The femtosecond laser has been applied to create the corneal flap in the established laser-assisted in situ keratomileusis (LASIK) procedure ever since 2003.1 It was further introduced in the femtosecond laser lenticule extraction (FLEx) procedure because of the high predictability and milder corneal healing responses than the excimer laser.2,3 However, the corneal flap is still needed, and it may induce higher-order aberrations (HOAs).4,5 Corneal HOAs were associated with night vision disturbance, glare, halo, and degraded optical quality.6 Many efforts have been made to reduce corneal HOAs to obtain premium postoperative visual qualities.7–9
Small incision lenticule extraction (SMILE) is a new procedure that removes the refractive lenticule with an incision ranging from 2 to 5 mm,10,11 and flap-induced aberrations are subsequently avoided.4,5 Although previous studies investigated the changes of corneal HOAs after the SMILE procedure, they mainly focused on the anterior corneal HOAs or total corneal HOAs12–21; whether the SMILE procedure has an effect on posterior corneal HOAs is still not well understood, and only a few studies have systematically compared the anterior, posterior, and total corneal HOAs among SMILE, femtosecond laser-assisted in situ keratomileusis (FS-LASIK), and FLEx.
The posterior corneal HOAs are important for the evaluation of corneal biomechanical responses22 and could provide valuable information in determining the cause of poor visual quality after corneal refractive surgery.23 Hence, we performed the investigation on the characteristics of the anterior corneal HOAs, posterior corneal HOAs, and total corneal HOAs in myopic eyes before and after FS-LASIK, FLEx, and SMILE surgeries to have a better understanding of postoperative visual qualities.
MATERIALS AND METHODS
Subjects and Examinations
This study followed the tenets of the Declaration of Helsinki; written informed consent was obtained from the subjects after explanation of the nature and possible consequences of the study. The Institutional Review Board of Tianjin Eye Hospital approved this study. This retrospective study included a total of 160 eyes: 73 myopic eyes of 42 subjects who underwent SMILE surgery, 52 myopic eyes of 32 subjects who underwent FS-LASIK surgery, and 35 myopic eyes of 29 subjects who underwent FLEx surgery in our department. We reviewed the clinical charts of patients who underwent SMILE, FS-LASIK, or the FLEx procedure in our department, and only patients who underwent corneal aberration measurements preoperatively and at 3 months postoperatively were included. Exclusion criteria entailed eyes with postoperative complications, such as dry eye (corneal/conjunctival staining, tear film breakup time <10 sec, and Schirmer I test <10 mm/5 min) or steroid-induced high intraocular pressure (IOP) because these changes may influence the corneal HOAs.24–26 All subjects had mesopic pupil size less than 6.0 mm preoperatively to avoid postoperative glare or halo. We performed SMILE, FS-LASIK, or FLEx surgery according to the patients' own choice, regardless of the amount of preoperative manifest refraction spherical equivalent (MRSE) or HOAs. Detailed clinical data are shown in Table 1. There are no statistical differences among the three groups regarding the patients' age, preoperative MRSE, spherical diopter, cylindrical diopter, central corneal thickness, IOP, and the mean keratometric readings, which provided a fair comparison of the corneal HOAs.
The same surgeon (Y.W.) performed all surgeries. Three drops of oxybuprocaine hydrochloride (Benoxil; Santen, Inc., Osaka, Japan) were applied 3 min before surgery as topical anesthesia.
Femtosecond Laser-Assisted In Situ Keratomileusis Procedure
After the surface was anesthetized preoperatively, patients were told to fixate on the target light so that suction could be initiated. The flaps were created with the 500-kHz femtosecond laser system (VisuMax; Carl Zeiss Meditec AG, Jena, Germany). After the corneal flap was created and lifted, the surgeon confirmed that the center of the ablated zone was aligned with the center of the pupil; the corneal stroma was ablated with a 400-kHz Allegretto excimer laser system with the wavefront-optimized mode (Wavelight Allegretto, WaveLight AG, Erlangen, Germany). The flap was repositioned in a similar fashion as in routine LASIK. The corneal flap thickness was 110 μm, the flap diameter was 7.9 to 8.0 mm, and the hinge of the corneal flap was in the superior position. The target refraction was emmetropia (±0.25 D). The optical zone was 6.0 to 6.5 mm surrounded by a transition zone of 1.0 mm.
Femtosecond Laser Lenticule Extraction Procedure
After the surface was anesthetized preoperatively, patients were told to fixate on the target light so that suction could be initiated. The surgeon confirmed that the center of the ablated zone was aligned with the center of the pupil. After that, the surgery was performed using a 500-kHz VisuMax femtosecond laser (Carl Zeiss Meditec AG). The femtosecond laser created the posterior surface of the lenticule, the lenticule border, the anterior surface of the lenticule, and the sidecut of the corneal flap successively. The flap diameter was 7.0 mm with a thickness of 105 μm. The hinge of the corneal flap was in the superior position. The lenticule diameter (optical zone) was 6.0 to 6.5 mm. The target refraction was emmetropia (±0.25 D). Once the femtosecond laser cutting procedure was finished, the suction switched off, and the corneal flap lifted, the refractive lenticule was extracted from the cornea.
Small Incision Lenticule Extraction Procedure
After the surface was anesthetized preoperatively, patients were told to fixate on the target light so that suction could be initiated. The surgeon confirmed that the center of the ablated zone was aligned with the center of the pupil. After that, the surgery was performed using a 500-kHz VisuMax femtosecond laser (Carl Zeiss Meditec AG). Four cleavage planes were created including anterior and posterior surfaces of the refractive lenticule, the vertical edge of the refractive lenticule, and a single sidecut incision with a circumferential length of 2.0 to 5.0 mm at the 12-o'clock position. Once the femtosecond laser cutting procedure was finished, the suction was switched off and the refractive lenticule was extracted from the small incision. For all myopic corrections, the lenticule diameter (optical zone) was 6.0 to 6.5 mm. The cap thickness was 110 μm. The target refraction was emmetropia (±0.25 D).
Measurement of Corneal Higher-Order Aberrations
Corneal HOAs were obtained using a rotating Scheimpflug Camera (Pentacam HR; Oculus, Wetzlar, Germany), which has been used to measure the corneal aberrations of the anterior surface, posterior surface, and total cornea.18,23,27
The Pentacam is a noninvasive and reproducible diagnostic method. However, corneal scars and light reflections impair the reliability of measurements.28 To avoid these misleading effects, the corneas with preoperative scars were excluded from the study, and measurements were made in a dark room. The corneal HOAs of the anterior surface, posterior surface, and total cornea were analyzed over a 6.0-mm central diameter preoperatively and at 3 months postoperatively. We analyzed the root mean square values of the corneal HOAs including the third to eighth HOAs, spherical aberration, coma, trefoil, quadrafoil, and secondary astigmatism.
Statistical analysis was performed using SPSS version 19.0 (SPSS, Chicago, IL). The Kolmogorov–Smirnov test was used to evaluate the normality of the sample. The analysis of variance test was used for normally distributed data among the SMILE, FS-LASIK, and FLEx groups, and the Kruskal–Wallis H test was used to determine significant differences among the three groups for the nonnormally distributed corneal HOA data. The differences of corneal HOAs before and after surgery were evaluated by the Wilcoxon test. The surgically induced aberrations were calculated through the comparison of the postoperative corneal HOAs with the preoperative corneal HOAs in each group. The Spearman rank correlation test was used to evaluate the relationship between induced aberrations and the preoperative spherical equivalent (SE). P<0.05 was considered statistically significant.
Changes in Anterior, Posterior, and Total Corneal Higher-Order Aberrations After Small Incision Lenticule Extraction
Corneal third to eighth HOAs, coma, quadrafoil, secondary astigmatism, and spherical aberration of the anterior corneal surface and total cornea significantly increased after SMILE surgery (Fig. 1A,C). The posterior corneal third to eighth HOAs and coma slightly increased after surgery (Fig. 1B). Other HOAs remained statistically unchanged after SMILE surgery.
Changes in Anterior, Posterior, and Total Corneal Higher-Order Aberrations After Femtosecond Laser-Assisted In Situ Keratomileusis
The corneal third to eighth HOAs, coma, quadrafoil, secondary astigmatism, and spherical aberration of the anterior cornea and total cornea significantly increased after FS-LASIK (Fig. 2A,C). The trefoil and spherical aberration of the posterior cornea slightly increased after surgery (Fig. 2B). The other HOAs remained statistically unchanged after FS-LASIK.
Changes in Anterior, Posterior, and Total Corneal Higher-Order Aberrations After Femtosecond Laser Lenticule Extraction
In the FLEx group, the third to eighth HOAs, coma, quadrafoil, secondary astigmatism, and spherical aberration of the anterior cornea (Fig. 3A) and total cornea (Fig. 3C) significantly increased. The posterior third to eighth HOAs, coma, secondary astigmatism, and corneal trefoil slightly increased after surgery (Fig. 3B). Other HOAs remained statistically unchanged after FLEx.
Comparison of the Surgically Induced Anterior Corneal Higher-Order Aberrations Among SMILE, FS-LASIK, and FLEx
As seen in Table 2, significant differences of the anterior corneal HOAs among the three groups were observed on the third to eighth HOAs and spherical aberration postoperatively. The SMILE procedure induced significantly smaller third to eighth HOAs (P=0.002, Fig. 4A) and spherical aberration (P=0.001, Fig. 4B) than FLEx. There was no statistically significant difference in induced third to eighth HOAs or spherical aberration of the anterior surface between SMILE and FS-LASIK (P=0.130 and P=0.112) or between FLEx and FS-LASIK (P=0.119 and P=0.265).
Comparison of the Surgically Induced Posterior Corneal Higher-Order Aberrations Among SMILE, FS-LASIK, and FLEx
As seen in Table 3, there were no significant differences in postoperative corneal HOAs among the three groups except coma, postoperatively. The induced posterior corneal coma was significantly higher after FLEx than after SMILE (P=0.013) and FS-LASIK (P<0.001, Fig. 4C). There was no statistically significant difference in induced posterior coma between SMILE and FS-LASIK (P=0.114).
Comparison of Surgically Induced Total Corneal Higher-Order Aberrations Among SMILE, FS-LASIK, and FLEx
As seen in Table 4, there were significant differences in postoperative third to eighth HOAs and spherical aberration of the total cornea. Moreover, the SMILE procedure induced significantly less third to eighth HOAs (P=0.002, Fig. 4D) and spherical aberration (P=0.001, Fig. 4E) than FLEx. There was no statistical significant difference in induced third to eighth HOAs or spherical aberration in the total cornea between SMILE and FS-LASIK (P=0.129 and P=0.113) or between FLEx and FS-LASIK (P=0.407 and P=0.281).
The correlations between MRSE and surgically induced corneal HOAs are shown in Table 5. The MRSE is significantly correlated with the induced spherical aberration of the total cornea after SMILE, FS-LASIK, and FLEx and also is significantly correlated with surgically induced coma of the anterior surface and total cornea in eyes that underwent SMILE and FS-LASIK (Table 5).
This study evaluated the corneal HOAs of the anterior surface, posterior surface, and total cornea in myopic eyes after SMILE, FS-LASIK, and FLEx surgeries. It was believed that posterior corneal HOAs could complement corneal topography information to gain insight into the corneal biomechanical response22 and provide valuable information in determining the cause of poor visual quality after corneal refractive surgery.23
This study showed that the anterior and total corneal HOAs, especially coma and spherical aberration, significantly increased after each type of surgery. The total corneal aberrations are mainly determined by the anterior surface, in which the two surfaces are maintained parallel and similar to each other. Although the anterior and total corneal HOAs showed significant increases, most posterior corneal HOAs remained unchanged postoperatively. Juhasz et al.27 analyzed the changes of posterior corneal HOAs after photorefractive keratectomy and also found that the posterior corneal HOAs remained relatively stable after surgery. Maeda et al.23 analyzed the third to fourth HOAs of the posterior cornea and found that the posterior HOAs were 0.14±0.04 μm in normal eyes after LASIK, 0.83±0.57 in eyes with keratectasia after LASIK, and 1.18±0.65 in eyes with keratoconus. In this study, the posterior corneal third to eighth HOAs were 0.19±0.03 in each group postoperatively, which were similar to the posterior corneal HOAs in normal eyes after LASIK. Hence, it may suggest that these three procedures are safe without significant influences on the posterior cornea. Recently, Gyldenkerne et al.18 also found that the SMILE and FS-LASIK procedures induced negligible changes in the posterior corneal HOAs, which were consistent with our results.
This study not only found that the coma aberrations of the anterior surface and total cornea significantly increased after SMILE surgery but also found that the changes of the anterior and total corneal coma are significantly correlated with the SE in eyes that underwent SMILE surgery. As far as we can ascertain, this is the first study that reported the correlations. Some studies suggested that decentrations play a major role in the induction of coma aberrations after SMILE.8 However, Sekundo et al.9 found that more eyes that underwent SMILE showed smaller decentrations than in the FS-LASIK procedure. This may suggest that other reasons explain the induced coma aberrations in the SMILE procedure. We suppose the coma aberration may also be associated with the single incision in the SMILE procedure, which may cause imbalanced corneal healing responses and induce optical changes. In addition, Kamiya et al.7,16 found that induced third order aberrations are significantly correlated with SE in eyes that underwent LASIK, and there are no correlations between induced third order aberrations and the SE in eyes that underwent FLEx, which are also found in this study.
The SMILE procedure induced significantly lower optical changes than FLEx. This may result from the absence of flap in the SMILE procedure, which may avoid flap-induced HOAs.4,5 Ağca et al.12 and Vestergaard et al.13 found no significant differences in corneal HOAs between FLEx and SMILE12,13; the long-term observations in these studies (at 6 or 12 months) may made the eyes have similar tear film regularities,28 which may lead to similar wavefront properties between FLEx and SMILE procedure. The discrepancy of the results may also result from the different surgical parameters (i.e., flap and cap thicknesses) in these studies.
Femtosecond laser lenticule extraction and FS-LASIK are both flap-based procedures, and this may explain why the optical changes were similar between these two procedures. However, recently, Hjortdal et al.14 and Sekundo et al.15 found that FLEx induced less spherical aberration than FS-LASIK. We observed that they used the MEL-80 excimer laser system (Carl Zeiss Meditec AG) to perform the FS-LASIK surgery, whereas we used the Allegretto laser system. It should be noted that various excimer systems or even various ablation modes with the same excimer laser machine in the FS-LASIK procedure might cause significantly different corneal HOAs postoperatively.29 This may lead to discrepancies of the comparison results between FS-LASIK and other procedures. Tan's group recently published one article20 focused on the differences of the ocular HOAs between pseudo-SMILE surgery and the FLEx surgery. Peudo-SMILE surgery is a transition SMILE surgery with a lager incision than the SMILE surgery and smaller incision than the FLEx surgery. The authors found no significant difference of the HOAs between pseudo-SMILE surgery and FLEx surgery at 1 year postoperatively. The long-term observations may made the cornea heals after surgery and difficult to observe the differences at the early stages after surgery.29
This study found that the SMILE procedure induced similar optical changes with FS-LASIK. Recently, Gyldenkerne et al.18 also found that the SMILE procedure induced equal amount of the posterior corneal HOAs with FS-LASIK, which were consistent with our results. Some studies used the MEL-80 excimer laser17,18 or the SCHWIND AMARIS excimer laser (SCHWIND eye-tech-solutions, Kleinostheim, Germany)19 to perform FS-LASIK surgery and found that SMILE procedure induced significantly lower spherical aberration than FS-LASIK. However, the degrees of the treated myopia in the SMILE group were relatively lower than that in the FS-LASIK group in the study by Lin et al.17 (−5.13±1.75 D vs. −5.58±2.41 D) and Gyldenkerne et al.18 (−7.3±1.5 D vs. −7.6±1.3 D). Another reason for this is that a smaller optical zone was used in this study than previous studies. It was suspected that more tissues were saved with the smaller optical zone. Recently, Wu et al.21 published one article focused on the ocular HOAs of SMILE and LASIK and found the SMILE procedure induced smaller changes of the ocular HOAs. However, the ocular HOAs may be interfered by the intraocular aberrations. More prospective, randomized collateral studies are needed to compare the corneal HOAs between SMILE and FS-LASIK, especially FS-LASIK of different excimer laser systems and different ablation modes. This may help surgeons to have a more precise understanding of the benefits of SMILE versus FS-LASIK regarding postoperative optical qualities.
There are some limitations in this study. Because it was a retrospective study, the authenticity of the results would probably have been better confirmed with prospective randomized controlled studies. To better understand the three procedures on postoperative visual quality, more studies are needed to investigate their differences on tear film irregularities, backward scatter, and so on.
In summary, coma and spherical aberration of the anterior cornea and total cornea significantly increased after SMILE, FS-LASIK, and FLEx surgeries. The SMILE procedure induced less spherical aberration of the anterior cornea and total cornea than FLEx surgery. Posterior corneal spherical aberration significantly increased after FS-LASIK surgery, and the FLEx procedure induced higher posterior corneal coma than SMILE surgery. However, further investigation is still needed to confirm these findings insofar as the possible relationship between biomechanics and optical effects.
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Keywords:© 2016 Contact Lens Association of Ophthalmologists, Inc.
SMILE; FS-LASIK; FLEx; Corneal higher-order aberrations; Posterior cornea