One year refractive outcomes of Femtosecond-LASIK in mild, moderate and high myopia.

Purpose: To evaluate the safety, efficacy, predictability and stability for a cohort of myopic eyes treated by Femtosecond-LASIK procedure. Methods: 60 eyes (36 patients) with different degrees of myopia that underwent refractive surgery by using the Femtosecond-LASIK technique were prospectively evaluated for 12 months. The mean preoperative spherical equivalent value was -3.827 ± 1.410 diopters (D) (range: -8.125 to -1.375 D). VisuMax® femtosecond laser was used for cutting the corneal flap and then the Mel80® excimer laser for the stromal ablation. Results: Mean age was 30.80 ± 5.745 years (range: 21 to 46 years) with 75% female patients. Postoperative spherical equivalent at 12 months was within ±0.25 D of emmetropia in 90% of the eyes and within ±0.50 D of emmetropia in 100% of the eyes. All the eyes achieved an uncorrected distance visual acuity (UDVA) of 1.0 (decimal scale). No eye lost lines of preoperative corrected distance visual acuity (CDVA). No major intraoperative or postoperative complications were encountered. Conclusions: Femtosecond-LASIK seems to be a suitable option for the correction of mild, moderate, and high myopia, as the procedure showed to be safe, effective, and predictable for the treatment of myopic refractive errors.


Introduction
Femtosecond Laser-Assisted In Situ Keratomileusis (FemtoLASIK) is a modern method for the correction of refractive errors, being introduced in our country for the first time in September 2011. The procedure requires two lasers, a femtosecond laser (wavelength in infrared light at 1043 nm [1]) for flap creation and an excimer laser (wavelength in ultraviolet light at 193 nm [2]) for refractive ablation.
The purpose of our study was to evaluate the safety, efficacy, predictability, and stability for a cohort of myopic eyes treated by FemtoLASIK procedure.

Patients and Methods
66 eyes (40 patients) were treated for mild, moderate, and high myopia by FemtoLASIK technique. All the surgeries were performed between September 1, 2011 and October 31, 2015. After being informed about the benefits and risks of the procedure, all the patients signed an informed consent.
Inclusion criteria for surgery were the patients' wish of not wearing glasses for nearsightedness, age of 20 years or over, good general health, stable refraction for at least two years before surgery, no previous ocular trauma or ocular surgery, no ocular diseases. Exclusion criteria for surgery were estimated residual thickness of the stromal bed of less than 300 µm after the treatment, evidence, or suspect of keratoconus, severe dry eye syndrome, pregnancy, lactation, general diseases, and poor compliance. When the fundus examination revealed at-risk peripheral lesions, FemtoLASIK surgery was delayed until photocoagulation treatment was performed.
Patients had to discontinue contact lens wearing for two weeks prior to all the corneal investigations and then again, for two weeks before the surgery.
The surgical attempted result was emmetropia in all cases. The Femtosecond laser (VisuMax  , Carl Zeiss Meditec, Germany) treatment was done first. The corneal flap thickness varied between 100 to 120 µm and the hinge was located superiorly. After the flap dissection, an excimer laser (MEL  80, Carl Zeiss Meditec, Germany) treatment was performed. The mean ablation depth was 57.27 ± 23.032 µm (range: 21 to 130 µm). After laser ablation, a plano soft contact lens was applied.
The flap position was checked at the slitlamp 30 minutes after the surgery in all patients.
The first appointment was in the first day postoperative when the bandage contact lens was removed. Next follow-up visits were at one month, three months, six months, and one year. UDVA and CDVA, manifest refraction and noncontact tonometry were measured and slitlamp examination was performed at each visit. Topographies were performed at one month, six months, and one year.
Twelve patients who underwent FemtoLASIK for mild, moderate, and high myopia in one eye were also treated in the fellow eye but for compound myopic astigmatism.
Patient data were stored into an Excel  database (version 14.0, Microsoft Corp.). Statistical analysis was performed by using SPSS statistical software (version 20, IBM  SPSS  Statistics, IBM Corp.).
After testing the normality of continuous variables distributions with the Shapiro-Wilk test, statistical analysis evaluated the postoperative outcomes based on the Paired-Samples T-Test or the Wilcoxon Signed-Rank Test.
The statistically significance level was fixed at P-value ≤ 0.05.

Results
As three patients who were treated in both eyes were lost to follow-up after the first month postoperative visit, our study included 60 eyes (31 right eyes and 29 left eyes) from 36 patients (27 females and 9 males). The mean age of included patients was 30.80 ± 5.745 years (range: 21 to 46 years).
Visual acuity Preoperative CDVA was 1.0 (decimal scale) in all eyes.

Refraction
The preoperative manifest and cycloplegic refraction data are presented in Table 1.  As the cylinder value was low and the spectacle corrections for CDVA and laser treatment were performed only with spherical diopters, our statistical analysis was focused on the manifest spherical equivalent.
Comparing the preoperative manifest and the cycloplegic mean spherical equivalent refraction data, we concluded that there was a statistically significant difference from -3.827 ± 1.410 D to -3.435 ± 1.452 D (P<0.0005) according to the paired samples t-test, the hyperopic shift being 0.391 ± 0.468 D. In order to determine the relationship between the preoperative manifest and the cycloplegic mean spherical equivalent refraction, we performed a Pearson product-moment correlation and we found a positive correlation which was statistically significant (r=0.947, P<0.0005). The refractive data for laser ablation were set according to the manifest refraction, cycloplegic refraction, and spectacle correction for CDVA.
The refractive outcomes data reported below are in accordance with the Standard Reporting in Refractive Surgery [3,4]. Fig. 1 shows the evolution of manifest refraction (spherical equivalent) after the surgery and its stability in time.
The mean preoperative manifest spherical equivalent significantly decreased in the first postoperative day. Comparing refractive parameters as pairs of successive postoperative visits, we found no statistically significant difference until the 1-year follow-up, meaning that the spherical equivalent refraction remained stable ( Table 2). Table 2. Mean values ± SD of the manifest spherical equivalent (shown in diopters) preoperatively, and at one and 12 months after FemtoLASIK surgery, for the 60 myopic eyes of our study. P-values represent the statistical significance of the difference between two consecutive visits.
Manifest In order to analyze the predictability of the refractive results, we considered the attempted spherical equivalent refraction and the achieved spherical equivalent refraction. The Sperman's correlation coefficient showed a strong correlation (r=0.971, P<0.0005). Fig. 2 shows the linear correlation, based on the following formula: achieved spherical equivalent refraction = 0.1214 + 1.0158 * attempted spherical equivalent refraction (r 2 =0.943). The accuracy of postoperative 1-year spherical equivalent refractions compared to the intended preoperative spherical equivalent refractions is shown in Fig. 3. Postoperative spherical equivalent values 12 months after surgery were within ±0.25 D and ±0.5 D of emmetropia in 90% and 100% of the eyes, respectively.

Complications
Neither intraoperative events nor major postoperative complications such as flap dislocation, epithelial ingrowth, diffuse lamellar keratitis or flap melting, were encountered. After 12 months of follow-up, none of the eyes developed keratectasia.
The aim of our study was to report the refractive outcomes after laser surgery; therefore, the adverse events as haze, night visual disturbances, reduced corneal sensitivity, reduced contrast sensitivity, or dry eye syndrome were not analyzed.

Clinical Case
We Pachymetry for the right eye was 552 µm and for the left eye, it was 547 µm. We have chosen a flap thickness of 110 µm for both eyes and an optical zone for excimer ablation of 6.5 for both eyes. The estimated residual stromal bed was 348 µm for the right eye and 343 µm for the left eye. We further presented the preoperative corneal thickness and tangential anterior maps for the right eye (Fig. 4) and for the left eye (Fig. 5).      Postoperative corneal thickness and tangential anterior maps for the right eye (Fig.  6) and for the left eye (Fig. 7) showed a good ablation profile. Twelve months after laser surgery, there was no sign of corneal ectasia in both eyes, neither on the anterior elevation map nor on the posterior elevation map (Fig. 8,9).  The differential anterior elevation map between the postoperative visit at 1 month and the postoperative visit at 12 months are presented in Fig. 10 (right eye) and Fig. 11 (left eye), which best show no change in the corneal shape (no risk of anterior corneal ectasia) during one year follow-up.

Discussion
Compared to the other nowadays techniques of correcting myopic refractive errors, Femtosecond-LASIK was comparable with Small Incision Lenticule Extraction (SMILE) [5][6][7] and Transepithelial Photorefractive Keratectomy (Transepithelial-PRK) [8] in terms of safety, efficacy and predictability. After Femto-LASIK, the corneal sensitivity was lower and the dry eye syndrome was more frequent than after SMILE [5] but compared to Transepithelial-PRK had a shorter recovery time [8].
Referring to the flap cutting, either the Femtosecond laser (in Femto-LASIK procedure) or the mechanical microkeratome (in LASIK procedure) demonstrated to be safe and effective in correcting myopia, with stable results and no significant difference in postoperative UCVA and CDVA [9]. However, the femtosecond laser may have advantages over the microkeratome in better flap thickness predictability, fewer induced high order aberrations, and longer tear break-up time [9].
To the best of our knowledge, in our country, this is the first study on Femtosecond-LASIK refractive results for myopia correction.
We achieved maximum UDVA and manifest refraction close to emmetropia at all postoperative visits with no major intraoperative or postoperative complication for all the treated eyes in our study cohort.
Our study's weaknesses included the small number of study eyes and the short period of follow-up. According to J.Alió et al., myopic regression is possible up to 5 years of follow-up and it is correlated with the achieved correction [10]. Further reports will be salutary after following-up the patients for a longer period of time.
In conclusion, Femtosecond-LASIK seems to be a suitable option to correct mild, moderate, and high myopia, as the procedure showed to be safe, effective, and predictable for the treatment of myopic refractive errors.

Financial Disclosure
None of the authors has any financial or proprietary interests to disclose.