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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 22  |  Issue : 2  |  Page : 103-110

Clinical outcome of femtosecond laser flap formation versus mechanical microkeratome in laser in situ keratomileusis for treatment of myopia


1 Department of Ophthalmology, Faculty of Medicine for Girls, Al Azhar University, Cairo, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Al Azhar University, Cairo, Egypt

Date of Submission15-Nov-2020
Date of Decision10-Jan-2021
Date of Acceptance07-Feb-2021
Date of Web Publication24-Jun-2021

Correspondence Address:
MSc Dina F Rashad
Department of Ophthalmology, Faculty of Medicine, Al Azhar University, Nasr City 11754, Cairo
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/djo.djo_79_20

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  Abstract 


Purpose The aim of this study was to evaluate and to compare the thickness and morphology of femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and mechanical microkeratome flaps using anterior segment optical coherence tomography (AS-OCT).
Patients and methods This prospective, interventional, comparative clinical study was performed on 60 eyes of 30 patients. Flaps were created in 30 eyes using the WaveLight FS200 FS laser and in 30 eyes using the Moria M2 microkeratome. AS-OCT was used at 1 month postoperatively to evaluate the thickness of each flap at four points, which were 1 and 2 mm from the corneal vertex on the horizontal meridian.
Results At the sixth month postoperatively, the uncorrected visual acuity was 0.98±0.10 in the FS-LASIK group and 0.98±0.11 in the microkeratome group, which denotes the efficacy of both procedures. No vision-threatening complications occurred in either group. The average flap thickness at 1 mm was 114.93±3.04 µm in the FS-LASIK group compared with 128.57±3.98 µm in the microkeratome group, with a statistically significant difference (P=0.000). The difference between the achieved and intended flap thickness at 1 mm (accuracy) was 4.93±3.04 µm in the FS-LASIK group compared with 18.57±3.98 µm in the microkeratome group, with a statistically significant difference (P=0.000). At 2 mm, the average flap thickness was 115.22±3.34 µm in the FS-LASIK group compared with 139.00±4.75 µm in the microkeratome group (P=0.000). The flap accuracy at 2 mm was 5.22±3.34 µm and 29.00±4.75 µm in the FS-LASIK group and the microkeratome group, respectively (P=0.000). Flap morphology showed a planar shape (uniform) in the FS-LASIK group and a meniscus shape in the microkeratome group.
Conclusion Both FS-LASIK and microkeratome techniques were safe and efficient in correcting myopia and myopic astigmatism, achieving good visual outcome. AS-OCT showed that flaps created by the FS laser were more accurate and uniform than those created by the Moria M2 microkeratome.

Keywords: anterior segment optical coherence tomography, flap, femtosecond laser, laser in situ keratomileusis


How to cite this article:
Rashad DF, Khallaf ME, Khalil AM, Aly MM. Clinical outcome of femtosecond laser flap formation versus mechanical microkeratome in laser in situ keratomileusis for treatment of myopia. Delta J Ophthalmol 2021;22:103-10

How to cite this URL:
Rashad DF, Khallaf ME, Khalil AM, Aly MM. Clinical outcome of femtosecond laser flap formation versus mechanical microkeratome in laser in situ keratomileusis for treatment of myopia. Delta J Ophthalmol [serial online] 2021 [cited 2021 Sep 22];22:103-10. Available from: http://www.djo.eg.net/text.asp?2021/22/2/103/319186




  Introduction Top


Laser-assisted in situ keratomileusis (LASIK) is the most commonly used surgical procedure to correct refractive errors. The procedure consists of two main steps: creation of a flap and corneal stromal ablation using the excimer laser machine [1]. Flap creation is the main surgical step of this procedure and can result in complications. Flaps are traditionally created with mechanical microkeratomes, but femtosecond (FS) laser technology has emerged as an alternative for flap creation [2]. FS-LASIK allows tailoring of the parameters of the corneal flap, such as diameter, thickness, and hinge position, which are the main benefits of using the FS laser. Accordingly, it may lessen the risk of flap complications, such as irregular, buttonholed, or incomplete flaps [3].

Lamellar cuts created with the FS laser and microkeratome are different in its morphology. The microkeratome creates the LASIK flap with a single cut, generating a meniscus shape, whereas the FS laser creates a LASIK flap by laser photodisruption in a surface-parallel direction [4].

This study was conducted to evaluate the clinical outcome of FS laser versus mechanical microkeratome in LASIK surgery for treatment of myopia and to evaluate and to compare the thickness and morphology of FS-LASIK and mechanical microkeratome flaps using anterior segment optical coherence tomography (AS-OCT).


  Patients and methods Top


This is a prospective, interventional, comparative clinical study that included 60 eyes of 30 patients. The study followed the principles of the Declaration of Helsinki and was approved by the Ethics Board of Al Azhar University. All patients signed a written informed consent to participate in the study and for publication of data before being enrolled in the study.

The eyes were divided into two groups: group I included 30 eyes that were subjected to FS laser for flap creation, whereas group II included 30 eyes that were subjected to mechanical microkeratome for flap creation.

The study included patients with spherical myopia up to −10.0 D and myopic astigmatism up to −5.0 D, with a minimum age of 18 years with stable refraction. The best-corrected visual acuity (BCVA) was 0.8 (20/25) or better with no other ocular pathology except myopia and astigmatism and no history of previous eye surgery. The central corneal thickness was at least 500 µm, and the calculated residual stromal bed after treatment was at least 300 µm.

Before the LASIK procedure, patients were subjected to the following: cycloplegic refraction, determination of uncorrected visual acuity (UCVA) and BCVA, slit-lamp biomicroscopy, Goldmann applanation tonometry (Topcon Medical Systems, Paramus, New Jersey, USA), and binocular indirect ophthalmoscopy through dilated pupils. Corneal topography was performed using Pentacam (Wave-light Allegro occulyzer2 GmbH, Erlangen, Germany).

Surgical technique

All patients were prepared and draped in the routine fashion. Topical anesthesia was achieved by benoxinate hydrochloride 0.4% eyedrops (Benox, EIPICO, 10th of Ramadan, Egypt) that were instilled into the eyes before surgery. A lid speculum was placed. The Moria M2 microkeratome (Moria SA, Antony, France) head created a superior-hinge flap. The intended flap thickness was 110 μm in all cases. The flap was then lifted and directed toward the hinge. The stromal bed was ablated using an excimer laser (Wavelight EX 500 GmbH, Erlangen, Germany) followed by irrigation with balanced salt solution (BSS, Alcon Laboratories Inc., Fort Worth, Texas, USA) before the flap was replaced. The flap was then repositioned onto the bed. A drop of an antibiotic, a corticosteroid, and a lubricating agent were applied to the cornea before removal of the speculum.

Flaps in FS-LASIK group were created using the Wavelight FS200 (Wave-Light GmbH, Erlangen, Germany) FS laser with intended flap thickness of 110 μm in all cases. After the suction ring was applied to the eye and the suction was turned on, the laser system with the applanation cone was lowered into the suction ring. A standard flap was created in ∼6–7 s. The edge of the flap was located and lifted. Fine adhesions were dissected, and the flap was folded back on itself and the interface was cleared. The stromal bed was ablated using the excimer laser (Wavelight EX 500) with the treatment based on the manifest refraction. A Plano prescription was attempted in all cases.

The postoperative topical medication regimen consisted of prednisolone acetate 1% (Pred forte eyedrops; Allergan Ltd, Marlow, Bucks, UK) and gatifloxacin 0.3% (Tymer, eyedrops; Jamjoom Pharmaceuticals, Jeddah, Saudi Arabia) four times per day for 1 week and artificial tears (Systane ultra eyedrops; Alcon Laboratories Inc.) for 1–3 months.

Postoperative examinations were performed at 1, 3, and 6 months after surgery. At each follow-up visit, the patients were subjected to refraction, evaluation of UCVA and BCVA, and slit-lamp biomicroscopy. Flap thickness was measured 4–6 weeks postoperatively with the AS-OCT (Topcon 3D OCT-2000, Topcon Corporation, Tokyo, Japan). These OCT measurements were compared with the intended flap thickness. The OCT measurements were performed using the zoom function on the OCT software to magnify the corneal image and the contrast function to highlight the flap interface.

Statistical analysis

Data were collected, revised, coded, and entered to the Statistical Package for the Social Sciences (SPSS; IBM Corp., Armonk, New York, USA), version 23. Quantitative data were presented as mean and SD, whereas qualitative variables were presented as number and percentages. Comparison between groups with qualitative data was done by using χ2 test. Comparison between two groups with quantitative data and parametric distribution was done by using independent t test, whereas data with nonparametric distribution were done by using Mann–Whitney test. The comparison between two paired groups with quantitative data and parametric distribution was done by using paired t test, whereas data with nonparametric distribution were done by using Wilcoxon rank test. The comparison between more than two paired groups with quantitative data and parametric distribution was done by using repeated measure analysis of variance test, whereas data with nonparametric distribution were done by using Friedman test. The confidence interval was set to 95%, and the margin of error accepted was set to 5%. So, P value was considered nonsignificant if more than 0.05, whereas P value less than 0.05 was considered significant and P value less than 0.01 highly significant.


  Results Top


The age, sex, preoperative BCVA, central corneal thickness, and mean keratometric power were not statistically significantly different between the two groups. The patients prepared for FS-LASIK were more myopic than the patients in the microkeratome group, and thus, the preoperative spherical equivalent and UCVA were significantly different between both groups (P=0.000 and 0.005, respectively; [Table 1]).
Table 1 Patient demographics and preoperative characteristics

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No vision-threatening flap complications were found in either group. There was no statistically significant difference between the two groups regarding the mean preoperative k-readings (P=0.864), whereas there was a statistically significant difference between the two groups regarding the mean k-readings at the sixth month postoperatively (P=0.017). There was no statistically significant difference between the two groups in terms of postoperative UCVA (0.98±0.10 in the FS-LASIK group and 0.98±0.11 in the microkeratome group) and refraction (SE= −0.31±0.49 and −0.28±0.47 D, respectively) at the sixth month postoperatively, which denotes efficacy of both procedures ([Table 2]). There was no loss of BCVA in either group at the sixth month postoperatively.
Table 2 Comparison of clinical outcome between groups at the sixth month postoperatively

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The morphology of the flap showed by the AS-OCT was different between the two groups. The flap had a planar configuration ([Figure 1]) in the FS-LASIK group and a meniscus shape (the thickness increases gradually towards the periphery) in the microkeratome group. [Figure 2] provides the mean flap thickness in the different meridians of the four points. The FS-LASIK flap showed a more regular planar shape. The microkeratome flap showed a meniscus shape, which was thin in the center and became thicker toward the periphery ([Figure 3]).
Figure 1 Anterior segment optical coherence tomography revealed the profile of the LASIK flap created by (a) femtosecond laser. Anterior segment optical coherence tomography revealed the profile of the LASIK flap created by (b) the microkeratome. LASIK, laser-assisted in situ keratomileusis.

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Figure 2 Comparison between flap thicknesses in both groups at diffrent eccentricities.

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Figure 3 Comparison of flap thickness at each eccentricity between FS-LASIK group and the microkeratome group. FS, femtosecond; LASIK, laser-assisted in situ keratomileusis.

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The mean central thickness (at 1 mm radius) of the FS-LASIK flaps was significantly thinner than that of the microkeratome flaps (114.93±3.04 and 128.57±3.98 μm, respectively, P=0.000). In addition, the mean peripheral thickness (at 2 mm radius) of the FS-LASIK flaps was significantly thinner than that of the microkeratome flaps (115.22±3.34 and 139.00±4.75 μm, respectively, P=0.000). In terms of flap creation predictability, the deviation from the intended thickness (110 μm) was more in the microkeratome group than the FS-LASIK group. Regarding the center–peripheral disparity (the difference between flap thickness at 1 and 2 mm from the center), there were significant differences between the two groups at 1-mm radius and 2-mm radius. There was no significant difference among the two eccentricities in the FS-LASIK group (P=0.708). On the contrary, the microkeratome flaps were thicker at the 2-mm radius than at the 1-mm radius (P=0.000, [Table 3]).
Table 3 Comparison of flap thickness at each eccentricity between femtosecond laser-assisted in situ keratomileusis group and the microkeratome group

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Comparison between nasal and temporal flap thickness at the first month revealed no statistically significant difference among them in both groups (P=0.509 in FS-LASIK group and P=0.485 in microkeratome group; [Table 4]).
Table 4 Comparison between the two groups regarding flap thickness at the first month

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  Discussion Top


LASIK has become the most popular procedure for the surgical management of myopia. During LASIK surgery, creation of a corneal flap is the first and the most critical step, as it influences the visual outcome. Flap formation has progressed from using mechanical microkeratomes to using the FS laser [5]. Many studies have compared FS lasers and mechanical microkeratomes for corneal flap creation in LASIK. Creating a flap with the intended thickness is crucial for obtaining an appropriate residual stromal thickness [6]. Postoperative refractive stability remains a concern, despite the passage of more than 30 years since the first excimer corneal refractive surgery was first conducted [7].

The current study showed no significant difference between both groups regarding UCVA and BCVA at 1, 3, and 6 months, postoperatively. This proves excellent and comparable effectiveness and stability of visual acuity in both techniques. Similar results were published in the study by Calvo et al. [2], which showed no significant difference between the microkeratome group and the FS-LASIK group in UCVA and BCVA at any examination and remained stable postoperatively through 3 years. The minimum detectable difference in visual acuity between treatments was less than or equal to 0.1 logMAR (≤1 line of vision).

No significant difference was noticed, in the present study, between both groups regarding correction of spherical equivalent at the first, third, and sixth month (−0.31±0.49 D for the FS-LASIK group vs. −0.28±0.47 D for the microkeratome group at the sixth month) and regarding the spherical error (−0.16±0.50 D for the FS-LASIK group vs. −0.09±0.47 D for the microkeratome group at the sixth month) and the cylindrical error (−0.36±0.23 D for the FS-LASIK group vs. −0.47±0.43 D for the microkeratome group at the sixth month).

The study by Kymionis et al. [8] showed comparable results. The mean preoperative BCVA was 1.02±0.06, whereas the UCVA at 6 months postoperatively was 1.01±0.09. Six months postoperatively, no eye lost lines of BCVA in their study. The results of Hashemi et al. [9] were comparable to the current study results in the FS-LASIK group, having the spherical equivalent reduced from −8.19±2.05 D preoperatively to −0.09±0.24 D at 6 months, postoperatively. The spherical error was reduced from −7.61±1.61 D preoperatively to 0.09±0.25 D at 6 months postoperatively, whereas the cylindrical error was reduced from 1.16±1.52 D preoperatively to 0.35±0.39 D at 6 months postoperatively. Their results confirmed the stability and predictability of FS-LASIK procedure.

Similar results were also published in a meta-analysis study by Chen et al. [10]. In the microkeratome group, they showed improvement of spherical equivalent from −3.77±1.40 D preoperatively to −0.20 (0.31) D 12 months postoperatively. After 12 months, all eyes in the microkeratome group were within 1 D of intended refractive change. They found no significant differences between the FS laser and mechanical microkeratome with respect to loss of two lines of BCVA, patients achieving UCVA of 20/20 or better, final UCVA, final astigmatism, or changes in higher order aberrations.

Regarding keratometric readings, the mean preoperative corneal refractive power (Km) was 43.82±1.29 D in the FS-LASIK group and 43.75±1.82 D in the microkeratome group, whereas the mean postoperative Km was 39.47±2.55 D in the FS-LASIK group and 41.09±2.58 D in the microkeratome group. The current study showed a significant difference between the two groups regarding keratometric readings at the sixth month postoperatively. This difference was owing to the difference in preoperative refractive error, which was higher in the FS-LASIK group (SE −5.42±2.25 D) compared with the microkeratome group (SE −3.19±2.36 D).

In the study by Igarashi et al. [11], the keratometric readings and the corneal astigmatism were determined in consecutive patients who had LASIK for moderate to high astigmatism (−2.00 D). There were changes in corneal refractive powers after LASIK. In their study, the preoperative corneal refractive power (K1) was 42.91±1.20 D (range, 40.50–45.25 D). K1 increased significantly from 38.46±2.04 D (range, 34.50–43.50 D) 1 week postoperatively to 39.17±1.77 D (range, 36.00–43.75 D) at 1 year postoperatively (P<0.001). The preoperative corneal refractive power (K2) was 45.87±1.45 D (range, 42.75–48.7 D). The mean K2 increased significantly from 39.89±2.26 D (range, 35.75–44.50 D) at 1 week to 40.72±2.19 D (range, 36.75–45.25 D) at 1 year postoperatively (P<0.001).

Regarding the flap thickness, in the present study, there was no statistically significant difference between the means in the nasal and temporal positions in the FS-LASIK group and the microkeratome group. The mean flap thickness measurements at each point showed a planar shape in the FS-LASIK group, with no statistically significant difference between average measurements of flap thickness at 1 and 2 mm from the corneal vertex, whereas it showed a meniscus shape in the microkeratome group. The center–peripheral disparity was nonsignificant in the FS-LASIK group, but it was significant in the microkeratome group. The average central thickness (at 1 mm) of the FS-LASIK flaps was significantly thinner than that of the microkeratome flaps (114.93±3.04 and 128.57±3.98 μm, respectively). The average peripheral thickness (at 2 mm) of the FS-LASIK flaps was also significantly thinner than that of the microkeratome flaps (115.22±3.34 and 139.00±4.75 μm, respectively). Regarding flap creation predictability, the deviation from the intended thickness at each measuring point in the FS-LASIK group was 4.93±3.04 µm at 1 mm from the corneal vertex and 5.22±3.34 µm at 2 mm from the corneal vertex, which was significantly less than the microkeratome group (18.57±3.98 at 1 mm and 29.00±4.75 at 2 mm, respectively).

In the study by Xia et al. [12], it was found that flap thickness was highly predictable with the VisuMax FS laser. The mean achieved central flap thickness was 113.05±5.89 μm for an attempted flap thickness of 110 μm. The variability of flap thickness was 3.05 μm with a narrow SD (5.89 μm), whereas the mean achieved central flap thickness with the mechanical microkeratome was 148.36±21.24 μm for an intended flap thickness of 140 μm and the variability of flap thickness was 8.36 μm, with a wider SD (21.24 μm).

The results in the study by Zhang et al. [13] were comparable to the current study. They compared the WaveLight FS200 flaps with Moria flaps. The intended flap thickness was 110 µm. The mean central flap thickness was 105.53±5.86 µm in the WaveLight group and 132.96±13.91 µm in the Moria group (P<0.001). The difference between the achieved and intended flap thickness (accuracy) was 6.17±3.98 and 23.60±12.64 µm, respectively (P<0.001). The SD within individual flap (uniformity) was smaller in the WaveLight group. The symmetry and regularity were also better in the WaveLight group. Flap morphology showed a more regular planar shape in the WaveLight group and a meniscus shape in the Moria group.

Kanellopoulos and Asimellis [14] investigated in 2013 the morphology of LASIK flaps created by two different FS lasers (IntraLase FS60 kHz and WaveLight FS200) and compared them with microkeratome (Moria M2). They found that both FS lasers were able to produce thinner and reproducible flap thickness with a reduced variability, compared with the microkeratome flaps. In the Moria M2 group, the average postoperative flap thickness was 138.83±12.38 μm (range, 114–159 μm) for an intended thickness of 130 μm. IntraLase FS60 group had an average flap thickness of 128.46±5.72 μm (range, 119–137 μm), with an intended thickness of 120 μm. Finally, the WaveLight FS200 group had an average flap thickness of 122.00±5.64 μm (range, 94–135 μm).

The meta-analysis study by Chen et al. [10] compared the IntraLase FS laser and mechanical microkeratomes for flap creation in LASIK and found that the flap thickness was more predictable in the IntraLase group. The microkeratome group had more epithelial defects, whereas the IntraLase group had more cases of diffuse lamellar keratitis.

The limitations of the current study include the relatively small number of enrolled patients and the short follow-up time. Therefore, further long-term studies on a larger number of patients are required to detect and compare the late complications especially myopic regression and corneal ectasia.


  Conclusion Top


Both FS-LASIK and microkeratome techniques were safe and efficient in correcting myopia and myopic astigmatism, achieving good visual outcome. AS-OCT showed that the flaps created by the FS laser were more accurate and uniform than those created by the Moria M2 microkeratome.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Kohnen T, Schwarz L, Remy M, Shajari M. Short-term complications of femtosecond laser-assisted laser in situ keratomileusis cuts: Review of 1210 consecutive cases. J Cataract Refract Surg 2016; 42:1797–1803.  Back to cited text no. 1
    
2.
Calvo R, McLaren JW, Hodge DO, Bourne WM, Patel SV. Corneal aberrations and visual acuity after laser in situ keratomileusis: femtosecond laser versus mechanical microkeratome. Am J Ophthalmol 2010; 149:785–793.  Back to cited text no. 2
    
3.
Ye MJ, Liu CY, Liao RF, Gu ZY, Zhao BY, Liao Y. SMILE and Wavefront-guided LASIK out-compete other refractive surgeries in ameliorating the induction of high-order aberrations in anterior corneal surface. J Ophthalmol 2016; 2016:8702162.  Back to cited text no. 3
    
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Santhiagoa MR, Juniorb NK, Waring GO. Microkeratome versus femtosecond flaps: accuracy and complications. Curr Opin Ophthalmol 2014; 25:270–274.  Back to cited text no. 4
    
5.
Liu Q, Zhou YH, Zhang J, Zheng Y, Zhai CB, Liu J. Comparison of corneal flaps created by Wavelight FS200 and Intralase FS60 femtosecond lasers. Int J Ophthalmol 2016; 9:1006–1010.  Back to cited text no. 5
    
6.
Ahn H, Kim JK, Kim CK, Han GH, Seo KY, Kim EK et al. Comparison of laser in situ keratomileusis flaps created by 3 femtosecond lasers and a microkeratome. J Cataract Refract Surg 2011; 37:349–357.  Back to cited text no. 6
    
7.
Zhou J, Gao Y, Li S, Gu W, Wu L, Guo X. Predictors of myopic regression for laser-assisted subepithelial keratomileusis and laser-assisted in situ keratomileusis flap creation with mechanical microkeratome and femtosecond laser in low and moderate myopia. Ophthalmic Epidemiol 2020; 27:177–185.  Back to cited text no. 7
    
8.
Kymionis GD, Kontadakis GA, Grentzelos MA, Panagopoulou SI, Stojanovic N, Kankariya VP et al. Thin-flap laser in situ keratomileusis with femtosecond-laser technology. J Cataract Refract Surg 2013; 39:1366–1371.  Back to cited text no. 8
    
9.
Hashemi H, Miraftab M, Ghaffari R, Asgari S. Femtosecond-assisted LASIK versus PRK: comparison of 6-month visual acuity and quality outcome for high myopia. Eye Contact Lens 2016; 42:354–357.  Back to cited text no. 9
    
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Chen S, Feng Y, Stojanovic A, Jankov MR, Wang Q. IntraLase femtosecond laser vs mechanical microkeratomes in LASIK for myopia: a systematic review and meta-analysis. J Refract Surg 2012; 28:15–24.  Back to cited text no. 10
    
11.
Igarashi A, Kamiya K, Shimizu K, Komatsu M. Time course of refractive and corneal astigmatism after laser in situ keratomileusis for moderate to high astigmatism. J Cataract Refract Surg 2012; 38:1408–1413.  Back to cited text no. 11
    
12.
Xia LK, Yu J, Chai GR, Wang D, Li Y. Comparison of the femtosecond laser and mechanical microkeratome for flap cutting in LASIK. Int J Ophthalmol 2015; 8:784–790.  Back to cited text no. 12
    
13.
Zhang Y, Chen YG, Xia YJ. Comparison of corneal flap morphology using AS-OCT in LASIK with the WaveLight FS200 femtosecond laser versus a mechanical microkeratome. J Refract Surg 2013; 29:320–324.  Back to cited text no. 13
    
14.
Kanellopoulos AJ, Asimellis G. Three-dimensional LASIK flap thickness variability: topographic central, paracentral and peripheral assessment, in flaps created by a mechanical microkeratome (M2) and two different femtosecond lasers (FS60 and FS200). Clin Ophthalmol 2013; 7:675–683.  Back to cited text no. 14
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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