Journal of Refractive Surgery

Original Article Supplemental Data

Incidence and Risk Factors of Opaque Bubble Layer Formation According to Flap Thickness During 500-kHz FS-LASIK

Dong Hui Lim, MD, PhD; Joo Hyun, MD; Eunhae Shin, MD; Byung-woo Ko, MD; Eui-Sang Chung, MD, PhD; Tae-Young Chung, MD, PhD

Abstract

PURPOSE:

To present the incidence, risk factors, and effect of opaque bubble layer (OBL) formation during flap creation in laser-assisted in situ keratomileusis (LASIK) with a 500-kHz femtosecond laser on visual performance.

METHODS:

In this retrospective study, preoperative characteristics (age, sex, keratometric value, spherical equivalent, and central corneal thickness) and intraoperative surgical factors (used energy, docking type, and flap thickness) were compared between eyes with and without OBL formation during flap creation. Possible risk factors for specific types of OBLs were analyzed.

RESULTS:

One hundred thirty-five eyes of 71 patients underwent LASIK, and OBL developed in 98 eyes (72.59%). In the univariate analysis, the greater than 80-µm flap group was associated with a lower OBL occurrence than the 80-µm flap group (P = .0424, odds ratio [OR] = 0.481) and hard docking was associated with increased OBL formation (P = .0001, OR = 6.859). In the multivariate analysis, hard docking was a risk factor for OBL development (P = .0003, OR = 6.329). In the subgroup analysis, hard docking had a marginal effect on OBL occurrence in the 80-µm flap group (P = .086, OR = 3.564), but it had a strong effect in the greater than 80-µm flap group (P = .0018, OR = 10.210).

CONCLUSIONS:

Hard docking is a risk factor for OBL development. However, hard docking had a small effect on OBL occurrence in the 80-µm flap group during LASIK. OBL formation did not affect visual performance.

[J Refract Surg. 2019;35(9):583–589.]

Abstract

PURPOSE:

To present the incidence, risk factors, and effect of opaque bubble layer (OBL) formation during flap creation in laser-assisted in situ keratomileusis (LASIK) with a 500-kHz femtosecond laser on visual performance.

METHODS:

In this retrospective study, preoperative characteristics (age, sex, keratometric value, spherical equivalent, and central corneal thickness) and intraoperative surgical factors (used energy, docking type, and flap thickness) were compared between eyes with and without OBL formation during flap creation. Possible risk factors for specific types of OBLs were analyzed.

RESULTS:

One hundred thirty-five eyes of 71 patients underwent LASIK, and OBL developed in 98 eyes (72.59%). In the univariate analysis, the greater than 80-µm flap group was associated with a lower OBL occurrence than the 80-µm flap group (P = .0424, odds ratio [OR] = 0.481) and hard docking was associated with increased OBL formation (P = .0001, OR = 6.859). In the multivariate analysis, hard docking was a risk factor for OBL development (P = .0003, OR = 6.329). In the subgroup analysis, hard docking had a marginal effect on OBL occurrence in the 80-µm flap group (P = .086, OR = 3.564), but it had a strong effect in the greater than 80-µm flap group (P = .0018, OR = 10.210).

CONCLUSIONS:

Hard docking is a risk factor for OBL development. However, hard docking had a small effect on OBL occurrence in the 80-µm flap group during LASIK. OBL formation did not affect visual performance.

[J Refract Surg. 2019;35(9):583–589.]

A femtosecond laser is commonly used in laser-assisted in situ keratomileusis (FS-LASIK) surgery. It generates ultrashort pulses and creates microplasma at the corneal stroma, which induces a plane of separation with gas bubbles (photodisruption).1 The use of a femtosecond laser results in thin and consistent flap creation, which allows for the treatment of higher myopia with sufficient residual stroma, referred to as thin-flap LASIK.2–4 The thin flap can be made with either a microkeratome or a femtosecond laser. Flap creation using a femtosecond laser is superior for controlling the thickness of the flaps,5–7 and it results in better spherical equivalents and less surgically induced astigmatism compared to flap creation by mechanical microkeratome.8,9

Pulse rates and laser energy are key factors for creating the corneal flap in FS-LASIK surgery.1 A low pulse rate requires high energy to cut corneal tissue, and it produces excessive gas bubbles accompanied with cavitations, referred to as an opaque bubble layer (OBL). The OBL may interfere with flap lifting, prevent pupil tracking, and disturb precise excimer laser ablation.10 A higher pulse rate can create an even cleavage plane with decreased spot distance using lower pulse energy. Low laser energy with a high pulse rate can reduce excessive cavitation bubbles; therefore, it was hypothesized that it also could decrease OBL formation.

Several studies have reported the incidence and risk factors of OBL in FS-LASIK surgery (15, 60, and 200 kHz).10–13 However, there are few reports on the effects following use of the VisuMax 500-kHz femtosecond laser system (Carl Zeiss Meditec, Jena, Germany). Jung et al.14 reported that the incidence of OBL was 5%, and that a steep, thick cornea and the hard docking technique could affect OBL occurrence. However, to our knowledge, there have been no studies of OBL occurrence in ultrathin-LASIK using the VisuMax 500-kHz femtosecond laser. Therefore, the purpose of this study was to describe different clinical manifestations and risk factors associated with OBL in LASIK using the VisuMax 500-kHz femtosecond laser.

Patients and Methods

This retrospective study assessed data from 135 eyes of 71 patients who underwent LASIK for myopia (≤ −11.00 diopters [D]) at Samsung Medical Center in Seoul, South Korea, between August 1, 2010, and August 31, 2013. All patients had stable myopia for at least 1 year before surgery and an astigmatism of 3.75 D or less on manifest refraction. Patients with an abnormal keratometric value (> 47.00 or < 40.00 D) and an ocular pathology (cataract, glaucoma, retinopathy, or corneal dystrophy) or a history of ocular surgery were excluded. Surgery was performed by two surgeons (E-SC, T-YC) who used the same surgical technique. Approval was obtained from the Samsung Medical Center Institutional Review Board.

Surgical Technique

All procedures were performed with the VisuMax 500-kHz femtosecond laser system. The flap was created using a standardized nasal hinge: hinge angle of 50°, side cut angle of 55°, and track and spot distance of 3 µm. The laser was shot with a spiral pattern and an energy level between 20 and 33 (100 to 166 nJ). The flap diameter was set to 8 mm, and the flap thickness ranged from 80 to 100 µm. Laser ablation of the stromal bed was performed with the MEL 80 excimer laser (Carl Zeiss Meditec). Using the automatic record system on the VisuMax femtosecond laser, all surgical procedures were videotaped. Postoperatively, all patients were prescribed 0.5% moxifloxacin hydrochloride (Vigamox; Alcon Laboratories, Inc., Fort Worth, TX), to be used four times a day for 1 week. They were also given 1.0% prednisolone acetate (Pred Forte; Allergan Inc., Dublin, Ireland), to be used eight times a day for 3 days and six times a day for the next 4 days. In addition, 0.1% fluorometholone (Flumetholon; Santen Pharmaceutical Co. Ltd., Osaka, Japan) was subsequently prescribed for use four times a day for the next month. The observer grading the surgical videos was masked to the operators.

Patient Examinations

All patients underwent a full ophthalmologic examination before surgery that included the following assessments: uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest refraction, intraocular pressure using non-contact tonometry, corneal topography (Orbscan II; Bausch & Lomb, Rochester, NY), ultrasonic central corneal thickness (CCT) (SP-100 ultrasound; Tomey Corporation, Nagoya, Japan), and wavefront analyzer (WASCA; Carl Zeiss Meditec). Visual performance including UDVA and higher order aberrations (HOAs) were reexamined 6 months postoperatively.

All videotaped procedures were analyzed by one ophthalmologist (B-WK). Eyes with a frosted area with gas bubbles after flap creation were categorized into the OBL group. The parameters mentioned above were compared between the OBL and no OBL groups in addition to flap thickness, docking duration, total energy, keratometric value, CCT, and clinical outcomes (eg, visual acuity and HOAs). Because we observed a potential relationship between flap thickness and OBL frequency, we additionally analyzed the effect of flap thickness on the occurrence of OBL.

We also investigated the docking type (hard and soft docking).10 Hard docking was defined as an eye with an applanated area over 90% of the cornea under the microscope right before suction, and soft docking referred to eyes with an applanated area less than 90%. Thus, hard docking exerted more pressure on the cornea during flap creation. ImageJ software (LOCI, University of Wisconsin, Madison, WI, and National Institutes of Health, Bethesda, MD) was used to measure the area of applanation.

Statistical Analysis

Statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC). The Mann–Whitney U test and chi-square test were used to compare preoperative and postoperative data between the OBL and no OBL groups. To identify risk factors that could affect OBL occurrence, univariate and multivariate logistic regression analyses were conducted using the generalized estimating equation model because data from both eyes was used. Variables with a P value of .20 or less in univariate logistic regression and clinically important variables were selected for further multivariate regression analyses. A P value of less than .05 was considered statistically significant. We also performed subgroup analyses to measure the effects of docking type and flap thickness on OBL occurrence.

Results

This retrospective study included 135 eyes of 71 patients. The mean age was 28.85 ± 7.57 years (range: 19 to 47 years). The mean preoperative spherical equivalent was −5.76 ± 2.48 D (range: −0.60 to −11.00 D) and mean keratometric value was 43.38 ± 1.17 D (range: 40.30 to 46.40 D). Ninety-eight eyes had OBL, resulting in an incidence of 72.59%. There was no statistically significant difference in OBL occurrence between the two surgeons. The surgeons did notice difficulty in flap lifts in eyes with OBLs; however, there were no flap complications after OBL formation.

Age, sex, and preoperative UDVA and CDVA were not significantly different between the no OBL and OBL groups (P = .508, .660, .682, and .074, respectively, Table 1). Visual outcomes are shown in Figure 1. UDVA and CDVA were improved after surgery in both groups, and postoperative UDVA, CDVA, and spherical equivalent were not significantly different between groups (P = .580, .055, and .789, respectively). Additionally, HOAs showed no difference between the groups (P = .975).

Preoperative and Postoperative Data and Intraoperative Parameters in Eyes Without and With OBLa

Table 1:

Preoperative and Postoperative Data and Intraoperative Parameters in Eyes Without and With OBL

Visual outcomes. OBL = opaque bubble layer; UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters

Figure 1.

Visual outcomes. OBL = opaque bubble layer; UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters

There were no differences in preoperative spherical equivalent, corneal power, and CCT between the OBL and no OBL groups (P = .925, .650, and .055, respectively). Although the intraoperative parameters of mean flap thickness and energy were not significantly different between the two groups (P =.094 and .927, respectively), the proportion of eyes with hard docking was statistically significantly higher in the OBL group (Table 2). Five of 37 eyes (13.51%) in the no OBL group were categorized as having undergone the hard docking technique, whereas 52 of 98 eyes (53.06%) in the OBL group received hard docking (P = .000, odds ratio [OR] = 19.07, chi-square test). Therefore, OBL occurred in 52 of 57 eyes (91.2%) that underwent hard docking but only 46 of 78 eyes (58.97%) with soft docking (P = .000).

OBL Development According to Docking Type

Table 2:

OBL Development According to Docking Type

OBL incidence according to flap thickness was also assessed. OBL occurred in 44 of 54 eyes (81.5%) with a flap thickness of 80 µm, 41 of 62 eyes (66.1%) with a flap thickness of 90 µm, and 13 of 19 eyes (68.4%) with a flap thickness of 100 µm (Table 3). Therefore, there was a trend toward increased OBL with reduced flap thickness; however, this was not statistically significant (P = .164, Pearson's chi-square test). Because there were few eyes with a flap thickness of 100 µm, we analyzed OBL incidence between the 80-µm flap group and the greater than 80-µm flap group. There was a significant difference in OBL occurrence based on flap thickness (P = .044, OR = 3.70, Fisher's exact test) (Table 3).

OBL Development According to Flap Thickness

Table 3:

OBL Development According to Flap Thickness

Potential risk factors for OBL occurrence (eg, age, sex, preoperative spherical equivalent, keratometric value, CCT, flap thickness, and docking type) were analyzed in a univariate regression analysis (Table A, available in the online version of this article). Flap thickness was categorized into two groups: 80-µm flap group (reference) and greater than 80-µm flap group. Flap thickness and docking type were significantly correlated with OBL occurrence (P = .042, OR = 0.481; P = .0001, OR = 6.859, respectively). Further, we analyzed clinically important risk factors and variables with a P value of .20 or less in the univariate and subsequent multivariate analyses (Table 4). Hard docking resulted in a 6.329 times higher risk for OBL occurrence than soft docking (P = .0003, OR = 6.329). Other factors and parameters did not affect OBL development using this model.

Univariate Analysis Using Generalized Estimating Equation Model for OBL Occurrence

Table A:

Univariate Analysis Using Generalized Estimating Equation Model for OBL Occurrence

Multivariate Analysis Using Generalized Estimating Equation Model for Risk Factors Associated With OBL Occurrence

Table 4:

Multivariate Analysis Using Generalized Estimating Equation Model for Risk Factors Associated With OBL Occurrence

In a subgroup analysis (Table B, available in the online version of this article), hard docking had a marginal effect on OBL occurrence in the 80-µm flap group (P = .086, OR = 3.564), but it had strong effect in the greater than 80-µm flap group (P = .0018, OR = 10.210).

Subgroup Analysis of OBL Occurrence With Regard to Flap Thickness

Table B:

Subgroup Analysis of OBL Occurrence With Regard to Flap Thickness

Discussion

LASIK that creates a flap thickness between 90 and 110 µm2,3 is performed globally as an alternative to conventional LASIK. Ultrathin LASIK uses a flap less than 80 µm.15 Thin flap approaches combine the advantages of conventional LASIK, such as faster recovery16 and superior safety and efficacy,17 with the benefit of preserving more residual stroma, which allows for the treatment of higher myopia. However, thin flap techniques may have the potential risks of microstriae, flap tear, and interface haze. OBL occurrence may increase these risks and interfere with eye registrations and laser ablation and impede operation time. Therefore, surgeons attempt to avoid OBL formation.

In the current study, OBL incidence was 72.59%. Comparisons of preoperative characteristics and intraoperative parameters (age, sex, refractive error, keratometric value, CCT, intraocular pressure, and HOA) between eyes with OBL and eyes without OBL revealed no differences, with the exception of docking type. Eyes with hard docking had a higher risk of OBL occurrence than those with soft docking. The OBL incidence reported in the current study is higher than previously reported.10,11 Kaiserman et al.10 and Liu et al.11 reported that OBL occurred in 56.4% and 52.5% of surgeries using the IntraLase system (15 and 60 kHz), respectively, and demonstrated that eyes with a smaller flap and thicker cornea developed OBL more frequently. Using the 200-kHz femtosecond laser system, Courtin et al.18 reported an OBL incidence of 48% and demonstrated that elevated corneal hysteresis is a biomechanical risk factor for OBL formation (Table C, available in the online version of this article).

Comparison of Incidence and Risk Factors of OBL According to the Femtosecond Laser System

Table C:

Comparison of Incidence and Risk Factors of OBL According to the Femtosecond Laser System

Through multivariate analysis, the current study demonstrated hard docking as a risk factor for OBL development in 500-kHz FS-LASIK. Further, when we compared the 80-µm flap group to the greater than 80-µm flap group, the latter had a significantly lower OBL occurrence.

In a previous study published by our group,12 we reported that OBL development increased approximately two-fold in the 80-µm flap group compared to the 120-µm flap group when the flap was created with a 200-kHz VisuMax femtosecond laser system. The results we report in the current study are in accordance with those of our previous study; however, unlike our previous study, corneal thickness or keratometric value had no significant influence on OBL occurrence with this system.

Theoretically, low pulse energy with a high repetition rate minimizes bubble expansion1 and improves the quality of the cleavage plane. Therefore, it is possible that the 500-kHz femtosecond laser reduces OBL incidence, as suggested by Jung et al.14 In one of the few studies in which the 500-kHz femtosecond laser system was used, Jung et al.14 reported the lowest OBL incidence (5.0%). However, we report an OBL incidence much higher than theirs, regardless of flap thickness. In the study by Jung et al., the flap was created with a thickness of 110 µm, and most OBL studies were conducted using a flap thickness greater than 90 µm. It is possible that a thinner flap is more vulnerable to OBL occurrence.

In our study, there was no statistically significant difference in total energy between the no OBL and OBL groups. The P value of energy factor was greater than .05 in univariate analyasis of this study (Table A). Table C lists several articles that studied OBL risk factors, but none of them suggested energy as a risk factor. Although there was a study that presented a modified flap technique to reduce OBL occurrence, it also concluded that the energy level did not have an important effect on OBL incidence.19

The 80-µm flap group had a higher incidence of OBL (Table 3), and flap thickness was correlated with OBL occurrence (Table 4). The anterior stroma adjacent to Bowman's layer has a more compact structure and consists of more keratocytes and collagen fibrils than the posterior, deeper stromal tissue. Assuming that gas bubbles tend to spread into the less resistant corneal tissue,20,21 cavitation bubbles are trapped by the surrounding compact anterior stroma with high resistance, and additional laser pulses at these bubbles cause them to expand. Therefore, a thin flap produced in the dense anterior stroma may increase OBL formation. Additionally, the finding that hard docking had a marginal effect on OBL occurrence in the 80-µm flap group (Table B) supports this concept.

Jung et al.14 concluded that a steep, thick cornea and hard docking technique could be risk factors for OBL formation in VisuMax 500-kHz FS-LASIK. They did not explain the exact mechanism of reduction of OBL, but they posited that it could be due to differences between the VisuMax and other femtosecond laser systems, such as the size and shape of the patient interface, energy, and capacity to create a pocket. In the current study, we found that hard docking was a strong risk factor for OBL formation. Hard docking caused greater applanation pressure than the soft docking technique, and it may provide increased pressure to the corneal stroma near the cleavage plane, resulting in OBL creation.

In the greater than 80-µm flap group, hard docking was a strong risk factor for OBL occurrence. Because corneal thickness is known to be correlated with corneal hysteresis and corneal resistance factor,22 a thick corneal flap could have greater rigidity than a thin flap, possibly producing a larger counterforce to oppose applanation pressure and resulting in increased susceptibility to docking type. However, further studies should quantitatively measure the corneal rigidity by flap thickness to assess this possibility.

Finally, several features of the VisuMax system may have influenced OBL occurrence rate. The applanation cone of the VisuMax system enables curved applanation due to a spherical surface (keratometric value = 43.26 D), whereas other systems have a flat cone design.14,23 The curved applanation tip of the VisuMax laser allowed lower pressure on the corneal center (< 100 mm Hg) than other femtosecond laser systems (> 100 mm Hg) that have a flat applanation surface.20,22 Therefore, direct force to the corneal surface may have played a more important role in OBL formation than characteristics such as keratometric value or CCT in cases with a thicker flap. Recently, Wu et al. published an article on reduced OBL occurrence using a cone modification technique for flap creation using the VisuMax laser. In the study, a small cone was used under the medium cone program setting, inducing a larger flap diameter.19

In addition, the IntraLase and FS200 systems have unique programs to create exits for the gas bubbles that are not available on the VisuMax system, such as pocket or canal formation.24 This difference may have also affected OBL occurrence.

Our results showed no statistically significant differences in postoperative visual acuity, spherical equivalent, or HOAs between the OBL and no OBL groups. This is in accordance with previous studies that reported no effects of OBL on visual acuity.10,11,14

Although the current study is not about small incision lenticule extraction (SMILE) LASIK, there was a study on the independent risk factor of OBL occurrence in SMILE recently. According to Li et al.,25 the multivariate analysis suggested that myopia and astigmatism remained significantly associated with the risk of intraoperative OBL (P < .05). Titiyal et al.26 discussed using SMILE with the VisuMax laser system and suggested that thicker corneas and thinner lenticules should be avoided to prevent OBL formation.26 Future studies comparing conventional LASIK to SMILE with OBL occurrence would be interesting.

This study has several limitations. First, it is a retrospective study and has lower validity than a randomized control study. However, because the current study contains data from 135 eyes of 71 patients, the authors suggest the results of this study have significance in its large number of cases. Furthermore, this study deals with OBL occurrence in ultrathin-LASIK (flap thicknesses of all cases were less than 100 µm). Second, this study evaluated the data of the Visumax 500-kHz femtosecond laser, meaning its results may not be applicable to other femtosecond lasers with different docking type or machine design.

The hard docking technique was the risk factor for OBL development. In a subgroup analysis, hard docking had a more significant effect on OBL formation in the greater than 80-µm flap group. The thinner flap (80 µm) was a possible risk factor for the higher occurrence of OBL, with a marginal effect of hard docking. This is the first study to demonstrate higher OBL rates in thinner flaps when using 500-kHz high pulse energy, which is known to have lower OBL rates. However, OBL formation did not affect visual performance.

References

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  14. Jung HG, Kim J, Lim TH. Possible risk factors and clinical effects of an opaque bubble layer created with femtosecond laser-assisted laser in situ keratomileusis. J Cataract Refract Surg. 2015;41:1393–1399. doi:10.1016/j.jcrs.2014.10.039 [CrossRef]
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  24. Kanellopoulos AJ, Asimellis G. Three-dimensional LASIK flap thickness variability: topographic central, paracentral and peripheral assessment, in flaps created by a mechanical microkeratome (M2) and two different femtosecond lasers (FS60 and FS200). Clin Ophthalmol. 2013;7:675–683. doi:10.2147/OPTH.S40762 [CrossRef]
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Preoperative and Postoperative Data and Intraoperative Parameters in Eyes Without and With OBLa

Characteristic No OBL Group (n = 37) OBL Group (n = 98) P
Preoperative
  Age (years) 28.16 ± 7.48 (19 to 46) 29.11 ± 7.63 (19 to 47) .508b
  Sex (male:female), no. 6:31 13:85 .660c
  UDVA (logMAR) 1.05 ± 0.52 (0.22 to 2.00) 1.08 ± 0.52 (0.05 to 2.00) .682b
  CDVA (logMAR) 0.00 ± 0.00 (0.00 to 0.00) 0.01 ± 0.03 (0.00 to 0.15) .074b
  SE (D) −5.76 ± 2.30 (−1.75 to −9.20) −5.76 ± 2.55 (−0.60 to −11.00) .925b
  Sphere (D) −5.29 ± 2.28 (−1.50 to −9.20) −5.25 ± 2.50 (0.00 to −10.25) .996b
  Cylinder (D) 0.94 ± 0.70 (0.00 to 2.85) 1.02 ± 0.79 (0.50 to 4.00) .690b
  Mean K (D) 43.34 ± 1.32 (40.40 to 45.90) 43.40 ± 1.11 (40.30 to 46.40) .650b
  Steep K (D) 44.08 ± 1.37 (41.10 to 46.50) 44.15 ± 1.16 (41.00 to 47.00) .758b
  Flat K (D) 42.59 ± 1.38 (39.60 to 45.20) 42.64 ± 1.20 (39.60 to 45.80) .700b
  IOP (mm Hg) 16.86 ± 1.92 (13.00 to 20.00) 16.82 ± 2.34 (11.00 to 21.00) .978b
  Central corneal thickness (µm) 559.16 ± 29.34 (489 to 617) 549.35 ± 31.58 (499 to 650) .055b
  HOA 0.42 ± 0.18 (0.13 to 0.81) 0.40 ± 0.13 (0.19 to 0.75) .735b
Intraoperative
  Docking type (hard:soft), no. 5:32 52:46 < .001c
  Flap thickness (µm) 88.92 ± 6.58 (80.00 to 100.00) 86.84 ± 6.98 (80.00 to 100.00) .094b
  Total energy (nJ) (energy level * 5) 119.52 ± 18.90 (100 to 165) 120.41 ± 19.72 (100 to 165) .927b
Postoperative
  UDVA (logMAR) −0.03 ± 0.09 (−0.30 to 0.05) −0.01 ± 0.09 (−0.30 to 0.40) .580b
  CDVA (logMAR) −0.07 ± 0.09 (−0.30 to 0.05) −0.03 ± 0.07 (−0.30 to 0.15) .055b
  SE (D) −0.03 ± 0.15 (−0.50 to 0.25) −0.07 ± 0.29 (−1.5 to 0.75) .789b
  All HOAs (µm) 0.58 ± 0.17 (0.37 to 0.94) 0.58 ± 0.21 (0.29 to 1.41) .975b

OBL Development According to Docking Type

Docking Type No OBL Group OBL Group Total OR P
Hard 5 (8.8%) 52 (91.2%) 57 19.07 < .001a
Soft 32 (41.0%) 46 (59.0%) 78
Total 37 98 135

OBL Development According to Flap Thickness

Flap Thickness No OBL Group OBL Group Total OR P
80 µm 10 (18.5%) 44 (81.5%) 54 .164a
90 µm 21 (33.9%) 41 (66.1%) 62
100 µm 6 (31.6%) 13 (68.4%) 19

80 µm 10 (18.5%) 44 (81.5%) 54 3.70 .044b
> 80 µm 27 (33.3%) 54 (66.7%) 81
Total 37 98 135

Multivariate Analysis Using Generalized Estimating Equation Model for Risk Factors Associated With OBL Occurrence

Characteristic Odds Ratio (95% CI) P a
Age 1.013 (0.960 to 1.069) .814
Preoperative central corneal thickness (µm) 1.002 (0.988 to 1.015) .810
Preoperative mean K (D) 1.039 (0.707 to 1.527) .846
Flap thickness (µm) 0.614 (0.230 to 1.635) .329
Docking type (soft:hard) 6.329 (2.344 to 17.088) .0003

Univariate Analysis Using Generalized Estimating Equation Model for OBL Occurrence

Characteristic Odds Ratio (95% CI) P
Age (y) 1.021 (0.978 to 1.067) .820
Sex 1.323 (0.609 to 2.875) .346
Preoperative spherical equivalent (D) 0.970 (0.850 to 1.106) .644
Preoperative mean K (D) 1.047 (0.798 to 1.367) .751
Preoperative central corneal thickness (µm) 0.990 (0.981 to 1.000) .055
Preoperative intraocular pressure (mm Hg) 1.012 (0.890 to 1.150) .862
Energy (nJ) 1.002 (0.985 to 1.019) .820
Flap thickness (µm) 0.481 (0.237 to 0.975) .042
Docking type (soft:hard) 6.859 (2.542 to 18.508) .0001

Subgroup Analysis of OBL Occurrence With Regard to Flap Thickness

Characteristic Odds Ratio (95% CI) P
80-µm flap group
  Age (y) 1.005 (0.884 to 1.143) .937
  Preoperative central corneal thickness (µm) 0.993 (0.963 to 1.025) .677
  Preoperative mean K (D) 1.099 (0.445 to 2.716) .837
  Docking type (hard) 3.564 (0.837 to 15.177) .086
> 80-µm flap group
  Age 1.017 (0.956 to 1.081) .601
  Preoperative central corneal thickness (µm) 1.004 (0.989 to 1.020) .579
  Preoperative mean K (D) 0.989 (0.625 to 1.564) .962
  Docking type (hard) 10.210 (2.377 to 43.855) .0018

Comparison of Incidence and Risk Factors of OBL According to the Femtosecond Laser System

Study Year Femtosecond Laser Device Pulse Rate (kHz) Incidence Risk Factors Flap Thickness (µm) Bed Energy (µJ) Bed Spot & Line Separation (µm)
Kaiserman et al. 2008 IntraLase 15 56.4% Smaller flap, thicker cornea 100 to 120 1.0 to 1.4 9.0
Liu et al. 2014 IntraLase 60 52.5% Thicker cornea 90 to 120 0.85 6.0
Courtin et al. 2015 WaveLight FS200 200 48% Elevated corneal hysteresis, thicker cornea 120 to 135 0.79 to 0.85 8.0
Lim et al. 2013 VisuMax 200 More frequent in the 80-μm flap group (about two folds) Steeper cornea, thicker cornea 80 & 120 0.155, 0.18 4.5
Jung et al. 2015 VisuMax 500 5% Steeper cornea, thicker cornea, hard-docking technique 110 0.115 3.0
Current study 2019 VisuMax 500 72.59% 80-μm flap, hard-docking technique 80 to 100 0.100 to 0.165 3.0
Authors

From the Department of Ophthalmology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea (DHL, JH, ES, B-WK, E-SC, T-YC); the Department of Preventive Medicine, Graduate School, The Catholic University of Korea, Seoul, South Korea (DHL); the Department of Ophthalmology, Saevit Eye Hospital, Goyang, South Korea (JH); Apgujeong Eye Center, Seoul, South Korea (B-WK); and SNU Seoul Eye Clinic, Seoul, South Korea (E-SC).

Supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (Grant No. HI19C0577) and Samsung Medical Center (SMC) Research and Development Grant (Grant No. OTC1190641).

The authors have no financial or proprietary interest in the materials presented herein.

Drs. Lim and Hyun contributed equally to this work and should be considered as equal first authors.

AUTHOR CONTRIBUTIONS

Study concept and design (DHL, JH, B-WK, E-SC, T-YC); data collection (DHL, JH, ES); analysis and interpretation of data (DHL, JH, B-WK, E-SC, T-YC); writing the manuscript (JH, ES, B-WK); critical revision of the manuscript (DHL, E-SC, T-YC); statistical expertise (DHL, JH, ES); supervision (E-SC, T-YC)

Correspondence: Tae-Young Chung, MD, PhD, Department of Ophthalmology, Samsung Medical Center, Sungkyunkwan University School of Medicine, (135-710), 81 Irwon-ro, Gangnam-gu, Seoul, South Korea. E-mail: tychung@skku.edu

Received: May 16, 2019
Accepted: August 14, 2019

10.3928/1081597X-20190814-01

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