|Year : 2019 | Volume
| Issue : 3 | Page : 100-106
Evaluation of corneal flap symmetry in the optical center by anterior segment optical coherence tomography: mechanical versus femtolaser flaps
Abdel Rahman E Sarhan1, Mohamed S Abd Elaziz1, Marwa A Zaki1, Asmaa M Ibrahim1, Nehal G Youssef2
1 Ophthalmology Department, Menoufia University, Egypt
2 Faculty of Medicine, Menoufia University, Egypt
|Date of Submission||11-Dec-2018|
|Date of Acceptance||14-Mar-2019|
|Date of Web Publication||26-Sep-2019|
MD Mohamed S Abd Elaziz
Department of Ophthalmology, Faculty of Medicine, Menoufia University, Shebeen El-Kom 32511
Source of Support: None, Conflict of Interest: None
Objective The aim of this study was to evaluate corneal flap symmetry in the central 3 mm for subjects undergoing either laser in-situ keratomileusis or femtolaser in-situ keratomileusis for correction of myopia and/or astigmatism.
Patients and methods This prospective study enrolled 160 eyes of 80 patients with mild to moderate myopia (<–6.00 D) as well as astigmatism less than −3.00 D. The corneal flaps of 80 eyes were created with the VisuMax femtosecond laser for intended thickness of 100 μm, and the other 80 eyes flaps were created by Moria M2 microkeratome (90 μm head) with intended thickness of 90 μm. The flap thickness was measured with a noncontact spectral-domain anterior segment optical coherence tomography system at three months postoperatively. The flap thicknesses were measured at five points (center and ±1.5 mm from the center) on each of the vertical and horizontal meridian (90° and 180°).
Results The mean central thickness of the VisuMax and Moria M2 flaps was 102.06±5.37 and 130.25±12.61 μm, respectively (P<0.0001). In terms of mean flap thickness at each eccentricity, there were significant differences between the two groups, along the four measured eccentricities (P<0.0001).
Conclusion The mean flap thickness measurements at each point showed more accurate outcomes in the femtolaser group. Moreover, slight vertical as well as horizontal asymmetry was found in the Moria group, although statistically insignificant.
Keywords: anterior segment optical coherence tomography, corneal flap, femtolaser, microkeratome
|How to cite this article:|
Sarhan AE, Abd Elaziz MS, Zaki MA, Ibrahim AM, Youssef NG. Evaluation of corneal flap symmetry in the optical center by anterior segment optical coherence tomography: mechanical versus femtolaser flaps. Delta J Ophthalmol 2019;20:100-6
|How to cite this URL:|
Sarhan AE, Abd Elaziz MS, Zaki MA, Ibrahim AM, Youssef NG. Evaluation of corneal flap symmetry in the optical center by anterior segment optical coherence tomography: mechanical versus femtolaser flaps. Delta J Ophthalmol [serial online] 2019 [cited 2020 Apr 5];20:100-6. Available from: http://www.djo.eg.net/text.asp?2019/20/3/100/267943
| Introduction|| |
Laser ablation procedures, especially laser in-situ keratomileusis (LASIK), became one of the most currently performed corneal surgeries to correct refractive errors . Various options to create the LASIK flap are available nowadays, ranging from modern automated microkeratomes to femtolaser-created flaps. Choice depends on surgeon preference, and each affords different theoretical advantages .
The mechanical microkeratome uses shear force through the use of an oscillating blade, traveling across the cornea in a torsional or translational approach. The femtolaser system employs a solid-state laser with 1043-nm wavelength and a 3-μm spot size and uses brief laser pulses (220–580 femtosecond) to cause disruption in a lamellar plane. A minimum of laser energy is thus used to create the flap. Hinge placement and flap thickness can be set to exact specifications by the surgeon .
The theoretical advantages of femtosecond laser over microkeratome include a decrease in flap imperfections such as buttonhole flaps, decreased risk of infection, a lower peak of intraocular pressure during the procedure ,, less induction of dry eye , and less effect on corneal sensation . Still, mechanical microkeratomes remain a cost-effective method for LASIK flap creation.
In the current study, spectral-domain anterior segment optical coherence tomography (AS-OCT) was used to compare corneal flap thickness generated by mechanical microkeratome (Moria M2) and femtolaser system (VisuMax) regarding predictability as well as symmetry.
| Patients and methods|| |
This study included myopic patients with a refraction less than −6.00 D, astigmatism less than −3.00 D, age above 20 years, a stable refraction for at least 1 year, cessation of soft contact lens wear for at least 2 weeks or rigid gas-permeable contact lens wear for at least 4 weeks, and keratometric reading value ranging between 41.50 and 45.00 D. The patients never had a previous keratorefractive surgery, and the corneal pachymetry was at least 520 μm at the thinnest location.
Patients who do not fit the inclusion criteria or with a history of herpetic eye disease, corneal dystrophy, corneal scarring because of infection or trauma, keratoconus, severe dry eye, collagen vascular disease, cataract, retinal disease, diabetes mellitus, pregnancy, and who showed intraoperative or postoperative complications were excluded from the study.
Patients who received bilateral LASIK surgery for myopia or myopic astigmatism in Tiba Eye Center, Shebin El-Kom, Menoufia, between May 2016 and November 2016 participated in this prospective study.
Before initiating this study, all procedures were reviewed and approved by the Ethical Committee of Menoufia University Hospital. A written informed consent was obtained from each patient after full discussion of the procedure involved, duration of treatment, possible intraoperative maneuvers, and potential postoperative symptoms. Each patient was informed that participation is voluntary and that he or she may withdraw from the study at any time and without giving any reason. The withdrawal did not affect the subsequent medical treatment or relationship with the treating surgeon.
Patients were divided into two equal groups: Femto and Moria groups. Corneal flaps in the first group of patients were created by the VisuMax femtosecond laser (Carl Zeiss Meditec, Oberkochen, Germany) and those of the second group were created by a Moria M2 microkeratome (Moria SA, Antony, France). The patients had the right to choose whether to have the VisuMax femtosecond laser or the microkeratome to create their corneal flaps.
Preoperatively, a detailed history was taken to rule out any systemic disease that might contraindicate LASIK. All patients underwent full ophthalmological examination including determination of uncorrected and best spectacle corrected visual acuities measured by decimal fraction; manifest and cycloplegic refraction to exclude high myopia and high astigmatism; pachymetry, keratometry, and corneal topography by Allegro Oculyzer (Wavelight AG, Erlangen, Germany) to exclude corneal abnormalities such as thin cornea and keratoconus; intraocular pressure measurement to exclude glaucoma; and a slit lamp and fundus examination to exclude any pathology that might be a contraindication for surgery or have an effect on flap creation.
All of the surgical procedures were performed by the same surgeon (A.S.). Superior-hinged flap parameters were programmed for all eyes. The intended VisuMax flap thickness was 100 μm. The cone size choice depends on white to white (WTW), with small cone selection if WTW was less than 11.8 mm and medium cone if WTW was more than 11.8 mm. The flap diameter choice was 7.9 mm for small cone or 8.5 mm for medium cone. All VisuMax flaps were lifted 20–30 min after creation of the interface to allow time for the bubbles in the interface to disappear.
The Moria flaps were created by a Moria M2 Single Use microkeratome (90 μm head) with a desired flap thickness of 90±10 μm. One single disposable blade was used for each patient with suction-ring selection depending on keratometric reading as provided by the manufacturer. The flaps were lifted immediately after creation of the interface.
In all eyes, stromal ablation was performed with a WaveLight EX500 Excimer Laser system (Alcon Surgicals, Fort Worth, Texas, USA).
Postoperative treatment included moxifloxacin 0.5%, prednisolone acetate 1%, and lubricant eye drops. All patients were followed at the first day postoperatively to ensure the absence of a corrugated flap, at 1 week to exclude the presence of any postoperative complications, and after 3 months to measure the flap thickness using anterior segment OCT.
Optical coherence tomography measurements
The flap thickness was measured at 3 months postoperatively with a noncontact AS-OCT system, Cirrus HD-OCT 5000 (Carl Zeiss Meditec AG, Jena, Germany), which is a spectral-domain OCT. All measurements were performed by another ophthalmologist who was blinded to the treatment group.
The flap thicknesses were measured at five points (center and ±1.5 mm from the center) on each of the vertical and horizontal meridian (90° and 180°), using the measuring tool provided with the software.
The horizontal vector runs from patient’s right side to left side, with −1.5 mm to the right from the center and +1.5 mm to the left. The vertical vector runs from below up, with −1.5 mm below the center and +1.5 mm above ([Figure 1]).
|Figure 1 Diagram illustrating locations of the corneal center and the four points assessed with anterior segment optical coherence tomography with the direction of the vector (arrows).|
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Data were stored in an Excel spreadsheet (Microsoft Corp., New York, USA). The collected data were computerized and statistically analyzed using statistical package for the social science program (SPSS, version 18.0, released 2011; IBM Corp., Armonk, New York, USA). Qualitative data were represented as frequencies and relative percentages.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the Institutional Research Committee of Menoufia University and with the 1964 Helsinki Declaration and its later amendments.
| Results|| |
This study included 160 eyes of 80 (15 males and 65 females) patients. The corneal flaps of 80 eyes were created with the VisuMax femtosecond laser and the other 80 eyes flaps were created by Moria M2 microkeratome. The demographic data of the two studied groups are summarized in [Table 1].
There were no statistical significant differences between the two groups in terms of spherical equivalent either before or after treatment ([Table 2]).
The mean central thicknesses of the VisuMax and Moria M2 flaps are shown in [Table 3]. There was a statistically significant increase in the mean center thickness in the Moria group compared with the femtosecond laser group, as in femtosecond laser group the mean thickness was 102.06±5.37 μm (range: 94–115 μm), but in the Moria group, it was 130.25±12.61 μm (range: 108.5–161.5 μm).
|Table 3 Comparison of mean center flap thickness of the studied groups 3 months postoperatively|
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In terms of mean flap thickness at vertical meridian ([Table 4]), the mean at −1.5 mm in the femtosecond group was 108.75±5.21 μm, whereas in the Moria group it was 158.99±12.34 μm. In addition, the mean at +1.5 mm in the femtosecond group was 108.68±4.99 μm, whereas in the Moria group it was 154.9±11.77 μm. Moreover, a statistically insignificant difference was found in the Moria group between the two vertical points with the lower point being thicker by ∼5 μm.
|Table 4 Comparison of mean thickness at 90° −1.5 mm and +1.5 mm from the center in the two studied groups 3 months postoperatively|
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The horizontal meridian also showed that the mean at −1.5 mm in the femtosecond group was 109.11±5.28 μm, whereas in the Moria group, it was 159.92±11.91 μm. In addition, the mean at +1.5 mm in the femtosecond group was 108.55±4.78 μm, whereas in the Moria group, it was 156.31±12.15 μm. Moreover, a statistically insignificant difference was found in the Moria group between the two horizontal points with the right point being thicker by ∼4 μm ([Table 5]).
|Table 5 Comparison of mean thickness at 180° −1.5 mm and +1.5 mm from the center in the two studied groups 3 months postoperatively|
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As for flap thickness predictability, significant deviation from the intended flap thickness was found in the mechanical group (90 μm head) than the femtolaser flap in all five points of assessment which is summarized with corresponding percentage of deviation in [Table 6].
|Table 6 The mean differences between measured thickness and intended thickness at each measuring point|
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Those flap thickness measurements at each point showed a typical meniscus shape in the Moria group ([Figure 2]), with the peripheral flap area much thicker than the central flap. Flaps in femtosecond group also had a meniscus shape, although the asymmetry between the center and periphery was not as prominent as in the Moria group ([Figure 3]).
|Figure 2 Anterior segment optical coherence tomography image of mechanical laser in-situ keratomileusis flap in the vertical meridian with typical meniscus shaped flap.|
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|Figure 3 Anterior segment optical coherence tomography image of femtolaser laser in-situ keratomileusis flap in the vertical meridian with less asymmetry between upper and lower points.|
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| Discussion|| |
To our knowledge, this is the first study that compares mechanical (90 μm head) and femtolaser flap thickness in the center as well as 1.5 mm around the center in both vertical and horizontal meridians using spectral-domain AS-OCT.
Talamo et al.  compared flap thickness reproducibility of the femtosecond laser (IntraLase, IntraLase, Inc., Irvine, CA) and two mechanical microkeratomes (Moria LSK-1 and M2 microkeratomes). Flap thickness for all eyes was measured as the difference between the preoperative (day of surgery) full corneal thickness and post-flap creation central stromal bed thickness using ultrasonic pachymetry. For 99 flaps created using the IntraLase FS laser with an intended thickness of 110 μm, the mean achieved thickness was 119±12 μm (range: 82–149 μm). However, in 135 eyes treated with the Moria M2 microkeratome with an intended flap thickness of 130 μm, the mean thickness was 142±24 μm (range: 84–203 μm). These results were comparable to this study. However, using manual subtraction ultrasonic pachymetry is associated with several problems including difficult placement of the probe at the same location before and after flap lifting, bias associated with flap hydration, and tissue compression.
In a study conducted by Zhang et al. , AS-OCT (Visante, Carl Zeiss, Dublin, CA) was used to assess and compare the flap morphology in LASIK with Femto Leonardo Da Vinci (LDV) femtosecond lasers versus Hansatome mechanical microkeratome. The intended flap thickness was 110 and 160 μm, respectively. The thickness of 25 points across each flap, which were 0, 1.5, 2.5, and 3.5 mm to the corneal vertex on the horizontal, vertical, 45°, and 135° meridians, respectively, was evaluated. One month postoperatively, the central flap thickness in the Femto LDV group was 107.43±4.70 μm, whereas in the Hansatome group, it was 125.90±17.50 μm. However, spectral-domain OCT may be better than time-domain OCT for evaluating the finer details of LASIK flaps.
Another important study of Ahn et al.  evaluated the thickness and side-cut angle of LASIK flaps created by one of three femtosecond lasers or a microkeratome using Fourier-domain OCT. Flap creation was performed using an IntraLase, VisuMax, or Femto LDV femtosecond laser (intended flap thickness of 110 μm) or an M2 microkeratome (130 μm head). Flap thickness was determined at 14 points. Measurements were taken 2 months postoperatively using an RTVue Fourier-domain OCT device and integrated software. Their results were similar to this study in showing typical meniscus shape in the microkeratome group, with the flap periphery being much thicker than the center. Flaps in femtosecond groups also had a meniscus shape, although the asymmetry between the center and periphery was not as prominent as in the microkeratome group. Yet, the mean thickness at the center in the VisuMax group was 133.9±13.9 μm, which is much deviated from the intended thickness in their study. We did not face such deviation in this study which could be explained by variation in measurement method used.
In this study, the deviation of the central flap thickness from the target thickness in microkeratome group was huge compared with other studies ,. Flap thickness in microkeratome depends on many factors, which include preoperative corneal thickness, corneal diameter, consistency across the cornea, quality and entry angle of the blade, cutting mechanism, translation and oscillation rate, suction-ring pressure setting, suction duration, room humidity, first eye–second eye difference, and repeated use of the blade ,,,,,.
| Conclusion|| |
Predictability of corneal flap thickness and subsequently residual stromal bed is a crucial step for safe LASIK surgery. This study showed a huge difference between intended and actual flap thickness in the mechanical microkeratome group, whereas this variation in thickness was almost nil in the femtolaser group. Flap symmetry along different meridian was almost perfect in the femtolaser group, whereas in the mechanical group, slight asymmetry was noted along the vertical and horizontal meridians, which may be related to tissue compression during the passage of the microkeratome head, and this needs further evaluation on a large scale.
Clinical trial protocol number is: PACTR201708002498199.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]