The comparison of visual outcomes, aberrations, and Bowman's layer micro-distortions after femtosecond laser small-incision lenticule extraction (SMILE) for the correction of high and moderate myopia and myopic astigmatism

Differences in ocular high order aberrations before and after small incision lenticule extraction for correction of myopia: a systematic review and meta-analysis

Objective

To examine the causes and factors that lead to high order aberration (HOA) during the treatment of myopia using small incision lenticule extraction (SMILE), as well as the differences between SMILE and other corneal refractive surgeries through a systematic review and meta-analysis.

Methods

A systematic search was conducted from January 2015 to February 2023 in Pubmed, Embase, Web of Science, and Google Scholar databases to gather relevant studies on SMILE and HOA. Studies meeting specific criteria were chosen, and clinical data was retrieved for analysis.

Results

This meta-analysis resulted in the inclusion of 19 studies involving 1,503 eyes. Pooled results showed significant induction of total HOA (tHOA, d = -0.21, p < 0.001), spherical aberration (SA, d = -0.11, p < 0.001) and coma aberration (CA, d = -0.18, p < 0.001) after SMILE compared to pre-SMILE, while no significant change in trefoil aberration (TA) was observed (d = -0.00, p = 0.91). There was a significantly lower induction of tHOA after SMILE compared to femtosecond laser-assisted in situ keratomileusis (FS-LASIK, d = 0.04, p < 0.001), and no significant difference was observed compared to wavefront aberration-guided (WFG) refractive surgery (d = 0.00, p = 0.75). There was also a significant association between different levels of myopia and astigmatism, duration of follow-up, lenticule thickness, and preoperative central corneal thickness (CCT) on the induction of tHOA after SMILE (p < 0.05), while the higher preoperative myopia group (sphere > -5D), lower preoperative astigmatism group (cylinder ≤ -1D), larger lenticule thickness group (lenticule thickness > 100 μm), shorter follow-up group (follow-up 1 month postoperatively) and the thicker CCT group (CCT > 550 μm) brought a significant induction of tHOA compared to the opposite comparison group (p < 0.001).

Conclusion

While SMILE can induce HOA significantly, it induces less HOA than FS-LASIK. Postoperative HOA following SMILE can be affected by factors such as myopia, astigmatism, lenticule thickness, CCT, and duration of follow-up. Future research should continue to explore techniques to decrease the induction of HOA by using this methodology.

Systematic review registration

https://www.crd.york.ac.uk/prospero/.

A Literature Review of the Incidence, Management, and Prognosis of Corneal Epithelial-Related Complications After Laser-Assisted In Situ Keratomileusis (LASIK), Photorefractive Keratectomy (PRK), and Small Incision Lenticule Extraction (SMILE)

Our purpose is to provide a comprehensive investigation into the incidence, treatment modalities, and visual prognosis of epithelial-related complications in corneal refractive surgeries, including laser-assisted in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and small incision lenticule extraction (SMILE). A systematic search of multiple databases was conducted by two independent examiners using various search terms related to epithelial-related complications and corneal refractive surgeries. A total of 91 research articles were included, encompassing a sample size of 66,751 eyes across the three types of surgeries. The average incidence of epithelial-related complications varied across the different types of corneal refractive surgeries. LASIK had an average incidence of 4.9% for epithelial defects, while PRK and SMILE had lower rates of 3.3% and 3.9%, respectively. Our findings indicate that SMILE has a lower incidence of epithelial defects compared to LASIK, potentially due to the less invasive nature of lenticule incision in SMILE. Visual prognosis after epithelial complications (EC) is generally favorable, with various supportive care and surgical interventions leading to significant improvements in postoperative visual acuity and full recovery. Understanding the incidence rates and management approaches for epithelial-related complications can guide clinicians in enhancing patient safety, refining surgical techniques, and optimizing postoperative outcomes in corneal refractive surgeries.

Correlation between the ablation ratio and posterior corneal stability after small incision lenticule extraction for high myopia

PURPOSE

The aim of this study is to investigate changes in posterior corneal elevation and their correlations with the ablation ratio 3 years after small incision lenticule extraction (SMILE) for high myopia.

METHODS

Eighty eyes underwent SMILE were enrolled in this study. Eyes were classified into two groups based on the ablation ratio (AR, lenticule thickness from SMILE machine/thinnest corneal thickness): group A (< 25%, 40 eyes) and group B (≥ 25%, 40 eyes). Pentacam was used to measure the posterior corneal elevation at the central point, thinnest point, and posterior maximum elevation (PME) and the mean posterior elevation in the central 2-mm area (MPE-2 mm), 4-mm area, and 6-mm area at the 3-year follow-up.

RESULTS

More than 85% of the eyes had an AR of less than 27%, and no cases of iatrogenic keratectasia developed. In both groups, for central region, posterior elevation decreased implying backward displacement; for peripheral region, it increased indicating forward trend. There was no significant difference in changes in all determined parameters between the two groups (P ≥ 0.07). Moreover, no significant correlation was noted between AR and posterior elevation changes. In group A, decreasing changes in PME (r =  - 0.42, P = 0.01) and MPE-2 mm (r = 0.40, P = 0.01) demonstrated negative correlations with residual bed thickness.

CONCLUSION

Region-dependent changes were demonstrated in the eyes that underwent SMILE. The central area showed a subtly declining posterior elevation, and the peripheral area showed a slightly increasing elevation. The limited ablation ratio had no impact on the changes in posterior corneal elevation.

Corneal morphological changes after small incision lenticule extraction for myopic anisometropia

Purpose

This research aims to study the corneal morphological changes in adult patients with myopic anisometropia after small incision lenticule extraction (SMILE) and the safety, efficacy, and predictability of clinical outcomes.

Methods

This was a prospective cohort study. Patients with myopic anisometropia [refractive difference &gt;2.0 diopters (D)] were included in this study who underwent SMILE at our hospital from September 2019 to March 2021. For the two eyes of each patient, the one with higher myopia was defined as group A, and the fellow eye was group B. The follow-up time points were set as 1 week, 1 month, 3 months, and 6 months after the surgery. The data collected were uncorrected and best-corrected distance visual acuity (UDVA and CDVA), spherical equivalent (SE), efficacy and safety indexes, posterior corneal elevation (PCE), anterior and posterior corneal radius of curvature in the 3 mm area at the center of the thinnest point of the cornea (ARC and PRC), and higher-order aberrations (HOAs).

Results

The study included 36 patients (72 eyes), and the mean age was 25.2 ± 6.4 years. The preoperative SEs were −6.45 ± 1.25 D in group A and −3.76 ± 1.29 D in group B. Six months after surgery, the SEs in groups A and B were −0.09 ± 0.50 D and 0.07 ± 0.47 (P = 0.059), respectively. The efficacy indexes were 1.06 ± 0.16 in group A and 1.07 ± 0.14 in group B (P = 0.750). The safety indexes were 1.08 ± 0.14 in group A and 1.12 ± 0.15 in group B (P = 0.173). The PCE was significantly reduced at 6 months after surgery in pagebreak both groups (P &lt; 0.05). The ARC was significantly higher than before the surgery (P &lt; 0.05) in the two groups. The two groups showed significant increases in total HOAs, coma 90°, and spherical aberrations (P &lt; 0.05).

Conclusion

SMILE is predictable, effective, and safe in correcting myopic anisometropia. The postoperative changes in HOAs are characteristic.

Long-Term Observation of Higher-Order Aberrations and Microdistortions in Bowman’s Layer After Small Incision Lenticule Extraction for the Correcting Myopia With Spherical Equivalent Higher Than −9.0 Diopters

Purpose

To evaluate the outcomes in corneal higher-order aberrations (HOAs) and microdistortions in the Bowman’s layer after femtosecond laser small incision lenticule extraction (SMILE) for correcting extremely high myopia.

Methods

This prospective study included patients with myopia with spherical equivalent ≥ -9.0 Diopters (D). SMILE was performed in forty eyes of 40 patients. Pentacam was used to evaluate HOAs before and at 1 day, 3 months, 6 months, and 2 years after surgery. Fourier-domain optical coherence tomography was used to evaluate microdistortions at 2 years postoperatively. Thirty-two eyes of 32 patients receiving femtosecond laser-assisted in situ keratomileusis (FS-LASIK) were enrolled as the control group. HOAs were measured before, at 1 day and at least 1 year postoperatively.

Results

After SMILE, the long-term safety and effectiveness index was 1.25 and 0.85, respectively. Microdistortions were observed in 73.5% of the eyes at 2 years, with an average number of 1.20 ± 1.22 microdistortions and an average width of 287.37 ± 259.00 μm. We detected more microdistortions in the horizontal meridian than in the vertical meridian (p = 0.035). The average number and width of microdistortions were both higher in the central region (≤4 mm) than in the peripheral region (4–8 mm) (both p < 0.001). With the exception of horizontal trefoil in the SMILE group and vertical trefoil in the FS-LASIK group, significant changes over time were observed in all other HOAs (all p < 0.05). Meanwhile, we detected significant increases in the total corneal HOA, spherical aberration (SA), and coma at all time-points after both surgeries (all p < 0.01). Compared with FS-LASIK, SMILE induced less SA (p < 0.001) and more horizontal coma (p = 0.036). In the SMILE group, the HOA, SA, and trefoil were more in the small optical zone (≤6.0 mm) than in the large optical zone (>6.0 mm) (all p < 0.05). The increase in SA and most trefoil correlated with the mean number of central microdistortions number (all p < 0.05).

Conclusion

For myopia over −9.0D, the microdistortions in the Bowman’s layer were still detectable in most eyes long-term after SMILE. Both SMILE and FS-LASIK induced more HOAs, mainly HOA, SA, and coma. The small optical zone and microdistortions may affect postoperative aberrations.

Results of thin-cap small-incision lenticule extraction

PURPOSE

To evaluate the influence of a thin cap in small-incision lenticule extraction (SMILE) for the correction of myopia or myopic astigmatism.

SETTING

Tertiary care private practice.

METHODS

A chart review of 102 eyes of 51 patients was performed. The effect of 120 µm vs 100 μm cap thickness on postoperative spherical equivalent refraction (SEQ), cylinder, corrected and uncorrected visual acuities, and ease of lenticule separation was assessed in a contralateral manner, whereas all other parameters were identical between eyes (including optical zone, minimum lenticule thickness, incision size, and energy and spot settings) using paired t test.

RESULTS

At 3 months postoperatively, SMILE with 120 μm cap thickness was undercorrected in SEQ relative to SMILE with 100 μm cap thickness in a cohort of 102 eyes of 51 consecutive patients. The difference of 0.06 ± 0.39 diopter (or 0.7% ± 5.7%) did not reach statistical significance. Postoperative cylinder was not statistically different in both groups. Visual acuity was similar in both groups. Ease of lenticule separation was identical in both groups. Suction time was shorter with a 100 μm cap (P < .005). Postoperative central residual stromal thickness was 20 ± 15 μm thicker with a 100 μm cap (P < .0001). Adverse events were comparable.

CONCLUSIONS

Postoperative refraction, visual acuity, ease of lenticule separation, and incidence of adverse events were not significantly affected by cap thickness. Surgeons might safely use 100 μm instead of 120 μm caps without nomogram adjustment. Alternatively, after suction loss during the lenticule cut, a second docking with a programmed cap thickness of 100 μm (and a larger optical zone) might be a rescue technique enabling surgeons to still perform the intended SMILE procedure.

Clinical Outcomes of Small Incision Lenticule Extraction in Myopia: Study of Vector Parameters and Corneal Aberrations

PURPOSE

To investigate clinical outcomes of small incision lenticule extraction (SMILE) including vector parameters and corneal aberrations in myopic patients.

METHODS

This retrospective, observational case series included 57 eyes (29 patients) that received treatment for myopia using SMILE. Visual acuity measurement, manifest refraction, slit-lamp examination, autokeratometry, corneal topography, and evaluation of corneal wavefront aberration were performed preoperatively and at 1 and 3 months after surgery. We analyzed the safety, efficacy, vector parameters, and corneal aberrations at 3 months after surgery.

RESULTS

Preoperatively, mean manifest refraction spherical equivalent refraction was -4.94 ± 1.94 D (range, -8.25 to 0 diopters [D]), and the cylinder was -1.14 ± 0.82 D (range, -3 to 0 D). Mean manifest refraction spherical equivalent improved to -0.10 ± 0.23 D at 3 months postoperatively, when uncorrected distance visual acuity was 20 / 20 or better in 55 (96%) eyes. The linear regression model of target induced astigmatism vector versus surgically induced astigmatism vector exhibited slopes and coefficients (R²) of 0.9618 and 0.9748, respectively (y = 0.9618x + 0.0006, R² = 0.9748). While total corneal root mean square higher order aberrations, coma and trefoil showed statistically significant increase, spherical aberration did not show statistically significant change after SMILE.

CONCLUSIONS

SMILE has proven to be effective and safe for correcting myopia and astigmatism. We showed that SMILE did not induce spherical aberrations. A small increase in postoperative corneal higher order aberration may be associated with increase in coma and trefoil.

Matched comparison of corneal higher order aberrations induced by SMILE to femtosecond assisted LASIK and to PRK in correcting moderate and high myopia: 3.00mm vs. 6.00mm

Background

The refractive surgeries induce corneal higher order aberrations (C-HOAs). In this study, change of C-HOAs after small-incision lenticule extraction (SMILE) compared to femtosecond assisted laser in situ keratomileusis (femto-LASIK), and to photorefractive keratectomy with mitomycin-C (PRK) under photopic and mesopic conditions.

Methods

In this prospective study, age, gender, and apical corneal thickness (ACT) matched cases with moderate myopia [spherical equivalent (SE) 3.00 to 6.00D) to high myopia (SE > 6.00D)] were enrolled. In addition to visual acuity and refraction, total C-HOA, coma, spherical aberration (SA), and trefoil in the 3- and 6-mm zones were measured before and 3 and 6 months after surgery.

Results

Overall, 372 moderate myopia cases (124 eyes of 124 individuals in each surgical group) and 171 high myopia cases (57 eyes of 57 individuals in each surgical group) were enrolled. At baseline, the differences in age, gender, ACT, uncorrected and corrected visual acuity, and SE were not statistically significant between subgroups of surgical methods within each myopia group (all P > 0.05). At 12 months, in the moderate myopia group, there was less increase in 6-mm zone total C-HOA, coma, and SA with SMILE compared to the other groups (all P < 0.05). In the high myopia group, there was greater increase in photopic total C-HOA and trefoil and less increase in mesopic SA with SMILE (all P < 0.05).

Conclusions

In correction of moderate myopia, SMILE has better results in mesopic condition. In high myopia correction, femto-LASIK and PRK have better results in photopic and SMILE in mesopic condition.

Complications of small incision lenticule extraction

The procedure of small incision lenticule extraction (SMILE) was introduced in 2011, and since then there has been an increase in the number of cases undergoing this procedure worldwide. The surgery has a learning curve and may be associated with problems in the intraoperative and postoperative periods. The intraoperative problems during SMILE surgery include the loss of suction, the occurrence of altered or irregular opaque bubble layer and black spots, difficulty in lenticular dissection and extraction, cap perforation, incision-related problems, and decentered ablation. Most of the postoperative problems are similar as in other laser refractive procedures, but with decreased incidence. The identification of risk factors, clinical features, and management of complications of SMILE help to obtain optimum refractive outcomes.

Visual Outcomes after Small Incision Lenticule Extraction and Femtosecond Laser-Assisted LASIK for High Myopia

PURPOSE

The aim of this study was to compare visual outcomes after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for high myopia.

METHODS

In this prospective, comparative study, a total of 52 eyes of 34 consecutive highly myopic patients with spherical equivalent within the range of -8.00 to -10.00 diopters were recruited. Twenty-three eyes of 16 patients were in the FS-LASIK group and 29 eyes of 18 patients were in the SMILE group. Visual outcomes and wavefront aberrations were analyzed preoperatively and 6 months postoperatively.

RESULTS

At the postoperative 6-month visit, 96.6% in the SMILE group and 91.3% in the FS-LASIK group achieved unchanged or better corrected distance visual acuity (CDVA). As for uncorrected distance visual acuity (UDVA), 96.6% in the SMILE group and 95.7% in the FS-LASIK group achieved UDVA of 20/20 or better. As for wavefront aberrations, high-order aberrations (HOAs) and spherical aberrations increased significantly after surgery in both groups relative to corresponding preoperative values (p < 0.001), and vertical coma increased after SMILE (p < 0.001). No statistically significant differences in changes of HOAs (p =0.90), spherical aberrations (p = 0.07), horizontal coma (p = 0.56), coma (p =0.08), horizontal trefoil (p =0.19), vertical trefoil (p = 0.90), and trefoil (p = 0.45) were detected between the 2 groups, except for vertical coma (p < 0.01).

CONCLUSIONS

SMILE is as effective as FS-LASIK in correcting high myopia, but attention should be paid to the induction of vertical coma in highly myopic patients following a SMILE procedure.