As one of the major intraoperative complications of femtosecond laser–assisted lamellar refractive surgeries, opaque bubble layer (OBL) has aroused widespread concern.1–3 The OBL can be explained as temporary “debris” as a result of the intracorneal femtosecond laser photodisruption, which progressively generates gas bubbles that cannot escape in a timely manner.4 These gas bubbles generally do not last for a long time after the femtosecond laser photodisruption; however, they sometimes diffuse into the corneal stroma, subconjunctival space, and anterior chamber.3 Also, a dense OBL may interfere with the femtosecond laser photodisruption, flap lift, subsequent iris recognition, and tracking in femtosecond laser–assisted LASIK (FS-LASIK),5 and with the laser photodisruption, separation of corneal tissue, and subsequent steps in small incision lenticule extraction (SMILE). Several risk factors for OBL have been reported with FS-LASIK6–8; however, to our knowledge, there is only one study evaluating risk factors for OBL for SMILE.9
This study evaluated the independent effect of myopia and astigmatism on the risk of OBL occurrence in SMILE and further analyzed the relationship between scanning depth and OBL. We hypothesized that lower corrections with a shallower photodisruption plane would be independent risk factors for intraoperative OBL.
Patients and Methods
Study Design and Patients
We conducted a case–control study nested in the database of the Refractive Surgery Center in Tianjin Eye Hospital, Tianjin Medical University, Tianjin, China. This was a longitudinal population-based database from April 2015 to July 2016 and a total of 3,004 eyes that had undergone SMILE for myopia and astigmatism correction were included. Twenty-two of 3,004 eyes had OBL occur during the operation. Because the scanning depth is the sum of the cap thickness and the lenticular thickness, we only included the same cap thickness (120 μm) so that variations in the lenticular thickness, which are dependent on the intended correction, would represent the scanning depth. For all cases, 317 controls who underwent surgery on the same days as the cases were selected.
Inclusion criteria were age of at least 18 years, corrected distance visual acuity of 20/25 or better and a stable refraction in the past 2 years, myopic spherical correction of less than 10.00 diopters (D), myopic astigmatism correction of less than 4.00 D, cap thickness of 120 μm, and intraocular pressure between 10 and 21 mm Hg. Patients with rigid and soft contact lenses were required to cease wear for at least 4 and 2 weeks, respectively, before the surgery. Exclusion criteria were: active ocular disease, history of ocular surgery or ocular trauma, and keratoconus on corneal topography.
OBL in SMILE occurs in two planes.10 When OBL occurred at the posterior plane of the lenticule, we defined it as phase I. Phase II was defined as when OBL occurred at the anterior plane of the lenticule. Phase I was subclassified into four levels according to the maximum covered distance: “+” if it reached 0.5 mm, “++” if it reached 0.5 to 1 mm, “+++” if it reached 1 to 1.5 mm, and “++++” if it reached beyond 1.5 mm of the inside of the lenticular edge. Considering the different scanning mode and the pattern of manifestation, OBL occurring in phase II (anterior plane) was just defined as central and diffuse OBL. All of the eyes in this study were manually reviewed by two different observers (JM, RD) masked to the preoperative refractive status.
All surgeries were performed by the same surgeon (YW). Preoperative medications included 0.5% levofloxacin eye drops (Cravit; Santen, Osaka, Japan) four times a day for 3 days. Intraoperatively, 0.4% oxybuprocaine hydrochloride eye drops (Benoxil; Santen) were used for topical anesthesia. The VisuMax femtosecond laser platform (Carl Zeiss Meditec AG, Jena, Germany) (500 kHz) was used to create the refractive lenticule. The laser settings are shown in Table A (available in the online version of this article). The posterior of the stromal lenticule was scanned from the periphery to the central cornea. The anterior plane of the lenticule was subsequently created from the center to the periphery, which extended toward the corneal surface to create a 3-mm incision located at the 12-o'clock position, from which the lenticule was extracted. A blunt spatula was used to separate the stromal lenticule, and the surgeon then grasped the lenticule with a pair of forceps and removed it. Postoperative medications included 0.5% levofloxacin eye drops four times a day for 3 days and 0.1% fluorometholone eye drops (Santen) four times a day. The fluorometholone eye drops were in tapering doses over a period of 2 months.
Baseline Characteristics of the Study Participants
All analyses were performed using Empower (X&Y solutions, Inc., Boston, MA; http://www.empowerstats.com) and R (The R Foundation; http://www.Rproject.org) software. Descriptive statistics were used to summarize baseline characteristics. The logistic regression analyses were conducted after adjusting the confounding factors for analyzing the independent relationship between myopia, astigmatism, and the risk of intraoperative OBL. The identity regression analyses were performed after adjusting the variables for analyzing the independent association between myopia, astigmatism, and lenticular thickness. Multivariate adjusted odds ratio (OR) (95% confidence interval [CI]) and β (95% CI) for the associations between independent and dependent variables were assessed using generalized estimating equations, which take into account the correlation between the measurements from two eyes. A P value of less than .05 was considered statistically significant.
From April 2015 to July 2016, a total of 3,004 eyes had undergone SMILE for myopia and astigmatism correction. There were 22 eyes that had OBL during the surgery, including 10 (45.5%) eyes from males and 12 (54.5%) from females. The baseline characteristics of cases and controls are listed in Table A. Age, myopia, astigmatism, flat keratometry, CCT, and lenticule thickness were significantly different between the two groups preoperatively. The 3-month uncorrected distance visual acuity (UDVA) of cases and controls was 20/16 (range: 20/20 to 20/13) and 20/16 (range: 20/32 to 20/10), respectively.
All 22 eyes had OBL at both the posterior lenticular plane (phase I) and the anterior lenticular plane (phase II). The degree of + to ++++ accounted for 1 (4.6%), 6 (27.3%), 13 (59.1%), and 2 (9.1%) eyes, respectively. For phase II, 18 eyes (81.8%) had central OBL and 4 eyes (18.2%) had diffuse OBL.
Univariate analyses are shown in Table 1. Smooth curve fitting (Figure 1) was performed after the adjustment of relevant confounding factors, which were central corneal thickness (CCT), astigmatism, gender, flat keratometry, and energy for myopia and CCT, myopia, axial direction, and energy for astigmatism. Although the resultant curve demonstrated a curvilinear association of the risk of intraoperative OBL and the preoperative myopic spherical diopter, the horizontal portion of the curve (almost no risk of OBL) was due to the fact that there were no cases of OBL in this range (sphere > −6.00 D, astigmatism > 1.50 D). Figure 1 clearly illustrates that the risk of OBL was significantly decreased with the increasing myopic and astigmatic diopter.
Univariate Analysis of Possible Influencing Factors of the Risk of Intraoperative OBL and Lenticular Thickness
The smooth curve fitting showed the association between opaque bubble layer (OBL) and myopia and astigmatism diopter after adjusting the relative confounding factors, which were central corneal thickness, astigmatism, gender, flat keratometry, and age for myopia and central corneal thickness, myopia, axial direction, energy, and age for astigmatism. Dotted lines represented the upper and lower 95% confidence intervals. D = diopters
Multivariate regression analysis was used to explore the quantified relationship between myopia, astigmatic diopter, and the risk of OBL. The results of the multivariate analysis suggested that myopia and astigmatism remained significantly associated with risk of intraoperative OBL (P < .05) (Table 2) after the adjustment of the confounding factors. Covariate screening was analyzed using computer software. The screening criteria I included risk factors judged by clinical significance, and the screening criteria II included risk factors producing a greater than 10% change in the regression coefficient after introduction into the basic model. Myopic correction diopter, CCT, and astigmatism met criteria I and CCT, astigmatism, gender, flat keratometry, and age met criteria II. For astigmatism, CCT, and spherical diopter met criteria I and CCT, spherical diopter, axial direction, energy, and age met criteria II.
Multivariate Regression Analysis of Preoperative Diopters With the Risk of Intraoperative OBL and Lenticular Thickness
Multivariate regression analysis was also used to explore the relationship between diopter and lenticular thickness and reported in Table 2.
To our knowledge, the only published study about the risk factors of OBL in SMILE demonstrated that eyes with a thicker cornea or a thinner lenticule were more likely to develop OBL during SMILE.9 However, this conclusion was based on a univariate analysis and did not probe the data further for possible confounding factors. Because the preoperative parameters may be related and cause interference in the model, we analyzed the independent association between preoperative diopter and the risk of intraoperative OBL with multiple regression analysis after the adjustment of potential confounding factors and further analyzed the relationship between scanning depth and OBL to reduce the risk from changing the scanning depth.
The temporary accumulation of gas bubbles in the intrastromal interface creates transient opacity known as the OBL.6 However, there is no uniform standard for OBL classification, especially in SMILE. Previous studies have illustrated two different types of OBL in femtosecond laser–assisted LASIK.11–13 “Soft,” “delayed,” or “diffuse” OBLs have a more pellucid shape and often occur later, after accomplishing the laser photodisruption in a particular area. “Hard,” “early,” or “advancing” OBLs always appear earlier and have a denser shape.14 This definition may not fit the OBL in SMILE because we observed that OBL often had a diffuse but early appearance at the posterior plane of the lenticule and a compact but late appearance at the anterior plane of the lenticule. After LASIK, some authors13 have calculated the area of OBL using a software algorithm, which may not be suitable for SMILE because of the biplanar nature of the photodisruption pattern, which could confuse a computer analysis. This approach has been used in SMILE,9 but only the posterior plane was analyzed. Because OBL can affect both the posterior and anterior planes of photodisruption, our team proposes a new classification for OBL after SMILE: phase I (posterior plane, + to ++++) and phase II (anterior, central, and diffuse).10 The classification in our study placed more prominence on bubbles reaching to or beyond 0.5 mm of the lenticular edge because these OBLs may have a more practical effect on the subsequent surgical steps. As a matter of fact, separation of the lenticule in OBLs was more difficult than others, although there was no significant difference of 3-month UDVA between cases and controls in this study. Theoretically, some OBL developing at the posterior direction plane may interfere with laser spots directed to the anterior plane dissection, and this fact may increase the chances of retained fragments of the lenticule and further failed the operation. Thus, although there were no relative complications of OBLs in this study because of the practiced surgeon, the study of OBL risk factors still has important clinical significance, especially for inexperienced surgeons.
Smooth curve fitting (Figure 1) was performed after the adjustment of relevant confounding factors and it clearly illustrates that the risk of OBL was significantly decreased with the increasing myopia and myopic astigmatism. Although it was not observed in this study, it is possible that eyes with high myopia can have OBL, but the true risk is likely lower. The OR for OBL per diopter of myopia is 0.44, indicating the risk of OBL is reduced by 56% with each increase in diopter of myopic correction. Likewise, the OR of 0.1 for astigmatism in the final model indicates that the risk of OBL will be reduced by 90% for each diopter increase in astigmatism. We believe that the difference in risk reduction for OBL between these two factors is accounted for by the lenticule shape difference. The larger difference between the flat and steep keratometry values in astigmatic corrections may be more conducive to allowing gas to escape, making OBL less likely to occur. Because photodisruption shape may have an effect on the risk of OBL in SMILE,15 optimization of the lenticule shape may lead to less OBL and better outcomes. Wu et al.11 reported that the cone modification technique (larger flap diameter) was associated with a lower risk of OBL formation, even in eyes with significant risk factors for OBL using the original technique. These may also indicate that we could reduce OBL occurrence in SMILE by redesigning the photodisruption profile of the VisuMax laser or other parameters (vacuum of the suction pump, novel cap ventilation canal parameters, etc.) to facilitate the evacuation of the gas from the photodisruption area.
Because OBL occurred during the laser photodisruption, we considered that it may be related to scanning depth. In this study, the regression coefficient of refraction and lenticule thickness in three regression models was approximately 15 μm, which means the photodisruption plane will deepen by 15 μm per diopter of myopic or astigmatic correction. Vestergaard et al.16 reported that the mean decrease in CCT was 106 μm in their SMILE-treated eyes, which is equivalent to approximately 14 μm/diopters corrected and is consistent with our results. To assess the relationship between OBL and scanning depth strictly from the perspective of refraction, we selected patients with the same cap thickness (120 μm). Thus, we can indicate further from the OR and the regression coefficient (β) in this study that the risk of OBL will reduce 56% to 90% with photodisruption plane deepening of 15 μm. Morishige et al.17,18 assessed corneal stromal collagen organization using second harmonic signals combined with three-dimensional reconstruction and illustrated that human corneal collagen has two patterns of lamellar organization: highly interwoven in the anterior stroma and orthogonally arranged in the posterior stroma. Tightly arranged collagen in the anterior stroma may produce more resistance for bubble clearance and therefore increase the risk of OBL. Theoretically, we could possibly appropriately increase the cap thickness to reduce the risk of OBL in patients with lower corrections. Liu et al.19 found that a thicker cap would lead to a lower level corneal wound healing response after SMILE, which also supports a deeper scanning depth.
However, it is noteworthy that the premise should be to consider the safety of the cornea when we recommend a deeper scanning depth to reduce the risk of OBL. Previous studies20,21 had reported that percent tissue altered (PTA = [cap thickness + ablation depth] / CCT preoperatively) more than 35 to 40 may be strongly associated with ectasia after LASIK. There were also a handful of reports of ectasia developing after SMILE,22–24 but most of these cases have exhibited abnormal preoperative topographic patterns and these may alert surgeons to screen more carefully preoperatively.25 The PTA in patients who had mild refractive errors with 120 μm was far less than 35 and it may not be beyond the safety range of PTA in SMILE, although there was no evidence about the relative safety range of PTA in SMILE. The result of this study in which the risk of OBL will reduce 56% to 90% with photodisruption plane deepening of 15 μm also indicated that it may be an effective way to reduce the risk. Nevertheless, some studies26 reported that there was almost no significant difference of biomechanic changes between FS-LASIK and SMILE, whereas some27 still thought there were differences. With the development of measurement technology and the attention of biomechanics in refractive surgery, it still needs further investigation.
There are limitations to this study. Although we consider our OBL classification scheme to be more clinically relevant, it is not as precise as the area calculation using software. Also, our study had a case–control design and as such was dependent on the composition of the control population matching that of the general population of patients undergoing SMILE. However, our controls were selected from a well-defined cohort, reducing the possibility of selection bias. Additionally, our cases and controls were manually reviewed by two different observers blinded to the preoperative refractive status, which reduced the observational bias.
Mild myopia or astigmatism is an independent risk factor of intraoperative OBL in SMILE and the effect of astigmatism on OBL may be greater than that of myopia. Given the positive correlation between scanning depth and the amount of intended correction, these results may suggest that surgeons could reduce the risk of OBL by appropriately deepening the photodisruption plane, especially for patients who have mild refractive errors. This has the potential to have a significant clinical impact on the ability of surgeons to perform SMILE more effectively.
- Mastropasqua L, Calienno R, Lanzini M, Salgari N, De Vecchi S, Mastropasqua R, Nubile M. Opaque bubble layer incidence in Femtosecond laser-assisted LASIK: comparison among different flap design parameters. Int Ophthalmol. 2017;37:635–641. doi:10.1007/s10792-016-0323-3 [CrossRef]
- Marino GK, Santhiago MR, Wilson SE. OCT Study of the femtosecond laser opaque bubble layer. J Refract Surg. 2017;33:18–22. doi:10.3928/1081597X-20161027-01 [CrossRef]
- Lifshitz T, Levy J, Klemperer I, Levinger S. Anterior chamber gas bubbles after corneal flap creation with a femtosecond laser. J Cataract Refract Surg. 2005;31:2227–2229. doi:10.1016/j.jcrs.2004.12.069 [CrossRef]
- Hurmeric V, Yoo SH, Fishler J, Chang VS, Wang J, Culbertson WW. In vivo structural characteristics of the femtosecond LASIK-induced opaque bubble layers with ultrahigh-resolution SD-OCT. Ophthalmic Surg Lasers Imaging. 2010;41(suppl):S109–S113. doi:10.3928/15428877-20101031-08 [CrossRef]
- Consultation section. A dense opaque bubble layer (OBL) appeared to interfere with the laser dissection. J Cataract Refract Surg. 2009;35:1647–1649.
- Courtin R, Saad A, Guilbert E, Grise-Dulac A, Gatinel D. Opaque bubble layer risk factors in femtosecond laser-assisted LASIK. J Refract Surg. 2015;31:608–612. doi:10.3928/1081597X-20150820-06 [CrossRef]
- 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]
- Liu CH, Sun CC, Hui-Kang Ma D, et al. Opaque bubble layer: incidence, risk factors, and clinical relevance. J Cataract Refract Surg. 2014;40:435–440. doi:10.1016/j.jcrs.2013.08.055 [CrossRef]
- Son G, Lee J, Jang C, Choi KY, Cho BJ, Lim TH. Possible risk factors and clinical effects of opaque bubble layer in small incision lenticule extraction (SMILE). J Refract Surg. 2017;33:24–29. doi:10.3928/1081597X-20161006-06 [CrossRef]
- Ma J, Wang Y, Li L, Zhang J. Corneal thickness, residual stromal thickness, and its effect on opaque bubble layer in small-incision lenticule extraction [published online ahead of print August 8, 2017]. Int Ophthalmol. doi:10.1007/s10792-017-0692-2 [CrossRef].
- Wu N, Christenbury JG, Dishler JG, Bozkurt TK, Duel D, Zhang L, Hamilton DR. J Refract Surg. 2017;33:584–590. doi:10.3928/1081597X-20170621-06 [CrossRef]
- Soong HK, Malta JB. Femtosecond lasers in ophthalmology. Am J Ophthalmol. 2009;147:189–197. doi:10.1016/j.ajo.2008.08.026 [CrossRef]
- Kanellopoulos AJ, Asimellis G. Digital analysis of flap parameter accuracy and objective assessment of opaque bubble layer in femtosecond laser-assisted LASIK: a novel technique. Clin Ophthalmol. 2013;7:343–351. doi:10.2147/OPTH.S39644 [CrossRef]
- Sutton G, Hodge C. Accuracy and precision of LASIK flap thickness using the IntraLase femtosecond laser in 1000 consecutive cases. J Refract Surg. 2008;24:802–806.
- Gatinel D, Hoang-Xuan T, Azar DT. Three-dimensional representation and qualitative comparisons of the amount of tissue ablation to treat mixed and compound astigmatism. J Cataract Refract Surg. 2002;28:2026–2034. doi:10.1016/S0886-3350(02)01379-2 [CrossRef]
- Vestergaard AH, Grauslund J, Ivarsen AR, Hjortdal JØ. Central corneal sublayer pachymetry and biomechanical properties after refractive femtosecond lenticule extraction. J Refract Surg. 2014;30:102–108. doi:10.3928/1081597X-20140120-05 [CrossRef]
- Morishige N, Petroll WM, Nishida T, Kenney MC, Jester JV. Noninvasive corneal stromal collagen imaging using two-photon-generated second-harmonic signals. J Cataract Refract Surg. 2006;32:1784–1791. doi:10.1016/j.jcrs.2006.08.027 [CrossRef]
- Morishige N, Wahlert AJ, Kenney MC, et al. Second-harmonic imaging microscopy of normal human and keratoconus cornea. Invest Ophthalmol Vis Sci. 2007;48:1087–1094. doi:10.1167/iovs.06-1177 [CrossRef]
- Liu M, Zhou Y, Wu X, Ye T, Liu Q. Comparison of 120- and 140-μm SMILE cap thickness results in eyes with thick corneas. Cornea. 2016;35:1308–1314. doi:10.1097/ICO.0000000000000924 [CrossRef]
- Santhiago MR, Giacomin NT, Smadja D, Bechara SJ. Ectasia risk factors in refractive surgery. Clin Ophthalmol. 2016;20:713–720. doi:10.2147/OPTH.S51313 [CrossRef]
- Santhiago MR, Smadja D, Wilson SE, Krueger RR, Monteiro ML, Randleman JB. Role of percent tissue altered on ectasia after LASIK in eyes with suspicious topography. J Refract Surg. 2015;31:258–265. doi:10.3928/1081597X-20150319-05 [CrossRef]
- Sachdev G, Sachdev MS, Sachdev R, Gupta H. Unilateral corneal ectasia following small-incision lenticule extraction. J Cataract Refract Surg. 2015;41:2014–2018. doi:10.1016/j.jcrs.2015.08.006 [CrossRef]
- Mastropasqua L. Bilateral ectasia after femtosecond laser-assisted small-incision lenticule extraction. J Cataract Refract Surg. 2015;41:1338–1339. doi:10.1016/j.jcrs.2015.05.013 [CrossRef]
- Wang Y, Cui C, Li Z, et al. Corneal ectasia 6.5 months after small-incision lenticule extraction. J Cataract Refract Surg. 2015;41:1100–1106. doi:10.1016/j.jcrs.2015.04.001 [CrossRef]
- Randleman JB. Ectasia after corneal refractive surgery: nothing to SMILE about. J Refract Surg. 2016;32:434–435. doi:10.3928/1081597X-20160613-01 [CrossRef]
- Sefat SM, Wiltfang R, Bechmann M, Mayer WJ, Kampik A, Kook D. Evaluation of changes in human corneas after femtosecond laser-assisted LASIK and small-incision lenticule extraction (SMILE) using non-contact tonometry and ultra-high-speed camera (Corvis ST). Curr Eye Res. 2016;41:917–922. doi:10.3109/02713683.2015.1082185 [CrossRef]
- Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg. 2013;29:454–460. doi:10.3928/1081597X-20130617-03 [CrossRef]
Univariate Analysis of Possible Influencing Factors of the Risk of Intraoperative OBL and Lenticular Thickness
|OR (95% CI)||P||β (95% CI)||P|
| Females||1.89 (0.79, 4.51)||.1502||8.45 (2.73, 14.16)||.0040|
|Age (y)||1.08 (1.01, 1.15)||.0181||−0.50 (−1.03, 0.02)||.0593|
|CCT (μm)||1.02 (1.01, 1.03)||.0012||0.09 (0.00, 0.18)||.0501|
|Myopia (D)||0.60 (0.44, 0.81)||.0008||14.83 (14.18, 15.48)||< .0001|
|Astigmatism (D)||0.19 (0.06, 0.64)||.0070||13.07 (8.91, 17.22)||< .0001|
|Axial direction (°)||1.00 (0.99, 1.01)||.8314||0.05 (0.01, 0.09)||.0098|
|Flat keratometry (D)||1.19 (0.88, 1.61)||.2656||3.28 (1.30, 5.27)||.0013|
|Steep keratometry (D)||1.03 (0.79, 1.35)||.8273||4.35 (2.65, 6.05)||< .0001|
|Energy (nJ)||1.07 (0.77, 1.48)||.6846||−1.97 (−4.13, 0.19)||.0743|
Multivariate Regression Analysis of Preoperative Diopters With the Risk of Intraoperative OBL and Lenticular Thickness
|Parameter||OBL Group||Lenticular Thickness Group|
|OR (95% CI)||P||β (95% CI)||P|
| Non-adjusted||0.60 (0.45, 0.79)||.0004||14.83 (14.13, 15.53)||< .0001|
| Adjust I||0.53 (0.38, 0.73)||.0001a||14.86 (14.56, 15.15)||< .0001b|
| Adjust II||0.44 (0.30, 0.64)||< .0001c|
| Non-adjusted||0.19 (0.07, 0.53)||.0016||13.07 (9.04, 17.09)||< .0001|
| Adjust I||0.17 (0.06, 0.46)||.0006d||15.10 (13.96, 16.24)||< .0001e|
| Adjust II||0.10 (0.02, 0.42)||.0017f|
Baseline Characteristics of the Study Participantsa
|Characteristics||Cases (n = 22)||Controls (n = 317)||P|
|Males, no. (%)||10 (45.50%)||194 (61.20%)||.18|
|Age (y)||25.45 ± 6.09||22.56 ± 5.30||.02|
| Myopia (D)||−3.57 ± 1.08||−4.82 ± 1.66||< .01|
| Astigmatism (D)||−0.32 ± 0.27||−0.70 ± 0.66||< .01|
| Axial direction, median (range)||17.50 (0.00 to 180.00)||25.00 (0.00 to 178.00)||.83|
| Steep keratometry (D)||43.82 ± 1.41||43.75 ± 1.62||.83|
| Flat keratometry (D)||42.84 ± 1.34||42.49 ± 1.41||.27|
| CCT (μm)||581.09 ± 26.16||557.69 ± 31.32||< .01|
| Corneal radius (mm)||7.80 ± 0.25||7.84 ± 0.27||.48|
| Incision width (mm)||3.02 ± 0.04||3.01 ± 0.03||.16|
|Myopia (%)||< .01|
| < 3.00 D||6 (13.60%)||38 (86.40%)|
| 3.00 to < 6.00 D||16 (7.50%)||197 (92.50%)|
| ≥ 6.00 D||0 (0.00%)||82 (100.00%)|
| < 1.50 D||22 (7.40%)||276 (92.60%)|
| ≥ 1.50 D||0 (0.00%)||41 (100.00%)|
| Size (mm)||7.66 ± 0.12||7.64 ± 0.08|
| Cap thickness (μm)||120.00 ± 0.00||120.00 ± 0.00|
| Optical zone (mm)||6.60 ± 0.18||6.55 ± 0.10||.34|
| Lenticule thickness (μm)||76.09 ± 18.23||100.26 ± 26.35||< .01|
|Energy, mean (nJ)||141.14 ± 7.23||140.55 ± 6.49||.69|
|Preoperative UDVA (range)||20/200 (20/400 to 20/50)||20/200 (20/2000 to 20/25)||.30|