In life, when you find something that works, you stick with it. Maybe it’s a restaurant that never fails to deliver a good meal, or an automobile brand that’s never let you down. For me, it’s the SMILE procedure. For several years now, SMILE has been the predominant refractive procedure in my practice, comprising 98 percent of our surgeries. We’ve chosen it simply because of its great outcomes and good safety profile, even when compared to LASIK. Here, I’ll tell you what we like so much about it.
SMILE: The Benefits
From the surgery itself through the postop period, small-incision lenticule extraction has many things to recommend it:
• Fewer variables. Air quality. Temperature. Humidity. Tears. BSS irrigation. All of these variables come into play when you’re performing LASIK, and they can negatively affect your refractive outcome.1 With the SMILE procedure, however, they don’t matter nearly as much. This simplifies the procedure for the surgeon so he can focus on the things that do matter. With SMILE, you simply dock the laser with the suction ring, activate the suction and let the femtosecond laser perform the photodisruption in the cornea independent of these factors.
• Potential optical benefits. Since SMILE involves directly cutting a lenticule within the cornea, you have the opportunity to achieve better optics postoperatively. In excimer-laser surgery, however, there’s some eccentricity to the ablation when the laser hits peripheral positions, and this eccentricity can affect the refractive outcome. SMILE directly cuts the tissue because you’ve drawn the cornea into the patient interface; therefore, in theory, you have a more reproducible lenticule removal.
In terms of postop vision, in the FDA trial of SMILE, none of the 328 patients saw 20/40 or better uncorrected preop. At the six-month visit, however, 98 percent (327 eyes) and 88 percent (287 eyes) saw 20/40 or better and saw 20/20 or better, respectively. Results at the 12-month follow-up were similar. The manifest refraction spherical equivalent was within ±1 D of target in 98 percent of eyes. No fewer than 79 percent of eyes were within ±0.25 D, and 92 percent were within ±0.5 D of target from one week to 12-months postop.2
• Preservation of corneal nerves. With SMILE, the incision is smaller than LASIK’s; creating a LASIK flap involves a large incision that extends almost around the entire cornea. This large incision has the potential to influence, for a while at least, the sensitivity of the cornea—especially the nerves in the anterior cornea. In SMILE, however, the patient has better corneal sensitivity postop and less of a reduction in reflex tearing.
Though we haven’t compared SMILE directly to LASIK, we did perform a study comparing it with femtosecond refractive lenticule extraction—FLEX—which involves creating a full flap similar to a LASIK flap while creating a lenticule. We performed SMILE in one eye and FLEX in the fellow eye. When we subsequently measured sensitivity and measured corneal nerve endings using confocal microscopy, after six months we found that SMILE has an advantage in terms of better preservation of nerves and less of a reduction in corneal sensitivity. We didn’t find a change in the composition or quantity of the tears.3
Steps of SMILE. A: The posterior refractive side of the lenticule has been cut and the cap-cut is almost half done. B: Immediately after completion of the lenticule cut by the femtosecond laser. C: Manual dissection of the anterior and posterior side of the lenticule. D: The intrastromal lenticule is being removed with forceps.
• Retained corneal strength. Also, in theory, since you create a corneal cap in SMILE—not a flap as in LASIK—this should result in a stronger corneal stroma postop. Over the long term, having a small incision rather than a flap reduces the risk of flap slippage if the patient suffers even a small ocular trauma, because flaps can be dislocated even years postop. In a contralateral-eye study in which I participated (N: 10 eyes of 5 patients), we found that flap-based lenticule extraction produced a 49 percent greater reduction (range: 2 to 87 percent) in effective stromal collagen fiber stiffness within the flap region than SMILE procedures in the patients’ fellow eyes. There were also lower stresses and deformations in the residual stromal bed in the SMILE cases, and stromal bed displacements and stresses were more affected by a loading increase in eyes with flaps than in the flapless ones.4 SMILE also helps avoid—but doesn’t eliminate—one of the most dreaded complications associated with LASIK: ectasia.
Responding to Criticisms
Over the past year or so, some surgeons have leveled criticisms at the SMILE procedure. However, these issues aren’t as significant as they’re made out to be:
• SMILE is too difficult. Some surgeons say that working with the SMILE lenticule is much more technically difficult than performing the steps involved with LASIK. Though working with the lenticule is more involved than cutting and lifting a flap, LASIK still has a learning curve of its own. Also, after you’ve performed some SMILE procedures and gotten comfortable with it, you’ll appreciate benefits such as better corneal sensation, stronger corneal structure and excellent predictability, even at high levels of correction.
• Enhancements are a problem. Enhancements haven’t really been an issue for us due to the procedure’s high accuracy, at least in our hands. We haven’t had to do one in the past year.
If an enhancement is required though, it’s not an insurmountable obstacle. If we ever need to enhance a SMILE, our procedure of choice is a PRK procedure on top of the cap with adjunctive use of mitomycin-C. Zeiss has also developed a SMILE enhancement procedure known as CIRCLE, in which you convert the SMILE cap into a flap and then use the excimer laser to ablate the stromal tissue.
In a retrospective study of surface ablation enhancement after SMILE, surgeons performed PRK on 43 eyes out of 1,963 SMILE cases (2.2 percent). Spherical equivalent was -6.35 ±1.31 D before SMILE and -0.86 ± 0.43 D before the PRK. Surface ablation was performed after a mean of 9.82 ±5.27 months, and resulted in a spherical equivalent of 0.03 ±0.57 D at three months (p<0.0001). The number of patients within ±0.5 and ±1 D of their target refraction respectively increased from 23 percent to 80 percent and from 73 percent to 93 percent. In these 43 enhancement eyes, mean uncorrected distance acuity improved from 0.23 ±0.20 logMAR (a little worse than 20/32) to 0.08 ±0.15 logMAR (about 20/20) (p<0.0001). The average best-corrected acuity remained unchanged overall (p=0.99), with 65 percent of patients gaining at least a line. Six eyes (15 percent) lost a line of BCVA, but the final BCVA was 20/20 in four of these cases and 20/25 in the other two.5
Ultimately, for us, SMILE is the effective, safe procedure we’ve relied on for the vast majority of cases for several years and, at this point, it’s hard to argue with 3,000 happy patients. REVIEW
To see the counterpoint, "LASIK: The Clear Choice" click here.
Dr. Hjortdal is a consultant and clinical professor of ophthalmology at Aarhus University Hospital in Denmark and medical director of the Danish Cornea Bank. Aarhus University Hospital has specified research support agreements with Carl Zeiss Meditec.
1. Walter KA, Stevenson AW. Effect of environmental factors on myopic LASIK enhancement rates. J Cataract Refract Surg 2004;30:4:798-803.
2. Summary of safety and effectiveness from the SMILE FDA trial. https://www.accessdata.fda.gov/cdrh_docs/pdf15/P150040B.pdf. Accessed 5 January 2019.
3. Vestergaard AH, Grønbech KT, Grauslund J, Ivarsen AR, Hjortdal JØ. Subbasal nerve morphology, corneal sensation, and tear film evaluation after refractive femtosecond laser lenticule extraction. Graefes Arch Clin Exp Ophthalmol. 2013;251:11:2591-600.
4. Seven I, Vahdati A, Pedersen IB, et al. Contralateral eye comparison of SMILE and flap-based corneal refractive surgery: Computational analysis of biomechanical impact. J Refract Surg 2017;33:7:444-453.
5. Siedlecki J, Luft N, Kook D. Enhancement after myopic small incision lenticule extraction (SMILE) using surface ablation. J Refract Surg 2017;33:8:513-518.