European Journal of Cardiovascular Medicine

In the early 1990s, combined Automated lamellar keratoplasty with the excimer laser into a procedure known as LASIK. It was proposed that a mechanical microkeratome to be used to make a hinged flap of the cornea. Excimer laser reshaping could then be done on the exposed corneal stroma, and finally the hinged flap could be refloated back on the cornea, and it allowed patients to heal in place without any sutures. ^[1] As the first cut with the microkeratome was only to expose the corneal stroma, its accuracy was not very critical; and because the refractive lenticule was ablated with the help of the excimer laser, its shape and size could be accurately controlled. As a result, the accurate correction of all types of refractive errors became possible. ^[2] Most of the disadvantages of ALK could be overcome by using the excimer laser to ablate the refractive lenticule. ^[3]

LASIK quickly became the default choice of treating refractive errors. Its advantages—short period of patient discomfort, rapid visual recovery, and minimal wound healing reaction—led to very quick clinical acceptance of the procedure, and millions of LASIK procedures have been performed in the last 20 years. ^[4]

Although LASIK with the mechanical microkeratome is still popular, the mechanical microkeratome is also the reason behind the majority of LASIK complications. ^[5] Some of these complications include free caps, incomplete flaps, irregular flaps, and flap displacements. Also, mechanical microkeratomes sometimes make flaps which are thicker than intended, which in some cases leads to keractesia, a progressive thinning and subsequent irregular steepening of the cornea. Many surgeons have therefore shifted to the femtosecond laser as their primary means for making LASIK flaps. ^[6]

In recent years, a new femtosecond laser is available, which can carve out a lenticule within the cornea. This procedure of carving out a refractive lenticule within the cornea, and its subsequent mechanical extraction from the cornea to perform corneal reshaping is called femtosecond lenticule extraction, or ReLEx. ^[7] The lenticule can then be extracted from within the corneal stroma, either by creating and lifting a hinged flap as in ALK or LASIK, or by extricating it from within the cornea through a small incision in the cornea. The former technique is usually called FLEx, and the latter is called Small incision lenticule extraction (SMILE). Most experienced surgeons use SMILE only, and FLEx is generally a technique used during the learning curve. ^[8]

SMILE is therefore akin to ALK, in that a whole refractive lenticule is created in the corneal stroma, and then removed from the cornea to achieve corneal reshaping. However, the femtosecond laser has several advantages over mechanical microkeratome for the lenticule creation procedure. ^[9] Unlike a mechanical microkeratome, a femtosecond laser can scan the cornea in all 3 dimensions, and thus the refractive lenticule created is more physiological in shape. Second, the femtosecond laser is more accurate. Studies indicate that for Femto-LASIK, the standard deviation of the flap thickness is around 10 to 12 μm. ^[10] In contrast, the standard deviation of mechanical microkeratomes used in ALK or LASIK was around 24 μm. ^[11] Third, with the femtosecond laser, the shape of the lenticule can be varied in thickness or diameter along a particular axis. Thus, astigmatic corrections are possible. Finally, the femtosecond laser offers a better safety profile than a mechanical microkeratome. ^[12]

In this study, we compared retrospectively, SMILE and FS-LASIK procedures in terms of dry eye.

This is a prospective study was conducted in the Department of Ophthalmology, Mamata Academy of Medical Sciences. Before the surgery, patients read and signed an informed written consent. Forty seven patients with myopia and/or myopic astigmatism who had undergone SMILE procedure. Forty three patients with myopia and/or myopic astigmatism who had undergone FSLASIK procedure.

All of the surgeries were performed by a single surgeon (MG). Patients did not have any ocular or systemic diseases like Diabetes Mellitus, Connective tissue diseases and chronic dry eye syndrome. SMILE procedures were performed by Visumax femtosecond laser system. Under topical anesthesia, patient was focused on a target light, then corneal suction was commenced. Posterior and anterior surfaces of the lenticule were cut and separated, then the lenticule was extracted. FS-LASIK procedures were performed by Visumax Femtosecond laser system for creation of flap. The ablation was done by Wavelight EX500 Laser system.

Postoperatively, patients used topical antibiotic (Moxifloxacin 0.5%) four times a day for a week, topical steroid (Dexamethasone Na Phosphate 0.1%) four times a day for two weeks and a preservative-free topical lubricating drop (Na Hyaluronate 0.15%) six times a day for three months. Follow-up examinations containing uncorrected distance visual acuity (UDVA), corrected distance visual acvity (CDVA), intraocular pressure measurement, fundus examination, topographic measurements, fluorescein staining, TBUT, ST1 and OSDI questionnaire were applied preoperatively and 1st day, 1st week, 1st month, 3rd month and 6th month postoperatively.

Statistical analysis

For statistical analysis, SPSS version 22 programme was used. For comparison of percentages and means, Chi-square test, t-test and paired-t test were used. A p value smaller than 0.05 was accepted as significant statistically

In total, 90 patients were recruited for the study, with a total of 90 eyes (the first eye to have surgery performed for each patient) included in the analysis. There was a total of 47 eyes in the SMILE group and 43 eyes in the FS-LASIK group. There were no significant differences between the two groups preoperatively in terms of age, SE refractive error, central corneal thickness (CCT), or preoperative TBUT. Demographic data for all subjects included in this study is outlinedinTable1.

Table1: Demographic data of the subjects included in this study

Table2: Lenticule thickness/ablation depth

Table3: TBUT between SMILE and FS-LASIK.

Table4: SEEQ scores between SMILE and FS-LASIK.

Objective surgical changes in corneal parameters were similar between the two groups, with no significant difference in lenticule thickness/ablation depth between the two groups (Table2).

Tear-Film Breakup Time (TBUT). Preoperatively, there was no significant difference in TBUT between the SMILE and FS-LASIK groups (11.92 ± 1.62 seconds and 11.61 ± 1.39 seconds, resp., 𝑝 = 0.318). One-way ANOVA showed that there was a statistically significant difference in TBUT between preoperative values and the different follow-uptime periods, for both SMILE (𝐹 (4, 230) = 79.673, 𝑝 < 0.001) and FS-LASIK (𝐹 (4, 210) = 55.531, 𝑝 < 0.001).

Post hoc tests showed that, at 1 and 3 months after operation, there was a statistically significant decrease in TBUT from preoperative values in the SMILE group (8.32 ± 1.39, 𝑝 < 0.001, and 10.25 ± 0.99, 𝑝 < 0.001, resp.), before returning to preoperative values by 6 and 12 months (11.62 ± 0.99, 𝑝 = 0.740, and 11.88 ± 0.98, 𝑝 = 1.00, resp.). In the FS-LASIK group, TBUT was statistically significantly reduced from preoperative values at 1 month, 3 months, and 6 months postoperatively (8.58 ± 1.28 𝑝 < 0.001, 9.48 ± 0.99, 𝑝< 0.001, and 10.24 ± 1.49, 𝑝 < 0.001, resp.), before returning to preoperative values at 12 months (11.35 ± 0.94, 𝑝 = 0.826).

Between the two procedures, TBUT was not statistically significantly different at 1 month postoperatively (𝑝 = 0.354); however, at 3, 6, and 12 months postoperatively, TBUT was statistically significantly greater in the SMILE group than the FS-LASIK group (𝑝 < 0.001, 𝑝 < 0.001, and𝑝 = 0.008, resp.) (Table3).

Salisbury Eye Evaluation Questionnaire. The Salisbury Eye Evaluation Questionnaire (SEEQ) was used to assess a patient’s subjective reporting of dry eye symptoms, with a higher score indicating a greater degree of experienced dry eye symptoms. Preoperative scores were 0, per the inclusion criteria. One-way ANOVA testing found a statistically significant difference in SEEQ scores within groups over the time period of review for both SMILE (𝐹 (4, 230) = 23.127, 𝑝 < 0.001) and FS-LASIK (𝐹 (4, 210) = 91.161, 𝑝 < 0.001).

Post hoc tests showed that, in the SMILE group, SEEQ scores were statistically significantly higher at 1 month (𝑝< 0.001) and 3 months (𝑝 = 0.003) after operation than preoperative values. By 6 and 12 months, this difference was no longer statistically significant (𝑝 = 0.640 and 𝑝= 0.991, resp.). For FS-LASIK, post hoc test evaluation found that SEEQ scores at 1 month, 3 months, and 6 months after operation were statistically significantly higher than preoperative values (𝑝 < 0.001 for all 3 follow-up time periods). By 12 months, this difference was no longer found (𝑝 = 0.636).

Postoperatively at the 1-, 3-, and 6-month follow-up intervals, the SEEQ score was higher in the FS-LASIK group than the SMILE group (𝑝 < 0.001 for all 3 follow-up time intervals). 12 months after operation, this difference was no longer statistically significant (𝑝 = 0.109) (Table4).

SMILE & FS-LASIK were extremely effective, safe and predictable. The effectiveness and safety indices for SMILE & Fs-LASIK were identical. ^[13-18]

Regarding UDVA of 20/20 or higher, we placed results just like the ones of previous research with SMILE13 and Fs-LASIK. Concerning CDVA and quality, dry eye is probably the main cause for decreasing of CDVA. As far as predictability is concerned, every SMILE and FS-LASIK had three months excessive emeteropia tendency. However, Vestergaard, confirmed, that SMILE was in the direction of emmetropia more than FS-LASIK. Upon activity, the refraction among both groups that fulfill Gertnere, was non-significant different. Dry eye condition is an acute post-refractive issue because the eye comfort determines its satisfaction. The dry eye disorder is a multifactorial eye surface disorder involving changes in the tear film and an improvement in tear osmolarity with or without the corneal injury. ^[19]

Golas, suggested that the tear secretion characteristic have become suppressed within the course of the first 3 months and returned back  to normal levels with the beneficial resource of 6 months after surgical treatment, whilst HorwathWinter, confirmed that Schirmer test results had no variations. Toda, concept that dry eye symptoms become parallel to that of corneal sensitivity. LASIK-precipitated dry eye is based mainly, on the breakdown of sub-basal nerves in addition, the SMILE protects the innervation of the cornea by producing just a small penetration tunnel for 40 to 60 degrees in contrast to the wide penetrating tunnel for LASIK, around 300 degrees. Reports have accepted reduced corneal sensitivity following LASIK, which is likely to last months or maybe years while a Fs laser is used in the manufacturing of flaps. Further information about mechanisms in post-refractive dry eye disease pathogenesis is a big issue. ^[20]

The induction of giant quantities of HOAs has been defined as a crucial side impact. It was related to a number of factors including biomechanics of the cornea, ablation algorithms, flap formation and ablation decentration. The use of the Fs-laser still delivered about ordinary HOAs, coma and spherical aberrations. No statistically huge difference among groups in RMS of established corneal higher order aberrations, with more induction of coma aberration in SMILE group and spherical aberrations in FS-LASIK age. For qualitative purposes, Ye, Vestergaard and Gartner, agreed with SMILE that the marginal growth in spherical aberration relative to the Fs-LASIK was simultaneously seen.

Spherical aberration in myopic patients with a greater pupil diameter caused more problem than coma-likes aberration. ^[21] Our literature was used in the aspheric ablation model in SMILE and Fs – LASIK groups, so that corneal curvature did not lead to any major spherical aberration changes. The formation of a flap, however, is another important explanation for HOAs. By maintaining the biomechanical balance, SMILE minimizes the alternative form of a cornea. In addition, spherical post-operative aberration correlating with optical and ablation zones. Coma aberration is greater with SMILE, as it does no longer encompass the iris registration technology. In addition, the SMILE concentration is much less accurate than the excimer laser attention tracker. Furthermore, coma aberration represents irregularity, propensity and concentration. ^[22] For SMILE, the correction of astigmatism results in an oval image of the lenticule that can be one of the key causes for asymmetry. However, the postoperative coma may also be caused by the vertical end of the Refractive Lenticular in SMILE compared with the 2.0 mm transition quarter in the FS-LASIS optical quarter. No discrepancy between SMILE and Fs-LASIK was noticed in the implemented average HOAs or SA.

As regards the corneal asphericity; we did not Δ Q rate among both groups, however SMILE tended to produce more oblate surface, also it was reported a statistically significant more ablation of corneal surface with wave fronts guided ablation in comparison to flex procedure. There was no statistically significant difference any more as we used the LASIK aspheric Q-adjustment profile in our study.

SMILE was more effective, safe and predictable manner as Fs-LASIK.  It had better outcomes in dry eye parameters at 3 months post-operative. Both techniques improved the overall corneal aberrations with no difference between both groups. SMILE probably caused additional coma aberration due to decentration, but Fs-LASIK introduced spherical aberrations, however all these adjustments had been statistically not big among all patients.

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