Impact of Treatment Decentration on Higher-Order Aberrations after SMILE

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/.

Comparing Effective Optical Zones After Myopic Ablation Between LASIK, PRK, and SMILE With Correlation to Higher Order Aberrations

PURPOSE

To explore size, decentration, and eccentricity of effective optical zones (EOZs) in laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and small incision lenticule extraction (SMILE) and correlate them to higher order aberrations (HOAs).

METHODS

This was a retrospective chart review of 188 eyes that underwent refractive surgery for compound myopia (61 LASIK, 84 PRK, 43 SMILE). EOZ measurements were determined using 1-year postoperative Pentacam (Oculus Optikgeräte GmbH) tangential difference maps. HOA data were measured using Pentacam wavefront aberration Zernike polynomials. Correlations between EOZs and HOAs were analyzed.

RESULTS

The EOZs of LASIK and PRK are smaller than SMILE at 19.54 ± 1.44, 19.39 ± 1.66, and 22.18 ± 2.61 mm2, respectively (P < .001). No difference existed in absolute decentration from corneal vertex (P = .078) or pupil center (P = .131), but horizontal and vertical components differed significantly (P < .001). Smaller EOZ areas were correlated with greater spherical aberration induction (rLASIK = -0.378, rPRK = -0.555, rSMILE = -0.501) and total HOA induction in all groups. Absolute decentration from corneal vertex positively correlated with total HOA (rLASIK = 0.396, rPRK = 0.463, rSMILE = 0.399) and directional vertical coma induction negatively correlated with vertical decentration from the corneal vertex (rLASIK = -0.776, rPRK = -0.665, rSMILE = -0.576) in all groups.

CONCLUSIONS

SMILE results in a larger EOZ than LASIK and PRK, and absolute decentration remains comparable regardless of surgical reference center, despite horizontal/vertical differences. Surgical planning to ensure adequate EOZ size and centration may reduce induction of HOAs, including spherical aberrations and vertical coma. [J Refract Surg. 2023;39(11):741-750.].

Methods of Corneal Vertex Centration and Evaluation of Effective Optical Zone in Small Incision Lenticule Extraction

Inappropriate small incision lenticule extraction (SMILE) centration methods can affect the decentration of the effective optical zone (EOZ) after operation, which can subsequently lead to the decline of postoperative visual quality. We aimed to provide an overview of corneal vertex (CV) centration methods and an evaluation of the size and decentration of the EOZ in SMILE. We described the CV centration methods for patients with myopia, myopic astigmatism, hyperopia, and large kappa angle. The measurement methods of the EOZ were evaluated from the aspects of corneal morphology and corneal refractive power. Additionally, we summarized the advantages and disadvantages of measuring decentration based on topographic mapping and intraoperative video-captured images. Finally, we discussed the relationship between the EOZ and visual quality. Based on our review, clinicians should consider the following when choosing CV centration methods and evaluating EOZ postoperatively. First, the tear film mark center or topographic map comparison method is preferred for the correction of myopia, low myopic astigmatism, hyperopia, and large kappa angle (>0.2 mm). Triple marking centration is recommended for high myopic astigmatism (-3.5 diopters). Second, the total corneal power better reflects the change in refractive power than the topographic method. The measurement of the area rather than the diameter of the total corneal refractive power is more suitable for the evaluation of noncircular EOZs after high myopia astigmatism (<-2.0 diopters). Third, for the evaluation of decentration, the tangential curvature difference map method is preferred as it is not influenced by offset pupils. Finally, a large EOZ after SMILE may improve patient tolerance to decentration.

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.

Two-year outcomes after full-thickness astigmatic keratotomy combined with small-incision lenticule extraction for high astigmatism

Background

To evaluate clinical outcomes after full-thickness astigmatic keratotomy (FTAK) combined with small-incision lenticule extraction (SMILE) in eyes with high astigmatism.

Methods

This study comprised 75 eyes of 43 patients with over 4.0 diopters (D) of astigmatism who were treated with SMILE after FTAK. Visual acuities and refractive measurements were evaluated at 1 month after FTAK, and 1, 6, 12, and 24 months after SMILE. Vector analysis of the astigmatic changes was performed using the Alpins method.

Results

Twenty-four months after the combined procedure, the average spherical equivalent was reduced from − 6.56 ± 2.38 D to − 0.36 ± 0.42 D (p < 0.001). The uncorrected and corrected distance visual acuities improved from 1.54 ± 5.53 to − 0.02 ± 0.09 and from − 0.03 ± 0.07 D to − 0.07 ± 0.08 D (both p < 0.001), respectively. The preoperative mean astigmatism was − 5.48 ± 1.17 D, which was reduced to − 2.27 ± 0.97 D and − 0.34 ± 0.26 D at 1 month after FTAK and 24 months after SMILE, respectively (p < 0.001). The surgically-induced astigmatism after FTAK, SMILE, and FTAK and SMILE combined was 3.38 ± 1.18 D, 2.22 ± 0.84 D, and 5.39 ± 1.20 D, respectively. Furthermore, the correction index of FTAK, SMILE, and FTAK and SMILE combined was 0.63 ± 0.17, 0.90 ± 0.40, and 0.98 ± 0.06, respectively. There were no intraoperative or postoperative complications.

Conclusion

Our surgical procedure combining FTAK and SMILE showed good and stable clinical outcomes during two-year follow-up for the treatment of high astigmatism.

Comparison of refractive outcomes and high-order aberrations after small incision lenticule extraction and wavefront-guided femtosecond-assisted laser in situ keratomileusis for correcting high myopia and myopic astigmatism

PURPOSE

To compare the results of using small incision lenticule extraction (SMILE) and wavefront-guided femtosecond-assisted laser in situ keratomileusis (WFG FS-LASIK) to correct high myopia and myopic astigmatism.

MATERIAL AND METHODS

The 94 eyes of 47 patients with high myopia or myopic astigmatism, if not both, who had undergone SMILE were compared with the 94 eyes of 47 patients with high myopia or myopic astigmatism, also if not both, who had undergone WFG FS-LASIK. Only eyes with high myopic or myopic astigmatism errors greater than - 6.0 diopter (D) spherical refraction and 0-3 D cylindrical refraction were included. Values of uncorrected distance visual acuity, corrected distance visual acuity, efficacy index, safety index, predictability, and high-order aberration between the patient groups were compared.

RESULTS

The SMILE and WFG FS-LASIK groups did not significantly differ according to sex or age. Values of preoperative and postoperative spherical refraction, cylindrical refraction, spherical equivalent, uncorrected distance visual acuity, and corrected distance visual acuity between the groups also did not significantly differ nor did values of predictability, the efficacy index, or the safety index. SMILE induced more coma and trefoil (p < 0.001), whereas WFG FS-LASIK induced more spherical aberration (p < 0.001).

CONCLUSION

Both SMILE and WFG FS-LASIK are efficient, safe, predictable procedures for correcting high myopia and myopic astigmatism. SMILE may induce more coma and trefoil, whereas WFG FS-LASIK may induce more spherical aberration.

Six modes of corneal topography for evaluation of ablation zones after small-incision lenticule extraction and femtosecond laser-assisted in situ keratomileusis

PURPOSE

To evaluate the ablation zone diameter (AZD) using six modes of corneal topography after small-incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for myopia and to compare the programmed and postoperative AZDs METHODS: This retrospective comparative study included 86 right eyes in 86 patients who underwent SMILE or FS-LASIK at the Shandong Eye Institute between June 2016 and August 2017. Data were collected preoperatively and at 1, 3, and 6 months postoperatively using the Pentacam system. The AZD was determined using six modes of corneal topography: postoperative axial curvature topography (PACT), postoperative tangential curvature topography (PTCT), difference-axial curvature topography (DACT), difference-tangential curvature topography (DTCT), postoperative front elevation topography (PFET), and difference-corneal thickness topography (DCTT). The difference between the programmed and postoperative AZDs was compared between the groups of two surgical procedures.

RESULTS

At 3 months, the AZDs measured by PTCT, DTCT, and PFET after SMILE showed a significant decrease, while the AZD by DACT revealed an increase (P < 0.001), and the AZDs measured by PACT, PTCT, DACT, DTCT, PFET, and DCTT after FS-LASIK were all significantly decreased (P < 0.001), as compared with the programmed ablation zone values. The AZDs measured by DACT and DTCT after 3 months were larger in the SMILE group (P < 0.001); likewise, the differences between the programmed and postoperative AZDs were lower in the SMILE group (P < 0.001). Furthermore, PACT, PTCT, DACT, DTCT, and PFET indicated acceptable inter-observer reliability (ICC > 0.75).

CONCLUSIONS

In the treatment of myopia, the achieved topographic ablation zones after FS-LASIK were smaller than the theoretically designed ones. The AZDs after SMILE were larger than those after FS-LASIK, with an equivalent programmed value. The DACT may be more suitable for measuring postoperative ablation zones in clinic with clearer borders and favorable repeatability. This method may be potential for the measurement of ablation zones and further research on the visual quality after refractive surgery.

Review on Centration, Astigmatic Axis Alignment, Pupil Size and Optical Zone in SMILE

The advent of "flapless" small-incision lenticule extraction (SMILE), employing all-in-one technology, has resulted in a revolutionary breakthrough in refractive surgeries. SMILE has been gaining popularity due to fewer potential complications, such as postoperative dry eyes and greater biomechanical stability, etc. However, attention must be given to 1) the centration on the corneal vertex, 2) the proper alignment of the astigmatic axis, and 3) the relationship between pupil size and treatment diameter, to achieve good SMILE results. There is no pupil-tracking system to ascertain the accuracy of centration during the SMILE surgery. To improve the centration accuracy, our center uses two corneal topographers (Pentacam and Sirius) to measure and determine corneal vertex. Proper predicted optical zone diameter is not clearly defined yet in SMILE. Some scholars insist that mesopic pupil size should be taken into consideration when setting the predicted optical zone. Meanwhile, the issue of "functional optical zone" still has many unresolved issues and warrants further studies.

SMILE intraoperative complications: incidence and management

AIM

To report the intraoperative complications associated with small incision lenticule extraction (SMILE) and their management.

METHODS

This was a retrospective consecutive interventional clinical study, carried out on patients with myopia and myopic astigmatism, who underwent SMILE procedure. Type of intraoperative complications and their management were recorded.

RESULTS

Our study comprised 282 eyes of 141 patients who were enrolled for SMILE surgeries. The intraoperative complications included lost vacuum (18 eyes, 6.38%), treatment decentration (6 eyes, 2.12%), wound bleeding (21 eyes, 7.45%), incomplete bubble separation (black islands) (3 eyes, 1.06%), the epithelial defects (15 eyes, 5.32%). Incision tear (27 eyes, 9.57%), lenticule adherence to the cap (6 eyes, 2.12%), and cap perforation occurred in 2 eyes (0.7%).

CONCLUSION

Although SMILE is a promising technique for the correction of myopia and myopic astigmatism with predictable, efficient, safe refractive and visual outcomes, complications can occur. However, most of them are related to inexperience and are included in the learning curve of the technique. More studies with a bigger number of eyes are required to efficiently evaluate the intraoperative complications and standardize their management strategies.

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

Background

To investigate corneal higher-order aberrations (HOAs) of the anterior surface, posterior surface, and total cornea after small incision lenticule extraction (SMILE) in high myopic and mild to moderate myopic patients.

Methods

This retrospective study included 197 eyes (101 patients) undergoing SMILE surgery. According to the preoperative spherical equivalent (SE), treated eyes were divided into two groups: a high myopic group (more than − 6.0 D, Group H) and a mild to moderate myopic group (less than − 6.0 D, Group M). Corneal HOAs of the anterior surface, posterior surface, and total cornea were measured using a Scheimpflug camera preoperatively and 3 months postoperatively. Pearson’s correlation analysis was conducted to determine relationships between corneal aberrations and the SE.

Results

There were no significant differences in third-order to eight-order aberrations (RMS HOAs) of the anterior surface, posterior surface, and total corneal between the two groups before SMILE surgery. However, after SMILE, anterior and total corneal HOAs, especially vertical coma and spherical aberrations, significantly increased in both groups (p < 0.0167), whereas posterior corneal HOAs remained relatively stable (p > 0.0167). The induction of HOAs was significantly greater in Group H than Group M postoperatively (p < 0.0167). Changes in anterior surface and total corneal HOAs, especially vertical coma and spherical aberrations, were related to the SE (p < 0.05).

Conclusions

Anterior and total corneal HOAs, particularly vertical coma and spherical aberrations, significantly increased after SMILE in both groups, whereas posterior corneal HOAs remained stable. Aberration changes were related to SE.

Trial registration

Retrospectively registered. ChiCTR-ORC-17011040. Registered 1 April 2017. Name of registry: The observation of clinical results after corneal refractive surgery. Data of enrolment of the first participant to the trial: 15 December 2016.

Lamellar keratoplasty using femtosecond laser intrastromal lenticule for limbal dermoid: case report and literature review

Small incision refractive lenticule extraction (SMILE) is a femtosecond laser technique to correct myopia and myopic astigmatism. Herein, we report a technique where intrastromal lenticule obtained from the SMILE procedure served as a graft for lamellar keratoplasty in the management of a limbal dermoid. An 18-year-old woman presented to the clinic with a corneal-limbal mass in the right eye. Slit-lamp examination revealed a vascularized circular mass of approximately 6 mm × 5 mm, which was attached at 7 o’clock in the inferotemporal region of the corneal limbus; this suggested limbal dermoid. Anterior segment optical coherence tomography revealed superficial involvement of the cornea. The patient was treated with excision and lamellar keratoplasty by using femtosecond intrastromal lenticule. The lenticule was sutured over the cornea with 10-0 interrupted nylon sutures. On postoperative follow-up, best-corrected visual acuity was 20/20; there was no corneal neovascularization and no sign of rejection. This case of limbal dermoid was managed by simple surgical excision and lamellar keratoplasty with a SMILE-extracted lenticule. This method may serve as an alternative surgical approach for management of limbal dermoid.

Corneal Spherical Aberration and Corneal Asphericity after Small Incision Lenticule Extraction and Femtosecond Laser-Assisted LASIK

Purpose

To investigate corneal spherical aberration and corneal asphericity after small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK).

Methods

This study enrolled 70 patients having SMILE and 64 subjects receiving FS-LASIK. The preoperative spherical equivalent (SE) was −5.83 ± 1.23 diopters (D) and −6.20 ± 1.52 D, respectively. The uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), SE, corneal spherical aberration, and asphericity over the 6.0 mm cornea were evaluated preoperatively and postoperatively.

Results

At 6 months, the UDVA, CDVA, and SE were −0.12 ± 0.11, −0.05 ± 0.05, and −0.16 ± 0.19 D in SMILE and −0.10 ± 0.06, −0.03 ± 0.06, and −0.08 ± 0.25 D in FS-LASIK. There was no difference between groups in the postoperative UDVA, CDVA, or SE (P > 0.05). SMILE showed lower inductions of spherical aberration along the anterior surface and the total cornea and less increases in corneal asphericity of the anterior surface postoperatively than FS-LASIK (P < 0.01). There were significant correlations between the changes in spherical aberration and corneal asphericity (P < 0.001).

Conclusions

SMILE and FS-LASIK exhibited excellent visual results and refractive outcomes. SMILE induced less increase in corneal spherical aberration and better preserved the corneal asphericity of the anterior corneal surface than FS-LASIK. Corneal asphericity changes contributed to the corneal spherical aberration changes following SMILE and FS-LASIK.

Comparison of visual results and higher-order aberrations after small incision lenticule extraction (SMILE): high myopia vs. mild to moderate myopia

Background

To compare the refractive results and higher-order aberrations (HOAs) after small incision lenticule extraction (SMILE) in high myopia and mild to moderate myopia patients.

Methods

This prospective study included 165 eyes (86 patients) undergoing SMILE. According to the preoperative spherical equivalent (SE), treated eyes were divided into two groups: the high myopia group (more than -6.0 D, group-H) and the mild to moderate group (less than -6.0 D, group-M). Follow-up intervals were at 1 day, 10 days, 1 month and 3 months postoperatively. We obtained the following parameters: uncorrected (UDVA) and corrected distance visual acuity (CDVA), SE, efficacy and safety index, and HOAs.

Results

Preoperative SE was -7.16 ± 0.93 D in group-H and -4.34 ± 0.97 D in group-M. At 3 months postoperatively, the SE in group-H and group-M was -0.20 ± 0.37 D and 0.01 ± 0.19 D (t = - 4.11, P<0.05), respectively. It was found that 77% and 98% had an UDVA of 20/20, 98% and 99% had a CDVA of 20/20 in group-H and group-M, respectively, while 87% and 95% had a SE within ±0.5 D and ±1.0 D in group-H, and 98% and 100% in group-M. The efficacy indexes were 0.98 ± 0.18 in group-H and 1.05 ± 0.10 in group-M (t = - 3.084, p < 0.05). The safety indexes were 1.06 ± 0.09 and 1.06 ± 0.09 (t = 0.153, p > 0.05), respectively. There were significant increases in total HOAs, 3^rd-order coma, and 4^th-order spherical aberrations.

Conclusions

SMILE is an effective and safe surgery for correcting myopia. But the target correction amount in high myopia patients should be adjusted to avoid undercorrection and acquired more satisfaction. SMILE induced increases of HOAs.

Trial registration

ChiTrial registration number: ChiCTR-OON-16009164. Retrospectively registered: 06.September.2016