The femtosecond laser became available for LASIK surgery approximately 10 years ago. Creating the LASIK corneal flap with a femtosecond laser rather than with a mechanical microkeratome has yielded better safety and reproducibility of flap and flap-associated complications such as incomplete flap, button hole, and epithelial defects have been expected to be reduced.1,2
The advantages of surface ablation, including laser-assisted sub-epithelial keratectomy and photorefractive keratectomy, compared to LASIK have been delineated.3–5
These include long-term safety, rapid rate of corneal nerve regeneration associated with dry eye symptoms, and postoperative biomechanical stability associated with leaving sufficient stromal tissue.6,7
However, when performed properly in appropriate eyes, LASIK has several advantages over photorefractive keratectomy, such as faster visual recovery, less discomfort after surgery, and milder and more predictable wound healing with less risk for corneal stromal haze8
because it preserves corneal epithelium and epithelial basement membrane.
The goal of creating a thin flap is to maximize residual bed thickness. Sufficient residual stromal bed thickness is considered important in preventing corneal ectasia, and the femtosecond laser is designed to make thinner and more uniform flaps with less variability and better predictability in thickness. Subepithelial haze has been reported as a potential complication of thin-flap LASIK.9
Comparable outcomes of femtosecond LASIK with a 90- or 100-μm flap have been reported.10–13
The current study was undertaken to compare short-term clinical results including intraoperative and postoperative complications and reproducibility of flap thickness in patients with moderate to high myopia contralaterally treated with ultrathin 80- and 120-μm flap LASIK using the 200-KHz VisuMax femtosecond laser.
Patients and Methods
This prospective, randomized, contralateral study comprised patients who had LASIK for moderate to high myopia (range: −2.00 to −10.00 diopters [D]) at the Eye Clinic of Samsung Medical Center in Seoul, Korea, between July and December 2009. Institutional review board approval was obtained from the Samsung Medical Center Institutional Review Board, and all procedures adhered to the tenets of the Declaration of Helsinki.
All patients who qualified for LASIK and showed stable refraction for more than a year were enrolled and only myopic eyes with a plano target refraction and pre-operative manifest astigmatism of 3.50 D or less at the spectacle plane were included. All eyes were predicted to have postoperative residual stromal bed thickness of greater than 350 μm. Eyes with ocular pathologies such as keratoconus, corneal scars, corneal dystrophies, and previous ocular surgery were excluded.
All procedures were performed by the same experienced surgeon (E-SC) and the standardized nasal-hinged flaps were created with the 200-KHz VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany). All patients underwent 80-μm flap LASIK in one eye and 120-μm flap LASIK in the fellow eye in a random fashion. Randomization was performed by a computer-generated list.
The femtosecond laser parameters were 7.85- to 8.25-mm diameter determined by pupil sizes of patients, hinge angle of 45°, side cut angle of 55°, spiral pattern energy of 0.18 μJ (120-μm flap) and 0.155 μJ (80-μm flap), spot and track distances of 4.5 μm, and side cut energy of 0.18 μJ (120-μm flap) and 0.155 μJ (80-μm flap). The flap diameters on VisuMax are smaller than those published with other femtosecond lasers because VisuMax uses a curved contact glass rather than fully planar applanation. Ablation of the stromal bed was performed with the MEL 80 excimer laser (Carl Zeiss Meditec). Intraoperative complications, if any, and stromal bed evaluation were analyzed and recorded through an automatic recording system on VisuMax. An internal video camera is permanently installed on the surgical microscope and automatically records all surgical procedures showing the sequence of treatment. Because the video camera is automatically controlled by the system, it cannot be controlled manually and these data can be exported from the hard drive to data storage media.
Preoperative assessment included uncorrected and corrected distance visual acuity (UDVA and CDVA), slit lamp and fundus evaluation, manifest refraction, ultrasound pachymetry (SP-100 ultrasound; Tomey Corp., Nagoya, Japan), manual keratometry, wavefront analyzer (WASCA; Carl Zeiss Meditec), contrast sensitivity (VCTS 6500; VisTech Consultants, Inc., Dayton, OH), and topography (Orbscan II; Bausch & Lomb, Rochester, NY). All visual acuity measurements were performed using Snellen charts, and we did not measure them beyond 20/20.
Patients were reviewed at postoperative 1 day, 1 week, and 1, 3, and 6 months. All postoperative follow-up visits included measurement of UDVA, CDVA, and manifest refraction. At 1 week and 1 month postoperatively, all eyes were evaluated with Visante optical coherence tomography (OCT) (Carl Zeiss Meditec). The same independent examiner who was not involved in the analysis of flap thickness performed the high-resolution corneal scans for both eyes along a 45° (including 225°) and a 135° (including 315°) meridian. During analysis of the scanned image, measurement points were manually located on the center of the cornea (0 mm), 1.5 mm left and right from center, and 3.0 mm left and right from center, using the cursor. The horizontal measurement arm of the cursor was then moved up or down to the location of the flap interface. In this way, each flap’s thickness was measured at a total of 10 points (Figure A
, available in the online version of this article).
Figure A. (Top) Method of measuring corneal flap thickness with anterior segment optical coherence tomography (OCT). Flap thickness was measured at the flap center, 1.5-mm zone (inner circle), and 3.0-mm zone (intermediate circle) along a 45° (including 225°) meridian and a 135° (including 315°) meridian from the flap center, for a total of 10 points. (Bottom) Measurement of a 120-μm flap thickness setting at 1 week postoperatively.
At postoperative 1, 3, and 6 months, contrast sensitivity in photopic condition was checked. The clinical outcomes were examined by evaluating the efficacy, predictability, and safety calculated from the preoperative to the 6-month postoperative data. Efficacy was assessed by the percentage of eyes achieving postoperative UDVA of 20/20 or better and by determining the efficacy index, which is defined as the mean postoperative UDVA over the mean preoperative CDVA. Predictability was evaluated by assessing the percentage of eyes within 0.25 and 0.50 D of emmetropia. Safety was assessed by the change in the number of lines of CDVA from the preoperative level to the 6-month follow-up visit and by determining the safety index, which is defined as the mean postoperative CDVA over the mean preoperative CDVA.
Main outcomes including visual acuity, contrast sensitivity, and complications and achieved flap thickness were compared to intended flap thickness. All data were entered into the database using Microsoft Office Excel 2007 (Microsoft Corporation, Redmond, WA), and statistical analyses were performed using SPSS version 15.0 for Windows (SPSS, Inc., Chicago, IL). At each of the 10 measurement points of the individual flaps, the means and standard deviations were calculated and the mean ranges of each individual flap were obtained. The independent t, paired t, and chi-square tests and multiple logistic regressions were applied for analysis.
This study included 72 eyes of 36 patients (11 men and 25 women) with a mean age of 26.5 years (range: 18 to 40 years). Preoperative sphere ranged from −2.0 to −9.5 D and cylinder ranged from 0 to −3.25 D. There were no statistically significant differences in preoperative parameters between the two groups in relation to intended thickness (Table 1
Table 1: Preoperative Data of Eyes That Underwent LASIK With an Intended Flap Thickness of 80 and 120 μm
Visual Acuity and Residual Refractive Errors
Mean UDVA at postoperative 1 day was significantly better in the 120-μm flap group (100% 20/25 or better) than the 80-μm flap group (91.7% 20/25 or better) (P = .01). At 1 week postoperatively, UDVA was 20/25 or better in all eyes of both groups.
For efficacy, 100% of the 120-μm flap group and 95.8% of the 80-μm flap group had 20/20 or better UDVA, which showed no statistical difference between groups (P = .151), and all eyes treated had at least 20/25 UDVA (Figure 1
). For predictability, 97.1% of eyes in the 120-μm flap group and 100% of eyes in the 80-μm flap group were within ±0.25 D of emmetropia. There was no significant difference between groups (P = .314). For safety, all eyes in both groups maintained or gained one line of Snellen CDVA postoperatively. The safety and efficacy index improved in both groups during the follow-up periods (Figure 2
). No eye in either group lost one or more lines of CDVA.
Figure 1. Postoperative uncorrected visual acuities of patients according to flap thickness. All P values during postoperative follow-up periods (except for postoperative 1 day) > .05.
Figure 2. Postoperative visual indices of patients according to flap thickness. All P values during postoperative follow-up periods (except for postoperative 1 day) > .05.
Higher residual refractive errors occurred more frequently in the 80-μm flap group than in the 120-μm flap group at postoperative 1 day and 1 week, but this was not statistically significant (Figure 3
Figure 3. Residual refractive errors (spherical equivalents) of patients according to flap thickness. All P values during postoperative follow-up periods > .05.
There was no significant difference in higher-order aberrations between groups (P = .923, independent t test). All higher-order aberrations including coma (P = .683), trefoil (P = .885), and spherical aberration (P = .649) were not significantly different between groups.
There were no significant differences in the contrast sensitivities through all frequencies at photopic condition between the groups preoperatively and postoperatively. Compared to preoperative contrast sensitivity, there was a significant reduction at 6 cycles per degrees (CPD) in the 80-μm flap group at postoperative 1 month (P = .01) and 3 months (P = .01), but this difference disappeared at postoperative 6 months.
Anterior segment OCT (AS-OCT) analysis was performed in 24 patients (48 eyes) at postoperative 1 week and 20 patients (40 eyes) at postoperative 1 month in both groups. Mean achieved flap thickness was 84.94 ± 3.80 μm (range: 76 to 94 μm) and 83.46 ± 3.50 μm (range: 73 to 95 μm) at postoperative 1 week and 1 month, respectively, in the 80-μm flap group and 123.97 ± 3.16 μm (range: 114 to 135 μm) and 122.93 ± 3.55 μm (range: 107 to 131 μm) at postoperative 1 week and 1 month, respectively, in the 120-μm flap group (Table 2
Table 2: Flap Thickness at 10 Measurement Points in the 80- and 120-μm Groups at Postoperative 1 Week and 1 Month
Opaque bubble layer (OBL) occurred more frequently in the 80-μm flap group. Except OBL, no flap-associated complications such as suction loss or gas breakthrough were found, and there were no postoperative complications including diffuse lamellar keratitis, haze development, microstriae, and epithelial ingrowth for all follow-up periods. Multiple logistic regression was done and we found that steeper mean keratometric value (P = .026, odds ratio = 1.877) and thicker central cornea (P = .024, odds ratio = 1.048) were associated with development of OBL (97.5% Wald confidence limits) (Table 3
). However, development of OBL did not affect postoperative visual outcomes such as UDVA, residual refractive error, and contrast sensitivity, even at 1 week.
Table 3: Risk Factors Associated With Development of Opaque Bubble Layer
It is important to make a LASIK flap that is ultrathin with high predictability. Durrie et al.10
performed LASIK with a thin (90 to 110 μm) uniform flap adjacent to the Bowman layer. Prandi et al.11
demonstrated that thin flaps were associated with better UDVA at 1 month and better residual spherical equivalent at 6 months postoperatively. With thinner flaps, faster visual recovery and lower postoperative myopic spherical equivalent were reported by Eleftheriadis et al.,12
and better contrast sensitivity and lower re-treatment rates were demonstrated by Cobo-Soriano et al.13
There have been a few reports on femtosecond LASIK with a 90- or 100-μm flap.10–13
VisuMax allows creation of flaps as thin as 80 μm. Because a greater residual bed thicknesses is desirable, the ability to predictably and safely create a thin flap is important.
Introduction of AS-OCT made it possible to evaluate the quality of the LASIK flap. There are some reports that the femtosecond laser makes a more uniform (planar) flap than the mechanical microkeratome, and the uniformity of the flap would influence postoperative corneal architecture and biomechanical stability significantly.14,15
In practice, AS-OCT has not been widely accepted as a standard method to measure the flap thickness, but flap thickness measurement comparisons between AS-OCT and ultrasonic pachymetry reported good correlation between techniques.16
In the current study, ultrasonic pachymetry was used to measure corneal thickness preoperatively and AS-OCT was used to measure flap thickness postoperatively. However, this discrepancy does not influence the results because the preoperative data were not compared with the postoperative data.
Rocha et al.16
found excellent correlation between flap thickness measured intraoperatively with ultrasound pachymetry and postoperatively with high-resolution OCT. Murakami and Manche17
reported no statistically significant difference between intraoperative ultrasonic subtraction pachymetry and postoperative AS-OCT measurements of femtosecond flaps. It was observed that highly predictable and uniform flaps could be obtained in both 120- and 80-μm flaps created by femtosecond laser in the current study.
Slade et al.18
reported faster visual recovery in 100-μm flap LASIK than in photorefractive keratectomy at postoperative 1 month in their contralateral eye study using femtosecond laser. However, according to Azar et al.,19
there were similar visual outcomes between femtosecond laser-based thin-flap LASIK and mechanical microkeratome-based LASIK at final follow-up. In our study, mean UDVA of the 80-μm flap group was slightly worse than that of the 120-μm flap group at postoperative 1 day, which might be attributable to mild higher residual refractive errors in the 80-μm flap group. At postoperative 1 month, 100% of the 120-μm flap group and 91.7% of the 80-μm flap group gained UDVA of 20/20, and visual outcome improved with time such that 94.5% of the 80-μm flap group gained UDVA of 20/20 at postoperative 6 months.20
In terms of postoperative contrast sensitivity, Patel et al.21
compared results of patients with femtosecond laser-assisted (15-KHz IntraLase; Abbott Medical Optics, Santa Ana, CA) 120-μm flap LASIK and mechanical microkeratome-assisted (Hansatome; Bausch & Lomb) 180-μm flap LASIK under mesopic conditions (6 cd/m2) at postoperative 6 months, and there was no difference in contrast sensitivity for five spatial frequencies (1.5, 3, 6, 12, and 18 CPD). However, Cobo-Soraino et al.13
reported that patients with thinner (< 100 μm) flaps had a better contrast sensitivity result, especially in lower spatial frequencies compared to medium (100 to 129 μm) and thick (130 μm) flap at postoperative 3 months in their mechanical microkeratome (Moria LSK-one microkeratome) LASIK results. In our study, contrast sensitivity was not different between the two groups at postoperative 1, 3, and 6 months. Compared with preoperative contrast sensitivity, reductions of contrast sensitivity only at 6 CPD in the 80-μm flap group were found at postoperative 1 and 3 months, but this reduction disappeared at postoperative 6 months. Although it does not seem to be significant, it might be attributable to relatively higher preoperative contrast sensitivity scores, and it could explain the slightly slower visual function recovery of the 80-μm flap group than the 120-μm flap group.
There have been some reports of unique complications associated with femtosecond laser, such as transient light sensitivity syndrome,22,23
increased corneal backscatter,21,24
The photodisruptive process generated by the femtosecond laser produces cavitation bubbles that expand to create a cleavage plane in the corneal tissue, and these high-pressure bubbles tend to expand into a path of least resistance that results in the production of OBL. Typically, expansion of the bubbles produced during the central cut takes place in a naturally occurring stromal interlamellar space of least resistance.26
Kaiserman et al.26
reported the incidence of OBL in 56.4% of 149 LASIK cases with femtosecond laser (15-KHz Intralase). They concluded that smaller flap diameter and thicker central cornea were associated with development of OBL, and that the sizes of OBL were larger because preoperative keratometric values were flatter and central cornea value were thicker in multivariate logistic regression. Also, they suggested that tight applanation during suction in the process of flap creation (so-called “hard docking technique”) might be the reason for OBL development. Although we used lower energy in the 80-μm flap group, OBL developed more frequently (above two folds) in patients receiving the 80-μm flap than with the 120-μm flap. In analysis of cases with and without OBL, we found that central cornea was thicker in cases with OBL than in cases without OBL (P = .024), favoring the results of Kaiser-man et al.26
However, according to multivariate logistic regression, OBL developed more frequently (odds ratio = 1.88, 97.5% Wald confidence limits) because preoperative mean keratometric values were steeper, and this result was opposite to that of Kaiserman et al.26
Assuming that almost the same forces occurred during applanation (all surgeries were done by the same surgeon) and considering the previous reports that cavitation bubbles produced by femtodissection spread into less resistant interlamellar corneal spaces or even into the anterior chamber,27
we could make a hypothesis that more compact anterior stroma may have a higher resistance. This higher resistance may interfere with dispersion of cavitation bubbles to surrounding tissue. If the cavitation bubble is larger than the microplasma bubble, the incoming laser pulse will land inside the cavitation bubble created by the previous pulse. Rather than vaporizing tissue, this laser pulse simply transfers the heat to the existing microcavitation bubble, increasing its size.28
A large cavitation bubble will generate a large shock wave and cause further spread of collagen fibrils, increasing the OBL.28
Thus, higher resistance in anterior stroma may result in more frequent OBL in thinner flap thickness settings. Although OBL did not affect the clinical results in the current study, large OBL could interfere with flap creation and eye tracking and delay excimer laser treatment.28
The results of our 6-month postoperative analysis indicate that LASIK using the VisuMax femtosecond laser supplied good clinical results and flap reproducibility in both the 80- and 120-μm flap in the treatment of myopia, with 100% of eyes in the 120-μm flap group and 94.5% of eyes in the 80-μm flap group achieving UDVA of 20/20 or better. We did not record visual acuity better than 20/20; we may have missed the ability to detect differences between groups. More comprehensive analysis including the mesopic contrast sensitivity function after treatment is recommended.
- Chen S, Feng Y, Stojanovic A, Jankov MR 2nd, Wang Q. IntraLase femtosecond laser vs mechanical microkeratomes in LASIK for myopia: a systematic review and meta-analysis. J Refract Surg. 2012;28:15–24 doi:10.3928/1081597X-20111228-02 [CrossRef] .
- Sutton G, Hodge C. Accuracy and precision of LASIK flap thickness using the IntraLase femtosecond laser in 1,000 consecutive cases. J Refract Surg. 2008;24:802–806.
- Ghadhfan F, Al-Rajhi A, Wagoner MD. Laser in situ keratomileusis versus surface ablation: visual outcomes and complications. J Cataract Refract Surg. 2007;33:2041–2048 doi:10.1016/j.jcrs.2007.07.026 [CrossRef] .
- Randleman JB, Loft ES, Banning CS, Lynn MJ, Stulting RD. Outcomes of wavefront-optimized surface ablation. Ophthalmology. 2007;114:983–988 doi:10.1016/j.ophtha.2006.10.048 [CrossRef] .
- De Benito-Llopis L, Alió JL, Ortiz D, Teus MA, Artola A. Ten-year follow-up of excimer laser surface ablation for myopia in thin corneas. Am J Ophthalmol. 2009;147:768–773 doi:10.1016/j.ajo.2008.12.022 [CrossRef] .
- Lee SJ, Kim JK, Seo KY, Kim EK, Lee HK. Comparison of corneal nerve regeneration and sensitivity between LASIK and LASEK. Am J Opthalmol. 2006;141:1009–1015 doi:10.1016/j.ajo.2006.01.048 [CrossRef] .
- Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Biomechanical and wound healing characteristics of corneas after excimer laser keratorefractive surgery: is there a difference between advanced surface ablation and sub-Bowman’s keratomileusis?J Refract Surg. 2008;24:S90–S96.
- Ambrósio R Jr, Wilson SE. LASIK vs LASEK vs PRK: advantages and indications. Semin Ophthalmol. 2003;18:2–10 doi:10.1076/soph.22.214.171.12474 [CrossRef] .
- Hafezi F, Seiler T. Persistent subepithelial haze in thin-flap LASIK. J Refract Surg. 2010;26:222–225 doi:10.3928/1081597X-20090930-02 [CrossRef] .
- Durrie DS, Slade SG, Marshall J. Wavefront-guided excimer laser ablation using photorefractive keratectomy and sub-Bowman’s keratomileusis: a contralateral eye study. J Refract Surg. 2008;24:S77–S84.
- Prandi B, Baviera J, Morcillo M. Influence of flap thickness on results of laser in situ keratomileusis for myopia. J Refract Surg. 2004;20:790–796.
- Eleftheriadis H, Prandi B, Diaz-Rato A, Morcillo M, Sabater JB. The effect of flap thickness on the visual and refractive outcome of myopic laser in situ keratomileusis. Eye (Lond). 2005;19:1290–1296 doi:10.1038/sj.eye.6701775 [CrossRef] .
- Cobo-Soriano R, Calvo MA, Beltrán J, Llovet FL, Baviera J. Thin flap laser in situ keratomileusis: analysis of contrast sensitivity, visual, and refractive outcomes. J Cataract Refract Surg. 2005;31:1357–1365 doi:10.1016/j.jcrs.2004.12.058 [CrossRef] .
- Stahl JE, Durrie DS, Schwendeman FJ, Boghossian AJ. Anterior segment OCT analysis of thin IntraLase femtosecond flaps. J Refract Surg. 2007;23:555–558.
- von Jagow B, Kohnen T. Corneal architecture of femtosecond laser and microkeratome flaps imaged by anterior segment optical coherence tomography. J Cataract Refract Surg. 2009;35:35–41 doi:10.1016/j.jcrs.2008.09.013 [CrossRef] .
- Rocha KM, Randleman JB, Stulting RD. Analysis of microkeratome thin flap architecture using fourier-domain optical coherence tomography. J Refract Surg. 2011;27:759–763 doi:10.3928/1081597X-20110812-03 [CrossRef] .
- Murakami Y, Manche EE. Comparison of intraoperative subtraction pachymetry and postoperative anterior segment optical coherence tomography of laser in situ keratomileusis flaps. J Cataract Refract Surg. 2011;37:1879–1883 doi:10.1016/j.jcrs.2011.05.024 [CrossRef] .
- Slade SG, Durrie DS, Binder PS. A prospective, contralateral eye study comparing thin-flap LASIK (sub-Bowman keratomileusis) with photorefractive keratectomy. Ophthalmology. 2009;116:1075–1082 doi:10.1016/j.ophtha.2009.01.001 [CrossRef] .
- Azar DT, Ghanem RC, de la Cruz J, et al. Thin-flap sub-Bowman keratomileusis versus thick-flap laser in situ keratomileusis for moderate to high myopia: case-control analysis. J Cataract Refract Surg. 2008;34:2073–2078 doi:10.1016/j.jcrs.2008.08.019 [CrossRef] .
- Bansal AS, Doherty T, Randleman JB, Stulting RD. Influence of flap thickness on visual and refractive outcomes after laser in situ keratomileusis performed with a mechanical keratome. J Cataract Refract Surg. 2010;36:810–813 doi:10.1016/j.jcrs.2009.12.025 [CrossRef] .
- Patel SV, Maguire LJ, McLaren JW, Hodge DO, Bourne WM. Femtosecond laser versus mechanical microkeratome for LASIK: a randomized controlled study. Ophthalmology. 2007;114:1482–1490 doi:10.1016/j.ophtha.2006.10.057 [CrossRef] .
- Stonecipher KG, Dishler JG, Ignacio TS, Binder PS. Transient light sensitivity after femtosecond laser flap creation: clinical findings and management. J Cataract Refract Surg. 2006;32:91–94 doi:10.1016/j.jcrs.2005.11.015 [CrossRef] .
- Haft P, Yoo SH, Kymionis GD, Ide T, O’Brien TP, Culbertson WW. Complications of LASIK flaps made by the IntraLase 15- and 30-kHz femtosecond lasers. J Refract Surg. 2009;25:979–984 doi:10.3928/1081597X-20091016-02 [CrossRef] .
- Rocha KM, Kagan R, Smith SD, Krueger RR. Thresholds for interface haze formation after thin-flap femtosecond laser in situ keratomileusis for myopia. Am J Ophthalmol. 2009;147:966–972 doi:10.1016/j.ajo.2009.01.010 [CrossRef] .
- Binder PS, Sarayba M, Ignacio T, Juhasz T, Kurtz R. Characterization of submicrojoule femtosecond laser corneal tissue dissection. J Cataract Refract Surg. 2008;34:146–152 doi:10.1016/j.jcrs.2007.07.056 [CrossRef] .
- Kaiserman I, Maresky HS, Bahar I, Rootman DS. Incidence, possible risk factors and potential effects of an opaque bubble layer created by a femtosecond laser. J Cataract Refract Surg. 2008;34:417–423 doi:10.1016/j.jcrs.2007.10.026 [CrossRef] .
- Srinivasan S, Rootman DS. Anterior chamber gas bubble formation during femtosecond laser flap creation for LASIK. J Refract Surg. 2007;23:828–830.
- Faktorovich EG. Opaque bubble layer. In: Faktorovich EG, ed. Femtodynamics: A Guide to Settings and Procedure Techniques to Optimize Outcomes With Femtosecond Lasers. Thorofare, NJ: SLACK Incorporated; 2009:22–24.
Preoperative Data of Eyes That Underwent LASIK With an Intended Flap Thickness of 80 and 120 μm
||Intended Flap Thickness (μm)
||Mean ± SD
|Spherical equivalent (D)
||−5.25 ± 1.53
||−2.75 to −9.75
||−4.89 ± 1.40
||−3.13 to −8.45
||−4.73 ± 1.60
||−2.50 to −9.25
||−4.98 ± 1.45
||−2.50 to −8.25
||−0.97 ± 0.86
||−3.15 to 0.00
||−0.98 ± 0.70
||−3.00 to 0.00
|Central corneal thickness (μm)
||554.06 ± 26.17
||520 to 622
||553.67 ± 18.78
||520 to 601
|Mean keratometric value (D)
||43.15 ± 1.68
||40.85 to 47.10
||43.34 ± 1.58
||40.90 to 46.95
Flap Thickness at 10 Measurement Points in the 80- and 120-μm Groups at Postoperative 1 Week and 1 Month
|Intended Flap Thickness (μm)
||Mean Achieved Flap Thickness (μm)
||76 to 94
||73 to 95
||114 to 135
||107 to 131
Risk Factors Associated With Development of Opaque Bubble Layer
||95% CI for EXP(B)
|Preop mean K