The Small Incision Lenticule Extraction (SMILE) procedure can be an attractive procedure for patients, as there is no flap, no sound, no smell, no pressure, and the duration of the procedure is short. In addition, because there is no flap and the incision is so small (i.e., 60 degrees in the U.S.), SMILE places virtually no postoperative restrictions on lifestyle. Also, there is strong evidence that the severity and duration of dry eye symptoms post SMILE is decreased.1,2 Another benefit: improved early uncorrected visual acuity, as reported in the recent study, “Comparison of Early Visual Outcomes Following Low-Energy Small Incision Lenticule Extraction (SMILE), High-Energy SMILE and LASIK for Myopic Astigmatism,” published in the Journal of Cataract and Refractive Surgery.3
Here I discuss the study itself, of which I was a part, and its implications for practice.
The FDA approved SMILE for the treatment of myopic astigmatism (i.e., -1.0 D to -10.0 D sphere and 0.75 D to 3.00 D cylinder) in October 2018, giving surgeons access to lower-energy settings on the VisuMax (Zeiss) laser used to perform the procedure.
Our study compared the un-corrected distance visual acuities (UDVA) and induced higher-order aberrations (HOAs) in the early postoperative period among those who underwent low-energy SMILE, high-energy SMILE, and femtosecond (FS) LASIK.
Results showed that SMILE patients whose surgeries were performed using low energy had significantly better postop day 1 (POD1) vision (20/19.86) compared with those who underwent high-energy SMILE (20/27.67) (P<0.001). Moreover, the mean UDVA on POD1 for the low-energy SMILE group was equivalent to that of the FS LASIK group (20/19.50) (P=0.498). Importantly, the percent of patients with UDVAs of 20/20 or better on POD1 was equivalent when comparing the low-energy SMILE group to the FS LASIK group. Furthermore, induced HOAs were equivalent between low-energy SMILE and FS LASIK at postoperative month 1 (POM1), except for induced spherical aberration. This was lower in the low-energy density SMILE group (0.136 µm) compared to those in the FS LASIK (0.186 µm, P=0.034) group.
Implications for Practice
Choosing the appropriate settings on the VisuMax (Zeiss) device has been paramount in achieving an easy dissection and rapid vision recovery following SMILE. Initially approved for spherical treatments in 2016, the SMILE laser settings were limited to high energy. SMILE using femtosecond energy of 125 nJ (or lower outside U.S.) facilitates better visual acuity with less induction of corneal aberrations early postoperatively.4 The resulting prolongation of corneal healing leads to relatively poor uncorrected visual acuity in the early postoperative period. Our study showed that low-energy SMILE results in much less opaque bubble layer formation (Figure 1). This directly corre-lates with easier lenticule dissection and improved early uncorrected visual acuity.
Zeiss recently announced the upcoming release of the VisuMax 800, a second-generation FS laser for SMILE that has a 2 GHz firing speed (4x faster than the existing 500 kHz VisuMax). This faster speed can be used to shorten the time for lenticule dissection and further adjust energy settings to additionally improve the ease of dissection and speed visual recovery. CP
- Kobashi H, Kamiya K, Shimizu K. Dry eye after small incision lenticule extraction and femtosecond laser-assisted LASIK: meta-analysis. Cornea. 2017;36(1): 85-91.
- Zhang Y et al. Clinical outcomes of SMILE and FS-LASIK used to treat myopia: A meta-analysis. J Refract Surg. 2016;32(4): 256-65.
- Hamilton DR, AC Chen, R Khorrami, M Nutkiewicz, Nejad M. Comparison of early visual outcomes following low-energy small incision lenticule extraction (SMILE), high-energy SMILE and LASIK for myopic Astigmatism. J Cataract Refractive Surg. 2021;47: 18-26.
- Woo YJ, Kim M, Sung Yong Kang D, Keun Lee H, Yul Seo K, Kim TI. Lower laser energy levels lead to better visual recovery after small-incision lenticule extraction: prospective randomized clinical trial. Am J Ophthalmol. 2017;179: 159-170.