For more than 30 years, corneal refractive laser surgery has been widely used to correct vision and help patients reduce or eliminate their dependence on glasses. While advances in technology continue, three generations of techniques have shaped the field of laser vision correction. In the early 1990s, excimer laser vision correction with photorefractive keratectomy (PRK) revolutionized refractive surgery with its precision and accuracy in reshaping the cornea and improving patients’ vision. Soon after, laser-assisted in situ keratomileusis (LASIK) was approved, which combined the accuracy of the excimer laser with accelerated visual recovery. In its current configuration, LASIK involves two laser platforms and is a two-step procedure: a femtosecond laser or microkeratome blade creates a corneal flap, and an excimer laser performs the stromal ablation to change the curvature of the cornea, resulting in the targeted refractive outcome.
The refractive surgery revolution continues with small-incision lenticule extraction (SMILE) for the correction of myopia. SMILE combines many of the features of LASIK and PRK in a single treatment step that requires only one laser. Today, SMILE has turned into a compelling alternative in refractive correction, and its popularity continues to grow worldwide. Most importantly, many believe that the excellent patient outcomes coupled with fewer side effects make it more advanced compared to PRK and LASIK.
The SMILE Procedure
More than 750,000 SMILE procedures have been performed internationally since its introduction in 2011. SMILE became available to U.S. surgeons in 2016 when the U.S. Food and Drug Administration (FDA) approved the VisuMax femtosecond laser (Carl Zeiss Meditec) for SMILE to reduce or eliminate myopia.
In the SMILE procedure, the femtosecond laser is used to create a lenticule of stromal tissue within the cornea without lifting a flap or removing the epithelium. The laser photoablates the refractive lenticule with a diameter of 6.0mm to 6.8mm at a depth of 100µm to 120µm. The same femtosecond laser then creates a single incision with a width of 2.5mm to 4.0mm at the edge of the lenticule. Once the laser treatment is complete, the surgeon extracts the lenticule through the side-cut with forceps, removing the small piece of corneal tissue. Removal of the lenticule produces the same effect as myopic LASIK in that the surgeon removes the same amount of tissue and flattens the cornea.
LASIK and SMILE Outcomes
The true judge of any surgical procedure is safety and efficacy. Visual and refractive outcomes for SMILE have been shown to be similar to those achieved with femtosecond LASIK (FS-LASIK) (Reinstein et al, 2014). This study showed that safety in terms of change in corrected distance visual acuity is similar to LASIK. In some circles, SMILE is thought to be potentially more accurate compared to LASIK, as it is not associated with the variability of environmental factors that can influence excimer stromal ablation, such as laser fluence and differences in stromal hydration (Aristeidou et al, 2015).
With initial clinical experience, it appears that SMILE compares favorably with both wavefront-guided and -optimized LASIK. Results of the first clinical study performed in the United States and submitted to the FDA were impressive, with 88% of eyes achieving 20/20 or better vision (Carl Zeiss Meditec, 2016). However, in comparison, the FDA trial for the iDesign advanced aberrometer and wavefront-guided procedure (Abbott Medical Optics) had 93.4% of patients achieving 20/20 and 79% 20/16 (Shallhorn et al, 2014). The Alcon FDA filing for the topography-guided procedure, Contoura Vision, demonstrated similar results, with 92.7% of Contoura Vision patients achieving uncorrected vision of 20/20, while 68.8% of eyes reached 20/16 or better (WaveLight GMBH, 2013).
SMILE is FDA approved for the correction of myopia of –1.00D to –8.00D, with –0.50D or less of cylinder and with manifest refraction spherical equivalent of –8.25D, in patients at least 22 years of age who have documentation of stable manifest refraction over the past year (FDA, 2015).
Screening criteria for SMILE are similar to those used for FS-LASIK. As with any refractive procedure, it is important to make sure that the ocular surface is healthy and that there are no clinical signs or symptoms of dry eye. A thorough pre-operative assessment should also include corneal topography to rule out any evidence of keratoconus or forme fruste keratoconus. There should be no sign of early cataract, retina, or glaucoma problems. Patients who have done well with spherical contact lenses are generally good candidates for SMILE, considering that astigmatism is not indicated in its FDA clearance.
SMILE offers several theoretical advantages over LASIK. Because the SMILE technique does not require the creation of a flap, many of the known flap-related early or late complications, such as displaced flap, flap striae, and epithelial ingrowth, are eliminated. In addition, the amputation of corneal nerve fibers during creation of a flap and subsequent laser ablation reduces corneal sensitivity (Piñero and Teus, 2016). Dry eye is believed to be, in part, caused by the decreased corneal sensation from those severed corneal nerves. Because no flap is needed in the SMILE procedure, it theoretically severs less anterior stromal corneal nerves and causes less dry eye.
A recent meta-analysis found that SMILE may create fewer dry eye symptoms compared to FS-LASIK, and corneal sensitivity was restored faster after SMILE compared with FS-LASIK (Zhang et al, 2016). Schirmer test data obtained at any time after surgery did not differ significantly between the two procedures. However, the tear breakup time (TBUT) was longer in the SMILE group than in the FS-LASIK group at one and six months after surgery. Also, the corneal sensitivity was lower in the FS-LASIK group than in the SMILE group at any time after surgery. It should be noted that the TBUTs after surgery were usually lower than normal in both groups, indicating that the TBUT fell after surgery in both groups, but to a greater extent in the FS-LASIK group than in the SMILE group (Zhang et al, 2016).
The absence of flap creation with minimal disruption of the anterior stromal architecture as the corneal lenticule is extracted from the mid-stroma allows for much greater preservation of the structural integrity of the cornea (Reinstein et al, 2014). Theoretically, leaving the cornea in a stronger state could reduce the possibility of corneal ectasia. However, it is important to understand that SMILE still loosens the fibers in the anterior part of the cornea, and a post-SMILE cornea may still be weakened compared to a non-dissected cornea. Rare cases of post-SMILE ectasia have been reported. However, it appears that the patients in question had some signs of subclinical keratoconus before the SMILE procedure, such as corneal asymmetry (Mattila and Holopainen, 2016).
SMILE also has some other disadvantages. Under the current FDA approval, treatment parameters are narrower compared to those for LASIK and PRK and are only FDA-approved to treat spherical myopia. In Europe, SMILE is approved to treat up to –5.00D of cylinder.
A second disadvantage of SMILE is in the rare case that a patient requires an enhancement. Currently, there is not an accepted standard for performing an enhancement or a secondary procedure when the primary procedure was SMILE; however, removing a lenticule does not appear to be an option at this time. Therefore, most surgeons currently prefer to perform the secondary procedure as PRK on the SMILE cap. This would be similar to performing PRK over a LASIK flap.
Lastly, SMILE demonstrates a slightly slower visual recovery when compared to FS-LASIK. Refinement of the laser settings, specifically the energy and spot spacing settings, has made this difference very small; it is approximately one to two lines in uncorrected distance visual acuity at one day post-op, which typically resolves after two to three weeks (Reinstein, 2015).
Is This the End of LASIK?
While some feel that the SMILE procedure threatens the current LASIK or PRK market, most see it as a next-generation procedure that could stimulate and invigorate the U.S. refractive surgery market; many believe SMILE is a complementary innovation to what already exists. In all likelihood, refractive surgeons will continue to offer all three corneal procedures, depending on an individual patient’s requirements and which procedure will provide the best possible success.
While SMILE is an exciting approach, and the idea of having only one laser is quite attractive, some surgeons may be hesitant to perform the relatively new refractive procedure until more long-term safety data is available. The SMILE procedure does show promise to increase the number of people who will undergo refractive surgery. As with all refractive surgeries, the ideal procedure for each individual depends on a number of factors and is determined after comprehensive testing. Also, clinicians should make sure that patients’ expectations are in line with what the surgical procedure can provide. CLS
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