Will the SMILE procedure replace LASIK?

Dr. Tulio

The femtosecond laser has brought many significant advances to eye surgery. For more than a decade, it has been used to create lamellar corneal flaps for laser in situ keratomileusis (LASIK), and more recently this laser is used to precisely perform several steps in cataract surgery.

Additionally, it is used to create matching perforations and buttons in corneal transplant surgeries.

The ability of the laser to accurately create a separation in corneal tissue makes it a very versatile laser. We will describe a new application of the femtosecond laser in refractive surgery: a procedure called small-incision lenticule extraction for moderate to high myopia (SMILE).

Currently, LASIK is a two-step procedure in which a femtosecond laser creates a corneal flap and an excimer laser ablates tissue to change the curvature of the cornea, resulting in the desired refractive outcome.

SMILE is a one-laser, femtosecond-only procedure. The femtosecond laser creates a lenticule of tissue, which is removed via a “channel” in the cornea, eliminating the flap in the cornea.

This procedure is CME approved in Europe and is under FDA investigation in the United States and not yet approved.

The SMILE procedure

When performing the SMILE procedure, the surgeon applies suction to the eye and “docks” the femtosecond laser. The laser photo-ablates a refractive lenticule with a diameter of 6.0 mm to 6.8 mm at a depth of 100 µm to 120 µm.

A single side-cut is made at the superior position, which has a width of 2.5 to 4.0 mm. Once the laser treatment is complete, the surgeon dissects the lenticule through the side-cut with forceps, removing the small piece of corneal tissue. Postoperative treatment includes the use of antibiotics and steroids similar to LASIK.

There are several theoretical advantages of this new procedure. Several of the known complications of LASIK are flap related, such as displaced flap, flap striae, and epithelial ingrowth.

While many of these complications are eliminated with a flapless procedure, some cases of epithelial ingrowth have been reported. The procedure is likely to sever fewer corneal nerves-possibly resulting in less dry eye-and the treatment removes less tissue per diopter, allowing for safely treating higher refractive errors and creating more corneal biomechanical stability.

Finally, only one laser is used completely to treat the patient; therefore, there may be less capital and per procedure costs to the surgeon and ultimately the patient.

SMILE outcomes

Several published studies have shown refractive results approaching those of LASIK. Published studies show that between 77 percent and 100 percent of moderate myopes achieve within ±0.50 D of their attempted target refraction.¹

Also, the results for higher myopic patients are promising. In a study of 144 patients with a mean spherical equivalent of -7.18 D ± 1.57 SD, 73 percent achieved 20/20 uncorrected distance visual acuity, and 25 eyes gained at least one line of corrected distance visual acuity.²

In a prospective study, Lui compared the refractive outcomes of 113 SMILE patients to those of 84 matched LASIK patients with moderate myopia. There was not a statistical difference in the outcomes, except Lui found that the SMILE group had a lower amount of spherical aberration at six months vs. the LASIK cohort.  

A five-year study conducted by Blum shows that SMILE is a stable, predictable procedure over that period of time. The overall efficacy of SMILE appears to be determined by the precision of the femtosecond to create a pristine lenticule.³

The comparison should be made to the most recent LASIK data from the Alcon FDA filing for the topography-guided procedure Contoura Vision. The FDA results found that nearly 69 percent of Contoura Vision patients achieved uncorrected vision of 20/16 or better.

Additionally, at 12 months, slightly over 31 percent of patients gained one line of uncorrected vision over their previous best-corrected vision.⁴

Dry eye side effects

The most common side effect of LASIK is dry eye, which is believed to be, in part, caused by the decreased corneal sensation from severed corneal nerves. Theoretically, the SMILE procedure severs less anterior stromal corneal nerves, resulting in less post-operative dry eye.

In a contralateral eye study, Demirok evaluated 28 patients for corneal sensitivity and dry parameters, including tear osmolarity, tear break-up time (TBUT), and Schimer’s at one week, one month, three months, and six months after LASIK and SMILE procedures.

While corneal sensitivity was statistically less at all four time periods than baseline and was statistically less in the LASIK eyes than the SMILE eyes, there was not any difference in the dry eye parameters.⁵

Wang evaluated 93 patients in a contralateral eye study in which he measured dry eye parameters including TBUT along with the Salisbury Eye Evaluation Questionnaire (SEEQ), which is used to evaluate a patient’s dry eye symptoms.

Tear break-up time (TBUT) was statistically significant at 12 months but not likely clinically significant at 9.83 (SMILE) vs. 9.30 (LASIK) seconds. The SEEQ was statistically significant up to 12 months possibly indicating a faster recovery of corneal sensation as evidenced by other studies.⁶

SMILE does not appear to provide overwhelming benefits regarding dry eye, but certainly is not worse. With improved techniques, it may provide a decrease in dry eye signs and symptoms in refractive patients.

Other complications

The SMILE procedure delivers energy to the cornea, and that energy can create diffuse lamellar keratitis (DLK). Zhao and colleagues enrolled 1,112 eyes to evaluate the incidence DLK with SMILE.

They reported an incidence of 1.6 percent, with all cases sporadic and significantly more with thinner and larger lenticules.⁷ This compares to an incidence of 0.20 percent in LASIK patients reported by Schallhorn in a study of 32,569 eyes.⁸

One paper proposes a more stable corneal structure with SMILE,⁹ but there are also reported cases of post SMILE ectasia.¹⁰ I believe we will still need to diligently evaluate the cornea structure in our selection of these patients.

Enhancing SMILE

Currently, there is not an accepted standard for performing enhancement or secondary procedures when the primary procedure was SMILE. Given similar refractive outcomes as LASIK, it is expected that some patients who have the SMILE procedure will need a secondary procedure.

Removing a second lenticule does not appear to be an option at this time.

There are surgeons who are using the femtosecond laser to dissect the “cap” of the SMILE procedure, use that a LASIK flap, and then treat as a LASIK procedure. There is the risk of a sliver of corneal tissue with this type of a procedure.

Therefore, currently most surgeons prefer to perform the secondary procedure as PRK on the SMILE cap. This would be similar to performing PRK over a LASIK flap.

The idea of having only one laser and eliminating many of the flap complications is quite attractive. Presently, the refractive outcomes for SMILE are approaching that LASIK, and there may be the potential of fewer dry eyes and a more stable cornea.

The SMILE procedure shows promise, and it will be interesting to follow its progress once it has FDA approval within the United States.

References

1. Kamiya K, Shimizu K. Visual and refractive outcomes of small incision lenticule extraction for the correction of myopia: 1-year follow-up. BMJ Open. 2015 Nov 26;5(11):e008268.

2. Vestergaard A, Ivarsen A, Asp S, et al. Small-incision lenticule extraction for moderate to high myopia: Predictability, safety, and patient satisfaction. J Cataract and Ref Surg. 2012 Nov; 38(11):2003−10.

3. Blum M, Täubig K, Gruhn C, et al. Five-year results of Small Incision Lenticule Extraction (ReLEx SMILE). Br J Ophthalmol. 2016 Jan 8.

4. FDA Clinical Trials. Allegretto Wave Eye-Q Addendum Procedure Manual T-CAT Topography-Guided Treatments. Available at: http://www.accessdata.fda.gov/cdrh_docs/pdf2/P020050S012d.pdf. Accessed: 04/06/2016.

5. Demirok A, Ozgurhan EB, Agca A, et al. Corneal sensation after corneal refractive surgery with small incision lenticule extraction. Optom Vis Sci. 2013 Oct;90(10):1040−7.

6. Wang B, Naidu RK, Chu R, et al. Dry Eye Disease following Refractive Surgery: A 12-Month Follow-Up of SMILE versus FS-LASIK in High Myopia. J Ophthalmol. 2015;2015:132417.

7. Zhao J, He L, Yao P, et al. Diffuse lamellar keratitis after small-incision lenticule extraction. J Cataract Refract Surg. 2015 Feb;41:400-7.

8. Shallhorn, S, Venter JA. One-month Outcomes of wavefront-guided LASIK for low to moderate myopia with the VISX STAR S4 Laser in 32,569 Eyes. J Refract Surg. 2009 Jul;25 (Suppl): S634−41.

9. Wu W, Wang Y. The Correlation Analysis between Corneal Biomechanical Properties and the Surgically Induced Corneal High-Order Aberrations after Small Incision Lenticule Extraction and Femtosecond Laser In Situ Keratomileusis. J Ophthalmol. 2015;2015:758196.

10. El-Nagger MT. Bilateral ectasia after femtosecond laser assisted small incision lenticule extraction. J Cataract Refract Surg. 2015 Apr;41(4):884−8.