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Comparison of corneal flaps created by
Wavelight FS200
and Intralase FS60 femtosecond lasers
Qian Liu, Yue-Hua Zhou, Jing Zhang, Yan
Zheng, Chang-Bin Zhai, Jing Liu
Ophthalmic
Center, Beijing Tongren Hospital, Capital Medical University, Beijing
Ophthalmology & Visual Sciences Key Lab, Beijing 100730, China
Correspondence to: Yue-Hua
Zhou. Ophthalmic Center, Beijing Tongren Hospital, Capital Medical University,
Beijing Ophthalmology & Visual Sciences Key Lab, No.1 Dongjiaomin Ln, Dongcheng
District, Beijing 100730, China. shouyiliuqian@163.com
Received:
2015-01-31
Accepted: 2015-08-26
Abstract
AIM: To assess and
compare the morphology of corneal flaps created by the Wavelight FS200 and
Intralase FS60 femtosecond lasers in laser in situ
keratomileusis (LASIK).
METHODS: Four hundred
eyes of 200 patients were enrolled in this study and divided into Wavelight FS200 groups
(200 eyes) and Intralase FS60 groups (200 eyes).
Fourier-domain optical coherence tomography (RTVue OCT) was used to measure the
corneal flap thickness of 36 specified measurements on each flap one week after
surgery. Results were used to analyze the regularity, uniformity and accuracy
of the two types of LASIK flaps.
RESULTS: The mean thickness of corneal flap and central flap was
105.71±4.72 μm and 105.39±4.50 μm in Wavelight FS200
group and 109.78±11.42 μm and 109.15 ±11.59 μm in Intralase FS60 group,
respectively. The flaps made with the Wavelight FS200 femtosecond laser were
thinner than those created by the Intralase FS60 femtosecond laser (P=0.000). Corneal flaps in the 2 groups
were uniform and regular, showing an almost planar configuration. But the
Wavelight FS200 group has more predictability and uniformity of flap creation. The
mean deviation between achieved and attempted flap thickness was smaller in the
Wavelight FS200 group than that in the Intralase FS60 group, which were 5.18±3.71 μm and 8.68±7.42 μm respectively.
The deviation of more than 20 µm was 0.2% measurements in Wavelight FS200 group and 8.29%
measurements in Intralase FS60 group.
CONCLUSION: The
morphologies of flaps created by Wavelight FS200
are more uniform and thinner than those created by Intralase FS60.
KEYWORDS: Fourier-domain
optical coherence tomography; laser in
situ keratomileusis; flap; femtosecond laser; Wavelight FS200; Intralase
FS60
DOI:10.18240/ijo.2016.07.12
Citation: 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(7):1006-1010
INTRODUCTION
Laser in situ
keratomileusis (LASIK) has become the most popular approach in the world for
the correction of refractive error[1-2].
The popularity of LASIK is due to many factors including the lack of
postoperative pain, quick vision recovery, and the refractive accuracy of the
procedure[3]. The
femtosecond laser has been used widely in recent years, which has provided an
alternative option for flap creation since the introduction of the IntraLaseTM
(Abbott Medical Optics, Santa Ana, CA, USA) in 2001[4]. It is important to
produce a uniform flap with a narrow standard deviation (SD) from the intended
thickness to obtain an appropriate residual stromal thickness during LASIK[5-7]. Sufficient residual
stromal bed (RSB) thickness (exceeding 250 mm) is important to reduce the
likelihood of corneal ectasia[8].
Ultrasound (US) pachymetry is considered the
gold standard method for measuring corneal center thickness (CCT). But mechanical pressure of the contact probe and
flap surface humidity can affect the accuracy of pachymetry flap thickness
measurement. In this prospective study, Fourier-domain optical coherence tomography
(OCT) (RTVue OCT; Optovue Inc, Fremont, California, USA) was used to compare
the dimensions of flaps created with the Intralase FS60 femtosecond
laser and the Wavelight FS200 femtosecond laser.
SUBJECTS AND METHODS
Subjects In
this prospective study, four hundred eyes of two hundred consecutive patients
were scheduled bilateral LASIK treatment from July 2012 to February 2013 in the
Tongren Ophthalmic Center of Capital Medical University (Beijing, China).
Patients with ocular pathologies such as keratoconus, corneal scars, corneal
dystrophies, previous ocular surgery, glaucoma, diabetes, or other systemic
diseases known to affect the eye were excluded. Based on the random and voluntary principle, 200 eyes of 100 patients underwent
LASIK with Wavelight FS200 femtosecond laser and 200 eyes of 100 patients with Intralase FS60 femtosecond laser.
All patients were informed about the surgical procedure and subscribed written
consents.
Methods This case series
study received approval by the Ethics Committee of our institution, adherent to
the tenets of the Declaration of Helsinki. All the patients underwent a series examination before surgery. The
examination included uncorrected distance visual acuity, slit-lamp microscopy,
corneal topography, manifest and cycloplegic refraction, corrected distance
visual acuity, fundus examination and US pachymetry. All the surgeries were
performed by Zhou YH. Both of the femtosecond lasers are flat applanation
systems and programmed to a nominal flap thickness of 110 μm. In WaveLight FS200 group, the
flap was created using WaveLight FS200 femtosecond laser. Superior hinged flaps
were created with 8.5 mm diameter for both of the two groups. In WaveLight
FS200 group the laser energy was set at 0.8 µJ and the repetition frequency at
200 kHz. The pulse duration was about 350 femtosecond. The line and spot
separations were 8.0 µm. The side cut angle is 90°, canal width of 1.5
mm, hinge width of 0.6 mm and hinge angle of 60°. In Intralase FS60
group, the flap was created using IntraLase FS60 femtosecond laser. The laser
energy was set at 0.75 µJ and the repetition frequency at 60 kHz. The pulse
duration was between 600 and 800 femtosecond. Superior hinged flaps were
created with 8.5 mm diameter. The line and spot separations were 8.0 µm. The
side cut angle is 90°, hinge angle of 50°. The start
depth of the pocket is 180 μm and width of the pocket is 0.2 mm. After the flap
was lifted, ablations were performed using the Visx S4 excimer laser (VISX
Inc., Santa Clara, USA) in the Intralase FS60 group and Alcon WaveLight EX500
excimer laser (Fort Worth, TX, Germany) in the Wavelight FS200 group with a 0.5-mm
transition zone and 6.0-mm optical zone.
Imaging and
measurement of the LASIK flaps were performed using RTVue OCT(Optovue Inc,
Fremont, California, USA) system.We used the CAM-L scan pattern of
the cornea anterior module to acquire scans of the cornea across a diameter of
8.0 mm for crossline analysis (Figure 1). The scan was centered on the vertex
reflection, and 0°, 45°, 90°, 135° meridian OCT images
were acquired and analyzed by the same ophthalmologist (Zhang J) 1wk after
surgery, who was masked to the type of flap creation (Figure 2). Flap thickness
was measured at 9 points in each meridian in 4 separate OCT images obtained for
each eye (center, 1.0, 2.0 and 3.0 mm from the center and the peripheral zone
of the flap). Center was defined as the range of ±0.5
mm from the flap vertex and peripheral zone, as the range within ±0.5 mm from
the flap edge (i.e. approximately
3.75 to 4.25 mm from the center of the flap).
Figure 1 Example of frame-averaged horizontal line scan of corneal
images For each image,
the upper numbers represent distances from the central cornea (mm).
The lower numbers represent flap thickness and RSB thickness (mm), respectively.
Figure 2 Pupil image showing
0°, 45°, 90°, 135° meridional scans
and flap thickness.
Statistical Analysis Data were expressed
as the mean±standard deviation (SD) and analyzed with SPSS software
(version13.0, SPSS, Inc.). An independent samples t-test and one-way
analysis of variance (ANOVA) were used to analyze data consistent with normal
distribution. The Wilcoxon signed-rank test was applied to identify data not
consistent with normal distribution. A P value
less than 0.05 was considered statistically significant.
RESULTS
Baseline Demographics Four hundred eyes of 200 patients were
evaluated. Table 1 shows the baseline demographics of the patients. No
significant differences were observed between the two groups (All P＞0.05).
Table 1 Baseline
patient characteristics
|
Parameters |
Wavelight FS200 group (n=200) |
Intralase FS60 group (n=200) |
aP |
|
Age (a) |
25.14±5.88 |
26.19±5.59 |
0.198 |
|
SE (D) |
-6.04±1.36 |
-6.25±2.13 |
0.290 |
|
CCT (μm) |
551.59±22.48 |
546.32±31.86 |
0.075 |
|
Corneal curvature (D) |
43.89±1.03 |
43.65±1.32 |
0.170 |
|
Intended flap (μm) |
110 |
110 |
- |
CCT: Central
corneal thickness; SE: Spherical equivalent; aIndependent-samples t-test.
Visual and Refractive Outcomes The uncorrected
visual acuity outcomes were 1.34±0.17 and
1.19±0.27 in Wavelight FS200 and Intralase group respectively 1wk after surgery (P=0.000) and the
manifest refraction spherical equivalent were 0.00±0.48 and 0.11±0.80 diopters,
respectively (P=0.104).
Corneal Flap Thickness One week postoperatively, the mean flap thickness was 105.71±4.72 μm in WaveLight
FS200 group and 109.78±11.42 μm in Intralase FS60 group (P=0.000; Figure 3). The mean central
flap thickness was 105.39±4.50 μm in WaveLight FS200 group and 109.15±11.59 μm in Intralase FS60 group (P＜0.001; Figure 4).
Figure 3 The mean flap
thickness The mean
flap thickness was 105.71±4.72 μm in WaveLight FS200 group and 109.78±11.42 μm in Intralase FS60 group.
Figure 4 The mean central flap thickness The mean central flap thickness was 105.39±4.50 μm in
WaveLight FS200 group and 109.15±11.59 μm
in Intralase FS60 group.
Corneal Flap Shape and
Regularity Table 2 shows the mean and SD of the flap thickness at
each of the 36 locations measured in each eye for both groups. The maximum
difference of the mean thickness at different measurement points was ±5 μm in WaveLight FS200 group and ±14 μm in Intralase FS60 group. Corneal flaps in
both groups were uniform and regular, showing an almost planar configuration. Figure
5 provided the mean flap thickness in different meridians of the 36 points.
Table 2 Flap
thickness at each measuring point
, μm
|
Locations |
-Peri |
-3.0 mm |
-2.0 mm |
-1.0 mm |
Center |
1.0 mm |
2.0 mm |
3.0 mm |
+Peri |
|
Wavelight FS200 group |
|
|
|
|
|
|
|
|
|
|
0° |
106±5 |
104±5 |
105±4 |
104±4 |
105±4 |
104±4 |
106±4 |
105±4 |
106±4 |
|
45° |
106±4 |
105±5 |
105±4 |
104±5 |
105±4 |
105±4 |
106±5 |
106±4 |
106±4 |
|
90° |
105±4 |
105±4 |
106±4 |
105±4 |
104±4 |
104±4 |
105±4 |
106±4 |
107±4 |
|
135° |
106±4 |
105±4 |
105±4 |
105±5 |
105±4 |
104±5 |
106±4 |
106±4 |
106±4 |
|
Intralase FS60 group |
|
|
|
|
|
|
|
|
|
|
0° |
109±10 |
107±11 |
110±14 |
109±11 |
108±11 |
109±11 |
111±11 |
110±11 |
111±10 |
|
45° |
108±9 |
106±11 |
111±11 |
110±11 |
110±11 |
109±11 |
111±11 |
109±11 |
111±10 |
|
90° |
109±9 |
108±10 |
110±12 |
109±12 |
108±11 |
108±10 |
110±11 |
108±11 |
110±10 |
|
135° |
109±10 |
107±11 |
110±11 |
110±11 |
109±11 |
110±11 |
110±11 |
109±10 |
110±9 |
±1.0 mm: Point 1.0 mm from flap vertex; ±2.0 mm: Point 2.0
mm from flap vertex; ±3.0 mm: Point 3.0 mm from flap
vertex; Center: Range of ±0.5 mm from flap
vertex; ±Peri: Range within 0.5 mm from the flap rim (approximately 3.75 to 4.25
mm from flap vertex).
Figure 5 Comparison of flap thickness in the 0° (A), 45° (B), 90° (C) and 135° (D) meridian between Wavelight FS200 group and Intralase
FS60 group.
Uniformity of Flap on Every Meridians
The mean flap thicknesses were 105.57±2.90, 105.71±2.65, 105.83±3.14
and 105.75±3.16 μm on the 0°, 45°, 90°, 135° meridians in WaveLight FS200 group (P=0.859), and
109.94±10.43, 109.37±10.26, 110.03±10.28 and 109.80±9.99 μm in Intralase FS60
group (P=0.923). There was no
significant difference between the 2 groups.
Predictability of Flap-thickness
Table 3 shows the mean deviation between the measured and
the intended flap thickness at each measuring point. The mean deviation in flap
thickness was 5.18±3.71 μm in WaveLight FS200 group and 8.68±7.42 μm in Intralase FS60 group (P＜0.001). The maximum deviation from the intended 110 mm thickness of 36
measurements was 6.2 μm in WaveLight FS200 group and 9.8 μm in Intralase FS60 group.
WaveLight FS200 group has a better
predictability than Intralase FS60 group.
Table 3 Mean
deviations from intended flap thickness
, μm
|
Locations |
-Peri |
-3.0 mm |
-2.0 mm |
-1.0 mm |
Center |
1.0 mm |
2.0 mm |
3.0 mm |
+Peri |
|
Wavelight FS200 group |
|
|
|
|
|
|
|
|
|
|
0° |
5.0±3.5 |
5.8±4.1 |
5.1±3.7 |
5.9±3.8 |
5.0±3.6 |
5.9±4.1 |
4.6±3.3 |
4.8±3.4 |
4.7±3.4 |
|
45° |
4.8±3.2 |
5.3±4.2 |
4.9±3.5 |
6.0±4.0 |
5.4±3.9 |
5.4±4.0 |
5.1±3.5 |
4.7±3.3 |
4.4±2.8 |
|
90° |
4.9±3.5 |
5.6±3.8 |
4.9±3.5 |
5.5±3.5 |
5.4±3.6 |
6.1±4.0 |
4.7±3.4 |
4.6±2.9 |
4.0±3.2 |
|
135° |
4.8±3.3 |
5.6±4.0 |
5.0±3.8 |
5.9±3.8 |
5.1±3.5 |
6.2±4.4 |
4.7±3.8 |
4.8±3.3 |
4.4±3.0 |
|
Intralase FS60 group |
|
|
|
|
|
|
|
|
|
|
0° |
8.2±6.9 |
9.1±7.3 |
9.8±10.1 |
8.7±7.0 |
8.8±7.5 |
9.1±7.3 |
9.0±7.7 |
8.7±7.3 |
8.2±7.1 |
|
45° |
7.7±6.3 |
9.7±7.5 |
9.2±7.7 |
9.1±7.6 |
9.1±7.3 |
8.9±6.9 |
8.6±7.5 |
9.1±7.0 |
7.9±7.6 |
|
90° |
7.6±6.2 |
8.4±7.0 |
9.1±7.8 |
9.0±7.9 |
9.0±7.6 |
8.5±6.9 |
8.6±8.0 |
9.1±7.4 |
7.4±7.0 |
|
135° |
7.9±6.8 |
9.0±7.4 |
8.8±7.6 |
8.5±7.6 |
8.4±7.4 |
8.9±7.6 |
8.8±7.5 |
8.5±6.8 |
6.6±6.3 |
±1.0 mm: Point 1.0
mm from flap vertex; ±2.0 mm: Point 2.0 mm from flap
vertex; ±3.0 mm: Point 3.0 mm from flap vertex; Center: Range of ±0.5 mm from flap
vertex; ±Peri: Range within 0.5 mm from the flap rim (approximately 3.75 to 4.25
mm from flap vertex).
Accuracy of Flap Thickness Figure 6 shows
the distributions of the differences between the intended corneal flap
thicknesses and the measured flap thicknesses in the Wavelight FS200 group and Intralase
FS60 group. There are 4156 measurements (57.72%) on which the difference was
less than 5 µm in the Wavelight FS200 group and 2965 measurements (41.18%) in
Intralase FS60 group. But differences greater than 20 µm were observed in 15
measurements (0.2%) in the Wavelight FS200 group and 597 measurements (8.29%)
in the Intralase FS60 group 1wk after surgery.
Figure 6 Distribution of the
difference between the intended corneal flap thicknesses and the measured flap
thicknesses in the Wavelight FS200 group and Intralase FS60 group.
DISCUSSION
LASIK has become the most popular approach in the world
for the correction of refractive error due to many factors including the lack
of postoperative pain, quick vision recovery, and the refractive accuracy of the
procedure. The first step in LASIK is the preparation of the corneal flap,
which is a critical and important step during the procedure. At present, a
LASIK flap can be created with a mechanical microkeratome or femtosecond laser.
Several studies[9-12] have
compared femtosecond lasers and mechanical microkeratomes for corneal flap
creation and found that the former may yield better safety, reproducibility,
and predictability. A study in hyperopic patients showed significantly better refractive
results with femtosecond laser flaps than with microkeratome flaps[13]. Another study showed
that the incidence of epithelial ingrowth after femtosecond LASIK was less than
microkeratome LASIK[14]. It is important to produce a uniform flap with a narrow
SD from the intended thickness. A corneal flap, which is too thin, increases the incidence of flap related complications including a free, irregular,
incomplete, buttonhole, or lacerated flap. A corneal flap, which is too thick,
increases the likelihood of violating the minimum safe RSB thickness of 250 μm[15-16].
Although US pachymetry is considered the gold standard
method for measuring CCT, it cannot be used to measure all regions of the
cornea at once. Hence, several alternatives have been developed, including OCT.
In this study, a new generation of Fourier-domain OCT (RTVue OCT, Optovue,
Inc.) was used to compare the flap morphology. In this study, nominal flap
thickness was set to 110 μm in both of the 2 femtosecond laser groups. The mean
central flap thickness and the mean flap thickness in WaveLight FS200
group were thinner than in Intralase FS60
group. Several studies have evaluated femtosecond laser-created
LASIK flaps. Zhou et al[12] enrolled 720 eyes of 360
patients to measure the flaps created by the Intralase FS60 femtosecond and
found that Intralase flaps with an intended thickness of 110 μm were actually a
mean of 110.5±2.9 μm. Zhang et al[17] enrolled 408 eyes of
204 patients to measure the flaps created by the Intralase FS60 femtosecond
laser using the RTVue OCT and found that Intralase flaps with an intended
thickness of 110 μm were actually a mean of 109.34±7.57
μm. Our results are consistent with findings in the above studies. In our study
the mean flap thickness was 105.71±4.72 μm in Wavelight FS200 group and
109.78±11.42 μm in Intralase FS60 group. The mean central flap thickness was
105.39±4.50 μm in Wavelight FS200 group and 109.15 ±11.59 μm in Intralase FS60
group. This difference may be related to the calibration of the different
femtosecond laser systems. It may also be due to the difference of energy or
frequency of laser we used. The mean thickness of the different research may be related to the kind of OCT used for measurement, and
different number of eyes enrolled.
Previous studies found that flaps created by femtosecond
laser had uniform thickness and resulted in planar shaped flaps compared with
microkeratome-created flaps. A uniform and regular flap may contribute to more
predictable refractive and wavefront outcomes[18-19]. Zhang et al[17] found that the mean deviation of flap created by Intralase
FS60 between the measured and the intended thickness was 6.59 μm. Our results
are consistent with findings in the above studies. The deviation of
less than 5 µm in the Wavelight FS200 group was 57.72% and 41.18% in Intralase
FS60 group. The deviation of more than 20 µm was 0.2% measurements in Wavelight
FS200 group and 8.29% measurements in Itralase FS60 group.
In summary, the flaps made with the Wavelight FS200
femtosecond laser were thinner than those created by the Intralase FS60. The
Wavelight FS200 femtosecond laser group has more predictability and uniformity
of flap creation. Further studies are needed to determine whether the thinner
and regular corneal flap contributes to improving postoperative biomechanical
reaction and reducing higher-order aberrations. In recent years, many kinds of
femtosecond laser as Intralase FS150, VisuMax, Femto LDV and Wavelight FS200
were used to create flaps of LASIK and more research with large sample
size needed to be done to analyze
the regularity, uniformity and accuracy of the different types of flaps. This is only a short-term study of corneal flap thickness between the 2
femtosecond lasers. Long-term outcomes needed to be done to explore the
consequence of the different corneal flap created by the 2 femtosecond lasers.
ACKNOWLEDGEMENTS
Conflicts of Interest: Liu Q, None; Zhou YH, None; Zhang J, None;
Zheng Y, None; Zhai CB, None; Liu J,
None.
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