After its approval in the United States in 2016, small manual incision lenticule extraction (SMILE) was believed to be the next big thing in laser refractive surgery. Experts touted the procedure as an alternative to laser-assisted in situ keratomileusis (LASIK) that could deliver reduced dry eye, greater biomechanical stability, and faster recovery time.1 Five years later, however, LASIK remains the most popular laser refractive surgery.2 Will SMILE ever catch up?
Unlike LASIK, SMILE is a flapless technique that uses a single femtosecond laser system to create a smaller incision than the standard 8 mm to 9 mm flap for LASIK. Because it avoids a corneal flap, SMILE patients have no risk of dislocation. For this reason, SMILE may be preferred by athletes, military service members, and others with active lifestyles or occupations.
At the time of its market debut, researchers called SMILE “the future of refractive surgery,” predicting that the 1 millionth procedure would be performed by the end of 2017.1 In 2018, when it was approved by the U.S. Food and Drug Administration (FDA) as a treatment for patients with myopia with astigmatism, the procedure’s potential patient pool was expected to jump from 30% of patients seeking laser corrective surgery to 80%.3,4 Today, approximately 3 million SMILE procedures have been performed so far.5
In the Shadow of LASIK
Research shows that 1-step SMILE produces similar efficacy, safety, predictability, and stability as LASIK.1 Because clinical results are so similar, it can be difficult for surgeons and patients to choose between them. SMILE is approved to treat myopia up to -10.0 D and myopic astigmatism up to -3.00 D, for a total spherical equivalent of no more than -10.0 D.2 However, it isn’t approved to treat hyperopia or mixed astigmatism.5
Arguably, LASIK still dominates the field, with more than 16 million surgeries performed worldwide since the 1990s.2 Interest in the procedure improved — as did patient outcomes — with the advent of femtosecond lasers for flap creation. The procedure is an established, safe, and effective treatment for patients with myopia, delivering rapid postoperative visual improvement, minimal patient discomfort, and good long-term refractive stability.6 However, LASIK has had its ups and downs, according to Rolando Toyos, MD, of the Toyos Clinic in Nashville.
“When LASIK first came out, it was predicted that millions of people would have the procedure,” explains Dr Toyos, who says he has performed thousands of LASIK surgeries. “But with any surgical procedure, there are people who love it and think it’s the best thing ever, and then there are some who don’t. You still have a percentage of people who won’t like the results and will be vocal about it.”
And with any elective procedure, funding is also a factor. LASIK cases all but dried up during the financial crisis of 2008, according to Dr Toyos. Since then, he says the procedure has made a slow comeback bolstered during the pandemic, when patients had more time — and more disposable income.
The Data Supporting SMILE
A significant body of research illustrates the potential benefits of SMILE over LASIK, including:1-3,,6,7
• Reduced iatrogenic dry eye
• Better recovery of corneal sensation
• Biomechanically stronger cornea
• No flap-related complications
• Smaller incision
• Quicker recovery
• Minimal corneal nerve interruption
• Fewer variables, such as air quality, temperature, humidity, tears, and irrigation
• Less laser energy required
• No moving from one laser platform to another
There are several potential advantages of SMILE over LASIK, according to Edward E. Manche, MD, director of research, cornea, and refractive surgery at the Byers Eye Institute at Stanford. Dr Manche, who says he has performed more than 45,000 laser vision correction procedures, explains, “SMILE causes less denervation of the cornea, which results in a lower incidence of dry eye symptoms in the early postoperative period. In addition, SMILE surgery has been shown in modeling studies to have superior biomechanical strength compared with LASIK surgery. The superior biomechanical properties of SMILE may allow us to perform surgery on eyes with high myopia with less risk of developing corneal ectasia. Lastly, there is no flap with SMILE, so there is little to no chance of traumatic dislocation of a SMILE cap. This makes SMILE more appealing in certain professions, such as professional boxers, mixed martial artists, and others who may be at risk for ocular trauma.”
In a 2019 study, 70 patients received LASIK in 1 eye and SMILE in the other. At 3 months, 99% of SMILE eyes and 97% of LASIK eyes achieved spherical equivalent within 1.0 D of attempted correction.6 Researchers concluded that SMILE delivered predictability, efficacy, and safety at 3 and 12 months of follow-up.
In a 2021 study, investigators reviewed several studies on SMILE, including 16 articles regarding dry eye outcomes, 6 articles on corneal sensation, and 11 articles on corneal biomechanical stability. Most studies in the review found that dry eye worsened after both procedures, but SMILE tended to induce less iatrogenic dry eye than LASIK.2
Regarding corneal sensitivity, the 2021 review found that most studies reported reduced corneal sensitivity in the LASIK group compared with SMILE.2 Researchers hypothesized that SMILE patients experienced heightened sensitivity because there’s less damage to the sub-basal nerve plexus and corneal stromal nerves during surgery.2
Dr Toyos agrees with the data: “It’s a game-changer for me that I can offer SMILE to my patients and not make their dry eye worse.”
SMILE Isn’t Without Its Drawbacks
A variety of complications can occur during and after SMILE surgery, including:6-8
• Diffuse lamellar keratitis
• Interface debris
• Interface haze
• Corneal edema
• Epithelial ingrowth
• Interface fluid syndrome
• Slower visual recovery in initial phases
• Difficulty removing corneal tissue
• Loss of suction
• Dry eye
• Moderate to severe glare
• Moderate to severe halos
More technically challenging, SMILE requires manual lamellar dissection within the cornea, followed by a smooth lenticule extraction. This process can take twice as long as LASIK, by some accounts.7 During this initial learning curve, complications related to lenticule dissection and removal may occur, including unsuccessful lenticule removal, retention of corneal lenticule fragments, iatrogenic stromal scarring, and interface irregularities — all of which can lead to inferior visual outcomes.2,6
A 2018 study examined the intraoperative complications reported during SMILE surgeries. Complications reported in the 282 eyes (141 patients) included lost vacuum, treatment decentration, wound bleeding, incomplete bubble separation, epithelial defects, incision tear, lenticule adherence to the cap, and cap perforation.9 These complications, according to investigators, are primarily related to surgeon inexperience.
“LASIK is an easy procedure. SMILE is more surgical-technique dependent, so adoption has been slower,” explains Dr Toyos.
Dr Manche adds: “Currently, there is only one manufacturer that produces a femtosecond laser for SMILE surgery in the United States. The VisuMax (Carl Zeiss Meditec AG) uses a curved patient interface with corneal suction. It also utilizes low suction, which has several advantages over systems that use scleral suction: patients never go dark or lose sight of the fixation light, they experience no pain during applanation, and there are no subconjunctival hemorrhages. However, a disadvantage of this system is that lower suction can be associated with suction loss more frequently than with scleral suction devices.”
When it comes to enhancements, some physicians contend that future procedures may be an issue with SMILE. With LASIK, if an enhancement is needed, surgeons can lift the flap and perform retreatment. With SMILE, however, surgeons can’t perform a repeat SMILE surgery on an eye that’s already had one, and there’s little consensus on which retreatment option is best.7
In his 2 years of performing SMILE procedures, Dr Toyos has only needed to perform 2 enhancements. In those cases, he performed surface ablations. If an enhancement is necessary, Dr Manche performs photorefractive keratectomy (PRK). Other options include performing a side cut and lifting the SMILE cap, or cutting a thin LASIK flap in the SMILE cap, according to Dr Manche.
Experts believe that poor marketing is a factor in SMILE’s slow adoption. The refractive option must make up for LASIK’s 20-year head start in the marketplace. For Dr Manche, LASIK is still king; 70% of his patients receive LASIK surgery, 20% receive PRK, and 10% receive SMILE. But, he says, that mix is evolving.
“LASIK has a long track record and is well-known to the general public,” explains Dr Manche. “One of the biggest drivers of laser vision correction procedures is word of mouth. Since SMILE is a fairly new procedure, fewer patients have undergone this procedure, and there are fewer people aware of this surgery. Therefore, one of the biggest challenges is educating potential patients about SMILE when they have come in for LASIK surgery. It requires additional chair time to explain the pros and cons of the various procedures.”
Dr Toyos, who markets SMILE as “flapless LASIK,” agrees: “LASIK is part of our nomenclature, part of our culture. SMILE has a long way to catch up.” And he is doing his part; 98% of his patients receive SMILE instead of LASIK.
And, as with any decision to invest in new technology, ophthalmologists must weigh patient benefit with cost, says Dr Manche. Purchasing a new system is a significant capital expenditure, since most centers already have another manufacturer’s femtosecond laser for LASIK.
What’s Ahead for SMILE
Researchers believe that as femtosecond laser technology improves and lenticule dissection becomes less tricky, more physicians — and patients — will choose SMILE.2 And, with anticipated nomogram adjustments, software enhancements, and other refinements, clinical outcomes will improve, too.5
“Whereas I’m not certain that SMILE surgery will ever completely replace LASIK surgery, I believe that it is here to stay,” says Dr Manche. “I believe it will continue to evolve and become more refined.”
Dr Toyos agrees. “Once you adopt this procedure and go through the barrier to entry, you’ll feel satisfied that this is the better way. SMILE is the future of refractive surgery.”
1. Doane JF, Cauble JE, Rickstrew JJ, Tuckfield JQ. Small incision lenticule extraction smile – the future of refractive surgery is here. Mo Med. 2018;115(1):82-84.
2. Huang G, Melki S. Small incision lenticule extraction (SMILE): myths and realities. Semin Ophthal. 2021;36(4):140-148. doi:10.1080/08820538.2021.1887897
3. VisuMax Femtosecond Laser – P150040/S003. United States Food and Drug Administration website. Updated October 29, 2018. Accessed June 18, 2021. https://www.fda.gov/medical-devices/recently-approved-devices/visumax-femtosecond-laser-p150040s003
4. Doane JF. FDA approves ReLEx SMILE for myopia with astigmatism. Ocul Surg News. Published online October 5, 2018. Accessed June 18, 2021. https://www.healio.com/news/ophthalmology/20181005/fda-approves-relex-smile-for-myopia-with-astigmatism
5. McKenna J. Zeiss reaches milestone of 3 million SMILE procedures. News release. Market Scope. Published online June 14, 2020. Accessed June 18, 2021. https://www.market-scope.com/pages/news/4559/zeiss-reaches-milestone-of-3-million-smile-procedures.
6. Ang M, Farook M, Htoon HM, Mehta JS. Randomized clinical trial comparing femtosecond lasik and small-incision lenticule extraction. Ophthalmol. 2020;127(6):724-730. doi:10.1016/j.ophtha.2019.09.006
7. Hjortdal, J, Wilson, S. Point-Counterpoint: SMILE vs. LASIK. Rev Ophthalmol. Published online February 5, 2019. Accessed June 18, 2021. https://www.reviewofophthalmology.com/article/pointcounterpoint-smile-vs-lasik.
8. FDA approves VisuMax Femtosecond Laser to surgically treat nearsightedness. US Food and Drug Administration. Updated September 13, 2016. Accessed June 16, 2021. https://www.fda.gov/news-events/press-announcements/fda-approves-visumax-femtosecond-laser-surgically-treat-nearsightedness
9. Hamed AM, Heikal MA, Soliman TT, Daifalla A, Said-Ahmed KE. SMILE intraoperative complications: incidence and management. Int J Ophthalmol. 2019;12(2):280-283. doi:10.18240/ijo.2019.02.15