Risk factors for opaque bubble layer in femtosecond-laser-assisted laser in situ keratomileusis (an Egyptian study)

Ahmed S Abdel-Wanes; Abelrahman G Salman; Azza M.A Said; Basem F.A Riad; Marwa A Karim

doi:10.4103/ejos.ejos_24_20

ORIGINAL ARTICLE

Year : 2020 | Volume : 113 | Issue : 3 | Page : 91-96

Risk factors for opaque bubble layer in femtosecond-laser-assisted laser in situ keratomileusis (an Egyptian study)

Ahmed S Abdel-Wanes, Abelrahman G Salman, Azza M.A Said, Basem F.A Riad, Marwa A Karim
Department of Ophthalmology, Faculty of Medicine, Ain Shams University, Cairo, Egypt

Date of Submission	15-Apr-2020
Date of Acceptance	20-Apr-2020
Date of Web Publication	07-Sep-2020

Correspondence Address:
MD, FRCS (Glasgow) Marwa A Karim
13/1 Block, 10th District Nasr City, Cairo 11765
Egypt

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/ejos.ejos_24_20

Abstract

Aim To determine the risk factors responsible for opaque bubble layer (OBL) formation in femtosecond-laser-assisted in situ keratomileusis (FSL-LASIK).
Patients and methods A retrospective, comparative study was carried out from January 2015 to January 2017 in El-Watany Eye Hospital. Two hundred eyes who were eligible for FSL-laser-assisted in situ keratomileusis (FSL was done using FS200, wavelight, Germany SR/1025-1-380) were classified into two groups. Group I: the study group with OBL formation during surgery (100 eye) and group (II): the control group without OBL formation during surgery (100 eye). Preoperative best-corrected visual acuity, spherical equivalent, and dilated fundus examination were done and corneal tomography including keratometric readings: flat keratometric readings (K1), steep keratometric readings (K2), and central corneal thickness (CCT) measurements. Programmed flap parameters were collected.
Statistical analysis Statistical analysis was done using IBM SPSS, version 24.
Results Among the studied 101 patients, OBL occurs bilaterally in 58 eyes among 29 patients and OBL occurs unilaterally among 42 patients. There was a statistically significant difference between OBL and non-OBL regarding preoperative steep keratometry (K2) (P<0.01) and CCT (P=0.03), where the corneal pachymetry was more than 545 µm in 57 (57%) eyes of OBL group versus 42 (42.0%) eyes in non-OBL eyes. There was no statistically significant difference between both groups regarding canal length offset (P=0.123), corneal flap thickness (P=0.489), corneal flap diameter (P=0.064), and flap hinge angle (P=0.074).
Conclusion Increased corneal thickness, corneal canal length offset, and steep keratometry were significant risk factors of OBL formation. Regarding regression analysis, CCT and corneal astigmatism were statistically significant independent predictors of OBL formation.

Keywords: corneal thickness, keratometry, opaque bubble layer

How to cite this article:
Abdel-Wanes AS, Salman AG, Said AM, Riad BF, Karim MA. Risk factors for opaque bubble layer in femtosecond-laser-assisted laser in situ keratomileusis (an Egyptian study). J Egypt Ophthalmol Soc 2020;113:91-6

How to cite this URL:
Abdel-Wanes AS, Salman AG, Said AM, Riad BF, Karim MA. Risk factors for opaque bubble layer in femtosecond-laser-assisted laser in situ keratomileusis (an Egyptian study). J Egypt Ophthalmol Soc [serial online] 2020 [cited 2023 Apr 2];113:91-6. Available from: http://www.jeos.eg.net/text.asp?2020/113/3/91/294446

Introduction

Laser-assisted in situ keratomileusis (LASIK) is a generally acknowledged method for refractive error correction [1]. Recently, the use of the bladeless LASIK surgery for lamellar flap creation, depending on a femtosecond laser (FSL) (advantaged by its ultrashort pulses, with femtosecond duration), act as a superior option to the mechanical microkeratome [2]. Excimer lasers in refractive surgery provides customized ablation, including aspheric ablation profiles, topography-guided or wave front-guided treatments [3]. FSL has valuable advances in refractive surgery by increasing the predictability, precision, and accuracy of LASIK flap creation. Moreover, flap customization (thickness, diameter, and side cut angle) is preferred for high astigmatism, thin cornea, and high spherical ametropia [4]. Ionization occurs when FSL pulses are focused in the corneal tissue. Plasma expansion within the tissue creates cavitation bubbles with stromal lamellae separation [1]. These gas bubbles produced, travel through the least resistance pathway, and under elevated pressure with high vacuum, corneal compression, and corneal rigidity can produce a counterbalance force opposing the applanation pressure. Thicker corneas provide greater resistance and more rigidity, thus restricting the cavitation bubble clearance and increasing the incidence of opaque bubble layer (OBLs) [5].

A flap with oval-shaped and a larger hinge angle resulted in less OBL formation in femtoLASIK [6]. Moreover, the customized denser spot application and wider canal settings has significant less risk of OBL [7].

Jung et al. [8] and Mastropasqua et al. [1] had less incidence rates of OBL formation (5.0 and 33.3%, respectively) compared with the previous results [6],[7]. This was explained by the use of new laser systems with reduced spot distance and higher pulse rate (500 kHz). With a resultant complete and regular interface with reduced OBL incidence, the aim of the study was to determine risk factors responsible for OBL formation in FSL-assisted LASIK.

Patients and methods

A retrospective study was conducted on patients who were eligible for FSL-assisted LASIK at El-Watany Eye Hospital in the period from January 2015 to January 2017. An individual patient‘s written consent was obtained after explaining the procedure with possible complications. Patient selection include the following:

Age: 18–40 years, myope up to −10 D or hyperope up to +6 D with or without astigmatism. Excluded candidates: previous refractive procedures, corneal pathologies, corneal surgeries, local or systemic diseases or medications, and eyes at risk of developing postrefractive corneal ectasia, such as keratoconus and keratoconus suspect.

Those patients were divided into two groups:

The study group included 100 eyes with OBL formation during surgery.
Control group included 100 eyes without OBL formation during surgery ([Figure 1]).
Figure 1 OBL formed intraoperatively in three patients of the current study. OBL, opaque bubble layer.

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This study followed the tenets of the Declaration of Helsinki on Ethical Principles for Medical Research Involving Humans. Study protocol was approved by the local research ethics committee.

Preoperative data analyzed were: best-corrected visual acuity expressed in (log.MAR), spherical equivalent (SE) powers, keratometric readings including flat keratometric reading (K1), steep keratometric readings (K2), and central corneal thickness (CCT) measurements using corneal tomography (Pentacam R HR type 70900; OCCULUS-Optikgerate Gmbh, Münchholzhäuser Str. 29, 35582 Wetzlar, Germany).

FSL was done using FS200, WaveLight, Germany SR/1025-1-380. Parameters of flap creation were recorded and are shown in [Table 1].

Table 1 Femtosecond laser flap creation settings

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Statistical analysis

The collected data were analyzed by a computer using the Statistical Package for Social Sciences (IBM SPSS), version 24 (SPSS Inc., Chicago, Illinois, USA). Data were represented in tables and graphs, continuous quantitative variables, for example, age, were expressed as the mean±SD and median (interquartile range), and categorical qualitative variables were expressed as absolute frequencies (number)) and relative frequencies (percentage). The Mann–Whitney U test was used to compare variables between two groups. χ² test was used for comparing numerical variables between the two groups. Spearman’s correlation coefficient was used to assess the correlation between two variables in the study group. Logistic regression analysis was used for modeling and analyzing several variables. The level of significance was set at P value less than or equal to 0.05.

Results

The present study was conducted on 200 eyes among 101 patients with a median age of 30 years. Sixty-four (63.4%) patients were women and 37 (36.6%) patients were men, who were divided into two groups:

Group I: 100 eyes had OBL during FSL-assisted LASIK.

Group II: 100 eyes had no OBL. Preoperative corneal parameters among the studied eyes are shown in [Table 2].

Table 2 Preoperative corneal parameters among the studied eyes

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Among the studied 101 patients, OBL occurred bilaterally in 58 eyes among 29 patients and OBL occurred unilaterally among 42 patients.

[Table 3] demonstrated a comparison between OBL and non-OBL regarding age and sex of the studied patients.

Table 3 Comparison between opaque bubble layer and non-opaque bubble layer regarding age and sex of the studied patients

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Being a retrospective study, it was difficult to differentiate the type of OBL formed, either soft or hard.

Preoperative variables:
1. Preoperative SE, there was no statistically significant difference between OBL and non-OBL (P=0.91), where SE was more than −2.25 in 23 (23%) eyes of OBL group versus 26 (26.0%) eyes in non-OBL eyes.
2. Same for preoperative astigmatism among the studied 200 eyes; there was no statistically significant difference between both groups (P=0.60).
3. Refraction in OBL group, 98 (98.0%) eyes were myopic, and only two (2%) eyes were hypermetropia while in non-OBL 96 (96.0%) eyes were myopic, and four (4%) eyes were hypermetropia with no statistically significant difference between OBL and non-OBL regarding refraction (P=0.7).
4. Regarding the distribution of flat keratometric reading (K1), there was no statistically significant difference between OBL and non-OBL (P=0.15), where K1 was more than 44.1 D in 19 (19%) eyes of OBL group versus 27 (27.0%) eyes in non-OBL eyes.
5. Preoperative steep keratometry (K2) regarding OBL formation among the studied 200 eyes, there was a highly statistically significant difference between both groups (P=0.01). Graph 5 is a column chart for comparison between the OBL group and the non-OBL group regarding distribution of steep keratometry (K2); there was no statistically significant difference between OBL and non-OBL (P=0.57).
6. Regarding preoperative CCT (µm) in relation to OBL formation among the studied 200 eyes, there was a statistically significant difference between both groups (P=0.03), it being thicker in the OBL group (median, interquartile range=551.0, 44.0 μm vs. 537.5, 44.0 μm in the non-OBL group).
Intraoperative data in relation to OBL formation:
1. There was no statistically significant difference between both groups regarding canal length offset (mm) (P=0.123), corneal flap thickness (P=0.489), corneal flap diameter (P=0.064), and flap hinge angle (P=0.074) ([Table 4] a,b).On the contrary, regarding canal length offset (mm), there was a statistically significant difference between OBL and non-OBL (P=0.04), where the canal length offset (mm) was more than 0.8 mm in 28 (28%) eyes OBL group versus 12 (12.0%) eyes in non-OBL eyes. Graph 1: column chart for comparison between the OBL group and non-OBL group regarding distribution of canal length offset.
Correlation was done between the development of OBL and each of study variables and revealed no statistically significant differences except weak significant correlation with the CCT (r=0.15, P=0.03) and also with K2, steep meridian (r=0.29, P<0.01). Linear regression analysis: [Table 5] represents the best fitting logistic regression model for OBL formation; the table displays that CCT, corneal astigmatism, flat, and steep corneal meridians were statistically significant independent predictors of OBL formation:
1. There was no statistically significant difference as regards best-corrected visual acuity and refraction between the two groups at the postoperative follow-up period of 3 months.
Table 4 Detailed preoperative corneal flap parameters in relation to OBL formation among the studied eyes

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Table 5 Logistic regression of opaque bubble layer depending on the presence of risk factors

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Discussion

LASIK is a generally acknowledged method for refractive error correction [1]. Recently, the use of the bladeless LASIK surgery for lamellar flap creation, depending on a FSL (advantaged by its ultrashort pulses, with femtosecond duration), acts as a superior option to the mechanical microkeratome [2]. Our goal of this study was to determine the risk factors responsible for OBL formation in FSL-assisted LASIK among Egyptian patients. We concluded that thick cornea, steep keratometry, and increased corneal canal length offset were significant risk factors for OBL formation during surgeries.

Moreover, CCT and increased corneal astigmatism were statistically significantly independent predictors of OBL formation. This could be because of thicker corneas provide more resistance and greater rigidity, steeper corneas might be affected by higher pressure with resultant escape of intrastromal gas bubbles. Less astigmatism results in more radially uniform applanation pressure, with no meridian allowing easier gas bubbles outflow to the conjunctival space versus high astigmatism where there is a meridian with lower applanation pressure. This was stated by Wu et al. [9]. Further studies are needed to justify the direction and magnitude of astigmatism as a risk factor for OBL formation.

This is concordant with the findings of Courtin et al. [10] in a retrospective study which included 198 eyes which underwent FSL-assisted LASIK by WaveLight FS200 laser. They concluded a significant correlation between CCT and OBL formation.

Also, Liu et al. [5] assessed the impact on visual outcomes, incidence, risk factors of OBL produced by IntraLase FSL (60 kHz) during LASIK. They concluded that OBLs occur more in thicker corneas even with a 60 kHz FSL.

Moreover, in a retrospective study Wu et al. [9] on 198 eyes that underwent FSL-assisted LASIK, OBL occurred in 118 eyes. They found that OBL had a higher incidence in less astigmatic, smaller, and thicker corneas with the use of the original technique for flap creation.

Mastropasqua et al. [1] noticed that OBL incidence was significantly reduced with less extension with increased LASIK flap setting diameter, and the flap edge was closer to contact glass border. They use Visumax Carl Zeiss (500 kHz) FSL, with different flap diameters (7.9, 8.0, 8.2 mm) and with same power settings and laser energy.

In this study, a wider flap diameter than the contact glass that ranged from 8.6 to 9.0 mm was used and there were no significant correlation between OBL formation and different flap diameters.

Wei et al. [11] had a case–control study on 60 consecutive patients (120 eyes) with unilateral OBL during FS-LASIK. The incidence of OBL was significantly reduced when the outlet of gas diffusing canal slit was at the posterior border of the corneoscleral limbus. This could be explained by a more effective gas diffusing canal. Corneal astigmatism was also an independent protective factor for OBL formation; this contradicts our finding of increased rate of OBL with astigmatism.

Mastropasqua et al. [1] studied 216 eyes for 108 patients (with bilateral hyperopia), OBL occurred in 33% overall. They concluded that higher astigmatism and a hard-docking technique are risk factors for OBL formation, and could relate laser energy and pulse rate in flap creation by an FSL with OBL incidence.

Serious problems of OBL were not encountered. However, after flap creation complicated with OBL, flaps were more difficult to lift. If OBL was excessive and flap lifting was difficult, the surgeon could wait for few minutes until the trapped gas dissolved or by pressing smoothly near this emphysematous-like pockets, the OBL disappeared.In some cases, OBL limited a patient’s ability to fixate the target so that it may interfere with pupil tracker during excimer laser part.

Added to the risk factors for OBL formation in FSL emission, the key point here is to allow the generated gas to diffuse outside of the stroma. This could be achieved in IntraLase, LDV, and WaveLight FS200 FSL machines, which have already taken special methods to drain the gas responsible for OBL [11].

Conclusion

This study confirms the result of the previous studies. It was found that increased corneal thickness, corneal canal length offset, and steep keratometry were significant risk factors of OBL formation. Regarding regression analysis, CCT and corneal astigmatism were statistically significant independent predictors of OBL formation.

Recommendations

If OBL was excessive and flap lifting was difficult, the surgeon could wait for few minutes until the trapped gas dissolved or by pressing smoothly near this emphysematous-like pockets, the OBL disappeared.
Further researches and studies are needed to follow up the effect of OBL on postoperative results.
More studies and innovations in technology are needed to prevent the appearance of OBL.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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