Many studies have investigated whether flap creation using a femtosecond laser (femto-LASIK) is more effective than that using a microkeratome (flap-on epi-LASIK) [6-9]. However, in the present study, we compared the outcomes between femto-LASIK and flap-off epi-LASIK. In previous studies, Kalyvianaki MI et al.  reported that flap-on epi-LASIK and flap-off epi-LASIK produced equivalent visual and refractive results for the treatment of low and moderate myopia. Furthermore, Na KS et al.  found that flap-off epi-LASIK yielded superior visual recovery and corneal re-epithelialization than flap-on epi-LASIK surgery in the early postoperative period.
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 [17,18]. Sia RK et al.  and Chen J et al.  reported that MMC was beneficial for reduction of corneal haze, without delaying epithelialization. The present study demonstrated little difference between the two techniques. Significantly better visual and refractive outcomes were associated with femto-LASIK than with the flap-off epi-LASIK at 1 day and 1 week postoperatively, with no additional significant differences during the remaining follow-up.
Myopic or hyperopic refractive surgery aims to correct the corneal shape by changing the keratometric power [4,21]. Huang J et al.  and Jain R et al.  confirmed obtaining highly repeatable results after LASIK using a Scheimpflug camera, with no significant difference between the automatic and manual keratometric readings . In this study, we used the Scheimpflug camera to evaluate the outcomes after refractive surgery. We found that both procedures showed a statistically significant decrease in CCT, keratometry readings, and ACD values after surgery. Dai ML and associates  reported that the anterior chamber depth was shallower in LASIK than in non-operated myopic eyes.
However, the surface ablation technique can help avoid numerous surgical complications arising from the creation of a lamellar corneal flap required in LASIK and can theoretically provide more stable corneal biomechanics. Shih PJ et al.  demonstrated that the corneal 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 concept of CTSP was first introduced by Ambrosio R Jr et al. . Moreover, Buhren J et al.  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 changes in corneal thickness at different corneal diameters, and found that CTSP changes were significantly smaller in the flap-off epi-LASIK group than the femto-LASIK group at a corneal ring diameter of 6-mm; the CTSP changes in the central region were greater than at the mid-periphery. In addition, the corneal HOAs at the 6.5-mm diameter were 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 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 is focused in front of the retina rather than on it. However, corneal refractive surgery is the kind of refractive surgery that ablates the corneal tissue to change the accommodation power. Wallace HB et al.  found that ACD was significant reduced by 0.10 mm with accommodation, and statistically significant change in corneal curvatures were seen in all participants with accommodation.
The principle of refractive surgery is to induce positive SA shifts for the correction of myopia, and negative shifts for hyperopic correction [31,32]. 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. Although the amount of corneal SA and asphericity are intrinsically related, they provide a 2:1 correspondence between corneal and ocular SA . However, in the present study, there were significant and slight inductions of SA before and after LASIK surgery in patients with low and moderate myopia (0.123 ± 0.217 μm in femto-LASIK and 0.124 ± 0.218 μm in epi-LASIK, respectively; data not shown), and much more significant induction of SA in patients with high myopia (0.305 ± 0.131 μm in femto-LASIK and 0.459 ± 0.149 μm in epi-LASIK, respectively; data not shown).
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 two groups in patients with low and moderate myopia (femto-LASIK, 0.417 ± 0.140 μm; epi-LASIK, 0.419 ± 0.137 μm; P = 0.504, data not shown); however, there were statistically significant differences in patients with high myopia, and the postoperative SA was markedly higher in the flap-off epi-LASIK group (femto-LASIK, 0.550 ± 0.106 μm; flap-off epi-LASIK, 0.661 ± 0.158 μm; P = 0.013, data not shown).
The induced changes in corneal asphericity (Q) and SA after laser ablation are key factors associated with the selection of 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 .
Corneal aberrations are usually positive; aberrations of the lens are usually negative, and the total 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 two different surgical techniques, but found no clinically significant difference for 2 years postoperatively; femto-LASIK produced superior visual outcomes to flap-off epi-LASIK in the early postoperative stage.
A meta-analysis shows that there were no statistically significant differences in either visual outcomes or visual quality between different corneal refractive surgery techniques and that femto-LASIK shows a better predictability than any other type of surgery. However, this study was limited by the small sample size; therefore, studies involving a larger population of patients are necessary to ensure more dependable results .