Corneal higher-order aberrations of the anterior surface, posterior surface, and total cornea after small incision lenticule extraction (SMILE): high myopia versus mild to moderate myopia

Visual acuity is the most commonly used parameter for assessments of overall visual function in the clinical setting. However, under dim light conditions or low contrast sensitivity conditions, visual acuity does not truly reflect a patient’s subjective vision function [1], especially after corneal refractive surgeries. Higher-order aberrations (HOAs) increase after successful refractive surgery and influence the vision quality of patients. Measurements of changes in wavefront aberrations are used to evaluate the impact of refractive surgery on vision quality [1]. Previous studies indicated that posterior corneal HOAs influenced corneal biomechanical responses and provided valuable information in determining the cause of poor vision quality after corneal refractive surgery [2, 3].

Small incision lenticule extraction (SMILE) surgery is a newly developed corneal refractive technique, which is less invasive than LASIK (Laser-assisted in situ keratomileusis) and FS-LASIK (femtosecond laser-assisted LASIK), with only a small incision required [4, 5]. Recent studies suggested that the SMILE procedure provided excellent clinical outcomes, considering its safety, efficacy, predictability, and postoperative ocular surface health [6–8]. Therefore, SMILE is considered to be a good choice for refractive surgery. Previous studies of changes of corneal HOAs after the SMILE procedure focused mainly on anterior corneal HOAs or total corneal HOAs [8–11]. Only a few studies have compared anterior, posterior, and total corneal HOAs following different refractive surgery procedures [2, 3, 12]. The potential effect of the SMILE procedure on posterior corneal HOAs is not well understood. In this study, we compared HOAs of the anterior surface, posterior surface, and total cornea after SMILE surgery for high myopia and mild to moderate myopia.

This study is the first to compare corneal HOAs of the anterior surface, posterior surface, and total cornea in high and mild to moderate myopic eyes after SMILE surgery. The results indicated that HOAs of the anterior cornea surface and total cornea significantly increased after SMILE surgery in both groups. However, HOAs of the posterior corneal surface remained relatively stable. These results are consistent with those of other studies, which indicated that posterior corneal HOAs could complement corneal topography information, provide insight into the corneal biomechanical response, and provide valuable information that could be used to determine the cause of poor vision quality after corneal refractive surgery [2, 3].

This study revealed no significant differences in anterior cornea surface, posterior cornea surface and total corneal HOAs between high and mild to moderate myopic groups before SMILE surgery. However, significant between-group differences in anterior cornea surface HOAs and total corneal HOAs were detected postoperatively. In terms of SMILE-induced HOAs, there were significantly more HOAs in the high myopic group than mild to moderate group postoperatively, whereas HOAs of the posterior cornea remained almost unchanged postoperatively in both groups. The lack of significant changes in the posterior corneal surface is in agreement with that of previous studies, which examined the posterior corneal surface after PRK (Photo Refractive Keratectomy), FS-LASIK, and SMILE using Pentacam [3, 12, 13]. Wu et al. compared HOAs after SMILE, FS-LASIK, and FLEx (Femtosecond Lenticule Extraction) surgeries [3]. They reported a value of 0.19 ± 0.03 μm for third-order to eight-order aberrations of the posterior corneal surface in each surgical group postoperatively. They found that these HOAs not only remained stable postoperatively but were similar to values for HOAs obtained prior to surgery. In the present study, the number of third-order to eight-order aberrations of the posterior corneal surface in each group preoperatively and postoperatively was similar to that found by Wu [3]. Gyldenkerne et al. reported that the posterior corneal surface showed almost no change after FS-LASIK and SMILE procedures, which was consistent with our study [13]. Juhasz et al. analyzed changes in anterior and posterior corneal surface HOAs after PRK [12]. They found that PRK-induced HOAs increased significantly more than 76.78 μm ablation depths. And the HOAs on the anterior corneal surface increased total corneal aberrations. However, posterior corneal surface HOAs remained relatively stable after surgery. They also pointed out that aberrations of the anterior corneal surface were statistically significantly higher than that of the total cornea, indicating that the posterior corneal surface plays a compensatory role in the balance of corneal aberrations in myopic eyes.

This study also found that spherical aberrations induced by the SMILE procedure increased in the anterior corneal surface and total cornea. The induction of spherical aberrations was significantly greater in the high myopic group than mild to moderate group. However, spherical aberrations of the posterior corneal surface remained almost unchanged in both groups after surgery. SMILE-induced coma aberrations, especially vertical coma aberrations of the anterior surface, posterior surface, and total cornea changed in both groups, with a significantly greater increase in vertical coma aberrations in the high myopic group than mild to moderate group. These results were similar to study of Wu et al. [3] Their study indicated that FS-LASIK, FLEx, and SMILE surgeries induced spherical aberrations and coma of the anterior surface and total cornea. The SMILE surgery induced fewer spherical aberrations of the anterior cornea and total cornea than FLEx procedure, and posterior corneal spherical aberrations significantly increased after FS-LASIK surgery. Thus, the SMILE surgery seems to induce fewer posterior corneal coma aberrations as compared with the FLEx surgery. Another study that compared aberrations induced by FLEx and wavefront-guided LASIK procedures indicated that FLEx induced fewer spherical aberrations but the same amount of coma aberrations as compared with that observed using wavefront-guided LASIK1 [14]. Gertnere et al. found a reduced incidence of spherical aberrations in a FLEx group but a higher incidence of induced coma aberrations in an FS-LASIK group [15]. Lin et al. compared changes in aberrations induced by FS-LASIK and SMILE surgery and found a significantly lower incidence of spherical aberrations in the SMILE procedure [16]. The aforementioned studies used different LASIK ablation techniques and different microkeratomes for cutting the LASIK flap. The use of different LASIK ablation techniques and diverse methods of flap creation may result in substantial differences in HOAs [13]. The increased number of induced aberrations observed with the FS-LASIK procedure seems to be primarily associated with the ablation of corneal tissue rather than the creation of the flap, as the flap-dependent FLEx procedure does not seem to be different from that used in SMILE [13]. In the present study, there were significantly more surgically induced aberrations in the high myopia group than mild and moderate myopia group, and more central corneal tissue was removed in the high myopia group than mild to moderate myopia group. The correlation study also indicated changes in the induction of aberrations in the two groups were related to the preoperative SE, which was consistent with the findings of the study by Chen et al. [17].

Previous studies indicated that centeration and wound healing might influence the induction of coma aberrations [18, 19]. Li et al. demonstrated that horizontal decentration induced horizontal coma aberrations but that there appeared to be no association between the magnitude of vertical decentration and induction of vertical coma aberrations [20]. In our previous study, we reported that spherical aberrations and horizontal coma aberrations increased significantly after SMILE surgery and that the increase of spherical aberrations was higher in Group H than that in Group M [8]. This result was similar to findings presented by Liang et al. [21]. The difference in these results may be due to different methods used to evaluate corneal aberrations. In our previous study, we used a Hartmann–Shack WASCA aberrometer (Carl Zeiss Meditec AG, Jena, Germany). To optimize postoperative vision quality, more studies are needed to investigate the resources of the vertical coma and horizontal coma.

It is known that tear film problems that might influence the measurement of the aberrations of the anterior corneal surface, especially in dry eyes. Recently, Jung reported that total HOA RMS, coma and trefoil significantly increased at 10 s after blinking compared with those measured immediately after blinking in dry eye patients after LASEK (Laser Assisted Subepithelial Keratomilesusis) [22]. In the study of Elmohamady [23], they evaluated dry eye after LASIK, FS-LASIK, and SMILE. They found the mean ocular surface disease index (OSDI) scores were significantly elevated in all groups postoperatively but were significantly lower in the SMILE group 3 months postoperatively. The mean tear breakup time (TBUT) was significantly decreased in all groups postoperatively but was significantly higher in the SMILE group 6 months postoperatively. This result indicated the influence on dry eye was minimal after SMILE surgery. In our study, there were no significant differences in aberrations in the two groups after SMILE surgery 1 and 3 months postoperatively. This result may be due to the relatively small interference of SMILE on the tear film. Of course, long-term follow-up is still needed.

There were some limitations in this study. First, this study included 197 eyes, the two groups were not of equal size, and available data covered only 3 months. Longer term follow-up visits would have been desirable. Second, for bilaterally treated patients, there may be a correlation between the two eyes of one patient. This is a common mistake in ophthalmology research, for the overall variance of a sample of measurements combined from both eyes is likely to be an underestimate of the true variance resulting in an increased risk of a Type 1 error [24]. Future studies of the association of HOAs and corneal biomechanics with vision quality are needed to shed light on.

In conclusion, third-order to eight-order aberrations, particularly spherical aberrations and vertical coma aberrations of the anterior cornea and total cornea significantly increased after SMILE surgeries. In contrast, posterior corneal surface HOAs remained relatively unchanged. The induction of aberrations postoperatively was related to the preoperative SE. Further and larger studies, with longer-term follow-ups are needed.