Delta Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 21  |  Issue : 4  |  Page : 242--248

Flap thickness and side-cut angle using sub-Bowman keratomileusis microkeratome, 200-kHz femtosecond laser system, and Moria M2 microkeratome


Sarah Azzam, Mohamed Hosny, Zahed Chehab, Mohamed Anis 
 Department of Ophthalmology, Faculty of Medicine, Cairo University, Cairo, Egypt

Correspondence Address:
MD, FRCS Sarah Azzam
Department of Ophthalmology, Cairo University, 50 Gezirat El Arab Street, Mohandeseen, Giza 12411
Egypt

Abstract

Purpose The aim of this study was to compare the thickness and side-cut angle of corneal flap in candidates undergoing laser-assisted in situ keratomileusis using sub-Bowman keratomileusis (SBK) microkeratome, femtosecond (FS) laser, or Moria M2 microkeratome (MK). Patients and methods This prospective clinical study included 60 eyes of 30 patients. They were divided into three groups (20 eyes in each group). The study was conducted from January 2019 till December 2019. Corneal flaps were created using Moria SBK microkeratome, Alcon Wavelight FS-200 FS laser, or Moria MK. Heidelberg anterior segment optical coherence tomography was done 1 month postoperatively to assess the central flap thickness and side-cut angle. Results The mean error cut angle was −39.00±19.00, 24.75±12.56, and −25.65±10.31° in the SBK, FS, and MK groups, respectively (P<0.001). The mean difference between the intended and achieved flap thickness was 1.50±8.06 μm in the SBK group, −1.20±6.86 μm in the FS group, and 16.25±29.73 μm in the MK group (P=0.340). Conclusion There was a statistically significant difference between the intended and achieved side-cut angle in the three groups. Flap thickness was more predictable in the SBK and FS group, with an insignificant difference between the intended and achieved flap thickness, whereas in the MK group, the difference was statistically significant.



How to cite this article:
Azzam S, Hosny M, Chehab Z, Anis M. Flap thickness and side-cut angle using sub-Bowman keratomileusis microkeratome, 200-kHz femtosecond laser system, and Moria M2 microkeratome.Delta J Ophthalmol 2020;21:242-248


How to cite this URL:
Azzam S, Hosny M, Chehab Z, Anis M. Flap thickness and side-cut angle using sub-Bowman keratomileusis microkeratome, 200-kHz femtosecond laser system, and Moria M2 microkeratome. Delta J Ophthalmol [serial online] 2020 [cited 2021 May 12 ];21:242-248
Available from: http://www.djo.eg.net/text.asp?2020/21/4/242/304942


Full Text



 Introduction



Laser-assisted in situ keratomileusis (LASIK) is one of the most popular refractive surgical procedures done for patients complaining of refractive errors [1]. A critical step of LASIK surgery is corneal flap creation. The accuracy of the LASIK flap thickness is a key risk factor for flap complications and ectasia following LASIK [2],[3]. Different ways are used to create the flap including the femtosecond (FS) laser and mechanical microkeratomes [4],[5].

Flap thickness and morphology became the main concern in LASIK surgery as corneal flap thickness is directly related to LASIK safety and predictability [6],[7]. Dry eye is a very common complaint of patients undergoing LASIK surgery. FS-LASIK flaps have shown longer tear break-up time and decreased dry eye symptoms when compared with microkeratomes [8]. In addition, it has been suggested that the LASIK flap biomechanics may depend on the side-cut angle of the flap. So, inverted side-cut flaps created with FS laser would result in better wound healing [9].

The purpose of this study was to compare the corneal flap thickness and side-cut angle between Moria sub-Bowman keratomileusis (SBK) microkeratome, FS, and Moria M2 microkeratome (MK) using anterior segment optical coherence tomography (AS-OCT).

 Patients and methods



This is a prospective nonrandomized interventional clinical trial that included 60 eyes (20 eyes in each group: SBK, FS, and MK groups) of 30 patients. The study was conducted from January 2019 till December 2019. The study was granted the approval of the Ethical Committee of the Faculty of Medicine, Cairo University, Egypt, and it followed the principles of the Declaration of Helsinki. All patients signed a written informed consent to participate in the study and for publication of data before being enrolled in the study.

The patients’ age ranged from 18 to 45 years, with refractive error either myope up to −8.0 diopters (D) without or with astigmatism up to −4.0 D, or hypermetrope up to +4 D, with central corneal thickness of 500 μm or more, normal corneal tomography with a D value less than 1.6 in the Belin/Ambrosio Enhanced Ectasia Display of the Pentacam HR (Oculus Inc., Lynnwood, Washington, USA), and an expected post-LASIK residual stromal bed of 300 μm or more. All patients meeting the inclusion criteria were subjected to full ophthalmic examination of both eyes in the form of measurement of uncorrected visual acuity and best-corrected visual acuity at baseline (using Snellen’s chart), full slit-lamp examination, and intraocular pressure measurement with Goldmann applanation tonometry. Dilated fundus examination was done by using slit-lamp biomicroscopy with +90 D Volk lens. Oculus Pentacam tomography was performed for both eyes before surgery.

Flap creation

The surgery was done in both eyes under topical anesthesia using two drops of topical benoxinate hydrochloride 0.4% (Benox; Eipico Inc., Cairo, Egypt) applied 2–3 min before surgery.

In the FS group, corneal flaps were done using Allegretto WaveLight FS-200 FS laser (Alcon, Fort Worth, Texas, USA). The device used a 200-kHz repetition rate, 1030 nm wavelength, pulse energy of 0.7 µJ, spot separation of 7.5 µm, and line separation of 7.5 µm for the flap bed and pulse energy of 0.85 μJ, spot separation of 5.5 μm, and line separation of 3.5 μm for the flap side cut. The flap diameter was adjusted according to the intended treatment zone. The intended flap thickness was 120 µm with a side-cut angle of 110°.

In the SBK group, the flap was created using Moria SBK microkeratome (Moria SA, Antony, France) with a nasal hinge, intended flap thickness of 110 μm, and side-cut angle of 90°.

In the MK group, corneal flaps were done using a Moria 2 Microkeratome (Moria SA). The flap had a superior hinge, with a flap thickness of 110 μm, and side-cut angle around 70° on average.

After flap creation, laser ablation was performed using Allegretto EX-500 excimer laser (Alcon).

Postoperatively, all patients received a topical antibiotic, moxifloxacin hydrochloride (Vigamox; Alcon), a topical steroid (Optipred; Jamjoom Pharma, Jeddah, Saudi Arabia), and artificial tears eye drops (Systane Ultra; Alcon) four times daily for 3 weeks and systemic steroids (Solupred 20 mg) for 5 days.

Flap assessment

AS-OCT was done to measure the central flap thickness and side-cut angle using Spectralis spectral domain OCT (Heidelberg Engineering, Heidelberg, Germany), 1 month after LASIK. Flap thickness was determined by measuring the distance between the corneal epithelium and the LASIK flap interface. For side-cut angles, scans were done across the temporal side of the flap, and then images were transferred to Draw editor software, and measurements were recorded and analyzed.

Statistical analysis

Data were coded and entered using the Statistical Package for Social Sciences (SPSS), version 26 (IBM Corp., Armonk, New York, USA). Data were summarized using mean, SD, median, minimum, and maximum in quantitative data and using frequency (count) and relative frequency (percentage) for categorical data. Comparisons between quantitative variables were done using the nonparametric Mann–Whitney test. For comparison of serial measurements within each patient, the nonparametric Wilcoxon signed rank test was used [10]. Correlations between quantitative variables were done using Spearman correlation coefficient [11]. P values less than 0.05 were considered statistically significant.

 Results



The mean preoperative manifest refraction spherical equivalent ranged from −5.00 to −1.25 D in the SBK group, −6.50 to +3.25 D in the FS group, and from −4.00 to +3.00 D in the MK group.

In the SBK group, the intended side-cut angle was 90°. However, the mean achieved side-cut angle was 51±19° (range, 30–85°), with a statistically significant difference between the achieved and intended side cut angle (P<0.001, [Table 1]). The mean deviation from the intended angle was −39.00° (range, −60.00° to −5.00°). In the FS group, the intended side-cut angle was 110°, and the mean achieved side cut angle was 134.75±12.56° (range, 110–155°), with a statistically significant difference (P<0.001, [Table 2]). The mean deviation from the intended angle was 24.75° (range, 0.00°–45.00°). In the MK group, the intended side-cut angle was 70°, and the mean achieved side cut angle was 44.35±10.31° (range, 30–65°), which is statistically significant (P<0.001, [Table 3]). The mean angle of deviation from the intended angle was −25.65° (range, −40.00° to −5.00°). The cut angle and the error cut angle in the threee groups are demonstrated in [Figure 1] and [Figure 2].{Table 1}{Table 2}{Table 3}{Figure 1}{Figure 2}

There was a statistically significant difference in the achieved cut angle on comparing SBK versus FS flap and on comparing MK versus FS flap (P<0.001). However, on comparing SBK versus MK flap, the difference was statistically insignificant (P=1.000). OCT images showing the side-cut angle in the three groups are represented in [Figure 3] [Figure 4] and [Figure 5].{Figure 3}{Figure 4}{Figure 5}

The intended flap thickness in the SBK group was 110 μm, whereas the achieved flap thickness was 111.50±8.06 μm (range, 100–128 μm), which is statistically insignificant (P=0.777, [Table 1]). In the FS group, the intended flap thickness was 120 μm, whereas the achieved thickness was 118.80±6.86 μm (range, 105–133 μm, P=0.406, [Table 2] and [Figure 6]). In the MK group, the intended flap thickness was 110 μm, whereas the achieved thickness was 126.25±29.73 μm (range, 70–158 μm), which is a statistically significant difference (P=0.030, [Table 3]).{Figure 6}

In the SBK group, the mean deviation from the intended flap thickness was 1.50 μm (range, −10.00 to 18.00 μm). In the FS group, the mean deviation from the intended thickness was −1.20 μm (range, −15.00 to 13.00 μm), but in the MK group, the mean deviation from the intended thickness was 16.25 μm (range, −40.00 to 48.00 μm).

A summary of the results is presented in [Table 4].{Table 4}

 Discussion



In this prospective study, the LASIK flap thickness and side-cut angle using three different machines; SBK, FS-assisted and MK-assisted flap creation, was compared by AS-OCT. To our knowledge, this is the first study comparing the corneal flap thickness and side-cut angle among the three methods.

Zhang et al. [12] used AS-OCT to study corneal flaps in patients undergoing both FS and mechanical microkeratome. Their study included 132 eyes of 61 patients. The mean central flap thickness was 105.53±5.86 µm in the FS group and 132.96±13.91 µm in the MK 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 was smaller in the FS group with a better flap regularity. Their study matches the results of the current study, as we found that the FS laser produced more predictable flap thickness with smaller deviations than mechanical microkeratomes.

Similarly, Zhou et al. [6] compared flap thickness between FS laser and Moria M2 MK with intended flap thickness of 110 μm for all patients. The mean deviation between achieved and attempted flap thickness was smaller in the FS laser group than in the microkeratome group. Such findings emphasize the higher accuracy of FS over MK, as shown in the present study. Meanwhile, in a study conducted by Liu et al. [13], comparing flap thickness created by Wavelight FS-200 (Alcon Laboratories) and Intralase (Johnson and Johnson Surgical Vision, Irvine, California, USA) FS60, where both systems were programmed to flap thickness of 110 μm, the mean central flap thickness was 105.39±4.50 μm in FS-200 group and 109.15±11.59 μm in FS60 group. Liu et al. [13] concluded that flaps created by FS-200 had a more predictable flap thickness with a small deviation between the achieved and attempted flap thickness. These results are comparable to the current study FS-200 results, with more predictable flap thickness and smaller deviations between the intended and achieved thickness.

In another study by Zhang et al. [14] comparing corneal flap thickness using FS-200 FS laser and Moria SBK microkeratome making 110-μm-thick corneal flap, the mean flap thickness was 114.0±6.6 μm (range, 98–126 μm) in the FS group, whereas in the SBK group, the mean flap thickness was 110.6±7.4 μm (range, 97–125 μm). The results of this study were similar to the current one, where we found a mean flap thickness of 111.50 μm in the SBK group; in the FS group, it was 118.80 μm; and in MK group, the flap thickness was 126.25 μm. SBK and FS-LASIK yielded a significantly more predictable flap thickness when compared with Moria MK.

The side-cut angle remains one of the most important factors in flap stability. FS-assisted flaps have the advantage to accurately determine the side-cut angle to ensure a stable flap. In the current study, in the Femto-LASIK group, using Wavelight FS-200, the flap side-cut angles were inverted in 100% of cases, with a minimum angle measure of 110° (the intended angle) and a maximum measure of 155°. On the contrary, in the microkeratome group patients, all flap side-cut angles were more acute than the intended angle (70°) in 100% of cases with a minimum angle of 30° and a maximum angle of 65°. In the SBK group, the mean measured flap side cut angle was 51°, with a minimum angle measure of 30° and a maximum measure of 85°, with more acute angle than intended (90°) in 100% of cases. A statistically significant difference was found between the intended and achieved side-cut angle in the three groups. However, it was found that the FS group had the least range of error between the intended and achieved angle.

In a study by Eldaly et al. [15] comparing LASIK flap morphology between Wavelight FS-200 FS laser and Moria microkeratome, a mean side-cut angle of 74.29±5.79° (range, 59–84°) was found in the FS group, whereas the MK group had a mean side-cut angle of 32.34±4.94° (range, 23–41°). In this study, the FS flap side cut was planned to be 70° also, with a mean difference of 6.12±3.73° (range, 0–14°) between the planned and measured postsurgery. However, this study is different from the current study in intended side-cut angle of FS flaps (70°) compared with 110° planned in the present study.

Knorz and Vossmerbaeumer [9] in their study compared the adhesion and the strength of flaps created with a mechanical microkeratome and IntraLase FS laser with side-cut angles of 70 and 140°. It was found that flap adhesion was stronger with FS laser than with mechanical microkeratome and the inverted side-cut increased the flap adhesion when compared with a standard side-cut.

 Conclusion



In conclusion, SBK and FS laser produced more predictable flap thickness than Moria MK. FS laser produced corneal flaps with obtuse side-cut angles which increases flap strength and adhesion. On the contrary, microkeratomes produced acute side-cut angles.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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