Purpose
To compare the outcomes of sub-Bowman keratomileusis (100-μm flap) and laser in situ keratomileusis (LASIK) (120-μm flap) using 150-kHz femtosecond laser.
Design
Randomized, double-masked, contralateral clinical trial.
Methods
One hundred patients (200 eyes) with myopia or myopic astigmatism were included. Postoperative examinations were performed at week 1 and months 1, 3, 6, and 12. Main outcome measures included postoperative uncorrected (UCVA) and best-corrected distance visual acuity (BCVA); manifest refraction spherical equivalent; efficacy and safety indices; corneal thickness; and complications.
Results
The mean age of patients was 33.9 ± 7.9 years. Overall, the preoperative UCVA, BCVA, and manifest refraction spherical equivalent were 1.349 ± 0.332, −0.022 ± 0.033, and −5.81 ± 1.61 diopters, respectively. No significant difference was observed in preoperative ( P ≥ .226) or intraoperative parameters ( P ≥ .452) between both groups, except residual stromal thickness ( P < .001). The UCVA, manifest refraction spherical equivalent, and central corneal thickness stabilized by 1 week, while the thinnest corneal thickness stabilized by 3 months postoperatively. There was no significant difference between both groups for any parameter during all follow-up visits ( P ≥ .132) except the 3-month safety index, which was better in the sub-Bowman keratomileusis group ( P = .007). Soft opaque bubble layer was noted intraoperatively in 12 cases (7, 100-μm group; 5, 120-μm group; P = .577). No postoperative complications were observed.
Conclusions
Our study did not find any differences in the visual and refractive outcomes between femtosecond-assisted sub-Bowman keratomileusis and LASIK. Both surgeries resulted in quick visual recovery as early as 1 week postoperatively.
Sub-Bowman keratomileusis is a variation of laser in situ keratomileusis (LASIK) with a targeted flap thickness between 90 and 110 μm. The most obvious advantage of sub-Bowman keratomileusis over conventional LASIK is that it allows treatment of higher refractive errors owing to a thicker residual stroma. Sub-Bowman keratomileusis combines the advantages of surface ablation, particularly the preservation of corneal biomechanical stability, and the merits of LASIK, including fast visual recovery and superior safety and efficacy profiles.
Retrospective studies utilizing microkeratome for flap creation suggested that thinner flaps were associated with faster visual recovery. Femtosecond laser allows the creation of thin and uniform LASIK flaps with better predictability of flap thickness. Earlier studies (level II evidence) have demonstrated comparable visual outcomes and patient satisfaction between sub-Bowman keratomileusis and LASIK with the use of a 60-kHz femtosecond laser for flap creation. A recent study reported better visual outcomes with a 120-μm flap compared to an 80-μm flap on the first postoperative day in patients undergoing femtosecond LASIK using 200-kHz laser.
In spite of the available evidence for its efficacy, sub-Bowman keratomileusis might be associated with a variety of complications, including intraoperative flap tear and vertical gas breakthrough and postoperative epithelial ingrowth and microstriae. It has been hypothesized that since the flaps are cut in close proximity to the Bowman layer in sub-Bowman keratomileusis, it can potentially damage the basement membrane of the corneal epithelium. The defects in the Bowman layer allow contact between proinflammatory epithelium-derived cytokines and the corneal stroma, which may lead to the formation of corneal haze. Furthermore, the source of currently available evidence on outcomes of sub-Bowman keratomileusis is limited to a few laser platforms.
The current study aimed to longitudinally compare the visual and refractive outcomes and changes in the corneal thickness in myopic patients who underwent femtosecond-assisted sub-Bowman keratomileusis with 100-μm flap and conventional LASIK with 120-μm flap in contralateral eyes using 150-kHz femtosecond laser.
Methods
A total of 100 consecutive patients (200 eyes) were recruited from the Refractive Surgery Clinic of Chinese University of Hong Kong Eye Centre between August 2012 and August 2013. A detailed informed consent was obtained from all the patients. The Institutional Review Board of the hospital (Kowloon Central Cluster Ethics Committee, Hospital Authority) approved the conduct of the study. The study adhered to the tenets of the Declaration of Helsinki. The study protocol was registered at the Chinese University of Hong Kong Clinical Trials Registry (CUHK_CCT00329) http://www.cct.cuhk.edu.hk/Registry/publictrialrecord.aspx?trialid=CUHK_CCT00329 . All patients underwent a complete ophthalmic examination and had no ocular abnormality except myopia or myopic astigmatism with a best-corrected distance visual acuity (BCVA) of 20/20 or better in both eyes. Patients with a stable refraction for more than 1 year, myopia of ≥3.0 diopters (D) bilaterally, and anisometropia of ≤1 D were included. Patients with suspicion of keratoconus on corneal topography (displacement of the corneal apex, decrease in thinnest-point pachymetry, asymmetric topographic pattern), cataract, ocular inflammation, and infection were excluded.
Randomization Method
Randomization was performed by a computer-generated list that generated an even number of blocks (100 μm and 120 μm) ( Supplemental Figure , available at AJO.com ). The allocation results were concealed from surgeons and patients. An independent, experienced technician entered the designated flap thickness setting into the femtosecond laser platform before the surgery.
Operative Procedure
Corneal flaps were created using a 150-kHz IntraLase femtosecond laser platform (Abbott Medical Optics, Chicago, Illinois, USA). All flaps had a superior hinge with an intended flap diameter of 9 mm. Other settings included: hinge angle, 55 degrees; bed energy, 0.75 μJ; spot separation, 6 μm; line separation, 6 μm; side-cut energy, 1.1 μJ; pocket width, 200 μm; pocket start depth, 210 μm; and both pocket tangent and radial spot separation, 4 μm. Stromal ablation was performed with Allegretto Wave & Eye-Q 400 Hz laser (Wavelight Technologie AG, Erlangen, Germany) using a 6.5 mm optical zone. Postoperatively, all patients received topical levofloxacin 0.5% eye drops 4 times a day for 1 week. Topical prednisolone acetate 1% eye drops were used 4 times daily for the first postoperative week and then tapered over 1 month. Preservative-free artificial teardrops were used for 6 months postoperatively.
All patients were examined preoperatively and at week 1 and months 1, 3, 6, and 12 postoperatively. Assessments included monocular uncorrected and best-corrected distance visual acuity (UCVA and BCVA), manifest refraction spherical equivalent, slit-lamp and fundus examination, and corneal thickness measurement (Orbscan II; Bausch & Lomb, Rochester, New York, USA). Efficacy index was calculated as the ratio of postoperative UCVA over preoperative BCVA. Safety index was determined as the ratio of postoperative BCVA over preoperative BCVA. The primary outcome measures included postoperative UCVA, BCVA, efficacy index, and safety index. The secondary outcome measures included change in manifest refraction spherical equivalent, change in central and thinnest corneal thickness, and complications.
Statistical Analyses
Statistical analyses were performed using R2.15.2 (R Foundation, Vienna, Austria). One-way analysis of variance (ANOVA) with repeated measures was used to compare the optical zone, transition zone, ablation zone, central depth of ablation, maximum depth of ablation, corneal thickness, residual stromal bed thickness, and flap diameter between the 100-μm and 120-μm flap LASIK groups. Two-way ANOVA with repeated measures was used to evaluate the change in central corneal thickness, thinnest corneal thickness, manifest refraction spherical equivalent, and logarithm of the minimal angle of resolution (logMAR) score in visual acuity among preoperative and postoperative week 1 and month 1, 3, 6, and 12 assessments, and between the 2 groups at different follow-up visits. Prevalence (odds ratios) of complications in both groups was compared using mixed-effect logistic regression to adjust the correlation between fellow eyes.
Bonferroni adjustment was applied for each measurement separately. A P value <.05/15 was considered to be statistically significant for 15 tests of longitudinal changes for each measurement, and P value <.05/5 was considered to be statistically significant for 5 tests for the difference between the 2 groups during the 5 follow-up visits.
Sample Size Calculation
According to the formula by Liu and Liang, the sample size in this study can detect a 10% change in 1-week post-LASIK efficacy index between both groups through repeated-measures ANOVA test with significance level of 5% and study power of 90%.
Results
Two hundred eyes of 100 patients were included in this study. All patients attended the designated follow-up visits at day 1, week 1, and month 1. Two patients did not attend the subsequent follow-up appointments. The mean age of the patients was 33.9 ± 7.9 years (range: 21–55 years). The overall mean preoperative logMAR UCVA and BCVA were 1.349 ± 0.332 (range: 0.7–2) and −0.022 ± 0.033 (range: −1 to 0.137), respectively. The manifest refraction spherical equivalent was −5.81 ± 1.61 (range: −12 to −2.5) D. Preoperative central and thinnest corneal thicknesses were 547.7 ± 23.0 μm and 541.8 ± 23.7 μm, respectively. No significant difference was observed in the preoperative UCVA, BCVA, manifest refraction spherical equivalent, central corneal thickness, and thinnest corneal thickness between the 2 groups ( P ≥ .226) ( Table 1 ).
| Mean | SD | Flap Thickness 100 μm | Flap Thickness 120 μm | P Value | |
|---|---|---|---|---|---|
| Mean | Mean | ||||
| Age (y) | 33.9 | 7.9 | |||
| CCT (μm) | 547.7 | 23.0 | 548.8 | 546.6 | .419 |
| TCT (μm) | 541.8 | 23.7 | 542.9 | 540.6 | .403 |
| MRSE (D) | −5.81 | 1.61 | −5.88 | −5.75 | .242 |
| UCVA (logMAR) | 1.349 | 0.332 | 1.353 | 1.343 | .693 |
| BCVA (logMAR) | −0.022 | 0.033 | −0.017 | −0.027 | .226 |
The intraoperative parameters (mean ± standard deviation) that were studied included overall optical zone (6.51 ± 0.07 mm), transition zone (1.09 ± 0.18 mm), ablation zone (8.70 ± 0.36 mm), ablation depth (86.3 ± 19.7 μm), residual stromal bed thickness (359.8 ± 29.8 μm), and flap diameter (8.85 ± 0.08 mm) ( Table 2 ). There were no significant differences in the intraoperative parameters between the 2 groups ( P ≥ .452), except in the residual stromal thickness ( P < .001) ( Table 2 ).
| Mean | SD | Flap Thickness 100 μm | Flap Thickness 120 μm | P Value | |
|---|---|---|---|---|---|
| Mean | Mean | ||||
| Optical zone (mm) | 6.51 | 0.07 | 6.51 | 6.52 | .993 |
| Transition zone (mm) | 1.091 | 0.182 | 1.091 | 1.090 | .965 |
| Ablation zone (mm) | 8.70 | 0.36 | 8.70 | 8.69 | .966 |
| Ablation depth (μm) | 86.3 | 19.7 | 86.8 | 85.9 | .452 |
| Residual stroma (μm) | 359.8 | 29.8 | 368.9 | 350.5 | <.001 |
| Flap diameter (mm) | 8.85 | 0.08 | 8.84 | 8.85 | .733 |
The results of postoperative UCVA and BCVA in both groups are shown in Tables 3 and 4 . At the end of 1 week postoperatively, the logMAR UCVA changed from 1.353 ± 0.022 to 0.102 ± 0.022 in the 100-μm group ( P < .001) and from 1.343 ± 0.022 to 0.089 ± 0.022 in the 120-μm group ( P < .001) ( Table 3 ). No significant changes were noted in the UCVA 1 week after surgery in both groups ( P ≥ .057). There was no statistically significant difference between the preoperative and postoperative BCVA at any of the follow-up visits in the100-μm group ( P ≥ .008) ( Table 4 ). However, the 1-week and 3-month postoperative BCVA was significantly better than preoperative BCVA in the 120-μm group ( P = .002 and P < .001, respectively) ( Table 4 ). The efficacy and safety indices are shown in Tables 5 and 6 . In the 100-μm group, the efficacy index was 0.80 at the end of 1 week and 1 month. It increased to 0.86 after 3 months and 0.87 at the end of 1 year after surgery. The efficacy index increased from 0.80 to 0.87 between 1 week and 1 year in the120-μm group ( Table 5 ). There was no statistically significant change in the efficacy index 3 months after surgery in both groups ( P ≥ .101). The safety index was 0.97 at 1 week and 0.98 at 12 months postoperatively in the 100-μm group. The corresponding values were 0.96 and 0.97, respectively, in the 120-μm group ( Table 6 ).
| Mean | SD | 95% LCI | 95% UCI | Compare to a | |||||
|---|---|---|---|---|---|---|---|---|---|
| Pre-LASIK | 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | |||||||||
| Pre-LASIK | 1.353 | 0.021 | 1.312 | 1.395 | |||||
| 1 week post-LASIK | 0.102 | 0.021 | 0.060 | 0.143 | <.001 | ||||
| 1 month post-LASIK | 0.102 | 0.021 | 0.061 | 0.143 | <.001 | .994 | |||
| 3 months post-LASIK | 0.070 | 0.021 | 0.029 | 0.111 | <.001 | .208 | .206 | ||
| 6 months post-LASIK | 0.065 | 0.027 | 0.013 | 0.117 | <.001 | .220 | .217 | .863 | |
| 12 months post-LASIK | 0.043 | 0.031 | −0.018 | 0.105 | <.001 | .088 | .087 | .433 | .562 |
| Flap thickness 120 μm | |||||||||
| Pre-LASIK | 1.343 | 0.021 | 1.302 | 1.385 | |||||
| 1 week post-LASIK | 0.089 | 0.021 | 0.047 | 0.131 | <.001 | ||||
| 1 month post-LASIK | 0.113 | 0.021 | 0.071 | 0.155 | <.001 | .344 | |||
| 3 months post-LASIK | 0.075 | 0.021 | 0.033 | 0.116 | <.001 | .571 | .130 | ||
| 6 months post-LASIK | 0.055 | 0.027 | 0.002 | 0.108 | <.001 | .265 | .057 | .521 | |
| 12 months post-LASIK | 0.070 | 0.025 | 0.021 | 0.120 | <.001 | .513 | .137 | .878 | .643 |
a Wald test P value based on 2-way repeated-measures analysis of variance. Bonferroni adjustment was applied for multiple testing. P value <.003 was considered to be statistically significant.
| Mean | SD | 95% LCI | 95% UCI | Compare to a | |||||
|---|---|---|---|---|---|---|---|---|---|
| Pre-LASIK | 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | |||||||||
| Pre-LASIK | −0.017 | 0.007 | −0.030 | −0.003 | |||||
| 1 week post-LASIK | 0.000 | 0.007 | −0.014 | 0.013 | .057 | ||||
| 1 month post-LASIK | −0.016 | 0.007 | −0.029 | −0.002 | .916 | .072 | |||
| 3 months post-LASIK | 0.007 | 0.007 | −0.007 | 0.020 | .008 | .441 | .010 | ||
| 6 months post-LASIK | 0.000 | 0.009 | −0.017 | 0.018 | .102 | .969 | .122 | .543 | |
| 12 months post-LASIK | −0.007 | 0.011 | −0.028 | 0.013 | .424 | .547 | .470 | .242 | .560 |
| Flap thickness 120 μm | |||||||||
| Pre-LASIK | −0.027 | 0.007 | −0.041 | −0.014 | |||||
| 1 week post-LASIK | 0.000 | 0.007 | −0.014 | 0.014 | .002 | ||||
| 1 month post-LASIK | −0.003 | 0.007 | −0.017 | 0.010 | .006 | .709 | |||
| 3 months post-LASIK | 0.018 | 0.007 | 0.004 | 0.032 | <.001 | .040 | .015 | ||
| 6 months post-LASIK | −0.006 | 0.009 | −0.023 | 0.012 | .038 | .608 | .840 | .026 | |
| 12 months post-LASIK | 0.000 | 0.008 | −0.016 | 0.017 | .006 | .973 | .716 | .075 | .610 |
a Wald test P value based on 2-way repeated-measures analysis of variance. Bonferroni adjustment was applied for multiple testing. P value <.003 was considered to be statistically significant.
| Mean | SD | 95% LCI | 95% UCI | Compare to a | ||||
|---|---|---|---|---|---|---|---|---|
| 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | ||||||||
| 1 week post-LASIK | 0.80 | 0.02 | 0.76 | 0.83 | ||||
| 1 month post-LASIK | 0.80 | 0.02 | 0.76 | 0.84 | .971 | |||
| 3 months post-LASIK | 0.86 | 0.02 | 0.82 | 0.89 | .005 | .006 | ||
| 6 months post-LASIK | 0.86 | 0.02 | 0.81 | 0.90 | .019 | .021 | .997 | |
| 12 months post-LASIK | 0.87 | 0.03 | 0.82 | 0.93 | .011 | .012 | .618 | .649 |
| Flap thickness 120 μm | ||||||||
| 1 week post-LASIK | 0.80 | 0.02 | 0.77 | 0.84 | ||||
| 1 month post-LASIK | 0.78 | 0.02 | 0.74 | 0.82 | .277 | |||
| 3 months post-LASIK | 0.84 | 0.02 | 0.80 | 0.88 | .096 | .006 | ||
| 6 months post-LASIK | 0.88 | 0.02 | 0.84 | 0.93 | .002 | <.001 | .101 | |
| 12 months post-LASIK | 0.87 | 0.02 | 0.83 | 0.91 | .009 | <.001 | .250 | .607 |
a Wald test P value based on 2-way repeated-measures analysis of variance. Bonferroni adjustment was applied for multiple testing. P value <.005 was considered to be statistically significant.
| Mean | SD | 95% LCI | 95% UCI | Compare to a | ||||
|---|---|---|---|---|---|---|---|---|
| 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | ||||||||
| 1 week post-LASIK | 0.97 | 0.02 | 0.92 | 1.01 | ||||
| 1 month post-LASIK | 1.05 | 0.02 | 1.00 | 1.10 | .010 | |||
| 3 months post-LASIK | 0.96 | 0.02 | 0.91 | 1.00 | .727 | .004 | ||
| 6 months post-LASIK | 0.97 | 0.03 | 0.91 | 1.03 | .958 | .035 | .728 | |
| 12 months post-LASIK | 0.98 | 0.04 | 0.91 | 1.06 | .714 | .121 | .529 | .771 |
| Flap thickness 120 μm | ||||||||
| 1 week post-LASIK | 0.96 | 0.02 | 0.91 | 1.00 | ||||
| 1 month post-LASIK | 0.96 | 0.02 | 0.92 | 1.01 | .840 | |||
| 3 months post-LASIK | 0.93 | 0.02 | 0.88 | 0.98 | .411 | .306 | ||
| 6 months post-LASIK | 0.98 | 0.03 | 0.92 | 1.06 | .490 | .603 | .167 | |
| 12 months post-LASIK | 0.97 | 0.03 | 0.92 | 1.03 | .667 | .803 | .243 | .789 |
a Wald test P value based on 2-way repeated-measures analysis of variance. Bonferroni adjustment was applied for multiple testing. P value <.005 was considered to be statistically significant.
The manifest refraction spherical equivalent changed significantly in the 100-μm group, from −5.88 ± 0.09 D before surgery to −0.25 ± 0.09 D at 1 week after LASIK ( P < .001), and remained stable up to 12 months postoperatively ( P ≥ .264) ( Table 7 ). Likewise, in the 120-μm group, the manifest refraction spherical equivalent increased from −5.75 ± 0.09 D preoperatively to −0.24 ± 0.09 D at 1 week after LASIK ( P < .001), and remained stable up to 12 months postoperatively ( P ≥ .354) ( Table 7 ).
| Mean | SD | 95% LCI | 95% UCI | Compare to a | |||||
|---|---|---|---|---|---|---|---|---|---|
| Pre-LASIK | 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | |||||||||
| Pre-LASIK | −5.88 | 0.09 | −6.05 | −5.70 | |||||
| 1 week post-LASIK | −0.25 | 0.09 | −0.42 | −0.07 | <.001 | ||||
| 1 month post-LASIK | −0.33 | 0.09 | −0.50 | −0.15 | <.001 | .479 | |||
| 3 months post-LASIK | −0.22 | 0.09 | −0.40 | −0.05 | <.001 | .813 | .345 | ||
| 6 months post-LASIK | −0.37 | 0.12 | −0.59 | −0.14 | <.001 | .358 | .745 | .264 | |
| 12 months post-LASIK | −0.27 | 0.14 | −0.54 | −0.01 | <.001 | .869 | .722 | .735 | .557 |
| Flap thickness 120 μm | |||||||||
| Pre-LASIK | −5.75 | 0.09 | −5.92 | −5.57 | |||||
| 1 week post-LASIK | −0.24 | 0.09 | −0.42 | −0.06 | <.001 | ||||
| 1 month post-LASIK | −0.31 | 0.09 | −0.48 | −0.13 | <.001 | .563 | |||
| 3 months post-LASIK | −0.25 | 0.09 | −0.43 | −0.07 | <.001 | .927 | .626 | ||
| 6 months post-LASIK | −0.36 | 0.12 | −0.59 | −0.13 | <.001 | .375 | .687 | .418 | |
| 12 months post-LASIK | −0.36 | 0.11 | −0.57 | −0.15 | <.001 | .354 | .677 | .397 | .993 |
a Wald test P value based on 2-way repeated-measures analysis of variance. Bonferroni adjustment was applied for multiple testing. P value <.003 was considered to be statistically significant.
In the 100-μm group, the central corneal thickness changed significantly, from 548.8 ± 4.0 μm preoperatively to 440.0 ± 4.0 μm at 1-week post LASIK ( P < .001). It further increased to 448.0 ± 4.0 μm at 1 month after LASIK ( P < .001), and remained stable thereafter ( Table 8 ). A similar trend was noted in the 120-μm group ( Table 8 ). Unlike central corneal thickness, the thinnest corneal thickness stabilized at the end of 3 months postoperatively in both groups ( Table 9 ).
| Mean | SD | 95% LCI | 95% UCI | Compare to a | |||||
|---|---|---|---|---|---|---|---|---|---|
| Pre-LASIK | 1 Week Post-LASIK | 1 Month Post-LASIK | 3 Months Post-LASIK | 6 Months Post-LASIK | |||||
| Flap thickness 100 μm | |||||||||
| Pre-LASIK | 548.8 | 4.0 | 541.0 | 556.6 | |||||
| 1 week post-LASIK | 440.0 | 4.0 | 432.2 | 447.8 | <.001 | ||||
| 1 month post-LASIK | 448.0 | 4.0 | 440.2 | 455.8 | <.001 | .003 | |||
| 3 months post-LASIK | 451.5 | 4.0 | 443.7 | 459.3 | <.001 | <.001 | .208 | ||
| 6 months post-LASIK | 452.5 | 4.4 | 443.9 | 461.2 | <.001 | <.001 | .172 | .745 | |
| 12 months post-LASIK | 452.2 | 4.8 | 442.8 | 461.6 | <.001 | .001 | .272 | .847 | .934 |
| Flap thickness 120 μm | |||||||||
| Pre-LASIK | 546.6 | 4.0 | 538.8 | 554.4 | |||||
| 1 week post-LASIK | 435.9 | 4.0 | 428.1 | 443.7 | <.001 | ||||
| 1 month post-LASIK | 451.4 | 4.0 | 443.6 | 459.2 | <.001 | <.001 | |||
| 3 months post-LASIK | 455.4 | 4.0 | 447.5 | 463.2 | <.001 | <.001 | .155 | ||
| 6 months post-LASIK | 455.0 | 4.4 | 446.3 | 463.7 | <.001 | <.001 | .295 | .908 | |
| 12 months post-LASIK | 454.3 | 4.3 | 445.8 | 462.7 | <.001 | <.001 | .373 | .738 | .851 |
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