Many studies have investigated whether flap creation using a femtosecond laser is more effective than flap creation using a microkeratome. However, in the present study, we compared the outcomes between femto-LASIK and epipolis LASIK. In addition, in previous studies, Kalyvianaki MI et al.  reported that epipolis LASIK and off-flap epipolis LASIK produced equal visual and refractive results for the treatment of low and moderate myopia. Another study by Na et al.  found that off-flap epipolis LASIK yielded superior visual recovery and corneal re-epithelialization than epipolis LASIK surgery in the early postoperative period. In addition, femto-LASIK surgery, despite allowing accurate, safe, and predictable in flap creation, occasionally involves either intraoperative or postoperative complications . Hence, in the current study, we clinically compared the visual and refractive outcomes between femto-LASIK and flap-off epipolis LASIK surgery in myopia or myopic astigmatism; however, we found no statistically significant differences in these outcomes for two years between the groups.
In generally, surface ablation techniques (such as photorefractive keratectomy [PRK], transepithelial photorefractive keratectomy [T-PRK], laser epithelial keratomileusis [LASEK], and epipolis laser in situ keratomileusis [epipolis LASIK]) results in less painful and offer faster visual rehabilitation than stromal ablation methods (such as laser in situ keratomileusis with a flap created either mechanically with a microkeratome or with a femtosecond laser-based microkeratome [femto-LASIK]).
Meanwhile, corneal haze with decreased corneal transparency is typically determined by corneal backward light scattering. It has been reported that the ablation volume may increase the degree of backscattering , and cases of severe myopia that require more ablation may require a higher dose of MMC during the refractive procedure [23,24]. Sia et al.  and Chen et al.  reported that MMC was beneficial for reduction of corneal haze, without delaying epithelialization. The present study demonstrated little difference. There was statistically significant improvement of visual acuity and refractive errors by 1 day and 1 week in the femto-LASIK group, as compared to the flap-off epipolis LASIK group; however, similar outcomes were achieved between the 2 different surgical techniques during the remaining follow-up period.
The principle of refractive surgery is to induce positive SA shifts for correction of myopia, and negative shifts for hyperopic correction [27,28]. Moreover, the concept of the SCHWIND Amaris 750S excimer laser involves using the optimized aspheric profile  to prevent the surgically induced HOAs, especially SA and coma aberration. However, there were statistically significant and slight induction of SA between preoperative and postoperative LASIK surgery in low and moderate myopic patients (0.123 ± 0.217 μm in femto-LASIK and 0.124 ± 0.218 μm in epipolis LASIK, respectively; data not shown), and much more significant induction of SA in high myopic patients (0.305 ± 0.131 μm in femto-LASIK and 0.459 ± 0.149 μm in epipolis LASIK, respectively; data not shown).
The effect of SA on the depth of focus has been investigated using adaptive optics systems . The depth of focus, by definition, is relatively insensitive to focal length and subject distance for a fixed f-number. Typically, myopia is a condition in which light focuses in front of the retina rather than on it. Myopic or hyperopic refractive surgery aims to correct the corneal shape by changing the keratometric power [11,30].
Huang et al.  and Jain et al.  confirmed obtaining highly repeatable results after LASIK using a Scheimpflug camera, with no significant difference in keratometry readings compared to those provided by manual keratometry . In this study, we also used the Scheimpflug camera to evaluate the outcomes after refractive surgery. We found that both procedures showed a statistically significant decrease in CCT and reduced the keratometry readings. We also found changes of keratometry due to ablation of the keratometry axis. This ablation technique was achieved by balancing the negative and positive cylinder ablations to create a more aspherical optical zone .
The induced changes in corneal asphericity (Q) and SA after laser ablation are key factors when selecting candidates for refractive surgery. Scheimpflug imaging provided reliable measurements, consistent with those reported in the literature; there was a positive change in the Q value of the anterior surface after myopic ablation and a negative change after hyperopic ablation .
Total corneal refractive power involves compensation for negative posterior refractive power by positive anterior refractive power. Steepening of the anterior corneal surface increases the positive refractive power; when both surfaces bulge similarly, the anterior surface induces far greater absolute refractive changes than the posterior surface. According to our results, there were no statistically significant differences in SA between the 2 groups in low and moderate myopic patients (0.417 ± 0.140 μm in femto-LASIK and 0.419 ± 0.137 μm in epipolis LASIK, respectively; P = 0.504, data not shown); however, there were statistically significant differences in high myopic patients, and the postoperative SA was markedly higher in the flap-off epipolis LASIK group (0.550 ± 0.106 μm in femto-LASIK and 0.661 ± 0.158 μm in epipolis LASIK, respectively; P = 0.013, data not shown).
However, Shih et al.  demonstrated that Bowman's membrane and Descemet's membrane, as a pair of forces, provided approximately 20% of the rigidity against bending, despite their being very thin. After refractive surgery, the disruption of Bowman's or Descemet's layer had been associated with corneal ectasia. Moreover, corneal posterior surface elevation can be used to diagnose keratoconus and forme fruste keratoconus (FFKC). In addition, previous studies have focused on the biomechinical simulation of stress concentration after refractive surgery , and they proposed that both surface and stromal ablation techniques caused stress in an obliquely downwards direction after surgery. The present study revealed that the posterior surface underwent more oblate changes after surgery, and there were no accidents of keratectasia after refractive surgery. Dai and associates  reported that the anterior chamber depth was shallower in LASIK patients than in non-operated myopic eyes. However, in our study results, although there were significant changes in anterior chamber depth after surgery, and there were no statistically significant differences between 2 groups.
The cornea is an elastic and pellucid connective tissue. After refractive surgery, corneal curvature and opacity may influence the postoperative visual outcomes. The concept of CTSP was first introduced by Ambrosio et al. . Buhren et al.  performed discriminant analysis in subclinical keratoconus and normal eyes by using corneal anterior and posterior surface aberrations and thickness spatial profiles. They found that the posterior aberrations and thickness spatial profile data did not markedly improve discriminative ability over that of anterior wavefront data alone. In our study, we used CTSP to evaluate corneal thickness changes with different corneal diameters. We found that CTSP changes were significantly smaller in the flap-off epipolis LASIK than the femto-LASIK group at the 6-mm ring of the cornea, and the CTSP changes in the central region were greater than at the mid-periphery. Zernike polynomial equation can be used for characterization of wavefront aberrations of the human eye and for complex corneal shapes. In this study, the corneal HOAs at the 6.5-mm diameter were statistically significantly different in the front and total HOAs of SA, while few significant differences were found in posterior HOAs of vertical coma aberration, oblique trefoil aberration, and oblique tetrafoil aberration. We postulated that these changes in the CTSP may influence the changes in corneal HOAs and may also affect the Q-value (8 mm) changes after LASIK, in a manner dependent on the size of the optical zone being treated. The principle of Scheimpflug imaging analysis systems involves examining slit images of light scattering of the anterior segment of the eyes light ; the different surgical techniques had different effects on corneal elasticity and backward light scattering after refractive surgery.
Aberrations include lower-order and higher-order components. Corneal aberrations are usually positive; aberrations of the lens are usually negative, and the total spherical aberration (SA) changed more than other HOAs with accommodation. Moreover, ocular wavefront aberrations are primarily created in the cornea and lens and are strongly affected by various factors, including the accommodative state , pupil diameter , tear film , age , and pupil entrance decentration . We found a statistically significant difference in postoperative SA between the 2 different surgical techniques, but found no clinically significant difference for 2 years postoperatively, and femto-LASIK produced superior visual outcomes to flap-off epipolis LASIK in the early postoperative stage.
Furthermore, the corneal epithelium comprises surperficial, wing, and basal cells. Flap-off epipolis LASIK involved separation using an Epi-KTM epikeratome, and the absence of the epithelium was a factor in corneal repairing. It has been verified that the epithelial flap acts as a barrier that protects the eye from inflammatory mediators and infectious bacteria, and stabilizes the tear film. It suggest that the epithelial layer played an important role in visual outcomes in case of myopia with low astigmatism, In addition, Zhou et al.  investigated the factors associated with optical and visual quality after corneal surface ablation in high myopic patients, and found that designing a larger optical zone diameter was recommended to achieve better visual quality after surgery.
This study was limited by the small sample size; therefore, studies involving a larger population of patients are necessary to ensure more dependable results .