Functional optical zone and visual quality after SMILE and FS-LASIK treatment for high myopia

Background We compared the functional optical zone (FOZ) and wavefront aberrations achieved after small-incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) in high-myopia patients. METHODS Thirty four and fourty high myopic (spherical equivalent refraction ≤ -6D) patients were respectively enrolled in FS-LASIK group and SMILE group, only right eye of each patient was analyzed. The planned FOZ (PFOZ), ablation depth/lenticule thickness and other data were acquired from medical records. The achieved FOZs (AFOZ) were assessed using a Pentacam at 1 month, 3 month, 6 month post-surgery. We defined the AFOZ diameter as the mean ring diameter corresponding to average total corneal refractive power (TCRP) in the central 4-mm zone plus 0.5D. The wavefront aberrations were obtained with WASCA system pre-operatively and 6-month-postoperatively. RESULTS The PFOZs in the SMILE and FS-LASIK groups were 6.57±0.22 mm vs 6.54±0.15 mm (P =0.350). In both groups, the AFOZ at every follow-up point was smaller than the PFOZ (P < 0.001). The AFOZ correlated negatively with the wavefront higher order aberrations (HOA), spherical aberration in both groups. HOA, spherical aberration (P<0.001), but not coma (P=0.477) and trefoil (P=0.812), differed significantly between the groups.


Background
Corneal refractive surgery corrects ametropia by removing the corneal stroma and accordingly changing the anterior corneal refractive power. However, due to the limited 3 corneal thickness, the treatment region, named the functional optical zone (FOZ), cannot cover the whole cornea. The FOZ is defined as the area with the best visual quality and least aberration typically associated with the anterior corneal power distribution. Some studies have defined the FOZ as the area represented by power within the ± 0.5 D range over the central 4 mm. (1) It is also regarded as the area with the best optical quality, providing visual acuity of no less than 20/32. (2) Small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) are widely used lamellar corneal refractive surgeries. SMILE creates a lenticule with a specific shape, according to the attempted correction, after two femtosecond scans at different depths.(3) LASIK aims to reconstruct the corneal anterior surface by ablating the corneal stroma. (4) While several studies have been conducted regarding FOZ after LASIK,(5) few such studies have been published on SMILE. As these procedures use different mechanisms, we considered whether there were differences in the achieved FOZ (AFOZ) between SMILE and LASIK and whether the AFOZ diameters correlated with visual quality results after these two surgeries.

Methods
Patients with an excessively thin cornea (< 480 µm); those diagnosed with or suspected of keratoconus; and those with other ocular diseases or systemic diseases were excluded.
Patients were required to stop wearing soft contact lens at least 1 week before surgery, rigid contact lens at least 2 weeks before surgery, and remain without orthokeratology at least 4 weeks before surgery. Antibiotics eyedrop were delivered 4 times daily for 3 days before surgery.

Small-incision lenticule extraction procedure
All the SMILE procedures were performed via VisuMax (Carl Zeiss Meditec AG), with the energy set at 130 nJ and a repetition rate of 500 kHz, with a spot spacing of 4.5 μm. The cap diameter was designed to be 7.5 mm, with thickness ranging from 110 to 120 μm. The lenticule diameter was 5.8-6.7 mm. The side-cut angle was 90°at the superior position.
The transition zone was 0.1 mm for astigmatism correction. The target refraction was set to 0 to + 0.75 DS on the basis of age and preoperative refractive power.
The patient was required to stare at the green light throughout the operation. The surgeon centered the corneal vertex by observing through the microscope and moving the joystick.
Suction was activated when the watermark exceeded 80% of the cone; then, the eyeball was fixed and photodisruption was created to generate the posterior surface and the anterior surface of the refractive lenticule, followed by a single side-cut incision. The lenticule was then dissected through the side-cut incision and removed manually using 5 microforceps.

FS-LASIK Procedure
Flaps were cut using the VisuMax femtosecond laser, with energy set at 180 nJ and a repetition rate set at 500 kHz. The flap diameter was intended to be 8.5 mm, and the thickness was 110 µm. The side-cut angle was 90°, with a 4-mm-wide hinge immediately above. The corneal stroma was ablated using the MEL 80 excimer laser (Carl Zeiss Meditec AG), with energy set at 0.9 mJ and a repetition rate set at 250 Hz. The targeted diopter was set as 0 to +0.75 DS. The ablation optical zone was centered on the intersection of the optical axis and the corneal plane, and its diameters were set at 5.75 mm to 6.75 mm, with a 2-mm transition zone. A bandage contact lens (ACUVEOASYS, Inc., Jacksonville, FL) was placed after the flap was well repositioned.
The bandage lenses were removed 1 day after FS-LASIK. Patients were followed up at 1 day, 1 month, 3 month, and 6 month postoperatively. Slit-lamp examination, UDVA, CDVA, and intraocular pressure measurement were performed at each visit. Corneal topography (Pentacam HR, Oculus, Germany) examinations were performed at 1 month, 3 month, and 6 month postoperatively. Wavefront aberrations was examed at 6 month after the sugery.

FOZ and wavefrontaberration measurements
Topography was performed by the same experienced technician using the same machine 6 (Pentacam) preoperatively, 1 month, 3 month, and 6 month postoperatively, and only scans marked "OK" by the instrument were saved and analyzed. Calculations were available in the "Power Distribution" scan in the Pentacam. Centered on the corneal apex, the average total corneal refractive power (TCRP) in the central 4-mm zone was regarded as the baseline. By adjusting the diameter of the ring, the AFOZ was acquired as the largest diameter when the mean ring TCRP did not exceed 0.5 D more than the baseline value ( Figure 1).. A threshold value of 0.5D was arbitrarily chosen, because a -0.5 D defocus generally induces a UDVA of 20/32, which allows for daily-life activities to be performed.(6) Wavefront aberrations with a 5-mm analysis diameter were performed preoperatively and 6-month-postoperatively. The present study analyzed three commonly examined aberrations: spherical aberration, coma, and trefoil, as well as the total highorder aberration (HOA) in OSA-VSIA Zernike polynomials.
For subjective alignment in SMILE and eye movement in FS-LASIK, decentration is inevitable. As one of the factors influencing the FOZ, we measured decentration using Lazaridis's method.(7) Using the thickness difference map in the Pentacam (postoperative 1 month -preoperative), the distance (r = √(x 2 + y 2 )) from the point with the largest value (x, y) to the corneal vertex (0, 0) was regarded as the decentration value. Because the maxium value point isn't the only one, three consecutive measurements were taken by the same observer and the average values were calculated.

Statistical Analysis
Using SPSS (24.0 IBM Corporation, Armonk, NY), the KolmogorovSmirnov test was firstly applied to establish whether the data followed a normal distribution or not. For values with a normal distribution, Student's t-test and the paired t-test were utilized for betweengroup and intra-group comparisons, respectively. The FOZs at different timepoints were compared by repeated-measures analysis of variance. Pearson's analysis was then used to assess the relationship between preoperative parameters and AFOZ. For abnormally distributed data, Kruskal-Wallis and Spearman analyses were used. P-values less than 0.05 were considered as indicating statistically significant differences.

Patient characteristics
In total, 74 participants (SMILE group, 40 eyes; FS-LASIK group, 34 eyes) were included in the study. The two groups were comparable in terms of age, spherical equivalent, intraocular pressure (IOP), corneal thickness, ablation depth or lenticule thickness, corneal front mean keratometry (Km), and planned FOZ (PFOZ) ( Table 1)..

Visual outcomes
All surgeries were uneventful, without suction loss, difficult dissection, incomplete scanning, wreck lenticule, and postoperative infection or diffuse lamellar keratitis.

Discussion
In the present study, we investigated the differences of AFOZ and wavefront aberrations after SMILE and FS-LASIK treatments for high myopia. Our results demonstrated that the AFOZ diameter for SMILE exceeded that of LASIK with equivalent PFOZ design on similar refraction correction. More induction of wavefront aberrations there was after FA-LASIK than SMILE.
To date, there were several methods about the AFOZ by topographic measurement.
Though different ways used, it was well accepted that AFOZ would be smaller than Besides, this study demonstrated SMILE would achieve larger AFOZ than FS-LASIK did on the premise of identical correction design. A possible reason for these findings may be the difference in the mechanism of tissue removal. The excimer laser changes the corneal shape by breaking down molecular bonds and ablating corneal tissues. The region in which the excimer laser releases is distant from the cornea. The excimer laser is perpendicular to the corneal plane when ablating the central cornea, but it is at an angle to the corneal plane when ablating the peripheral cornea. Due to the curvature of the peripheral cornea, part of the energy will be reflected by the cornea, leading to attenuation of the functional laser energy. (9) This effect is similar to the difference in heating effects produced by sunlight on the earth's equator and on the hemispheres. The energy attenuation in the peripheral area reduces the amount of tissue actually ablated and causes a corresponding reduction in the corrected diopter, so that the size of the central cornea with good vision will be reduced. In SMILE, the femtosecond laser scans the corneal stroma twice at different depths, thus isolate a lenticule with specific diopters. Then, the lenticule was extracted through a 2-mm incision, leading to a change in corneal shape.(4) In SMILE, the femtosecond laser is launched closely to the corneal plane, such that there is no peripheral energy attenuation as occurred in FS-LASIK. Moreover, the shape of lenticule is determined by the scan depth, which is not related to the energy value. Therefore, the distribution of corneal refractive power is not significantly different and a larger area of the central cornea with good vision, defined as AFOZ in this study, can be achieved. A continuous observation was performed at three postoperative timepoints, but we found no significant difference in the AFOZs among 1 month, 3month and 6month after both two surgeries. This finding was consistant with what Hou et al (10) reported, who outlined the zero diopter on the tangential curvature difference map as the rim of the effective optical zone measurement.
The present study also compared the wavefront aberrations at 6 month after high myopic correction by SMILE and FS-LASIK. Among the four analyzed wavefront aberrations components, HOA, spherical aberration, and coma were significantly increased in both groups. We also found that more HOA and spherical aberration were induced in FS-LASIK group than that in SMILE group. Our results agree with the results reported by both Ganesh et al (11) and Lin et al (12), which suggested that SMILE might provide better visual quality than FS-LASIK for high myopia. The curvature change of anterior corneal surface is the mechanism for the corneal refractive suguries. However, these procedures change the anterior corneal surface from a prolate shape to an oblate shape, increase corneal 11 asphericity and, consequently, break the physiological ocular aberration state and cause induction of higher order aberrations. Both Gyldenkerne et al (13) and Zhang et at (14) came to the same conclusion that SMILE preserves the physiological corneal asphericity better than FS-LASIK does, which is related with the tissue removal difference between excimer laser and femtosecond laser. Furthermore, the trasition zone and flap design (15,16), corneal wound healing response and remodeling difference(17) may attribute to more HOA induction in FS-LASIK. The increase in spherical aberration and coma is usually associated with treatment decentration (13,18). In current study, the decentration in the SMILE and FS-LASIK groups were comparable, suggesting that decentration has little effect on the conclusions of this study. Another important finding was that the increase of higher order aberrations and spherical aberration were correlated with AFOZ for both SMILE and FS-LASIK procedures, which may provide potential insights into predicting visual quality through AFOZ size.
The limitation of the present study is that it lacked other objective and subjective indicators, such as contrast sensitivity and subjective questionnaire-based visual quality evaluation. Further studies that combine these approaches with visual quality assessment are warranted.

Conclusion
In conclusion, this study compared the corneal power distribution and wavefront aberration after SMILE and LASIK, and identified that AFOZs are larger in SMILE group than FS-LASIK, which would be asscociated with better optical quality after surgery. •Consent for publication Written informed consent was obtained from the patients for publication of this study.
Copies of the written consent are available for review by the Editor of this journal.

•Availability of data and material
The datasets during and/or analyzed during the current study available from the corresponding author on reasonable request.

•Competing interests
No competing interests.

•Funding
No financial support was received for this submission.   Figure 1 An example of functional optical zone (FOZ) measurement