Low myopia correction: SMILE versus surface ablation

The use of small-incision lenticule extraction (SMILE) in low myopes is still controversial for two reasons: there might be greater cross-talk between the cap and the lenticule plane, and the challenging separation of a thin lenticule may lead to more complications. In Germany, SMILE is still not recommended for myopia correction <3 D. For this reason, my colleagues and I felt the need to compare the visual and refractive outcomes of SMILE and surface ablations (SAs).

At the Center for Refractive Surgery Muenster, Germany, we are conducting an ongoing observational case series of our first 123 consecutive SMILE procedures and 29 consecutive SAs for low myopia. Our inclusion criteria for this study are identical to those of the only published study on SMILE for low myopia correction by Dr Dan Reinstein and colleagues, who conducted a retrospective analysis of 120 SMILE procedures for low myopia.¹

Their inclusion criteria were preoperative spherical equivalent refraction of up to -3.50 D, cylinder of up to 1.50 D and corrected distance visual acuity (CDVA) of 20/20 or better. They reported preoperative mean spherical equivalent refraction of -2.61 ± 0.54 D and mean cylinder of 0.55 ± 0.38 D.

After one year, mean spherical equivalent refraction was -0.05 ± 0.36 D and mean cylinder was ± 0.50 D in 84% and ± 1.00 D in 99% of eyes. They reported that uncorrected distance visual acuity (UDVA) was 20/20 or better in 96% of eyes and 20/25 or better in 100% of eyes. Based on these results, they found SMILE for low myopia was found to be safe and effective with outcomes similar to LASIK.

Initial findings

When performing the SMILE procedure in our ongoing case series, we made a default lenticule side cut of 15 µm irrespective of dioptric correction, and found that the identification of lenticule surfaces was the same for both high and low corrections. However, in our first few cases, we increased the lenticule side cut to up to 30 µm,  but our preferred method for obtaining a thicker lenticule was to increase the diameter of the lenticule and not the side cut.

With this approach, we obtain a larger optical zone and better visual quality. We also use a dissector with a semi-sharp spoon-shaped tip (SMILE Double Ended Dissector; Duckworth & Kent) that facilitates the lenticule dissection in every direction, including reverse movements, because of its round shape.

After completing the procedures, visual recovery was much quicker following SMILE than after SA, with SMILE patients at 0.78 at one day postoperative and SA  patients at 0.32 at 1 day postoperative. The efficacy index after 3 months was 0.91 in SMILE and 0.84 in SA patients (Figure 1).

The comparison of postoperative UDVA and preoperative CDVA was slightly in favour of SMILE, with UDVA after 3 months being the same or better than preoperative CDVA in 65% of SMILE and 48% of surface ablation patients, respectively (Figure 2). Predictability was slightly better in SMILE as well, with 93% of SMILE patients being within +/- 0.5 D at 3 months and 88% of surface ablation patients being within +/- 0.5 D at 3 months (Figure 3).

We found that both SMILE and SA for low myopia patients brought a high level of safety, efficacy and predictability with visual outcomes. For SMILE patients, these results were at least as good as with surface ablation, if not better.

Of note, there is more tissue alteration in SMILE than in SA. However, patients appreciate having more comfort and faster visual recovery after SMILE than following SA. However, it is important that we do not throw away our excimer lasers because SAs are invaluable for corneal dystrophies, recurrent erosion syndrome and thin corneas.

In conclusion, we use SMILE as the preferred method in all myopic laser corrections and use SA in patients with thin corneas and therapeutic indications.

Reference

1. Reinstein DZ, Carp GI, Archer TJ, Gobbe M. Outcomes of small incision lenticule extraction (SMILE) in low myopia. J Refract Surg. 2014 Dec;30(12):812-8.

Professor Suphi Taneri, MD

Prof. Taneri is the director of the Center for Refractive Surgery Muenster, Germany and one of the directors and owners of the Eye Department at the St. Francis Hospital, Muenster, Germany. He is an associate professor at the Ruhr University Bochum.