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Comparison of the flaps made by femtosecond laser and automated keratomes
for sub-bowman keratomileusis
ZHAI Chang-bin1*, TIAN Lei1*, ZHOU Yue-hua1, ZHANG Qing-wei1 and ZHANG
Jing1
1
Ophthalmic Center, Beijing Tongren Hospital, Capital Medical University, Beijing
Ophthalmology & Visual Sciences Key Lab, Beijing 100730, China.
No author has a financial or proprietary interest in any material or method mentioned.
Correspondence to: Dr. ZHOU Yue-hua, Ophthalmic Center, Beijing Tongren Hospital,
No. 1 Dongjiaomin Ln, Dongchong District, Beijing 100730, China.
Phone: +8613910836019
Fax: +861058269072
Email: [email protected]
* The first two authors contribute equally to this article.
Background: To assess and compare the variations of LASIK flap created by the
IntraLase femtosecond laser, Moria One Use-Plus SBK and Moria M2 Single-Use 90
µm-head microkeratome using Anterior segment optical coherence tomography
(Visante® OCT).
Methods: 161 eyes of 81 consecutive patients were enrolled in this prospective study
and randomized divided into three groups depending on the flap creation method: flap
creation with the the IntraLase femtosecond laser (IntraLase group, 59 eyes), with the
Moria One Use-Plus SBK (SBK group , 44 eyes) and with the Moria M2 Single-Use
90 µm-head microkeratome (M2SU90 group, 58 eyes). Nominal flap thickness was
110 μm for all patients and for the three devices. One month after surgery, Visante®
OCT was used to measure flap thickness at 20 locations on each cornea and the
results were assessed for uniformity, regularity and accuracy.
Results: At one month after surgery, the mean central flap thickness was 111.18±3.33
µm in the IntraLase group, 113.85±8.07 µm in the SBK group and 117.96±13.14 µm
in the M2SU90 group, respectively. The flaps in the IntraLase group and the SBK
group were more regular, showing an almost planar configuration, than the
meniscus-shaped flaps in the M2SU90 group. The maximum deviation from the
intended flap thickness (110 µm) was 6.15 µm in the IntraLase group, 10.36 µm in the
SBK group and 19.75 µm in the M2SU90 group, respectively. A differences greater
than 20 µm was observed in 0.42% of measurements in the IntraLase group; 2.95% of
measurements in the SBK group and 21.12% of measurements in the M2SU90 group.
Conclusions: The flaps created by the IntraLase femtosecond laser and Moria One
Use-Plus SBK are more uniform; more regular and more accurate than those created
by the Moria M2 Single-Use 90 µm-head microkeratome. The first two methods can
make precise flaps for Sub-Bowman Keratomileusis.
Keywords: sub-bowman; keratomileusis; femtosecond laser ； microkeratome; flap
thickness;
Corneal ectasia is one of the most serious complications of Laser in situ
keratomileusis (LASIK). Sufficient residual stromal bed (RSB) thickness (exceeding
250 µm) is important to reduce the likelihood of corneal ectasia. Sub-Bowman
Keratomileusis is a LASIK procedure in which the flap is thinner (thin-flap LASIK)
with a planar flap architecture 1. A major advantage of creating a thin flap during
Sub-Bowman Keratomileusis is leaving sufficient stromal tissue to allow safer
excimer laser ablation in patients with moderate or high myopia.
Several years ago, doctors advocated that the ideal flap thickness in LASIK
should exceed 130 µm because thin flaps may be associated with a higher frequency
of potential complications, such as flap folds, striae, epithelial ingrowth, and irregular
astigmatism2. But more recent report advocates performing Sub-Bowman
Keratomileusis with flaps ranging from 90 µm to 110 µm in thickness3. Some reports
showed that thin flaps were associated with better early visual and refractive results
than thick flaps4.
There are three methods to achieve a thin flap in the most clinical center. The
traditional one is using Moria M2 microkeratome with plastic single use 90 µm-head;
the second method is using Moria One Use-Plus microkeratome with SBK head and
the most recent one: Femtosecond laser, which uses ultrafast pulses of energy to
induce photo-disruption of tissue with minimal collateral tissue damage and
inflammation at a preset depth.
The purpose of this study was to compare the thickness and uniform, regularity
and accuracy of the flaps using anterior segment optical coherence tomography
(Visante® OCT). In this study, we assessed and compared the configuration of LASIK
flap created by the IntraLase FS60 laser (Abbott Medical Optics, Santa Ana,
California, USA) and the Moria One Use-Plus microkeratome with SBK head (Moria,
Antony, France) and the Moria M2 microkeratome with single use 90 µm-head
(Moria, Antony, France).
METHODS
Patients
One hundred and sixty-one eyes of eighty-one consecutive patients who were
scheduled bilateral LASIK treatment from December 2011 to August 2012 in the
Tongren Ophthalmic Center of Capital Medical University (Beijing, China) enrolled
in this prospective study. Patients with ocular pathologies such as corneal scars,
corneal dystrophies, previous ocular surgery, keratoconus, glaucoma, diabetes, and
systemic diseases known to affect eyes were excluded. All the patients were told
about characteristics and indications about the three different methods for creating
corneal flaps particularly, they freely chose one from these three surgical procedures.
The first group had flaps with the IntraLase femtosecond laser (IntraLase group). The
second group made the flaps with the Moria One Use-Plus microkeratome with SBK
head (SBK group); the third group used Moria M2 microkeratome with plastic single
used 90 µm-head (M2SU90 group) for flap creation.
This study was a prospective, randomized, comparative clinical study. All patients
signed an informed consent in accordance with the tenets of the Declaration of
Helsinki.
Surgical Procedure
Preoperative examination included the following standard measurements: cornea
thickness by ultrasound pachymetry, corneal topography, silt-lamp biomicroscopy,
uncorrected visual acuity (UCVA), best corrected visual acuity (BSCVA), intraocular
tension measurement (NCT), applanation tonometry, fundus examination, manifest
refraction and cycloplegic refraction.
All surgeries were performed by the same surgeon (Y.H.Z.) under topical anesthesia.
The corneal flaps were created using the IntraLase FS60 laser or the Automated
Keratome. After lifting the flap, ablation was performed using the Visx S4 excimer
laser (VISX Inc., Santa Clara, USA). The corneal flap and stroma surface were
irrigated with balanced normal saline solution, and the flap was replaced.
Postoperative, patients were instructed to use 0.1％fluorometholone four times per
day for 3 days, and then tapered over for two weeks, levofloxacin eye drops and
artificial tears four times per day for 2 weeks. All patients were asked to have regular
follow-up visits.
IntraLase Femtosecond laser. In order to obtain a flap thickness of 110 μm, the
IntraLase femtosecond laser with the 60 kHz laser engine was programmed to a
thickness of 110 μm. The other settings were: 8.5 mm diameter, side-cut energy of
0.08 μJ, side-cut angle of 70º, raster pattern energy of 0.75 μJ, spacing between each
laser spot and raster line of 8x8 μm, a superior hinge with hinge angle of 45° and
the pocket enabled.
One Use-Plus SBK microkeratome. The Moria One Use-Plus microkeratome with
SBK head was used to create an 8.5-9.0 mm diameter corneal flap with a nasal hinge.
This device is used to obtain an attempted central flap thickness of 110 μm with a
calibrated 90 µm-head selecting the lowest speed (“Speed 1”) on the associated
Evolution 3 control unit.
M2 Single-Use 90 µm-head microkeratome. The Moria M2 microkeratome with a
plastic single use 90 µm-head was used to create an 8.5-9.0 mm diameter corneal flap
with a superior hinge. The calibrated 90 µm-head is a plastic, single used device. It is
used to obtain a central flap thickness of 110 µm selecting the lowest speed (“Speed
1”) on the associated Evolution 3 control unit.
Anterior Segment Optical Coherence Tomography Technology
The Visante® OCT (Carl Zeiss Meditec, Inc.) is a computerized instrument that
acquires and analyzes cross-sectional tomograms of the anterior eye segment (cornea,
anterior chamber, iris and the central portion of the lens). It employs non-invasive,
non-contact, low-coherence interferometry to obtain these high-resolution images. We
can obtain cross-sectional tomograms of the cornea at any appointed meridians using
this device (Figure 1). In our study, the 0°,45°,90°and 135°meridians OCT
images were acquired by the same skilled technician at one month after surgery
(Figure 2A). In the cross-sectional images, the LASIK flap was clearly visible by
increased reflection at the flap interface and increased internal reflectivity5. Then, the
flap thickness was measured using the semi-manual software’s flap tool by the same
technician who did not know the designed flap dimension. Flap thickness was
measured at 5 points in each meridian obtained for each eye (for a total of 20
measurements for each eye in the study). For each meridian, 1 location was at the
central zone (between ±0.5 mm from the flap vertex); 2 locations were at the
paracentral zone (±1 to 2 mm from the flap vertex); 2 locations were at the peripheral
zone (±3 to 5 mm from the flap vertex) (Figure 2B).
Statistical analysis
Data were expressed as mean ± standard deviation (SD) and analyzed with SPSS
17.0 software (SPSS Inc, Chicago, Illinois). An Independent-samples t test and
one-way analysis of variance (ANOVA) were used to analyze measurement data
conforming to normal distribution and a Wilcoxon signed ranks test was applied for
measurement data not conforming to normal distribution. A P value < 0.05 was
considered statistically significant.
RESULTS
Preoperative Characteristics
The preoperative characteristics of the patients are shown in Table 1. No significant
differences were observed among the three groups (P>0.05）.
Central Flap Thickness
The mean central flap thickness at one month postoperative were 111.18±3.33 µm
(range: 103.75 to 119.50 µm) in the IntraLase group, 113.85±8.07 µm (range: 97.50 to
130.00 µm) in the SBK group and 117.96±13.14 µm (range: 93.50 to 156.00 µm) in
the M2SU90 group. The distribution of central flap thickness for all eyes were
presented in Figure 3.
Flap Thickness Uniformity
Table 2 shows the mean standard deviation of the flap thickness measurements at each
of the 20 locations measured in each eye for the three groups. The IntraLase flaps and
the SBK flaps were more uniform, showing an almost-planar configuration. The
maximum deviations were 7.73 µm in the IntraLase group and 12.78 µm in the SBK
group, among the 20 different measurements of each flap. The flaps created with the
M2SU90 Keratome were thinner in the central zone and thicker in the periphery,
which provided a meniscus shape, with the maximum deviation of 19.42 µm.
Flap Thickness Regularity
The IntraLase flaps and the SBK flaps were more regular than the M2SU90 flaps
when measured from the center to the periphery. The average thickness values in the
central, paracentral and peripheral zones were not significantly different in the
IntraLase group (111.17±3.33 µm，111.10±2.85 µm and 110.34±3.42 µm, respectively.
P>0.05, by ANOVA) and the SBK group（113.85±8.07 µm，111.89±5.17 µm and
113.78±4.79 µm, respectively. P>0.05, by ANOVA）. In the M2SU90 group, the
central flap thickness was statistically significantly thinner than the peripheral zones
with the mean flap thickness of 117.96±13.14 µm ， 117.01±13.43 µm and
123.11±13.77 µm, respectively (P<0.05, by ANOVA).
Flap Thickness Accuracy
The mean deviation between the achieved and attempted flap thickness were
smaller in the IntraLase group and the SBK group than in the Moria M2SU90 group.
The IntraLase flap maximum deviation from the intended 110 µm of the 20
measurements was 6.15 µm. The SBK group was 10.36 µm, whereas the M2SU90
group from the intended 110 µm was 19.76 µm (Table 3).
Figure 4 shows the distributions of the differences between the intended corneal
flap thickness and the measured flap thickness in the IntraLase group (59 eyes, 1180
measurements), the SBK group (44 eyes, 880 measurements) and in the M2SU90
group (58 eyes, 1160 measurements). The differences were less than 5 µm in 682
measurements (57.79%) in the IntraLase group; in 363 measurements (41.25%) in the
SBK group and in 271 measurements (20.17%) in the M2SU90 group. Differences
greater than 20 µm were observed in 5 measurements (0.42%) in the IntraLase group;
in 26 measurements (2.95%) in the SBK group and in 303 measurements (21.12%) in
the M2SU90 group one month after surgery.
DISCUSSION
Thin flap LASIK (Sub-Bowman keratomileusis) is a safe technique to correct
myopic defects. Moreover, it achieves excellent refractive outcomes, a lower rate of
enhancements, and a good visual performance with better contrast sensitivity6. Precise
creation of the corneal flap is essential for successful Sub-Bowman keratomileusis.
With the improvement in traditional mechanical microkeratome system and the
development of the femtosecond laser, it is now possible to achieve clinically uniform
flaps in LASIK7. Femtosecond laser tend to create a planar flap with highly
predictable thickness and diameter. Stahl et al.8 demonstrated that a femtosecond laser
created a uniform flap with minimal variability, which made the process of creating a
thin LASIK flap safer and accurate.Main et al. 9 demonstrated that LASIK flaps
created with a femtosecond laser provide better astigmatic neutrality, induction of
fewer higher-order aberrations and decreased epithelial injury compared to traditional
mechanical microkeratomes.
In treating myopic refractive errors, the laser ablation is deepest in the central
cornea, which is also the area of the cornea that is thinnest. Most flap research is
mainly focused on this central portion of the flap and stromal bed10. However,
decreased post-operative visual performance such as glare and halo under dim
conditions, poor night vision and decreased contrast sensitivity values have been
reported and may be related to more peripheral corneal distortions11. Therefore,
analysis of central corneal parameters alone in LASIK may be less than optimal.
Fortunately, research on flap morphology and its effects on postoperative flap
performance have gained more attention in recent years.
In this study, we acquired thickness measurements at 20 points in the
0°,45°,90°and 135°meridians in each eye by Visante® OCT and divided these
points into the central, paracentral and peripheral zones. Traditionally, ultrasound
pachymetry has been used to measure corneal flap and stromal bed thickness, but this
technique has many potential pitfalls including the risk of damage of the corneal
epithelium, creating localized variations in corneal bed hydration which could affect
the laser ablation, and the risk of infection transmission with the ultrasound probe.
Furthermore, it is difficult to exactly correlate the position of the pre- and
intra-operative measurements, and it is very difficult to describe flap thickness
throughout the entirety of the flap12,13. Using the non-invasive, non-contact Visante®
OCT, we can obtain the high-resolution images of the cornea at any appointed
meridians, and can therefore precisely measure the flap thickness at any points14. Kim
et al.15 also reported that Visante® OCT is more reliable than ultrasound pachymetry
in measuring the central corneal flap thickness.
Our data revealed that the IntraLase flaps and the SBK flaps have more uniform
and regularity thickness than the M2SU90 flaps. In the M2SU90 group, the central
flap thickness was thinner than the paracentral and peripheral flap thickness
(P<0.05). Some clinical trials also revealed the differences16. The reason why Moria
One Use-Plus SBK can make a uniform thickness flap compared with Moria M2
Single-Use 90 μm-head may be because the working mechanisms are different. The
Moria M2 Single-Use 90 μm-head microkeratome has a hinge in the superior and the
One Use-Plus SBK gets a hinge at the nasal. Another possible reason may be that the
90 μm distance on the plastic head is not accurate.
The accuracy of the LASIK flap thickness is a key factor in security. In order to
maintain postoperative corneal strength, and prevent that corneal flap become thinner,
especially avoid the occurrence of iatrogenic keratoconus, the residual stromal
thickness must be more than 250 μm17. The residual stromal thickness was calculated
with the preoperative corneal thickness minus the prediction flap thickness and the
laser ablation flap thickness. If the difference between the attempted thickness and
achieved thickness is too large, patients will be greatly in danger, especially for those
with too-thin corneal thickness. In our trial, the IntraLase flap maximum deviation
from the intended 110 µm was 6.15 µm, the SBK was 10.36 µm and the M2SU90
maximum deviation from the intended 110 µm was 19.75 µm. The maximum
deviations in the three groups were produced in the peripheral zone, and in this region,
the accuracy of femtosecond laser group was better than the SBK group and the
M2SU90 group (Table 3).
It is reported that the corneal flap could affect low-level aberrations18, and
increase the higher-order aberrations19. These factors caused a negative impact to the
postoperative visual quality. Durrrie et al.20 reported that in 51 consecutive patients,
one eye was randomized to have the flap created with the IntraLase femtosecond laser
and the other flap using a standard microkeratome. The result showed that there was
significantly less astigmatism and trefoil in the IntraLase group.
Under these conditions that people pursuit higher visual quality, an uniform thin
corneal flap with high predictability and regularity attracted more and more attention.
The SBK flaps created by the femtosecond laser are safer, more accurate and have
better visual quality. It should be first choice for patients. But in some clinic without
femtosecond laser, Moria One Use-Plus SBK can also get a predictable uniform flap.
REFERENCES
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12. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ
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13. Eisner RA, Binder PS. Technique for measuring laser in situ keratomileusis flap
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Cataract Refract Surg 2005; 31: 120-126. PMID: 15721704
Figure 1. Example of 0° meridian cross-sectional. Visante® OCT image of the cornea.
Figure 2A. The 0°, 45°, 90°, and 135° meridians of the cornea.
Figure 2B. The five points in each meridian. From the corneal vertex, the “+” is in the positive X-axis
and “-” is in the negative X-axis direction of the image.
Figure 3. Distribution of mean central flap thicknesses at 1 month post-surgery for all eyes made with
IntraLase femtosecond laser; Moria One Use-Plus SBK and Moria M2SU90 Mechanical Keratome.
Figure 4. Distribution of the differenc in flap thickness among the IntraLase group, the SBK group and
the M2SU90 group.
Table1. Preoperative characteristics of patients.
Parameter
IntraLase FS
Moria OUP SBK
Moria M2SU90
No. of eyes
59
44
58
P Value
Age(y)
Mean±SD
Range
27.00±4.96
28.1±6.98
25.5±4.89
18 to 44
21 to 40
19 to 36
-7.15±2.87
-7.88±3.24
0.3725
SER (D)
Mean±SD
Range
-9.20±3.97
-11 to -3
-11.63 to -3.63
-13.88 to -3.63
514.24±34.56
519.50±22.89
506.83±26.78
0.0975
CCT (µm)
Mean±SD
0.4824
Range
442 to 603
474 to 602
470 to 550
44.29±1.82
43.68±0.89
43.79±1.73
39.72 to 47.98
41.79 to 46.61
40.45 to 46.82
Corneal curvature（D）
Mean±SD
Range
0.4458
SER=spherical equivalent refraction; CCT= central corneal thickness
Table 2. The mean flap thickness and standard deviation of the relevant 20 measurements
in the four
meridians among the IntraLase group, SBK and M2SU90 group.
Flap Thickness (Mean±Standard Deviation) (µm)
Measurement points
-Peripheral
-Paracentral
meridian
109.66±7.37
111.20±4.82
45° meridian
110.00±7.69
90° meridian
135°meridian
Central
Paracentral
Peripheral
110.49±5.90
110.64±5.91
108.42±7.54
110.22±5.17
112.78±6.68
113.69±5.45
109.93±6.37
110.75±7.26
110.54±6.39
109.58±6.30
110.39±7.22
110.53±6.50
112.05±7.73
110.85±6.02
111.86±6.36
111.29±5.93
111.42±6.89
114.77±9.69
109.59±10.48
112.22±9.97
111.61±8.06
111.77±7.53
45° meridian
113.73±11.52
109.82±8.47
114.70±8.97
110.86±9.40
113.39±9.19
90° meridian
116.02±9.07
114.48±8.04
114.93±12.64
11.95±8.47
115.45±6.73
135°meridian
110.00±8.47
112.39±8.92
113.55±12.78
112.43±8.51
115.14±8.27
meridian
125.17±20.39
120.45±14.08
116.60±14.98
115.62±16.82
120.72±16.90
45° meridian
126.64±18.15
118.03±16.82
118.86±14.12
114.76±14.71
120.81±19.42
90° meridian
124.53±16.94
116.57±13.33
118.71±16.11
116.45±15.68
120.40±18.06
135°meridian
125.53±15.94
115.66±16.29
117.69±15.72
118.56±15.11
126.64±18.06
IntraLase
0°
SBK
0°
meridian
M2SU90
0°
Table 3. The mean deviations from the intended flap thickness of the 20 corresponding measurements
among the IntraLase flaps, One Use-Plus SBK and the M2 Sing-Use 90 µm -head Microkeratome
flaps.
Measurement
points
Mean±Standard Deviation Flap Thickness (µm)
-Peripheral
-Paracentral
meridian
5.42±4.96
4.15±2.68
45° meridian
5.93±4.83
90° meridian
135°meridian
Central
Paracentral
Peripheral
4.59±3.72
4.58±3.75
6.22±4.47
4.22±2.95
5.93±4.08
5.93±3.42
4.88±4.04
5.66±4.55
5.32±3.52
5.00±3.80
5.98 ±3.98
5.34±3.67
6.15±5.05
4.78±3.70
5.29±3.94
4.92±3.51
5.39±4.47
8.90±6.00
8.32±6.26
8.00±6.24
6.80±4.51
6.09±4.69
45° meridian
10.36±6.09
6.59±5.23
8.70±5.06
6.86±6.40
7.84±5.77
90° meridian
8.75±6.41
7.16±5.71
10.11±8.95
7.91±4.87
7.00±5.06
135°meridian
7.14±4.43
7.39±5.44
8.64±9.99
6.70±5.70
7.55±6.09
meridian
19.76 ±15.90
14.13±10.30
12.60±10.36
13.31±11.61
15.34±12.76
45° meridian
19.36±15.15
15.00±10.99
12.24±11.26
11.93±9.73
17.36±13.76
90° meridian
17.74±13.48
11.50±9.34
13.57±12.22
13.52±10.11
15.91±13.36
135°meridian
17.91±13.16
13.62±10.44
12.83±11.82
13.71±10.57
18.33±15.27
IntraLase
0°
SBK
0°
meridian
M2SU90
0°