Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes

Abstract

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman’s layer and corneal wound healing responses are also described.

Refractive errors

“…This central removal of material also alters the biomechanical characteristics of the cornea, potentially affecting among other things the applanatory measurement of internal ocular pressure in the course of investigation of glaucoma (22)(23)(24). Kerato-refractive procedures are not discussed in more detail here owing to the limited scope of this article, but the relevant literature is cited in review articles (25,26).…”

Association Between Corneal Deformation and Ease of Lenticule Separation From Residual Stroma in Small Incision Lenticule Extraction

“…after treatment than after LASIK, lower decrease and faster recovery of corneal sensation than after LASIK, less theoretical reduction in the biomechanical strength of the cornea, and better wound-healing response than after LASIK. 5,6,12 The hypothesis of this study was that corneas with lower deformation may offer greater resistance to separation of the lenticule from the residual stroma during the surgery assuming that these corneas are biomechanically stiffer.…”

Safety of cornea and iris in ocular surgery with 355-nm lasers

“…Although the 2013 study by Trost et al 4 showed that 6.5-mm diameter corneal flaps could be created in rabbits with no unintended UV-induced damage to the cornea at a total radiant exposure of 6.9 J∕cm 2 , it did not establish an operational range and safety margin for 355-nm lasers. A measurement of the actual damage thresholds is important to establish the safety limits of 355-nm lasers for the full range of flap cutting parameters as well as other potential applications, such as refractive lenticule extraction, 6 lamellar keratoplasty, 7 or cataract surgery. 8 Of the three major damage mechanisms for any laser ocular surgery, photomechanical, photothermal, and photochemical, we are mostly concerned with photochemical damage for 355-nm lasers as it sets a hard limit on the total energy that can be delivered during a procedure.…”