Comparison of Clinical Outcomes of Small-incision Lenticule Extraction (SMILE) Between Different Cap Thickness.

In the past two decades, the femtosecond laser (FSL) technology has been introduced in the corneal refractive surgery filed, and brought a remarkable innovation. It can make tissue dissection through photodisruption and plasma cavitation. Initially, the FSL was used predominantly to make a corneal flap when performing laser in situ keratomileusis (LASIK), which is followed by stromal ablation using excimer laser. A new surgical technique called femtosecond lenticule extraction (FLEx) has been developed that uses only FSL to dissect two interfaces to create refractive lenticule and then remove it, which is very similar with LASIK. Small incision lenticule extraction (SMILE) which is the advanced form of all-in-one FSL refractive technique does not make a corneal flap rather make small incision where the separated refractive lenticule is removed through, and the upper part of the corneal tissue is called cap. Since the clinical outcomes of SMILE were firstly published in 2011, SMILE has been widely used for correction of myopia or myopic astigmatism worldwide. SMILE provides excellent visual outcomes and has advantages including a lesser decrease in corneal sensitivity and absence of flap related complications compared to LASIK. Because corneal ectasia after refractive surgery is the one of most terrifying complication, corneal biomechanics has been drawn interests to many researchers and clinicians. Theoretically, SMILE may preserve corneal biomechanics better than LASIK, because the anterior stroma which is stiffer than the posterior stroma remains intact in SMILE. However, there are some controversies, because previous studies investigating corneal biomechanics have been reported inconsistent outcomes, although SMILE has been reported equal to or better than LASIK. Weakening of corneal biomechanics and iatrogenic corneal ectasia have also been reported after SMILE. In addition, because the tensile strength of cornea gradually decreases as it goes backwards, creati