Various reasons have been attributed to the relatively slower visual recovery after small incision lenticule extraction (SMILE) compared to laser in situ keratomileusis (LASIK),1,2 such as experience of the surgeon, surgical technique, degree of manipulation, and energy settings.3–6 Energy optimization is considered to be a crucial factor determining the visual recovery following SMILE, and recent reports have suggested better visual outcomes with the use of lower energies.7 However, energy optimization is a stepwise approach, in which the surgeon needs to find the optimum energy at which the laser delivers an ideal bubble pattern with minimal opaque bubble layer, thereby facilitating smooth tissue dissection and potentially resulting in better first day postoperative visual results. The end point of energy optimization is usually subjective, and there is no clinical sign described to help a surgeon decide if an ideal bubble pattern has been achieved or not.
We describe a new intraoperative sign called the “gas bubble escape” (GBE) sign, which may signify a perfect bubble pattern and also predict the immediate postoperative quality of vision in patients undergoing SMILE. The current study was undertaken to study the incidence of the GBE sign and to compare the immediate visual recovery between eyes showing this sign versus those in whom this sign could not be appreciated.
Patients and Methods
This prospective study included 100 eyes from 50 consecutive patients undergoing bilateral SMILE surgery for correction of myopia or myopic astigmatism. The study was approved by our institutional ethics committee and adhered to the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients regarding participation in the study.
Inclusion criteria were age between 21 and 40 years, myopic refractive error within −3.00 to −6.00 diopters (D), spherical equivalent (SE) with astigmatism up to −1.00 D, stable refraction (< 0.50 D change in past 12 months), corrected distance visual acuity (CDVA) of 20/20 or better, healthy ocular surface, absence of corneal ectatic diseases or corneal scars, absence of any retinal pathology likely to affect visual outcomes, and completed follow-up visits.
Eyes with thin corneas (central corneal thickness < 480 µm), diagnosed or suspicious cases of corneal ectatic conditions, severe dry eyes, and contact lens–induced allergy, patients taking systemic steroids, immune suppressants, oral contraceptives, or antidepressants, and pregnant women were excluded from the study.
All patients underwent a thorough preoperative evaluation including anterior and posterior segment examination, cycloplegic and subjective refraction, assessment of uncorrected distance visual acuity (UDVA) and CDVA, corneal topography using the Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany) and Orbscan II (Bausch & Lomb, Rochester, NY), aberrometry (Hoya iTrace surgical workstation; Hoya, Tokyo, Japan), dry eye evaluation (Schirmer 1 and tear film break-up time tests), and the Optical Quality Analysis System (HD Analyzer; Visiometrics, Cerdanyola de Vallès, Spain) for optical quality of vision.
Patients using soft and rigid contact lenses were instructed to discontinue their lenses at least 1 and 3 weeks, respectively, prior to the topographic evaluation.
Description of the Sign
It is proposed that once the energy parameters and spot spacing of the femtosecond laser are optimized, the laser would deliver a uniform bubble pattern on the cornea with no or minimal opaque bubble layer or black spots. Immediately after the laser delivery, the corneal interface would appear nearly opaque, due to accumulation of the femtosecond laser bubbles within the planes of the lenticule. However, once the planes of the lenticule are accessed, the surgeon may visualize the escape of gas bubbles, followed by an immediate generalized clearing of the interface. This escape of bubbles, combined with a change in the morphology of the interface from a nearly opaque to a more transparent one, was referred to as the GBE sign (Video 1, available in the online version of this article). However, escape of gas bubbles without a change in interface appearance was referred to as partial GBE sign. The eyes in which there was neither escape of gas bubbles nor change interface appearance were classified as no GBE sign.
All surgeries were performed by a single experienced refractive surgeon (SG) under topical anesthesia using the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany), with a pulse repetition rate of 500 kHz, cut energy of 140 to 150 nJ, spot separation of 4.5 µm, cap thickness of 120 µm, 6.5- to 7-mm optical zone, and 2-mm superior incision. The treatment was centered on the visual axis.
The pointed side of the Reinstein separator (Malosa Medical, Elland, United Kingdom) was used to access the interface and delineate the planes of the lenticule. At this point, the surgeon carefully looked for the escape of bubbles through the incision and any immediate change in the appearance of the interface, and a note of the presence of GBE/partial GBE/no GBE sign was made. The surgeon then dissected the planes of the lenticule using the blunt spatulated side of the Reinstein separator. The superficial plane was dissected first, followed by the deeper plane. Once the lenticule was separated, it was extracted using the same instrument. At the end of the surgery, a minimal interface wash with balanced salt solution was given. Postoperative medications included 1% prednisolone acetate eye drops (Pred Forte; Allergan India Pvt. Ltd., Bangalore, India) four times a day, tapering over 4 weeks; 0.3% tobramycin eye drops (Toba; Sun Pharmaceutical Industries Ltd., Mumbai, India) three times a day for 1 week; and 0.4% polyethylene glycol 400 and 0.3% propylene glycol (Systane Ultra; Alcon Laboratories, Inc., Fort Worth, TX) four times a day for 4 weeks.
Postoperative examinations were carried out at 1 day and 1 month, which included assessment of UDVA, CDVA, subjective refraction, higher order aberrations, and optical quality evaluation using the OQAS double- pass system.8–10 This equipment provides an Objective Scatter Index (OSI) score, which is an objective measure of the patient's optical quality of vision. This index is computed by evaluating the intraocular light scatter using point spread function and thereby helps in analyzing the scatter produced by the interface in patients after SMILE.11 A higher score indicates poor visual quality.12,13 It also provides the modulation transfer function (MTF) cut-off value, which is quantified as a frequency at which MTF reaches a value of 0.01, which is proportionate to 1% contrast.12,13 MTF evaluates the ratio of contrast between the retinal image and the object; the higher the MTF, the better the contrast.
SPSS software for Windows (version 17.0.0; IBM Corporation, Armonk, NY) was used for statistical analysis. All values were expressed as mean ± standard deviation (SD). Data were checked for normality, and the paired t test was used for intragroup comparison of means. An independent t test was applied for intergroup comparison between the means. A P value of .05 or less was considered statistically significant.
A total of 100 eyes of 50 consecutive patients were included in the study, of which 68% of eyes showed classic GBE sign, 8% of eyes showed partial GBE sign, and 24% of eyes showed no GBE sign. Of the 50 patients, 40 patients demonstrated the same behavior bilaterally (GBE, partial GBE, or no GBE).
For the sake of simplicity of analysis, partial GBE eyes were grouped under the no GBE group. Thus, the no GBE group comprised 32 eyes (32%). Comparative analysis was performed between 32 eyes of the no GBE group and 32 randomly selected eyes of the GBE group (of the total 68 eyes).
Table 1 shows the baseline characteristics of both study groups. Preoperatively, there was no significant difference between the groups in terms of mean age, SE, cylinder, CDVA, corneal higher order aberrations, MTF cut-off, and OSI scores (P > .05, for all parameters).
Preoperative Baseline Characteristics
Postoperative Visual and Refractive Results
Day 1 cumulative UDVA analysis showed that all eyes in the GBE group had UDVA of 20/16 versus 20/20 in the no GBE group. A total of 12% eyes in the GBE group had UDVA better than 20/16, whereas no eye in the no GBE group had UDVA better than 20/16 (Figure 1).
Cumulative uncorrected distance visual acuity (UDVA) of eyes with and without gas bubble escape (GBE) sign at 1 day postoperatively.
On day 1, the mean UDVA and CDVA were significantly better in the GBE group compared to the no GBE group (P = .001 for UDVA, P = .01 for CDVA). However, at 1 month postoperatively there was no statistically significant difference for UDVA and CDVA between the two study groups (Table 2).
Postoperative 1 Day and 1 Month Visual and Refractive Outcomes
At 1 month, 84% of eyes in the GBE group had SE predictability within ±0.50 D versus 80% of eyes in the no GBE group (Figure 2). However, the mean SE was not significantly different between groups (P = .69; Table 2).
Spherical equivalent (SE) predictability in eyes with and without gas bubble escape (GBE) sign.
No eye lost lines of CDVA at 1 month of follow-up in either group. However, the GBE group saw a gain of one line in 28% of eyes versus the no GBE group, where only 8% of eyes gained one line of CDVA (Figure 3).
Safety at 1 month postoperatively in eyes with and without gas bubble escape (GBE) sign.
OSI and MTF Cut-off
On day 1, the mean OSI score was significantly lower in the GBE group (0.78) compared to the no GBE group (1.11) (P = .03, Figure 4). However, the mean MTF cut-off values did not show a significant difference at both the 1 day and 1 month postoperative follow-up (Table 2).
Comparison of mean Objective Scatter Index (OSI) scores between the gas bubble escape (GBE) and no GBE groups.
Postoperative Induced Aberrations
The mean corneal higher order aberrations measured with ray tracing at 4-mm scan size were also comparable at both postoperative 1 day and 1 month, with no statistically significant difference between both groups (Table 2).
No eye in either study group had any intraoperative or postoperative complications (eg, suction loss, lenticule tear, incision tear, epithelial defect, deep lamellar keratitis, or infection) affecting the visual outcomes.
Energy optimization of the femtosecond laser is a crucial factor that may potentially affect the immediate visual recovery after SMILE.7 We previously emphasized that the energy settings must be optimized to obtain an ideal bubble pattern, so that there is minimal resistance when the tissue is being separated. The bubble pattern should be free of black spots and opaque bubble layer, the former being associated with obstruction to the laser beam and low energies, whereas the latter is associated with higher energies. It is noteworthy that the optimal energy and spot settings vary between laser systems, so these values will not necessarily be optimal for all users.14
However, the process of energy optimization is a gradual process, wherein the surgeon tweaks the fluence of the laser to obtain the ideal bubble pattern. The GBE sign described above may help a surgeon to recognize this end point, especially after the installation or an overhaul of the femtosecond laser. Once the surgeon begins to visualize this sign more often, this may be an indication that the laser is delivering an optimal fluence, and no further optimization may be required.
The uniform bubble pattern obtained from an optimized laser creates minimum tissue bridges, leading to an uninterrupted escape of gas bubbles when the planes of the lenticule are accessed using the pointed side of the dissector (Figure 5). This is followed by an almost immediate, generalized clarity of the interface, potentially resulting in a smooth surgical dissection.2 However, when there is opaque bubble layer or black spots, these areas may entrap the gas bubbles, thus resisting their free escape (Figure 5). The appearance of the interface may also take longer to clear, which may be result in some amount of resistance to the dissection (Videos 2–3, available in the online version of this article).
Diagrammatic representation of the (A) gas bubble escape and (B) no gas bubble escape groups. OBL = opaque bubble layer
Even though the laser is optimized, one may not expect to observe this sign in all eyes. In the current study, 68% of eyes demonstrated the classic GBE sign, whereas the rest had either complete absence or showed the sign partially. This may be possibly explained by other factors that may affect the laser efficiency, including the tear film layer (coupling interface), which may vary from surgery to surgery, and the differences in the corneal density and biomechanics between different corneas. In our study, we observed that the fellow eye showed a similar behavior (GBE, partial GBE, or no GBE) as the first eye in the majority (40 of 50) of the patients, which may support that differences in corneal structural properties may be the determining factors for individual behavior. It may be interesting to study if there could be any differences between the eyes demonstrating this sign versus the eyes that do not demonstrate this sign intraoperatively, in terms of their preoperative corneal biomechanics and densitometry.
We would like to emphasize that, although we state that the GBE sign may be used to guide the optimization of energy settings of the laser, in the current study we did not use different energy parameters to test the correlation of the GBE sign. This is because our laser energy was already optimized at the time of the study and, thus, it was not deemed necessary to test different energy levels to see this correlation. The aim of the study was to compare the visual quality between the eyes that showed the sign and those that did not or partially showed this sign. Because all of the other variables (age, refractive error, method, time taken for preparation of the eyes, operating surgeon, surgical technique, and postoperative medications) were similar in both groups, the difference in the OSI score and other visual quality outcomes could be mainly attributed to the presence or absence of this sign.
One limitation of the study could be that the eyes were not randomly selected and there may be a potential bias in the results due to inclusion of both eyes of the same patient because they are expected to behave in a similar manner. However, there may be a preliminary evidence suggesting the significance of this sign, especially in relation to the immediate postoperative visual recovery, because the eyes showing this sign had significantly better OSI scores and UDVA at day 1 postoperatively. However, further research is recommended to study the correlation of this sign with ease of lenticule dissection, visual results, and individual corneal characteristics.
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- Jin Y, Wang Y, Xu L, Li H, Dou R, Zhang J. Comparison of the optical quality between small incision lenticule extraction and femtosecond laser LASIK. J Ophthalmol. 2016;2016:e2507973. doi:10.1155/2016/2507973 [CrossRef]
- Kamiya K, Shimizu K, Igarashi A, Kobashi H. Effect of femtosecond laser setting on visual performance after small-incision lenticule extraction for myopia. Br J Ophthalmol. 2015;99:1381–1387. doi:10.1136/bjophthalmol-2015-306717 [CrossRef]
- Yao P, Zhao J, Li M, Shen Y, Dong Z, Zhou X. Microdistortions in Bowman's layer following femtosecond laser small incision lenticule extraction observed by Fourier-domain OCT. J Refract Surg. 2013;29:668–674. doi:10.3928/1081597X-20130806-01 [CrossRef]
- Luo J, Yao P, Li M, et al. Quantitative analysis of microdistortions in Bowman's layer using optical coherence tomography after SMILE among different myopic corrections. J Refract Surg. 2015;31:104–109. doi:10.3928/1081597X-20150122-05 [CrossRef]
- Donate D, Thaëron R. Lower energy levels improve visual recovery in small incision lenticule extraction (SMILE). J Refract Surg. 2016;32:636–642. doi:10.3928/1081597X-20160602-01 [CrossRef]
- Güell JL, Pujol J, Arjona M, Diaz-Douton F, Artal P. Optical quality analysis system: instrument for objective clinical evaluation of ocular optical quality. J Cataract Refract Surg. 2004;30:1598–1599. doi:10.1016/j.jcrs.2004.04.031 [CrossRef]
- Saad A, Saab M, Gatinel D. Repeatability of measurements with a double-pass system. J Cataract Refract Surg. 2010;36:28–33. doi:10.1016/j.jcrs.2009.07.033 [CrossRef]
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Preoperative Baseline Characteristicsa
GBE (n = 32)
No GBE (n = 32)
||−4.125 ± 1.91
||−4.059 ± 1.73
||25.63 ± 3.12
||25.10 ± 2.99
||−0.725 ± 0.65
||−0.756 ± 0.82
||−0.038 ± 0.04
||−0.042 ± 0.04
||0.156 ± 0.08
||0.147 ± 0.06
|MTF cut-off (cpd)
||41.08 ± 8.90
||39.89 ± 9.12
|Optical zone (µm)
||6.76 ± 0.031
||6.76 ± 0.03
||0.33 ± 0.12
||0.34 ± 0.13
Postoperative 1 Day and 1 Month Visual and Refractive Outcomes
||−0.13 ± 0.36
||−0.16 ± 0.42
||−0.06 ± 0.41
||−0.08 ± 0.37
||−0.26 ± 0.34
||−0.24 ± 0.24
||−0.35 ± 0.28
||−0.28 ± 0.44
||−0.25 ± 0.40
||−0.22 ± 0.37
||−0.21 ± 0.38
||−0.19 ± 0.40
||−0.11 ± 0.05
||−0.07 ± 0.09
||−0.14 ± 0.05
||−0.12 ± 0.06
||−0.18 ± 0.10
||−0.15 ± 0.17
||−0.20 ± 0.08
||−0.18 ± 0.09
||0.78 ± 0.32
||1.11 ± 0.90
||0.53 ± 0.29
||0.76 ± 0.86
||37.98 ± 9.25
||36.80 ± 6.25
||41.23 ± 5.25
||39.82 ± 6.12
|HOA (RMS) (µm)
||0.45 ± 0.21
||0.48 ± 0.25
||0.36 ± 0.19
||0.38 ± 0.20