces of the flap and stroma bed. From the above, the irregularities of the interfaces are negligible and do not contribute much to a temporary increase in scattering. Agca et al.8 evaluated confocal microscopy images in eyes with SMILE and found that increased reflectivity was due to the extracellular matrix and activated keratocytes; thus, we speculate that the temporary scatter could be associated with the corneal lamellar wound-healing process. Overall, inflammatory and wound healing responses following SMILE tend to be minimal,16,17 but it remains to be seen whether these are the reasons for an increase in intraocular scattering.
This study is limited in that we evaluated the microdistortions and intraocular scattering in the central 4-mm zone of the cornea or pupil, an area that does not represent the whole optical zone or the whole eye but rather represents the situation under a natural pupil size. However, the microdistortions were reportedly more obvious in the central area than in the peripheral area3; therefore, microdistortions in a larger corneal zone would not necessarily affect overall optical quality. Another limitation of this study is that its measurements of the microdistortions are semi-quantitative; perhaps the use of better models and improved higher-resolution OCT in future research could lead to better (ie, more accurate and valid) assessments of Bowman's membrane layer microdistortions in future studies. Finally, eyes with higher myopia were found to yield more and wider Bowman's layer microdistortions after SMILE-CCL in the current study because thicker lenticules were extracted. However, another question raised is how the microdistortions distribute in the eyes with different astigmatism degrees and axis after SMILE. Theoretically, the lenticule is thicker in the meridian that is perpendicular to the astigmatism axis; therefore, more interlayer space will be released in the corresponding meridian after the lenticule is extracted. Whether this will lead to characteristic microdistortion distribution in certain meridians is worth exploring.
This study found temporary OSI increases and Bowman's layer microdistortions after SMILE-CCL, especially in eyes with higher myopia; both attenuated with time. The microdistortions were found not to affect intraocular scattering.
- Zhao Y, Li M, Yao P, Shah R, Knorz MC, Zhou X. Development of the continuous curvilinear lenticulerrhexis technique for small incision lenticule extraction. J Refract Surg. 2015;31:16–21. doi:10.3928/1081597X-20141218-02 [CrossRef]
- Zhao Y, Li M, Sun L, Zhao J, Chen Y, Zhou X. Lenticule quality after continuous curvilinear lenticulerrhexis in SMILE evaluated with scanning electron microscopy. J Refract Surg. 2015;31:732–735. doi:10.3928/1081597X-20151029-01 [CrossRef]
- Yao P, Zhao J, Li M, Shen Y, Dong Z, Zhou X. Microdistortions in Bowman's layer following femtosecond laser small incision lenticule extraction observed by fourier-domain OCT. J Refract Surg. 2013;29:668–674. doi:10.3928/1081597X-20130806-01 [CrossRef]
- Beerthuizen JJ, Franssen L, Landesz M, van den Berg TJ. Straylight values 1 month after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg. 2007;33:779–783. doi:10.1016/j.jcrs.2007.01.017 [CrossRef]
- Miao H, He L, Shen Y, Li M, Yu Y, Zhou X. Optical quality and intraocular scattering after femtosecond laser small incision lenticule extraction. J Refract Surg. 2014;30:296–302. doi:10.3928/1081597X-20140415-02 [CrossRef]
- Lee K, Ahn JM, Kim EK, Kim TI. Comparison of optical quality parameters and ocular aberrations after wavefront-guided laser in-situ keratomileusis versus wavefront-guided laser epithelial keratomileusis for myopia. Graefes Arch Clin Exp Ophthalmol. 2013;251:2163–2169. doi:10.1007/s00417-013-2356-x [CrossRef]
- Iijima A, Shimizu K, Yamagishi M, Kobashi H, Igarashi A, Kamiya K. Assessment of subjective intraocular forward scattering and quality of vision after posterior chamber phakic intraocular lens with a central hole (Hole ICL) implantation. Acta Ophthalmol. 2016;94:e716–e720. doi:10.1111/aos.13092 [CrossRef]
- Agca A, Ozgurhan EB, Yildirim Y, et al. Corneal backscatter analysis by in vivo confocal microscopy: fellow eye comparison of small incision lenticule extraction and femtosecond laser-assisted LASIK. J Ophthalmol. 2014;2014:265012. doi:10.1155/2014/265012 [CrossRef]
- Han T, Zhao J, Shen Y, Chen Y, Tian M, Zhou X. A three-year observation of corneal backscatter after small incision lenticule extraction (SMILE). J Refract Surg. 2017;33:377–382. doi:10.3928/1081597X-20170420-01 [CrossRef]
- Xu L, Wang Y, Li J, et al. Comparison of forward light scatter changes between SMILE, femtosecond laser-assisted LASIK, and epipolis LASIK: results of a 1-year prospective study. J Refract Surg. 2015;31:752–758. doi:10.3928/1081597X-20151021-04 [CrossRef]
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- Miao H, Tian M, Xu Y, Chen Y, Zhou X. Visual outcomes and optical quality after femtosecond laser small incision lenticule extraction: an 18-month prospective study. J Refract Surg. 2015;31:726–731. doi:10.3928/1081597X-20151021-01 [CrossRef]
- Luo J, Yao P, Li M, et al. Quantitative analysis of microdistortions in Bowman's Layer using optical coherence tomography after SMILE among different myopic corrections. J Refract Surg. 2015;31:104–109. doi:10.3928/1081597X-20150122-05 [CrossRef]
- Shroff R, Francis M, Pahuja N, Veeboy L, Shetty R, Sinha Roy A. Quantitative evaluation of microdistortions in Bowman's layer and corneal deformation after small incision lenticule extraction. Transl Vis Sci Technol. 2016;5:12. doi:10.1167/tvst.5.5.12 [CrossRef]
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- Liu YC, Teo EP, Lwin NC, Yam GH, Mehta JS. Early corneal wound healing and inflammatory responses after SMILE: comparison of the effects of different refractive corrections and surgical experiences. J Refract Surg. 2016;32:346–353. doi:10.3928/1081597X-20160217-05 [CrossRef]
|Parameter||Low to Moderate Myopia < −6.00 D (n = 36)||High Myopia ≥ −6.00 D (n = 57)||P|
|Age (y)||26.81 ± 5.15||25.63 ± 3.81||.211|
|Refractive sphere (D)||−3.59 ± 1.08||−7.35 ± 1.17||< .001|
|Refractive cylinder (D)||−0.78 ± 0.70||−0.85 ± 0.52||.215|
|Spherical equivalent (D)||−3.98 ± 1.12||−7.77 ± 1.10||.000|
|Corneal thickness (μm)||553.11 ± 19.73||547.00 ± 28.52||.130|
|Lenticule thickness (μm)||91.42 ± 19.51||141.19 ± 11.03||< .001|
Changes of Objective Scatter Index After SMILE-CCLa
|Group||N||Preoperative||20 Days Postoperative||3 Months Postoperative||P|
|Low to moderate myopia (<−6.00 D)||36||0.57 ± 0.40||0.97 ± 0.58||0.69b ± 0.30||< .001|
|High myopia (≥ −6.00 D)||57||0.73 ± 0.46||1.17 ± 0.64||0.93c ± 0.49||< .001|
|All||93||0.67 ± 0.44||1.09 ± 0.62||0.84d ± 0.44||< .001|
Mcount and Mwidth Changes in Patients With Corneal Bowman's Layer Microdistortions at 20 Days After SMILE-CCLa
|Parameter||Group||N||20 Days Postoperative||3 Months Postoperative||P|
|Mwidth (μm)||Low to moderate myopia (<−6.00 D)||11||120.27 ± 74.21||64.50 ± 91.50||.021|
|High myopia (≥ −6.00 D)||47||321.30 ± 197.87||177.42 ± 165.46||< .001|
|All||58||283.18 ± 197.19||156.00 ± 159.86||< .001|
|Mcount||Low to moderate myopia (<−6.00 D)||11||0.41 ± 0.26||0.23 ± 0.33||.031|
|High myopia (≥ −6.00 D)||47||1.05 ± 0.62||0.63 ± 0.52||< .001|
|All||58||0.93 ± 0.62||0.55 ± 0.51||< .001|
Preoperative Parameters in Eyes With and Without Microdistortions at 3 Months Postoperativelya
|Parameter||Without (n = 48)||With (n = 45)||P|
|Spherical equivalent (D)||−5.39 ± 2.23||−7.29 ± 1.60||< .001|
|Refractive cylinder (D)||−0.81 ± 0.63||−0.83 ± 0.56||.565|
|Corneal thickness (μm)||558.74 ± 24.24||539.14 ± 23.03||< .001|
|Postoperative OSI at 3 months||0.80 ± 0.47||0.88 ± 0.42||.132|
Multiple Linear Regression Analysis of Factors That Influence the OSI at 3 Months Postoperatively (N = 93)
|Preoperative SE (D)||−0.09||< .001|
|Mcount at 3 months||−0.23||.634|
|Mwidth at 3 months||0.00||.788|
Poisson Regression Analysis of Factors That Influence the Mcount at 3 Months Postoperatively (N = 93)
|Preoperative SE (D)||−0.28||.010|
|Corneal thickness (μm)||−0.02||.010|