Purpose
To compare the visual and refractive outcomes of femtosecond lenticule extraction (FLEx) and small-incision lenticule extraction (SMILE) in eyes with myopia.
Design
Prospective, randomized, masked clinical trial with intraindividual comparison.
Methods
This study evaluated 52 eyes of 26 consecutive patients with spherical equivalents of −4.19 ± 1.65 diopters (D) (mean ± standard deviation) who underwent FLEx in 1 eye and SMILE in the other eye by randomized assignment. Before surgery, and 1 week and 1, 3, and 6 months after surgery, we assessed safety, efficacy, predictability, stability, and adverse events of the 2 surgical techniques.
Results
LogMAR uncorrected and corrected distance visual acuity was, respectively, −0.17 ± 0.10, −0.20 ± 0.07 in the FLEx group and −0.15 ± 0.10, −0.19 ± 0.07 in the SMILE group 6 months postoperatively. In the FLEx and SMILE groups 6 months postoperatively, 96% and 100% of eyes, respectively, were within 0.5 D of the targeted spherical equivalent correction. Changes of −0.02 ± 0.39 D and 0.00 ± 0.30 D occurred in manifest refraction from 1 week to 6 months in the FLEx and SMILE groups, respectively. No clinically significant complications occurred in the FLEx or the SMILE group.
Conclusions
Both FLEx and SMILE performed well in the correction of myopia throughout the 6-month observation period. FLEx may be essentially equivalent to SMILE in terms of safety, efficacy, predictability, and stability, suggesting that the presence or absence of lifting the flap does not significantly affect these visual and refractive outcomes.
The corneal resectional refractive procedure for the treatment of myopia was developed to remove a layer of intrastromal tissue using a mechanical microkeratome, but the clinical results of this surgery were not entirely satisfactory, possibly because the mechanical microkeratome was not precise enough to accurately treat myopic refractive errors. In ophthalmology, femtosecond laser has mainly been used for the creation of corneal flaps for laser in situ keratomileusis (LASIK) as an alternative to the mechanical microkeratome, and it has rapidly gained acceptance as a safe and predictable way to create corneal flaps for this surgical technique. A recent breakthrough in this technology has resulted in a novel refractive procedure called refractive lenticule extraction (ReLEx), which requires neither a microkeratome nor an excimer laser, and uses only the femtosecond laser system as an all-in-one device for flap and lenticule processing. The first clinical results of laser-induced extraction of a refractive lenticule were reported in highly myopic eyes and in blind or amblyopic eyes. The ReLEx technique, which can be used for femtosecond lenticule extraction (FLEx) by lifting the flap and for small-incision lenticule extraction (SMILE) without lifting the flap, has been proposed as an alternative to conventional LASIK for the correction of refractive errors. However, as far as we can ascertain, no intraindividual comparison of visual and refractive outcomes after the 2 surgical procedures has so far been conducted. The purpose of the current study is to prospectively and intraindividually compare the 6-month visual and refractive outcomes of FLEx and SMILE for the equivalent correction of myopic refractive errors.
Methods
Study Population
The protocol was registered with the University Hospital Medical Information Network Clinical Trial Registry (000009489). This prospective randomized intraindividual comparative study examined 52 eyes of 26 consecutive patients (9 men and 17 women) who underwent bilateral ReLEx for the correction of myopia and myopic astigmatism using the VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany) with a 500 kHz repetition rate. The patients were recruited in a continuous cohort. The inclusion criteria for the procedures in our institution were as follows: corrected distance visual acuity (CDVA) of 20/20 or more, dissatisfaction with correction using spectacles or contact lenses for non-optical reasons, manifest spherical equivalent of −1 to −9 diopters (D), manifest cylinder of 0-4 D, sufficient corneal thickness (estimated total postoperative corneal thickness >400 μm and estimated residual thickness of the stromal bed >250 μm), and absence of a history of ocular surgery, severe dry eye, progressive corneal degeneration, cataract, or uveitis. Eyes with keratoconus were excluded from the study by using the keratoconus screening test of Placido disk videokeratography (TMS-2; Tomey, Nagoya, Japan). Using the envelope technique, eligible patients were randomly allocated to receive FLEx in 1 eye (26 eyes) and SMILE in the other eye (26 eyes). The surgeon confirmed the randomization table in sequence on the day of surgery. Patients and investigators remained masked to which surgery had been performed in which eye. After data entry, this information was unmasked, together with the randomization list, for statistical analysis. The sample size in this study offered 90.4% statistical power at the 5% level in order to detect a 0.10 logMAR difference in the logarithm of the minimal angle of resolution (logMAR) of visual acuity, when the standard deviation (SD) of the mean difference was 0.15. Patient demographics are summarized in the Table . Preoperatively, and 1 day, 1 week, and 1, 3, and 6 months postoperatively, we determined the following: logMAR of uncorrected distance visual acuity (UDVA), logMAR of CDVA, and manifest refraction (spherical equivalent and refractive cylinder). In all eyes, the preoperative manifest refraction was selected as the target myopic correction. The study was approved by the Institutional Review Board of Kitasato University and followed the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients after explanation of the nature and possible consequences of the study. The CONSORT flow diagram is available as Supplemental Material ( Supplemental Figure , available at AJO.com ).
| Mean ± SD (Range) | FLEx Group | SMILE Group | P Value |
|---|---|---|---|
| Age (y) | 31.5 ± 6.2 (21-41) | ||
| Sex (% female) | 65% | ||
| Manifest spherical equivalent (D) | −4.18 ± 1.72 (−1.50 to −8.00) | −4.21 ± 1.63 (−1.25 to −8.25) | .78 |
| Manifest cylinder (D) | −0.64 ± 0.76 (−0.00 to −2.75) | −0.54 ± 0.74 (−0.00 to −2.25) | .27 |
| LogMAR UDVA | 1.08 ± 0.27 (0.52 to 1.52) | 1.12 ± 0.23 (0.52 to 1.52) | .14 |
| LogMAR CDVA | −0.20 ± 0.07 (−0.30 to −0.18) | −0.19 ± 0.06 (−0.30 to −0.18) | .41 |
| Mean keratometric readings (D) | 43.3 ± 1.1 (41.5-46.1) | 43.3 ± 1.2 (41.1-45.9) | .86 |
| Central corneal thickness (μm) | 545.8 ± 31.3 (492-626) | 544.0 ± 31.7 (483-614) | .19 |
Femtosecond Lenticule Extraction and Small-Incision Lenticule Extraction Surgical Procedures
Both FLEx and SMILE were performed using the VisuMax femtosecond laser system (Carl Zeiss Meditec AG) with a 500 kHz repetition rate. The laser was visually centered on the pupil. A small curved interface cone was used in all cases. The main refractive and nonrefractive femtosecond incisions were performed in the following automated sequence: the posterior surface of the lenticule (spiral in pattern), then the anterior surface of the lenticule (spiral out pattern), followed by a side cut of the flap. The femtosecond laser parameters were as follows: 120 μm flap thickness, 7.5 mm flap diameter, 6.5 mm lenticule diameter, 140 nJ power for lenticule and flap, and a 310-degree side cut (superior hinge) with side cut angles of 90 degrees for FLEx; and 120 μm flap thickness, 7.5 mm diameter of anterior lenticule surface, 6.5 mm diameter of posterior lenticule surface, 140 nJ power for lenticule and flap, and a 50-degree side cut for access to the lenticule with angles of 90 degrees for SMILE. After the suction was released, the patient was moved toward the observation position under the VisuMax integrated surgical microscope. For FLEx, after completion of the laser sequence, a Siebel spatula was inserted under the flap near the hinge and the flap was lifted; the refractive lenticule was then grasped with forceps and extracted. The flap was then repositioned. For SMILE, a thin spatula is inserted through the side cut over the roof of the refractive lenticule dissecting this plane, followed by the bottom of the lenticule. The lenticule is subsequently grasped with modified serrated McPherson forceps (Geuder, GmbH, Heidelberg, Germany) and removed. After the removal of the lenticule, the intrastromal space is flushed using a standard LASIK irrigating cannula. After surgery, steroidal (0.1% betamethasone, Rinderon; Shionogi, Osaka, Japan) and antibiotic (0.5% levofloxacin, Cravit; Santen, Osaka, Japan) medications were topically administered 4 times daily for 2 weeks, and then the frequency was steadily reduced.
Statistical Analysis
All statistical analyses were performed using SPSS (SPSS Inc, Chicago, Illinois, USA). Sample size calculation was performed using PASS 2008 software (NCSS, Kaysville, Utah, USA). One-way analysis of variance (ANOVA) was used for the analysis of the time course of changes, with the Dunnett test for multiple comparisons. The Wilcoxon signed rank test was used for statistical analysis to compare the data between the 2 groups. Unless otherwise indicated, the results are expressed as mean ± SD, and a value of P < .05 was considered statistically significant.
Results
Study Population
The demographics of the study population are summarized in the Table . All surgeries were uneventful and no definite intraoperative complication was observed. There were no significant differences in terms of manifest spherical equivalent (Wilcoxon signed rank test, P = .78), manifest cylinder ( P = .27), logMAR UDVA ( P = .14), logMAR CDVA ( P = .41), mean keratometric readings ( P = .86), or pachymetry ( P = .19) in the FLEx and SMILE groups. No eyes were lost during the 6-month follow-up in this series.
Safety Outcomes
LogMAR CDVA was −0.20 ± 0.07 (range, −0.08 to −0.30) in the FLEx group and −0.19 ± 0.07 (range, −0.08 to −0.30) in the SMILE group 6 months postoperatively ( P = .33). Nineteen eyes (73%) showed no change in CDVA, 3 eyes (12%) gained 1 line, and 4 eyes (15%) lost 1 line in the FLEx group, while in the SMILE group 20 eyes (77%) exhibited no change in CDVA, 2 eyes (8%) gained 1 line, and 4 eyes (15%) lost 1 line 6 months postoperatively ( Figure 1 ). In all, 8 eyes lost 1 line, but the eyes had a CDVA of 20/16 or more.
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