Ever since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can then be removed in one piece manually, thereby circumventing the need for incremental photoablation by an excimer laser. This was achieved in 2005, presented at the American Academy of Ophthalmology (AAO) meeting in Las Vegas in 2006, and published in 2008 with the Femtosecond Lenticule Extraction procedure (FLEx) in which a lenticule was manually removed after lifting a flap,1 11 years after this had first been demonstrated in rabbit eyes with a picosecond laser.2 Following the successful implementation of FLEx, a new procedure called small incision lenticule extraction (SMILE) was developed; an all-femtosecond laser, keyhole, flapless procedure that is in the process of revolutionizing corneal refractive surgery and realizing Jose Ignacio Barraquer’s original concept of keratomileusis.3,4
The SMILE procedure is gaining popularity following the results of the first prospective trials5–7 and more recent reports that have demonstrated that the visual and refractive outcome is similar to laser in situ keratomileusis (LASIK),8–18 and there have now been over 140,000 procedures performed worldwide with more than 300 surgeons regularly doing SMILE. The feasibility of the procedure has been proved by studies on the surface quality of the lenticules,19,20 wound healing and inflammation,21–23 and the accuracy of the lenticule thickness parameters have been verified using very-high-frequency digital ultrasound24,25 and optical coherence tomography (OCT).26–29
The safety has also been demonstrated to be similar to LASIK30 and our recent publication has shown that there are no concerns in treating patients with SMILE for low myopia.17
In terms of safety, SMILE also brings two advantages over LASIK, relevant to the most common complication: dry eye, and the most serious complication: ectasia. Both of these advantages stem from the minimally invasive pocket incision nature of the procedure as this results in maximal retention of anterior corneal innervational as well as structural integrity.
It was expected that there would be less postoperative dry eye after SMILE. While the trunk nerves that ascend into the epithelial layer within the diameter of the cap will continue to be severed in SMILE, those that ascend outside the cap diameter, or that are anterior to the cap interface will be spared. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensitivity after SMILE than LASIK,31–39
with recovery to baseline after 3–6 months after SMILE compared with 6–12 months after LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in sub-basal nerve fiber density after SMILE than LASIK.34,38,40
The other major advantage of SMILE is the biomechanical profile as the anterior stroma above the lenticule remains uncut (except in the location of the small incision), unlike in LASIK where anterior stromal lamellae are severed by the creation of the flap. It has been shown that the vertical sidecut of a flap is responsible for almost all of the change in strain due to LASIK flap creation.41 It has also been shown that the anterior corneal stroma is the strongest part of the stroma,42–45 due to the greater interconnectivity of collagen fibers in the anterior stroma compared with the posterior stroma where the collagen fibers lie in parallel to each other.46 Therefore, SMILE must leave the cornea with greater biomechanical strength than LASIK for the same amount of visioncorrecting tissue removal. Not surprisingly, therefore, using a mathematical model based on the nonlinearity of tensile strength through the stroma, we have shown that SMILE leaves greater biomechanical strength even than photorefractive keratectomy (PRK) for the same amount of vision correcting tissue removal, as PRK involves ablating within the strongest anterior stroma.47 Surgeons are accustomed to calculating the residual stromal thickness in LASIK as the amount of stromal tissue left under the flap, and therefore the first instinct is to apply this rule to SMILE. However, for the reasons given above, the actual residual stromal thickness in SMILE should be calculated as the total uncut stroma (i.e., the stroma above and below the lenticule). This biomechanically allows for much higher corrections to be achieved by SMILE than either LASIK or PRK.