Comparison of surgical outcomes following femtosecond laser-assisted LASIK (Femto-LASIK) versus small incision lenticule extraction (SMIL) for myopia and astigmatism

doi:10.21203/rs.3.rs-3238999/v1

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Comparison of surgical outcomes following femtosecond laser-assisted LASIK (Femto-LASIK) versus small incision lenticule extraction (SMIL) for myopia and astigmatism

https://doi.org/10.21203/rs.3.rs-3238999/v1

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Purpose

To compare the visual, refractive, and topographic outcomes of patients who underwent Femto-LASIK and SMILE.

Methods

This historical cohort study selected 83 eyes of 83 patients with myopic and myopic astigmatism. Patients were divided into two groups based on the surgical procedure: femto-LASIK-treated (46 patients) and SMILE-treated patients (37 eyes). In all patients, corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA), spherical component, spherical equivalent (SE), cylindrical component, and some tomography readings were measured preoperatively and compared with the outcomes 45 days after surgery. In addition, the symptoms reported by patients were investigated in follow-ups on the 10th, 30th, and 45th days following the operation.

Results

Significantly better spherical refractive outcomes were associated with SMILE than Femto-LASIK at 45 days postoperatively (sphere component: -0.22 ± 0.61 D for Femto-LASIK and − 0.04 ± 0.17 D for SMILE, and P = 0.080; SE: -0.37 ± 0.72 D for Femto-LASIK and − 0.09 ± 0.21 D for SMILE, and P = 0.021). In addition, the SMILE procedure induced more flattening effect on steep-K readings than Femto-LASIK 45 days after the surgery (41.96 ± 2.23 D for SMILE and 40.65 ± 2.04 D for Femto-LASIK, and P = 0.007). No significant differences were found between the two study groups' postoperative measurements of flat-K and K-max. In addition, the higher proportions of patients who were treated with SMILE complained of severe dryness sensation at ten days postoperatively, no patient complained of this symptom at 45 days following both procedures.

Conclusions

By the end of the 45-day follow-up after the operation, SMILE would be a slightly better surgical procedure providing less postoperative myopic refraction and fewer symptoms.

Ophthalmology

Femto-LASIK

Small incision lenticule extraction

SMILE

Myopia

Refractive error

Myopia is a prevalent refractive error ranging from 14 to 80% (average 25%).(1–3) Surgical methods for treating refractive errors can be divided into intraocular and corneal procedures. Intraocular methods include lensectomy and the use of phakic intraocular lenses.(4, 5) Corneal methods are the most widely used, with the majority applying refractive laser surgery procedures.(6) They can be divided into three categories of surface ablations techniques (photorefractive keratectomy, epi-laser in-situ keratomileusis, laser-assisted subepithelial keratectomy, and radial keratotomy), intrastromal (refractive lenticule extraction), and flap (Laser in situ keratomileuses using the microkeratome or femtosecond laser) procedures.(7–10)

In recent years, corneal methods for correcting refractive errors have been noticeably more complex in technique and were used only for medical reasons (correction of a high degree of ametropia and anisometropia), and might not be able to save the patient from spectacle and contact lenses correction.(9, 11) In line with the scientific and technological advancements, as well as to improve the efficiency, safety, and predictability of the outcomes of corneal refractive surgeries, the use of a femtosecond laser for the formation of a corneal flap has begun in ophthalmic practice.(12) This laser acquired its name because of its extremely short duration of the laser pulse − 10–15 seconds, equal to one femtosecond. It has been shown that the frequency of intra- and postoperative complications of laser in-situ keratomileusis (LASIK) with femtosecond laser is less than with classical technology using a microkeratome.(13, 14) In the femto-LASIK technique, a thin corneal flap is formed using femto-laser exposure, and cornea is subsequently reshaped using an excimer laser. Clinically, femto-LASIK technology is based on a combination of femtosecond corneal flap formation and corneal surface ablation using excimer laser technology. At the stage of formation of a corneal flap with a diameter of 8.0 mm and an incision length of about 20 mm, complications are much less common, but at the same time, the biomechanical properties of the cornea may be weakened.(15–17). A new method of refractive surgery, which is becoming more widespread, is the refractive lenticule extraction (ReLEx) technology, which allows the use of various methods of femtosecond vision correction using the VisuMax® femtosecond laser (Carl Zeiss Meditec, Germany).(18) "ReLEx" is the brand name for a laser vision correction procedure of myopia and astigmatism (sphere from − 0.50 D to -10.00 D, cylinder up to 5.00 D and spherical equivalent from − 0.50 D to -12.50 D) that includes two evolutionary techniques: femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SMILE). In 2008 Sekundo et al. published a paper on the first clinical application of the FLEx technology.(19) When performing the FLEx procedure, a stromal lenticule corresponding to the diopter is first formed, then a corneal flap is created, and the hinged flap is lifted, and the lenticule is removed. In the next step, the flap is placed in its original position. The next generation of ReLEx technology was the SMILE (small incision lenticule extraction) technique, which differs from FLEx in that way the flap is formed incomplete, and the stromal lenticule is removed through a 3.2 to 4.2 mm tunnel incision. SMILE was first introduced in 2011, and was approved by the US Food and Drug Administration (FDA) in 2016 for the treatment of myopic refractive error from − 1.00 D to -8.00 D. With regard to the correction of astigmatism, the accuracy of the refractive outcome may cause some problems. In SMILE procedure, an intrastromal lenticule is created using a femtosecond laser, which is then removed through a small incision. It retains most of the nerve fibers of the cornea and theoretically has a better biomechanical strength, which leads to a decrease in the incidence of dry eye syndrome in the postoperative period and possible corneal ectasia.(20, 21) Compared to LASIK method, SMILE has also been shown to be highly effective and safe in correcting myopia and astigmatism.(22) In contrast to femto-LASIK, SMILE does not require the creation of a flap and subsequent tissue ablation. In addition, SMILE seems to have less postoperative halo and glare symptoms in the eyes of patients with large pupils.(23, 24) In 2018, SMILE was approved by the FDA for the treatment of myopic astigmatism up to 3.00 D.(25) However, astigmatism correction remains a challenge for SMILE, especially for inexperienced surgeons. A feature of the SMILE technique is the absence of a wide marginal incision of the flap, which reduces the impact of the operation on the biomechanical properties of the cornea by maintaining the integrity of the collagen fibers of the anterior layers of the cornea.(26) Despite the minimal biomechanical influence, some cases of the development of iatrogenic keratectasia after SMILE in patients with preoperative normal corneal topography have been reported.(27) It is notable that FLEx and SMILE include a more complicated surgical procedure than conventional methods of surface ablation. Clinically, during the removal of the lenticule, it can be fragmented and be remained in the intrastromal pocket or on the surface of the stromal bed (depending on the method performed), especially with a thin lenticule in the correction of mild myopia. This problem may subsequently lead to induced astigmatism.(28–30)

The use of a femtosecond laser brings refractive surgery to a new level of development and allows for safer, more accurate, and more efficient correction of refractive errors in various groups of patients. However, there are already some challenges for surgeons in selecting the best refractive surgery procedures for their patients. In addition, there is a lack of clarity due to limited studies comparing the surgical outcomes of a new SMILE procedure to a standard refractive procedure such as femto-LASIK.(31–34) Therefore, the main aim of the present study is to compare the surgical outcomes of patients who underwent femto-LASIK and SMILE procedures.

In this historical cohort study, 160 eyes of 83 patients were selected. The study was conducted in accordance with the tenets of the Declaration of Helsinki and was approved by the Research Ethics Committee, Mustaqbal College University of Medical Sciences ( Opt.7/2023). As this study was a retrospective study, informed consent was waived based on our university policy. The data was collected from privet clinical in Babylon- city of Iraq from 15 January 2019 to 25 January 2021.

The selection criteria were as follows: the stability of eye refraction over the past 12 months, the degree of myopia from − 2.5 to -8.0 diopters, astigmatism no more than − 1.0 diopters, the minimum thickness of the cornea in the center of at least 500 microns (according to the data of Galilei G6; Ziemer Ophthalmic Systems AG, Port, Switzerland). The study did not include patients with previous refractive surgeries, severe dry eye syndrome, as well as with concurrent ocular pathology.

Based on the surgical procedure, patients were divided into two groups. The first group was patients who underwent femto-LASIK procedure and included 46 patients, with an average myopia of -2.27 ± 1.49 diopters; the second group was patients who underwent SMILE procedure and included 37 patients, an average myopia of -3.26 ± 1.84 diopters.

Before and after surgery, all patients were evaluated by experienced optometrists in several short-term follow-ups. The examinations consisted of medical and ophthalmic history, corrected distance visual acuity (CDVA), slit lamp biomicroscopy, and fundus examination. Spherical equivalent (SE), spherical component, and cylindrical component were measured using the retinoscope (ParaStop HEINE BETA 200; HEINE Optotechnik, Herrsching, Germany). In addition, all patients underwent a Galilei G6 scan to determine the corneal thickness data and the feasibility of refractive surgery.

In both methods of SMILE and femto-LASIK, the flap was created with Ziemer FEMTO LDV femtosecond laser (Ziemer Ophthalmic Group, Port, Switzerland). Cornea specialists performed all the operations. In all cases, the operations were uneventful and without complications. A comparative analysis of visual acuity with and without correction, refractive error measurements, and tomography evaluation was carried out. The objective data were collected before the operation and compared with the outcomes 45 days after the operation. In addition, the symptoms reported by patients were investigated in follow-ups on the 10th, 30th, and 45th day following the operation.

Statistical data analysis was performed using the IBM SPSS Statistics, version 24.0 (SPSS, Inc., Chicago, IL, USA). The changes in variables were compared between the two groups using the student t-test. In addition, the changes in variables were assessed in each group. We used repeated measures analysis of covariance and the post hoc test. Results were considered significant at P < 0.05.

The mean age of 46 patients who underwent femto-LASIK surgery was 27.8 ± 6.5 (range, 19 to 43) years [28(60.9%) females and 18 (39.1%) males]. Also, the mean age of 37 patients who underwent SMILE surgery was 24.5 ± 6.8 (range, 18 to 41) years [19 (51.4%) females and 18 (48.6%) males].

The comparison of visual and refractive measurements between patients who underwent femto-LASIK and SMILE procedures is shown in Table 1. As reported in this table, in both preoperative and postoperative measurements, there were no significant differences in preoperative corrected distance visual acuity (CDVA), postoperative uncorrected distance visual acuity (UDVA), and cylindrical refractive error between femto-LASIK and SMILE groups (all P > 0.05). However, the spherical equivalent (SE) refractive error between femto-LASIK and SMILE groups showed statistically significant differences in comparing preoperative and postoperative measurements (P = 0.019 for preoperative and P > 0.021 for postoperative measurements). On the other hand, in both study groups, all refractive error components after surgery improved significantly (all P < 0.001). Also, in the SMILE group, the mean CDVA after the procedure improved significantly compared to the preoperative measurements (P = .023); however, the improvement in patients who underwent femto-LASIK was not statistically significant (P = 0.323).

Table 1

The comparison of visual and refractive parameters between femto-LASIK and SMILE surgery.
Parameter		Mean ± Standard Deviation		P-value
Parameter		Femto-LASIK (n = 46) (Mean ± SD)	SMILE (n = 37) (Mean ± SD)	P-value
Visual acuity	Pre-CDVA	0.09 ± 0.20	0.06 ± 0.10	.387
	Post-UDVA	0.08 ± 0.20	0.03 ± 0.08	.142
	P-value	.323	.023
Sphere	Pre	-2.27 ± 1.49	-3.26 ± 1.84	.008
	Post	-0.22 ± 0.61	-0.04 ± 0.17	.080
	P-value	< .001	< .001
Cylinder	Pre	-1.25 ± 1.42	-1.09 ± 1.21	.583
	Post	-0.30 ± 0.74	-0.10 ± 0.28	.130
	P-value	< .001	< .001
Spherical equivalent	Pre	-2.89 ± 1.64	-3.80 ± 1.82	.019
	Post	-0.37 ± 0.72	-0.09 ± 0.21	.021
	P-value	< .001	< .001

UDVA: uncorrected distance visual acuity; CDVA: corrected distance visual acuity.

The comparison of preoperative and postoperative tomographic data between femto-LASIK and SMILE surgery groups is shown in Table 2. Based on the obtained information from this table, the mean values of flat-K, steep-K, K-max, and central corneal thickness (CCT) were significantly improved after surgery (all P < 0.001). Moreover, in both preoperative and postoperative findings, there were no significant differences between SMILE and femto-LASIK for all measured tomographic data, except for postoperative steep-K (P = 0.007).

Table 2

The comparison of preoperative and postoperative tomographic data between femto-LASIK and SMILE surgery groups.
Parameter		Mean ± Standard Deviation		P-value
Parameter		Femto-LASIK (n = 46) (Mean ± SD)	SMILE (n = 37) (Mean ± SD)	P-value
Flat-K	Pre	43.21 ± 1.51	43.10 ± 1.41	.750
	Post	40.34 ± 2.18	39.82 ± 2.15	.281
	P-value	< .001	< .001
Steep-K	Pre	44.85 ± 2.23	43.96 ± 3.58	.173
	Post	41.96 ± 2.23	40.65 ± 2.04	.007
	P-value	< .001	< .001
K-max	Pre	42.92 ± 1.37	43.75 ± 2.10	.032
	Post	41.16 ± 2.10	40.34 ± 1.96	.071
	P-value	< .001	< .001
CCT	Pre	522.02 ± 83.04	535.63 ± 31.53	.361
	Post	430.00 ± 102.67	424.83 ± 7.96	.790
	P-value	< .001	< .001

K: simulated keratometry; CCT: central corneal thickness.

At ten days postoperatively, the most common reported symptom of patients in the femto-LASIK and SMLE groups was severe dryness (80.4%) and mild dryness (51.4%), respectively. In addition, the frequency of severe dryness ten days following surgery in the femto-LASIK group (80.4%) was significantly higher than in the SMILE group (18.9%) (P < .001). Furthermore, at 30 days postoperatively, the most common symptom in the femto-LASIK group was mild dryness (76.1%) which was significantly higher than in the SMILE group (32.4%) (P < .001). Evaluation of symptoms at 45 days following surgery showed that 97.3% of patients in the SMILE group did not complain of any symptom, whereas 78.3% of patients in the femto-LASIK group were asymptomatic (P < .001). The comparison of the percent frequency of different symptoms and patients who had no symptoms at 10-, 30- and 45-day following surgery is shown in Fig. 1.

The present study compares short-term refractive, visual, and tomographic measurements after refractive surgery procedures of femto-LASIK and SMILE. A significantly better spherical refractive outcomes were associated with SMILE than femto-LASIK at 45 days postoperatively (sphere component: -0.22 ± 0.61 D for femto-LASIK and − 0.04 ± 0.17 D for SMILE, and P = 0.080; SE: -0.37 ± 0.72 D for femto-LASIK and − 0.09 ± 0.21 D for SMILE, and P = 0.021). In addition, the SMILE procedure induced more flattening effect on steep-K readings than femto-LASIK 45 days after the surgery (41.96 ± 2.23 D for SMILE and 40.65 ± 2.04 D for femto-LASIK, and P = 0.007). In the present study, we applied the Scheimpflug technology to analyze the outcomes after both methods. We found that both procedures showed a statistically significant decrease in keratometry readings and CCT findings. However, no significant differences were found between the postoperative measurements of flat-K and K-max between the two study groups.

Investigation of refractive and visual outcomes in previous studies showed the SMILE procedure is as safe, efficient, and predictable as femto-LASIK for treating myopia and astigmatism over short-term follow-ups.(35–40) In the present study, both procedures showed excellent efficacy in treating myopic and astigmatism refractive error 45 days after the operation; however, the postoperative mean SE measurements of patients who underwent SMILE were significantly less than those patients who underwent the femto-LASIK procedure. It represents a higher efficacy of SMILE in correcting myopic refractive error of patients than femto-LASIK. At 45 days postoperatively, both femto-LASIK and SMILE groups had a good tendency toward emmetropia. However, patients who were treated with the SMILE procedure demonstrated closer measurements to emmetropia than the patients who were treated with the femto-LASIK procedure. This finding is in accordance with the previous reports.(33, 35, 41)

The primary superiorities of the SMILE surgical technique over the corneal flap procedures are the small incision and slight epithelium ingrowth that can result in postoperative acceleration of corneal healing and reduction of eye discomfort, respectively.(39, 42) In addition, the SMILE procedure minimizes the postoperative flap-related complications that is due to flap tearing, dislocation, or traumatic dissociation.(43) Several studies have compared the two surgery options regarding postsurgical outcomes, including refractive, visual, and tomographic findings. Some studies noted a superior availability of femto-LASIK in improving vision than SMILE, which can be attributed to the eye tracking technology, cyclorotation adjustment, and active control of centration during the operation.(44, 45) However, some studies reported fewer postoperative dry eye signs and symptoms in patients who were treated with the SMILE procedure.(39, 42, 46–48) As per in our study, most previous studies on postoperative parameters of SMILE and femto-LASIK were performed during a short-term period after the procedures. For example, in a prospective study, Chen et al. compared the visual and refractive outcomes of patients who underwent SMILE and femto-LASIK at one- and three-month follow-ups after the procedures.(49) They reported a better UDVA measurement in the femto-LASIK group than the SMILE group. Furthermore, the postoperative cylinder component was significantly higher in SMILE-treated eyes at three months postoperatively. One factor that can contribute to patient satisfaction with vision is that the SMILE procedure tends to induce slightly lower spherical aberrations,(38, 40, 50) as well as higher-order aberrations.(51, 52) The visual outcome after a SMILE procedure is also positively influenced by a lower induction of scattered light (53) and higher contrast sensitivity.(52, 54)In a long-term retrospective study on the visual and refractive outcomes of SMILE and femto-LASIK over five years, Breyer et al. compared 1192 eyes treated with SMILE to 404 eyes that were treated with femto-LASIK.(32) They reported favorable postoperative outcomes, safety, and predictability in both methods at five years postoperatively. The authors also emphasized better outcomes of SMILE for correcting myopia and astigmatism, as well as the greater level of patient comfort and satisfaction during and after the procedure associated with no dry eye and flap complications. These findings were consistent with the previous reports.(55–57) It is worth noting that the surgical procedure of the SMILE method is technically more complicated than femto-LASIK since it requires manual dissection of the lamella into the stromal tissue for extracting the lenticule. Accordingly, it forms a steeper surgeon learning curve and the probability of an unsuccessful surgery is increased.(58, 59) This surgeon-dependent property of SMILE may be the reason for some reports of inferior postoperative outcomes in SMILE-treated eyes.(44, 59) Development of adjusted surgery nomograms, optimization of software with capability of eye-tracking and correcting the static cyclotorsion might improve postoperative outcomes and experience of patients who are treated with SMILE method.(60) In the present study, although postoperative SE was significantly better in the SMILE group than the femto-LASIK group, the mean postoperative UDVA values showed no significant differences between the two groups. Accordingly, it seems that both methods are similar in visual acuity outcome during the short-term follow-up of 45 days postoperatively.

In terms of postoperative complications, it was also observed that dry eyes occur markedly less frequently after the SMILE procedure, and hence this procedure contributes to higher patient satisfaction.(50) The reason for this is the much smaller incision of only 2–3 mm, which means that remarkably fewer nerve fibers are severed than with femto-LASIK.(39, 42) In the present study, patients’ complaints of SMILE appear to be slightly better than femto-LASIK during the first 45 days following surgery. Although the higher proportions of patients who were treated with SMILE complained of severe dryness sensation at ten days postoperatively, no patient complained of this symptom at 45 days following both procedures. The less feeling of severe dryness in patients who underwent the SMILE procedures can be attributed to the minimal interruption of corneal innervation during this surgical option.(39, 42, 46–48) This finding is in line with the previous studies on patients who were treated with the SMILE procedure.(61–64) However, some studies reported almost the same postoperative symptoms and patient-reported experience after the two procedures.(40, 65, 66) In a prospective and comparative study, Li et al. compared the clinical signs and symptoms of dry eye in patients treated with SMILE and femto-LASIK one month after the operation.(67) They concluded that the SMILE method led to a short-term worsening in signs and symptoms of dry eye. This finding was in line with our results.

This study has a few limitations. First, only visual, refractive, and tomographic measurements were examined, and other ocular parameters, such as high-order aberrations and biomechanical properties of the cornea were not evaluated. Second, the present study was performed in a short-term period, and setting up future long-term studies are suggested. Also, we did not include patients who were treated with other surface ablation methods; for this reason, future studies should involve patients who will be treated with different strategies.

Our findings indicated that both refractive surgery procedures of SMILE and femto-LASIK were effective in correcting myopia and astigmatism during a short-term follow-up. By the end of the 45-day follow-up after the operation, SMILE would be a slightly better surgical procedure providing less postoperative myopic refraction and fewer symptoms. However, in the early postoperative days, a higher proportion of patients who were treated with SMILE reported transient complaints of severe dryness that was resolved in the following days in both groups.

Although the SMILE procedure made a substantial advance in the field of refractive surgery technology, femto-LASIK is already the procedure of choice for patients who do not meet indications of SMILE surgery, including hyperopia and high astigmatism.

Conflicting Interest: The authors declare that there is no conflict of interest.

Funding information: The authors did not receive financial support from public or private sources. The authors have no financial or proprietary interest in a product, method, or material described herein.

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Comparison of surgical outcomes following femtosecond laser-assisted LASIK (Femto-LASIK) versus small incision lenticule extraction (SMIL) for myopia and astigmatism

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