Comparison Between FS- LASIK and FS-SMILE for Correction of Myopia and Myopic Astigmatism

Rationale and Background

LASIK (laser in situ keratomileusis) is a surgical procedure designed to correct refractive errors. LASIK involves creating a corneal flap using a microkeratome followed by reshaping the cornea using an excimer laser to remove tissue from the underlying stromal bed and then replacing the corneal flap.

LASIK evolved from a variety of techniques in refractive surgery. Keratomileusis, with both freeze and non-freeze techniques was used in the USA in the 1970s.This procedure was followed by automated lamellar keratoplasty (ALK), in which a microkeratome was used to create either a free cap or a hinged corneal flap. Tissue from the corneal bed was removed to alter the refractive error and the flap was replaced. Keratomileusis and ALK were relatively imprecise mechanical techniques. After the ophthalmic excimer laser has been developed, it has been used to reshape the cornea in a technique called photorefractive keratectomy (PRK). LASIK combines the technique of creating a hinged corneal flap from ALK with excimer laser ablation from PRK. Potential advantages of LASIK over PRK include earlier postoperative stabilization and faster improvement of visual acuity, less postoperative patient discomfort, shorter duration of postoperative medication use and an easier enhancement procedure.

However LASIK -like any other surgical procedures- is not without complications. Complications of LASIK include dry eye, glare and night vision complaints and the most serious flap complications which include button hole, free cap, incomplete flap, infection and inflammation.

Femtosecond lasers have been widely used in LASIK to fashion a corneal flap instead of mechanical microkeratome, which is followed by corneal ablation using excimer laser. The advantages of femotosecond or 'bladeless' LASIK over microkeratome LASIK flaps are more precise flap thickness, reduced postoperative dry eye symptoms, reduced flap dislocation and reduced incidence of buttonholes or free caps.

Recently, refractive lenticule extraction (ReLEx) has been introduced as a single laser refractive procedure (all-in-one femtosecond LASIK) without the use of an excimer laser. Small incision lenticule extraction (SMILE) is a variation of ReLEx which requires no retractable flap so reducing the incidence of flap complications, dry eye, reducing the surgical time and patient discomfort through operating on only one device in addition to decreasing the risk of ectasia.

Patients and methods Type of study Our study is a prospective, non-randomized, comparative clinical study applied on patients with myopia or myopic astigmatism.

Treatment group The study will include 60 eyes of patients with myopia or myopic astigmatism presenting for refractive correction. Femtosecond assisted LASIK group will include 30 eyes and SMILE group will include 30 eyes.

Pre-operative examination

All patients will undergo complete ophthalmological examination before surgery, including:

• Uncorrected and best corrected visual acuity assessement using snellen visual acuity chart.
• Manifest refraction.
• Slit lamp examination.
• IOP measurement.
• Dilated fundus examination
• Assessment for dry eye using Schirmer test and tear break up time test (TBUT).
• Corneal topographic analysis using combined Schimpflug and placido disc imaging principles using CSO topography system (CSO, Firenze, Italy).
• Wavefront analysis.
• Corneal biomechanical parameters using Ocular Response Analyzer (ORA; Reichert Inc, Depew, NY).

Surgical procedure LASIK procedure Each LASIK procedure will be performed using a standard, established technique. Under topical anesthesia and standard draping, a lid speculum is used to retract the eyelids. A superiorly hinged 100 μm thick flap will be created using the WaveLight FS200 (Alcon Laboratories, Inc.) With inverted side cut (115 ̊) .Excimer laser ablation is then performed using WaveLight EX500 excimer laser (Alcon Laboratories, Inc.).After ablation, the flap will be carefully repositioned, and postoperative medications are commenced.

SMILE procedure Each SMILE procedure will be performed using an established, described technique. After application of topical anesthesia, standard sterile draping, and insertion of the speculum, the patient's eye will be centered and docked with the curved interface cone before application of suction fixation.

The laser will be activated for photo-dissection in the following sequence: first the posterior surface of the refractive lenticule (spiral in), then the lenticule border is created. The anterior surface of the refractive lenticule (spiral out) is then formed which extended beyond the posterior lenticule diameter by 0.5 mm to form the anterior flap and is followed by a rim cut.

After the suction is released, a Siebel spatula (Rhein Medical, Heidelberg, Germany) is inserted under the cap near the hinge before the cap is separated. The edge of the refractive lenticule is separated from the stromal bed with a sinsky hook and the posterior border of the lenticule gently separated with the Siebel spatula. The lenticule is then grasped with non-toothed serrated forceps through the small incision.

Postoperative management

Postoperative treatment:

• Topical tobramycin and dexamethasone eye drops (Tobradex; Alcon Laboratories, Inc., Fort Worth, TX) was used postoperatively every 6 hours for 1 week and stopped.
• Topical lubricants was also prescribed to be applied every 6 hours for 1 month (refresh liquigel, Allergan Laboratories, Inc.).

Postoperative assessment:

• Postoperative visits were scheduled for the first postoperative day, 1 month and 3 months after surgery. On the first postoperative day, UCVA measurement and slit-lamp examination (flap position and corneal integrity) will be performed. Three months after surgery, the same clinical examinations as preoperatively was performed.

Statistical analysis Data will be presented in terms of parametric and non-parametric data. Parametric data will be analyzed with paired student t test. Non-parametric data will be analyzed with Wilcoxon test. Statistical significance will be considered at 95% confidence interval. All tests will be two tailed and considered significant at p < 0.05 and highly significant at p < 0.01.