Journal of Refractive Surgery

Original Article Supplemental Data

Optimal Parameters of Scanning Mode in Small Incision Lenticule Extraction (SMILE): Clinical Results and Ultrastructural Analysis

Shengbei Weng, MD; Daoman Xiang, MD, PhD; Limian Lin, MD; Haiqin Lin, MM; Fang Liu, MD; Quan Liu, MD, PhD

Abstract

PURPOSE:

To compare the effect of two scanning modes with different femtosecond laser settings on clinical outcomes and ultrastructural analysis of the lenticule surface in small incision lenticule extraction (SMILE).

METHODS:

A total of 78 eyes from 39 consecutive patients were included with the standard scanning mode (energy level of 105 nJ vs spot distance of 3 mm) in one eye and the fast scanning mode (energy level of 110 nJ vs spot distance of 4.5 mm) in the other eye, all eyes being randomly assigned. Patients received complete ophthalmic examinations preoperatively and at different follow-up visits after SMILE. Ten human corneal lenticules were analyzed using scanning electron microscopy.

RESULTS:

Uncorrected distance visual acuity at 1 day and 1 week postoperatively was −0.06 ± 0.10 and −0.11 ± 0.05 logMAR, respectively, in the fast scanning group, which was significantly better than in the standard scanning group (−0.11 ± 0.08 and −0.12 ± 0.05 logMAR, respectively), and no significant difference was found between the groups at 1 and 3 months. The contrast sensitivity was higher at different time points in the fast scanning mode group compared with the standard scanning group (all P < .05). Ultrastructurally, the corneal lenticules from the fast scanning group showed a smoother surface than those from the standard scanning mode group qualitatively and quantitatively (all P < .001).

CONCLUSIONS:

SMILE using the fast scanning mode facilitates better visual acuity with improved contrast sensitivity in the early postoperative period. Experimental studies on the ultrastructure of corneal lenticules support these clinical results.

[J Refract Surg. 2020;36(1):34–41.]

Abstract

PURPOSE:

To compare the effect of two scanning modes with different femtosecond laser settings on clinical outcomes and ultrastructural analysis of the lenticule surface in small incision lenticule extraction (SMILE).

METHODS:

A total of 78 eyes from 39 consecutive patients were included with the standard scanning mode (energy level of 105 nJ vs spot distance of 3 mm) in one eye and the fast scanning mode (energy level of 110 nJ vs spot distance of 4.5 mm) in the other eye, all eyes being randomly assigned. Patients received complete ophthalmic examinations preoperatively and at different follow-up visits after SMILE. Ten human corneal lenticules were analyzed using scanning electron microscopy.

RESULTS:

Uncorrected distance visual acuity at 1 day and 1 week postoperatively was −0.06 ± 0.10 and −0.11 ± 0.05 logMAR, respectively, in the fast scanning group, which was significantly better than in the standard scanning group (−0.11 ± 0.08 and −0.12 ± 0.05 logMAR, respectively), and no significant difference was found between the groups at 1 and 3 months. The contrast sensitivity was higher at different time points in the fast scanning mode group compared with the standard scanning group (all P < .05). Ultrastructurally, the corneal lenticules from the fast scanning group showed a smoother surface than those from the standard scanning mode group qualitatively and quantitatively (all P < .001).

CONCLUSIONS:

SMILE using the fast scanning mode facilitates better visual acuity with improved contrast sensitivity in the early postoperative period. Experimental studies on the ultrastructure of corneal lenticules support these clinical results.

[J Refract Surg. 2020;36(1):34–41.]

Small incision lenticule extraction (SMILE), which is a flapless surgical technique for the correction of refractive errors, has been performed in a large number of cases.1,2 It has advantages over laser in situ keratomileusis (LASIK) in terms of dry eye in the early postoperative period.3,4

However, it has been postulated that a longer period of time is required for visual recovery after SMILE in comparison with LASIK.5,6 Moreover, recent research has confirmed that visual outcomes of SMILE for myopia were worse compared with femtosecond laser–assisted LASIK within 24 hours postoperatively.7 In this regard, previous studies have suggested that the interface quality of the corneal stroma depends on the influence of various combinations of pulse energy and spot distance of the femtosecond laser.8–10 A relatively high pulse energy combined with closer spot separations would reduce the regularity of the cut during LASIK.8,11 Currently, a rapid visual recovery could be obtained with a femtosecond laser energy of less than 115 nJ.12,13

The purpose of the current study was to investigate the optimal femtosecond laser parameters of the scanning mode, allowing maximization of visual performance after SMILE for the correction of myopia.

Patients and Methods

This prospective, randomized, intraindividual comparative study included 78 eyes of 39 consecutive patients who received bilateral SMILE for treatment of myopia or myopic astigmatism in both eyes at the Zhongshan Ophthalmic Center from December 2017 to March 2018.

All patients had a preoperative baseline evaluation including anterior and posterior segment examinations. The inclusion criteria were as follows: 18 years of age or older with a stable refraction for 1 year; preoperative manifest spherical equivalent ranging from −1.00 to −10.00 diopters (D) with or without myopic astigmatism (<−1.50 D); and minimum residual stromal thickness of 270 µm. The exclusion criteria were ocular surface disease and systemic diseases that did not qualify for laser refractive surgery for myopia and a history of previous intraocular or corneal surgery. All procedures were approved by the Research Ethics Board of the Zhongshan Ophthalmic Center of Sun Yat-sen University, China. An informed consent form was obtained from each patient after explanation of the nature and possible consequences of the treatment before participating in this study.

Clinical Examinations

A complete ophthalmic evaluation included the following tests: monocular uncorrected (UDVA) and corrected (CDVA) distance visual acuity by Snellen charts, objective and manifest refraction, ocular anterior segment by slit lamp, corneal topography (WaveLight Oculyzer II; Alcon Laboratories, Inc., Fort Worth, TX), central corneal thickness by ultrasound pachymeter (AC Master; Carl Zeiss Meditec AG, Jena, Germany), intraocular pressure measurement (noncontact tonometry, TX-20; Canon, Inc., Tokyo, Japan), and fundus evaluation.

Wavefront aberration analysis was performed under scotopic conditions using the WASCA wavefront analyzer (Carl Zeiss Meditec AG) to detect variations in ocular aberrations. The root mean square (RMS) was used to analyze higher order aberrations (HOAs) for 4- and 6-mm diameter zones. We measured contrast sensitivity by the CSV 1000E test (Vector Vision, Greenville, OH) at a spatial frequency of 3, 6, 12, and 18 cycles per degree (cpd) under different lighting conditions: photopic with and without glare and mesopic with and without glare. The amount of contrast sensitivity function (CSF) improvement was quantified by computing the change in the area under the log CSF (AULCSF). This examination was conducted at 2.5 m in the patient's CDVA conditions. All patients had routine examinations preoperatively and at 1 day, 1 week, and 1 and 3 months postoperatively, including UDVA, CDVA, manifest spherical equivalent, HOAs, and contrast sensitivity.

Randomization was performed by assigning a dominant eye using the Dolman method.14 Patients were asked to use their hands to create a frame around a far target and then report which eye was being used to assist under these circumstances (dominant eye). Each dominant eye was randomly assigned to receive the standard scanning mode or fast scanning mode, and the contralateral eye received the alternative scanning mode during the SMILE procedure. A total of 78 eyes from 39 patients were randomized to receive either the standard scanning mode or fast scanning mode. The randomization protocol was confirmed and implemented by the refractive surgeon on the day of surgery. Figure A (available in the online version of this article) shows the CONSORT flow diagram.

The CONSORT 2010 flow diagram showing the flow of patients participating in the study.

Figure A.

The CONSORT 2010 flow diagram showing the flow of patients participating in the study.

Surgical Techniques

All surgeries were performed by the same refractive surgeon (QL) using the VisuMax 500-kHz FS laser (Carl Zeiss Meditec AG) with SMILE system software (version 2; Carl Zeiss Meditec AG) for the current laser. The diameter of the lenticule ranged from 6.5 to 7.5 mm according to the patient's pupil size measured in the dark with less than 10 lux. All surgeries were performed with one of two different laser settings: standard scanning mode with a laser cut energy index of 105 nJ and spot distance of 3 µm or fast scanning mode with a laser cut energy index of 110 nJ and spot distance of 4.5 µm. Otherwise, the same femtosecond laser parameters were used as follows: 130-µm cap thickness and 6.5- to 7.5-mm lenticule diameter according to the patient's pupil size measured in the dark. A single small incision of 2 mm was set at the 90° meridian.

After the femtosecond laser cutting procedure, the patient was moved to the observation position under the VisuMax integrated surgical microscope. The corneal incision was opened using a hook with a 0.2-mm tip diameter (6 6 Vision Tech Co., Ltd., Suzhou, China). The anterior and posterior dissection planes of the lenticule were dissected with a corneal flap separator for femtosecond laser surgery (6 6 Vision Tech Co., Ltd.). The lenticule was grasped subsequently with a lens forceps for femtosecond laser surgery (6 6 Vision Tech Co., Ltd.) through a 2-mm incision superotemporally. After the removal of the lenticule, the intrastromal pocket was flushed with balanced salt solution for all eyes. Postoperatively, levofloxacin eye drops were applied four times a day for 2 weeks, tobramycin and dexamethasone eye drops (TobraDex; Alcon-Couvreur NV, Puurs, Belgium) were applied four times a day for 1 week, and sodium hyaluronate eye drops were applied four times a day for 1 month.

Microstudy of Lenticules Extracted From Smile

For this micromorphological procedure, 10 eyes of 5 patients were randomly chosen from all patients (N = 39). The extracted corneal lenticules were used to analyze the ultrastructure of the cut surface on two series of corneal lenticules in which the scanning mode varied. After extraction during the SMILE procedure, the lenticules were immediately immersed in 2.5% glutaraldehyde at room temperature for 2 hours and then placed in a refrigerator at 4°C overnight. The specimens were dehydrated in ascending concentrations of aqueous ethanol solution (50%, 70%, 85%, and 100%) and then in pure acetone. Finally, the solutions were replaced by amylacetate. Following the entire dehydration process, all specimens were treated with liquid carbon dioxide and critical-point dried by the Hitachi Model HCP-2 critical-point dryer (Hitachi Ltd., Tokyo, Japan). All specimens were then cut into two pieces with a double-edged stainless-steel blade (size: 40 × 20 × 0.1 mm) under scanning electron microscopy (SEM) (Quanta 200; FEI Co., Hillsboro, OR). After cutting, one piece was turned over using tying forceps and the other was not, so that different sides of the lenticule were represented. Samples were finally imaged with the SEM.

Surface Quality Analysis

Traditional Qualitative Roughness Grading. In the current study, each specimen was scanned at the center and several peripheral positions at 400- and 800-fold magnification. One scanning image from each sample showed the local characteristic of the lenticule.

An established scoring system was applied to evaluate the surface regularity of the lenticules.10,15 Four criteria were used to evaluate the surface relief (Table A, available in the online version of this article). All other criteria were analyzed at 400- or 800-fold magnification. Surface relief is generally evaluated at 400-fold magnification.15 Two observers (SW, LL) graded all images in a blinded fashion.

Criteria for Evaluating Cut Surface Characteristics

Table A:

Criteria for Evaluating Cut Surface Characteristics

Quantitative Roughness Grading. The same SEM images of each sample shown to the masked observers were analyzed with Image-Pro Plus software (version 6.0; Media Cybernetics, Inc., Rockville, MD). This new method was created by automated quantification of tissue bridges, which were believed to be the most important reason for surface irregularity on the lenticular surface.10,16 Three main steps were included in the image process: (1) setting the same grayscale in all images (Figure BA, available in the online version of this article), (2) thresholding and tissue bridge enhancement (Figure BB), and (3) final image for count analysis (Figure BC). The final image contained a black background with high-frequency elements in shades of gray and irregular structures (tissue bridges) approaching white, filled with red color to discriminate. The total number of tissue bridges was used to evaluate the lenticular surface regularity.

Representative images after each step of image processing: (A) setting the same grayscale in all images, (B) thresholding and tissue bridges enhancement, and (C) choosing the final image for count analysis.

Figure B.

Representative images after each step of image processing: (A) setting the same grayscale in all images, (B) thresholding and tissue bridges enhancement, and (C) choosing the final image for count analysis.

Statistical Analysis

Data were collected to compare the differences between specimens treated with two scanning modes based on a scoring system and were tabulated on a spreadsheet according to groups (Excel software 2010; Microsoft Corporation, Redmond, WA). The normality of all data samples was first tested by the Kolmogorov–Smirnov test. A two-tailed paired Student's t test was used for statistical analysis to compare the data between the two scanning modes at each examination time if the data were distributed normally, and Wilcoxon signed-rank tests if they were not. All quantitative results were presented as means ± standard deviation, and a P value of less than .05 was considered to be statistically significant.

Results

Study Population

A total of 78 eyes from 39 patients (9 men, 30 women) were recruited for the current study. The mean age of patients was 25.3 ± 5.0 years (range: 18 to 36 years). The preoperative and surgical parameters are summarized in Table B (available in the online version of this article). No significant differences were observed in preoperative spherical equivalent, cylindrical error, cap diameters, lenticule diameters, central corneal thickness, keratometry values, and lenticule thickness between the two scanning modes (all P > .05). The mean femtosecond laser scanning time was 34.23 ± 1.25 seconds (range: 33 to 37 seconds) in the standard scanning mode group and 23.41 ± 0.75 seconds (range: 22 and 25 seconds) in the fast scanning mode group (P < .001, Wilcoxon test).

Preoperative and Surgical Parameters of Patients in Both Groups

Table B:

Preoperative and Surgical Parameters of Patients in Both Groups

Efficacy and Safety

Figures CA–CB (available in the online version of this article) show the percentage of eyes in which there was a gain or loss of Snellen visual acuity lines compared with preoperative levels at 1 day, 1 week, and 1 and 3 months in the standard and fast scanning groups. At 1 day and 1 week, the fast scanning group had better UDVA than the standard scanning group. The CDVA of the fast scanning group was significantly better than that of the standard scanning group 1 day postoperatively (Table C, available in the online version of this article).

The visual and refractive outcomes for eyes with small incision lenticule extraction using different scanning modes. CDVA = corrected distance visual acuity; D = diopters; SEQ = spherical equivalent

Figure C.

The visual and refractive outcomes for eyes with small incision lenticule extraction using different scanning modes. CDVA = corrected distance visual acuity; D = diopters; SEQ = spherical equivalent

UDVA and CDVA After SMILE Using Two Scanning Modesa

Table C:

UDVA and CDVA After SMILE Using Two Scanning Modesa

Stability

The change in manifest spherical equivalent is shown in Figure CC. A significant difference was found in manifest spherical equivalent at 1 week postoperatively between the two groups (P = .028).

Wavefront Aberrations and Contrast Sensitivity Analysis

As shown in Table 1, there were no statistically significant differences between the two scanning modes in terms of coma, trefoil, spherical aberration, and total HOAs at 1 day, 1 week, and 1 and 3 months postoperatively (P > .05).

Comparison of Change in RMS Values of Different Aberrations Between Two Scanning Modes for 4-mm and 6-mm Analysis Diameters at Each Time Point After SMILEa

Table 1:

Comparison of Change in RMS Values of Different Aberrations Between Two Scanning Modes for 4-mm and 6-mm Analysis Diameters at Each Time Point After SMILE

The change in contrast sensitivity is shown inFigure D (available in the online version of this article). Significantly better contrast sensitivity was observed in the fast scanning mode group compared with the standard scanning group at 3, 6, 12, and 18 cpd under photopic or mesopic conditions with or without glare during the postoperative follow-up.

Contrast sensitivity at spatial frequencies of 3, 6, 12, and 18 cycles per degree under photopic, photopic with glare, mesopic, and mesopic with glare lighting conditions between the standard scanning and fast scanning groups preoperatively and postoperatively (*Significant difference between the two groups [P < .05]).

Figure D.

Contrast sensitivity at spatial frequencies of 3, 6, 12, and 18 cycles per degree under photopic, photopic with glare, mesopic, and mesopic with glare lighting conditions between the standard scanning and fast scanning groups preoperatively and postoperatively (*Significant difference between the two groups [P < .05]).

Ultrastructural Analysis of Lenticules

Figure 1 shows examples of images obtained from the different scanning mode groups.

Environmental scanning electron microscopy showing the lenticular surface of sides 1 and 2: (A, D) 50×, (B, F) 400×, and (C, F) 800× original magnification of the standard scanning and fast scanning groups, respectively.

Figure 1.

Environmental scanning electron microscopy showing the lenticular surface of sides 1 and 2: (A, D) 50×, (B, F) 400×, and (C, F) 800× original magnification of the standard scanning and fast scanning groups, respectively.

Qualitative Grading Method

A total of 30 images were collected for each group. The proportion of smooth surface as judged at 400-fold magnification was 30% in the fast scanning mode group and 3% in the standard scanning mode. Partial regularity or even worse surface regularity was observed at 800-fold magnification in the fast and standard scanning mode groups at 30% and 70%, respectively. The proportion of irregularities less than 10% of the cut surface area in the fast and standard scanning groups was 17% and 0%, respectively. A summary of the score results from the two scanning modes is provided in Table 2. The difference in the surface regularity score between the two scanning mode groups was significant (P < .001).

Surface Regularity Scorea

Table 2:

Surface Regularity Score

Quantitative Grading Method

A significant difference was found in the total number of tissue bridges between the fast and standard scanning groups at 400-fold magnification (paired t test; t = −3.980, P < .001). This finding was also confirmed at 800-fold magnification (paired t test; t = −6.846, P < .001) (Figure 2). Moreover, significant differences were noted in the total number of tissue bridges at 400-fold magnification between the two sides of the lenticules (paired t test; t = −4.648, P < .001), which was also confirmed at 800-fold magnification (paired t test; t = −2.708, P = .008).

Summary of quantitative data of the total number of tissue bridges at (A) 400× and (B) 800× original magnification.

Figure 2.

Summary of quantitative data of the total number of tissue bridges at (A) 400× and (B) 800× original magnification.

Discussion

For SMILE, the effect of the femtosecond laser is influenced by laser energy and spot distance settings. Kamiya et al.17 reported no significant differences in visual performance in refractive outcomes between 140 nJ (spot distance: 3 µm) and 170 nJ (spot distance: 4.5 µm) of eyes undergoing SMILE with the 500-kHz VisuMax laser. Hjortdal et al.18 suggested that visual acuity was better with a wider spot distance and higher energy (4.5 µm with 170 nJ vs 2.5 µm with 125 nJ by the 500-kHz VisuMax laser). However, the visual outcomes in our study were much better. We reduced the laser energy setting for the two scanning modes. Because each energy level of the femtosecond laser may have an individual optimized spot distance, the spot distance should be changed correspondingly as the femtosecond energy varies.19 The spot distances were commonly set as 4.5 and 3 µm for the fast and standard scanning modes, respectively.17,20

Our results showed that the UDVA in the fast scanning group was significantly better than that in the standard scanning group at 1 day and 1 week postoperatively, but there was no significant difference at 1 and 3 months. Moreover, there were no patients with loss of CDVA in the fast standard scanning group at 1 day postoperatively, unlike the standard scanning group, where we observed 2.6% with a loss of two lines or more of CDVA at 1 month postoperatively. Our results are similar to those of Donate and Thaëron,12 who found 3.8% and 2.7% with a loss of two lines or more of CDVA at 1 and 3 months postoperatively, respectively, in the group with 180 nJ energy versus 4.5-µm spot distance and no patients with loss of CDVA in the group with 100 nJ energy and the same spot distance. The refraction stabilized by 1 day postoperatively for both scanning mode groups, and fewer residual refractive errors were obtained in the fast scanning mode at 1 week postoperatively compared with the standard scanning mode. Our results for stability were similar to or even better than previously published results of SMILE.21,22

Contrast sensitivity and HOA are crucial parameters for patient satisfaction, and reflect the subjective quality of vision from the patient's perspective.23,24 In the current study, a faster recovery of contrast sensitivity at all spatial frequencies under different lighting conditions was observed in the fast scanning mode group compared with that seen from the standard scanning mode after SMILE. No significant difference was observed between the two groups in HOAs. Donate and Thaëron12 found the modulation transfer function value and the HOA root mean square were better in the group using the minimal energy necessary to induce plasma (100 nJ with 4.5 µm vs 180 nJ with 4.5 µm). Another study reported that no significant difference in optical quality was found between two groups (140 nJ with 3.0 µm vs 170 nJ with 4.5 µm), including the modulation transfer function cut-off frequency and the Strehl ratio.17

It is known that an irregular stromal surface induced by the surgery may be associated with decreasing visual and refractive outcomes following excimer laser refractive surgery.25,26 Previous studies also suggested that regular stromal surface induced by surgery would decrease the amount of HOAs and corneal scattering, which relate to visual performance after refractive surgery.17,27 Therefore, we analyzed the surface regularity of the extracted corneal lenticules. The SEM analysis demonstrated several significant observations. First, visible tissue bridges were observed in almost all specimens and were considered to be most related to the irregularity for evaluating surface morphology.10 A previous study suggested that they were the crucial reason for the surface irregularities.28 Second, the lenticule surface from the fast scanning group was smoother in comparison with the standard scanning group after SMILE. Several studies have evaluated corneal stromal bed quality of lamellar keratectomy in LASIK procedures and found that stromal bed quality can be improved with lower pulse energy and tighter spot separation settings.27,29,30 For SMILE, we found fewer tissue bridges in corneal lenticules from the fast scanning mode group than from the standard scanning group. Subsequently, we supposed that if the spot separation is too large for the chosen femtosecond energy, tissue bridges are left behind and must be broken mechanically, increasing surface irregularity. However, the cavitation bubbles would merge together and impair the subsequent laser beam with a too tight spot distance, resulting in an increase in the surface irregularity. Therefore, we proposed an optimal combination of pulse energy and the spot distance based on improved lenticule surface quality and maximized subsequent visual performance after SMILE.

Suction loss is a common intraoperative complication during SMILE.31 The longer duration of suction would induce patient anxiety and inability to follow instructions, leading to the greater possibility of suction loss compared with femtosecond laser–assisted LASIK.32 There is potentially a higher risk of suction loss with the longer duration of SMILE using the standard scanning mode compared with the fast scanning mode.

The use of the VisuMax laser operating at 500 kHz with a pulse energy of 110 nJ and a spot separation of 4.5 × 4.5 µm contributed to the rapid visual recovery with improvement of contrast sensitivity in the early postoperative period after SMILE for correction of myopia. It is also likely to produce a smoother optical surface with fewer tissue bridges using the fast scanning mode during the SMILE procedure.

Postoperative visual recovery and outcomes will be improved in the near future with further optimization in the femtosecond laser settings of the fast scanning mode. Further prospective studies with a large number of cases are needed to confirm these encouraging results.

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Comparison of Change in RMS Values of Different Aberrations Between Two Scanning Modes for 4-mm and 6-mm Analysis Diameters at Each Time Point After SMILEa

ParameterComaTrefoilSphericalTotal HOAs
1 day
  4-mm pupil diameter
    Standard scanning mode0.16 ± 0.090.14 ± 0.070.07 ± 0.030.13 ± 0.04
    Fast scanning mode0.15 ± 0.070.14 ± 0.060.06 ± 0.050.13 ± 0.04
     P.382.852.584.379
  6-mm pupil diameter
    Standard scanning mode0.44 ± 0.240.33 ± 0.190.29 ± 0.240.36 ± 0.34
    Fast scanning mode0.39 ± 0.210.34 ± 0.210.27 ± 0.180.34 ± 0.11
     P.384.590.662.375
1 week
  4-mm pupil diameter
    Standard scanning mode0.21 ± 0.130.16 ± 0.090.06 ± 0.050.15 ± 0.08
    Fast scanning mode0.18 ± 0.090.14 ± 0.060.06 ± 0.030.14 ± 0.04
     P.252.089.501.169
  6-mm pupil diameter
    Standard scanning mode0.50 ± 0.270.33 ± 0.170.36 ± 0.280.38 ± 0.15
    Fast scanning mode0.46 ± 0.260.32 ± 0.160.37 ± 0.260.37 ± 0.11
     P.572.712.873.945
1 month
  4-mm pupil diameter
    Standard scanning mode0.21 ± 0.100.14 ± 0.070.06 ± 0.040.15 ± 0.04
    Fast scanning mode0.20 ± 0.090.15 ± 0.060.06 ± 0.040.14 ± 0.04
     P.540.524.656.368
  6-mm pupil diameter
    Standard scanning mode0.63 ± 0.310.34 ± 0.210.44 ± 0.310.46 ± 0.16
    Fast scanning mode0.55 ± 0.270.32 ± 0.170.39 ± 0.300.42 ± 0.13
    P.233.503.177.145
3 months
  4-mm pupil diameter
    Standard scanning mode0.21 ± 0.090.14 ± 0.080.06 ± 0.030.15 ± 0.05
    Fast scanning mode0.21 ± 0.130.15 ± 0.070.08 ± 0.110.15 ± 0.07
     P.964.851.422.689
  6-mm pupil diameter
    Standard scanning mode0.62 ± 0.350.35 ± 0.170.44 ± 0.310.45 ± 0.16
    Fast scanning mode0.63 ± 0.260.34 ± 0.200.41 ± 0.270.44 ± 0.13
     P.865.654.546.674

Surface Regularity Scorea

ParameterStandard Scanning ModeFast Scanning ModeP
No.3030
Surface relief2.19 ± 0.793.03 ± 0.77
Regularity of surface2.92 ± 0.652.31 ± 0.52< .001
Extent of surface irregularity2.92 ± 0.652.28 ± 0.51< .001
Position of the irregular area2.78 ± 0.422.28 ± 0.51< .001
Total11.64 ± 2.269.06 ± 2.08< .001

Criteria for Evaluating Cut Surface Characteristics

Criterion (Original Magnification)AppearanceScore
A. Surface relief (×400)Very smooth4
Smooth3
Rough2
Very rough1
B. Regularity of surface structure (×800)Completely regular4
Regular3
Partially regular2
Not regular1
C. Portion of surface irregular (×800)< 10% of cut surface4
11% to 25% of cut surface3
26% to 50% of cut surface2
> 50% of cut surface1
D. Position of the irregular area (×800)No irregularities4
Peripheral only3
Large region2
All over1

Preoperative and Surgical Parameters of Patients in Both Groups

ParametersFast Scanning Mode (n = 39)Standard Scanning Mode (n = 39)P
Manifest spherical equivalent (D)−5.32 ± 1.74−5.46 ± 1.44.822
Manifest cylinder (D)−0.51 ± 0.41−0.44 ± 0.43.405
Cap diameter (mm)7.89 ± 0.227.87 ± 0.24.061
Lenticule diameter (µm)6.69 ± 0.216.69 ± 0.211.000
Central corneal thickness (µm)551.92 ± 25.35553.23 ± 27.36.218
Kmax (D)43.29 ± 1.3143.35 ± 1.39.451
Kmin (D)42.41 ± 1.1942.45 ± 1.24.509
Lenticule thickness (µm)111.54 ± 20.68111.87 ± 22.33.853

UDVA and CDVA After SMILE Using Two Scanning Modesa

ParameterFast Scanning Mode (n = 39)Standard Scanning Mode (n = 39)P
CDVA (logMAR)
  Preoperative−0.10 ± 0.06−0.10 ± 0.06.720
  1 day postoperatively−0.06 ± 0.10−0.11 ± 0.08.007
  1 week postoperatively−0.11 ± 0.05−0.12 ± 0.05.086
  1 month postoperatively−0.14 ± 0.05−0.15 ± 0.05.149
  3 months postoperatively−0.13 ± 0.05−0.13 ± 0.051.000
UDVA
  Preoperative
  1 day postoperatively−0.05 ± 0.10−0.10 ± 0.08.024
  1 week postoperatively−0.09 ± 0.06−0.12 ± 0.06.034
  1 month postoperatively−0.13 ± 0.05−0.14 ± 0.05.336
  3 months postoperatively−0.12 ± 0.05−0.12 ± 0.051.000
Authors

From the Department of Pediatric Ophthalmology, Guangzhou Women and Children's Medical Center, Guangzhou Medical University, Guangdong, China (SW, DX); and State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China (SW, LL, HL, FL, QL).

Supported by grants from the Natural Science Foundation of China (31671000).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (QL); data collection (SW, LL, HL, FL); analysis and interpretation of data (SW, DX); writing the manuscript (SW, LL); critical revision of the manuscript (SW, DX, HL, FL, QL); statistical expertise (SW, LL, HL, FL); administrative, technical, or material support (LL); supervision (DX, QL)

Correspondence: Quan Liu, MD, PhD, State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou 510060, China. E-mail: drliuquan@163.com

Received: August 13, 2019
Accepted: November 14, 2019

10.3928/1081597X-20191114-02

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