Comparison of risk factors for OBL in FS-LASIK and SMILE correction for myopia and myopia astigmatism

BACKGROUND

To find out the incidence and risk factors of opaque bubble layer (OBL) in eyes with myopia and myopic astigmatism following femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE).

METHODS

A total of 1076 eyes from 569 patients who had FS-LASIK or SMILE were included in the retrospective research. For each kind of surgery, eyes were separated into two groups: "OBL" groups and "no OBL" groups. In the FS-LASIK group, eyes that developed OBL were split into "hard OBL" and "soft OBL" groups. The incidence and size of OBL were analyzed after watching the surgical procedure videotaped during the operation and taking screenshots. Surgical parameters, including sphere, cylinder, keratometry, corneal thickness, flap thickness, cap thickness, lenticule thickness, and visual acuity, were compared.

RESULTS

In the FS-LASIK surgery, the incidence of OBL was 63.2% (347 eyes). A thicker central corneal thickness (CCT) was the only independent risk factor affecting the OBL area (β = 0.126, P = 0.019). One hundred and thirty of these eyes had hard OBL, and the flap thickness of these eyes was thinner than that of those with soft OBL (P = 0.027). In the SMILE group, 26.6% (140 eyes) developed OBL. A higher flat keratometry (K) and a thicker residual stromal thickness (RST) were risk factors affecting the OBL area (β = 0.195, P = 0.024; β = 0.281, P = 0.001).

CONCLUSION

The incidence of OBL differs between the FS-LASIK surgery and the SMILE surgery. There are differences in the factors influencing OBL between the two surgeries.

Possible risk factors of opaque bubble layer and its effect on high-order aberrations after small incision Lenticule extraction

Purpose

To evaluate the possible risk factors of opaque bubble layer (OBL) formation in small incision lenticule extraction (SMILE) surgery and its effects on visual quality.

Methods

Fifty-six eyes from 28 patients were included in this study. The preoperative parameters and intraoperative designs were recorded. Corneal high-order aberrations (HOAs), point spread function (PSF), and modulation transfer function (MTF) were measured using iTrace at pre-operation, 1 week, 1 month, and 3 months after SMILE. Generalized Estimating Equation and Linear Mixed Effects Model were employed for statistical analysis.

Results

The mean OBL area in SMILE surgery was 2.75% ± 1.25%. The patients were divided into groups based on whether the OBL was greater than the mean group or less than the mean group. Compared to the group with a smaller OBL area, the group with the larger OBL area had steeper corneal curvature and thinner cap thickness, the OBL area was positively correlated with the preoperative keratometry (r = 0.21, p = 0.04) and preoperative spherical value (r = 0.47, p = 0.01). The group with the larger OBL area induced more corneal SA and trefoil at 1 week postoperatively, but the difference was not significant at 1 month and 3 months postoperatively.

Conclusion

A steep corneal curvature, thin cap thickness, and high preoperative spherical value are possible risk factors for OBL formation in SMILE surgery. The OBL increased the ocular and corneal HOAs postoperatively for a short period (1 week), while it did not affect the long-term outcomes.

Effect of corneal flap thickness on opaque bubble layer formation in Visumax FS-LASIK using GEE analysis

Introduction

This study aimed to investigate two types of corneal flap thickness on opaque bubble layer (OBL) formation in Visumax femtosecond laser-assisted stromal for situ keratomileusis (FS-LASIK).

Methods

This retrospective study analyzed 203 eyes of 103 patients (32 men and 71 women) who underwent Visumax FS-LASIK between January 2020 and June 2020, and according to corneal flap thickness, they were divided into the 100-μm group (64 eyes) and the 110-μm group (139 eyes). Anterior-segment examination revealed no abnormal findings. Preoperatively, intraocular pressure (IOP), central corneal thickness (CCT), residual stromal thickness (RST), spherical power, cylindrical power, flat keratometry (K1), steep keratometry (K2), and biomechanical parameters including deformation amplitude (DA) ratio, Integrated Radius, stiffness parameter at first applanation (SP-A1), and Ambrosio relational thickness to the horizontal profile (ARTh) were evaluated. Primary outcomes were the incidence of OBL formation in the two groups compared by the Chi-square test and the correlation between the incidence of OBL and the above preoperative data by Spearman's Rho test. Secondary outcomes were the comparisons corrected by the generalized estimating equation (GEE) model.

Results

The incidence of OBL formation in the 100-μm group was 59.4 %, which was higher than that in the 110-μm group (23.0 %) with a significant difference (χ2 = 25.635, P < 0.001). The thinner corneal flap thickness (r = -0.355, P < 0.001) and higher spherical power (r = -0.142, P < 0.05) correlated with OBL formation. Higher K1 (r = 0.217, P < 0.01) and K2 (r = 0.198, P < 0.01) also correlated with OBL formation. The results of the GEE correction analysis showed higher rates of OBL formation in the 100-μm group (odds ratio [OR] = 4.704, 95 % CI 1.681-13.161, P < 0.01).

Conclusions

OBL was more likely to occur with the 100-μm corneal flap than with the 110-μm corneal flap in Visumax FS-LASIK. The risk of OBL formation in the 100-μm group was 4.704 times higher than that in the 110-μm group.

Flap-making patterns and corneal characteristics influence opaque bubble layer occurrence in femtosecond laser-assisted laser in situ keratomileusis

Background

Opaque bubble layer (OBL), which generates from photo-disruptive procedures on the cornea, has been a common phenomenon during femtosecond laser-assisted refractive surgeries and it would potentially impact eye tracking and flap lifting. And we have observed that an updated flap-making pattern could form less OBL clinically than the traditional pattern, which needed further approval. Thus, the purpose of this study is to prove our observation and investigate the possible risk factors related to the occurrence and type of OBL in laser in situ keratomileusis (LASIK) flaps using the Visumax laser system.

Methods

This prospective study included 167 eyes of 86 patients (mean age: 27.5 ± 6.1 years) undergoing bilateral femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) for myopia/myopic astigmatism by the same surgeon from April 2020 to August 2020. Preoperative data on refraction, central corneal thickness (CCT), and keratometry as well as intraoperative data were included for analysis. A new flap-making pattern creating an offset between flap-cut and side-cut was adopted to compare with the traditional pattern. The operation video of flap formation was analyzed to identify the existence and type of OBL. The area covered by OBL and the ratio of OBL to flap were calculated using Image J software.

Results

Among 167 eyes, 54 eyes (32.3%) developed OBLs, consisting of 31 as hard OBL coexisting with soft OBL, and 23 as soft OBL alone. The OBL incidence was significantly reduced in eyes with the new flap-making pattern compared with the traditional pattern (13.8% vs. 52.5%, P < 0.001). Hard OBLs had larger area ratios than soft OBLs (14.3 ± 8.3% vs. 1.1 ± 1.8%, P < 0.001). Univariate analyses revealed that eyes with more myopia, thicker CCT, and traditional flap-making patterns were more likely to develop OBLs. Multivariate analysis further confirmed that more myopia, thicker CCT, and traditional flap-making pattern were risk factors for OBLs. A Larger corneal diameter was associated with a higher incidence of hard OBL when applying the traditional flap-making process.

Conclusion

More myopia, thicker CCT, and larger corneal diameter were risk factors for OBL development during flap creation, whereas a flap-making pattern with an offset between flap-cut and side-cut could reduce the incidence of OBL.

Feasibility, Safety, and Outcomes With Standard Versus Differential Spot Distance Protocols in Eyes Undergoing SMILE for Myopia and Myopic Astigmatism

PURPOSE

To study the feasibility, safety, comparison of intraoperative ease of dissection, and immediate visual outcomes with standard versus differential small incision lenticule extraction (SMILE) spot settings in eyes undergoing bilateral SMILE for myopia or myopic astigmatism.

METHODS

One hundred eyes of 50 patients (mean age: 25.4 years) were randomized to receive standard settings (4.5-µm spot and track spacing in cap and lenticule interface) in one eye and differential settings (4.5-µm spot and track spacing in cap interface and 4.2 µm in lenticule interface) in the contralateral eye. Opaque bubble layer (OBL) was graded using a new grading system and a surgeon questionnaire was obtained to grade the ease of dissection (from 0 to 5, with 5 being the easiest) at the end of each surgery. Visual results and optical quality were analyzed at 1 day, 2 weeks, and 3 months postoperatively.

RESULTS

The mean OBL score was significantly lower in the differential group (26.3%) compared to the standard group (35.3%) (P < .01). Consequently, the mean dissection score was significantly higher in the differential group (4.01) compared to the standard group (3.57) (P < .01). The uncorrected distance visual acuity, Objective Scatter Index, modulation transfer function cut-off, and higher order aberrations were comparable with no statistically significant difference between both groups at 1 day, 2 weeks, and 3 months postoperatively (P > .05 for all parameters). No eye in either group had any intraoperative or postoperative complication affecting visual recovery.

CONCLUSIONS

Differential spot settings resulted in less intraoperative OBL and easy separability. However, the visual results and optical quality were comparable between groups. [J Refract Surg. 2021;37(5):294-302.].

Complications of small incision lenticule extraction

The procedure of small incision lenticule extraction (SMILE) was introduced in 2011, and since then there has been an increase in the number of cases undergoing this procedure worldwide. The surgery has a learning curve and may be associated with problems in the intraoperative and postoperative periods. The intraoperative problems during SMILE surgery include the loss of suction, the occurrence of altered or irregular opaque bubble layer and black spots, difficulty in lenticular dissection and extraction, cap perforation, incision-related problems, and decentered ablation. Most of the postoperative problems are similar as in other laser refractive procedures, but with decreased incidence. The identification of risk factors, clinical features, and management of complications of SMILE help to obtain optimum refractive outcomes.

Optical Vortex Beam for Gentle and Ultraprecise Intrastromal Corneal Dissection in Refractive Surgery

Purpose

We introduce a novel focus shaping concept for intrastromal corneal dissection that facilitates cleavage along corneal lamellae, and we analyze laser-tissue interactions governing cutting effectiveness and mechanical side effects.

Methods

Focus shaping was achieved by a spiral phase plate that converts an incident Gaussian beam into a Laguerre-Gaussian beam with a helical phase. Such vortex beams have zero intensity at their center, are propagation invariant, and possess a ring focus equal in length to the Gaussian focus but with a larger diameter. Cutting precision and the required absorbed energy for flap dissection were compared for Gaussian and vortex beams on ex vivo porcine corneal specimens at pulse durations between 480 fs and 9 ps. Cutting quality and bubble formation were characterized by scanning electron microscopy and macro photography.

Results

With the vortex beam, the cuts were much smoother. Bubble formation was markedly reduced because cutting can be performed close to the bubble threshold, whereas with the Gaussian beam energies well above threshold are needed. Although the incident energy at the flap dissection threshold was slightly larger for the vortex beam, the absorbed energy was much smaller and contributed more effectively to cutting. This reduced plasma-induced pressure more than sevenfold.

Conclusions

The vortex beam approach for corneal dissection is a simple, versatile, and cost-effective way of improving the precision of refractive surgery while reducing bubble formation and pressure-related mechanical side effects.

Translational Relevance

Phase plates for propagation invariant vortex beams are easily implemented in the beam path of next-generation clinical devices.

Influence of Cap Thickness on Opaque Bubble Layer Formation in SMILE: 110 Versus 140 µm

PURPOSE

To investigate the impact of cap thickness on the formation of an opaque bubble layer (OBL) during small incision lenticule extraction procedures.

METHODS

In total, 100 eyes from 50 patients were prospectively examined. One of two corneal cap thicknesses was randomly assigned to each eye and differed in the contralateral eye: 110 µm in one eye and 140 µm in the other. OBL area and density were quantitatively assessed.

RESULTS

The proportion of OBL areas in the anterior lenticule plane was 11.70% ± 7.35% in the 110-µm group, which was significantly higher than the 140-µm group (6.64% ± 4.68%, P < .001). For OBL areas located in the posterior lenticule plane, mean areas for the 110-µm group were also higher than those for the 140-µm group (1.32% ± 1.20% and 0.94% ± 0.59%, respectively; P = .002). Mean gray values of the OBL in the posterior lenticule plane were slightly different between the two groups (P < .001), but no significant difference in OBL of the anterior lenticule plane was observed (P = .055). Eyes with a 110-µm cap thickness had more focal OBLs, revealed by cap scanning (chi-square = 10.256, P = .001).

CONCLUSIONS

Corneal cap thickness is predictive of opaque bubble layer during small incision lenticule extraction procedures. [J Refract Surg. 2020;36(9):592-596.].

Quantitative Analysis of Human Corneal Lenticule Surface Microstructure Irregularity with 3D Optical Profiler Using White Light Interferometry

PURPOSE

To quantitatively evaluate the cutting quality of small incision lenticule extraction (SMILE) by measuring human corneal lenticule surface roughness in different areas with white light interferometer.

METHOD

A white light interferometer was used to quantitatively measure the corneal lenticule surface roughness in different areas. Sixty-three myopic patients (102 eyes) who underwent SMILE surgery were recruited. The surface roughness of the central, pericentral, and peripheral parts of the corneal lenticule surface was measured in both the anterior and posterior planes. Differences in corneal lenticule surface roughness were analyzed between different myopic groups.

RESULTS

The surface roughness of the anterior plane of the corneal lenticule was lower than the posterior plane in various areas (central, pericentral, and peripheral parts) (P < .01). Surface roughness gradually increased from the center to the periphery, in both the anterior and posterior planes (P < .01). There were no significant differences in the surface roughness of the central part in both the anterior and posterior planes between the low and high myopic groups. The surface roughness of the high myopic group was higher than that of the low myopic group in the peripheral part (P < .01). There were no significant differences among the three cylinder-specific groups. There was no significant correlation between two paired eyes. A positive correlation between posterior central surface roughness and the percentage tissue altered score (PTA) was found (r = 0.248, P = .012).

CONCLUSIONS

The cutting surface of the corneal lenticule performed by femtosecond laser was evaluated with a white light interferometer and displayed inhomogeneity. Deeper cutting with higher myopia and some intraoperative complications, such as suction loss, may result in larger irregularities.

Development of low laser energy levels in small-incision lenticule extraction: clinical results, black area, and ultrastructural evaluation

PURPOSE

To evaluate the outcomes of 4 low laser energy levels after small-incision lenticule extraction (SMILE) surgery.

SETTING

Zhongshan Ophthalmic Center, Guangzhou, China.

DESIGN

Prospective randomized clinical trial.

METHODS

This study evaluated consecutive patients who had SMILE to correct myopia or myopia with astigmatism. Eyes were placed into groups based on the laser energy used during surgery (ie, 105 nJ, 110 nJ, 115 nJ, or 120 nJ). All patients had a thorough ophthalmic examination preoperative and at 4 timepoints over 3 months postoperatively. Black areas and surface regularity of the extracted lenticules were observed and evaluated qualitatively and quantitatively.

RESULTS

The study comprised 124 eyes of 62 patients (40 women, 22 men), with 31 eyes in each laser energy group. The incidence of black areas was 45.16% (14 of 31 eyes), 12.90% (4 of 31 eyes), 16.13% (5 of 31 eyes), and 12.90% (4 of 31 eyes) for 105 nJ, 110 nJ, 115 nJ, and 120 nJ, respectively. The mean time for lenticule creation was the longest in the 105 nJ group (P = .015). The greatest increase in corneal thickness postoperatively occurred with 105 nJ (P < .05). Regression was highest in the 105 nJ group at 3 months (P < .01). However, corneal horizontal coma (C8) was lowest in the 105 nJ group at 1 week (P = .032). The lenticular surface in the 110 nJ group was the smoothest (P = .011). All contrast sensitivity values varied with time and recovered to preoperative levels by 1 week or 1 month. In all eyes, the uncorrected distance visual acuity and corrected distance visual acuity were good, with no statistically significant differences between the 4 energy groups.

CONCLUSIONS

The 105 nJ group, in which the lowest energy was used, had the highest risk for black areas, serious postoperative corneal edema, and a significant healing response.

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

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.].

Paired eye-control study of unilateral opaque bubble layer in femtosecond laser assisted laser in situ keratomileusis

AIM

To investigate the predictive factors of the opaque bubble layer (OBL) compared to the fellow eye of same patients in FS200 femtosecond laser assisted laser in situ keratomileusis (FS-LASIK).

METHODS

This study consisted of 60 consecutive patients (120 eyes) with unilateral OBL during FS-LASIK. Eyes were divided into OBL (the OBL eyes) and OBL-free groups (the fellow eyes) based on the occurrence of OBL. The preoperative demographic data, refraction, keratometry, corneal astigmatism, pachymetry, intraocular pressure and intraoperative data including the outlet location of gas diffusing canal were collected. Conditional logistic regression analysis was performed to find the associated factors with OBL in the two groups by determining odds ratios (OR) and 95%CI.

RESULTS

The preoperative demographic data, mean spherical errors, mean K value, suction time, intraocular pressure and central cornea thickness were not significantly different between the two groups. The outlet location of gas diffusing canal (P<0.01, OR 7.16, 95%CI 2.53-20.32) and the corneal astigmatism (P=0.013, OR 0.13, 95%CI 0.03-0.65) were significantly associated with the incidence of OBL by multivariate logistic regression analysis. Visual acuity, efficacy, and safety were comparable between the two groups two months after surgery except for a slightly lower predictability value for the hard OBL eyes.

CONCLUSION

The reduction of the incidence of OBL is obvious when the outlet of gas diffusing canal located at the posterior border of the corneoscleral limbus. This is probably consequent to more effectiveness of gas diffusing canal. Corneal astigmatism is also an independent protective factor for OBL formation.

Aborted small-incision lenticule extraction resulting from false plane creation and strategy for subsequent removal based on corneal layered pachymetry imaging

We describe a complication of false plane creation during small-incision lenticule extraction (SMILE) and the surgical plan for subsequent lenticule removal. During a primary SMILE procedure to treat high myopia, the separator instrument entered a false lamellar plane as a result of an area of resistance caused by an area of opaque bubble layer (OBL). The procedure was aborted to avoid removing an irregular lenticule. Based on measurements of the anatomic landscape, a new inferonasal small incision was created. The lenticule was separated and removed without further incidence. The patient recovered as normal and at 6 months, the uncorrected distance visual acuity was 20/16^-1. This case highlights the importance of monitoring the bubble layer creation and interface separation to avoid creating or removing an irregular lenticule. It also shows the importance of layered corneal imaging to analyze and diagnose complications as well as of aborting a procedure and planning lenticule removal at a later time if deemed appropriate.