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.
Highlights
Ever since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can be manually removed as a single piece thereby circumventing the need for incremental photoablation by an excimer laser
The refractive results were similar to those observed in laser in situ keratomileusis (LASIK), but visual recovery time was longer due to the lack of optimization in energy parameters and scan modes; further refinements have led to much improved visual recovery times [10]
In three contra-lateral eye studies there was no difference in either corneal hysteresis (CH) or corneal resistance factor (CRF) between the small incision lenticule extraction (SMILE) and LASIK groups [35,72,73], while one study found that CH and CRF were slightly greater after SMILE than LASIK (p < 0.02) [74]
Summary
Ever since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can be manually removed as a single piece thereby circumventing the need for incremental photoablation by an excimer laser. Demirok et al [58] performed a contra-lateral eye study comparing central corneal sensation after LASIK and SMILE in 28 myopic patients over a 6 months follow-up period. A similar result has been found in a study by Medeiros et al [65], who showed in pig eyes that there were significantly greater biomechanical changes following the creation of a thick flap of 300 μm compared to a thin flap of 100 μm Applying this finding to SMILE, since no anterior corneal sidecut is created, there will be less of an increase in corneal strain in SMILE compared to thin flap LASIK and a significant difference in corneal strain compared to LASIK with a thicker flap. The authors postulated possible causes as the greater femtosecond energy delivered to the cornea in SMILE, the fact that two femtosecond lamellar cut surfaces come face to face (as opposed to one surface being sculpted by an excimer laser), and the increased surgical maneuvers required in SMILE
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