One Step, One Laser

SMILE is a continuation of the refractive surgery revolution.

By Vance Thompson, MD; Keith Rasmussen, OD; and Doug Wallin, OD

In the early 1990s, excimer laser vision correction with PRK turned the heads of ophthalmologists and optometrists, revolutionizing refractive surgery with its precision and accuracy in reshaping the cornea. It set the tone for the birth of LASIK, which combined the accuracy of the excimer laser with accelerated visual recovery. Today, in most cases, LASIK involves the use of two lasers: a femtosecond laser to cut the corneal flap, and an excimer to perform the stromal ablation.

The revolution in refractive surgery is being carried on with the advent of small incision lenticule extraction (SMILE) laser vision correction. SMILE combines many of the features that we respect from LASIK and PRK in a single treatment step that requires only one laser. Having served as investigators in clinical trials for PRK, LASIK, and now SMILE, we have garnered a particular understanding of how SMILE compares with its predecessor.

WHAT IS SMILE?

SMILE is a femtosecond laser–driven procedure designed to correct nearsightedness. More than 600,000 procedures have been performed internationally,1 and SMILE became available to US surgeons in 2016 when the FDA approved the VisuMax (Carl Zeiss Meditec) femtosecond laser for SMILE to reduce or eliminate myopia.

Figure 1. A femtosecond laser is used to create a lenticule of tissue within the cornea without lifting a flap or removing the epithelium.

Figure 2. A 2.5- to 3-mm incision is then made at the edge of the lenticule.

Figure 3. The lenticule is removed through the incision, and its removal produces the same effect as myopic LASIK, in that the surgeon removes the same amount of tissue.

In the SMILE procedure, the femtosecond laser is used to create a lenticule of tissue within the cornea without lifting a flap or removing the epithelium (Figure 1). A 2.5- to 3-mm incision is then made at the edge of the lenticule (Figure 2). The lenticule is removed through the incision, and its removal produces the same effect as myopic LASIK, in that the surgeon removes the same amount of tissue (Figure 3). Just as with LASIK and PRK, in SMILE the Munnerlyn formula is used to calculate the exact amount of tissue to remove in order to achieve the right amount of curvature change to correct each patient’s refractive error.

In an unpublished, FDA-monitored study performed at our practice, 150 patients received LASIK in one eye and SMILE with the VisuMax laser in the other. We found that SMILE produced the same clarity and visual acuity as LASIK but without the large side cut necessary for LASIK. A separate clinical study of the safety and effectiveness of the VisuMax laser found that SMILE resulted in stable vision correction at 6 months. Of 328 participants evaluated in that study, all but one had uncorrected visual acuity (UCVA) of 20/40 or better postoperatively, and 88% had UCVA of 20/20 or better.2

PATIENT SELECTION

SMILE is approved by the FDA for correction of myopia of -1.00 D to -8.00 D, with -0.50 D or less of cylinder and with manifest refraction spherical equivalent -8.25 D, in patients at least 22 years of age with 1 year’s documentation of stable manifest refraction.

Watch it Now

SMILE combines many of the features that we respect from LASIK and PRK in a single treatment step that requires only one laser.

As with any refractive procedure, we want to make sure that the ocular surface is healthy and there is no untreated dry eye. Corneal topography should be normal with no evidence of preclinical keratoconus, and there should be no sign of early cataract, retina, or glaucoma problems. Screening is no different from screening for any other refractive surgery procedure.

Patients who have done well with spherical contact lenses are generally good candidates for SMILE, considering that astigmatism cannot currently be treated with SMILE in the United States.

NOTABLE DIFFERENCES

There appear to be some advantages to SMILE over LASIK. Studies have shown fewer symptoms of dry eye with SMILE, theoretically because there is no flap creation and therefore there are fewer transected corneal nerves.3 In addition, because the lenticule is removed from the deeper stroma without the broad side cut of LASIK, the only disruption of the anterior cornea is the small incision through which the lenticule is removed. Therefore, again, SMILE patients tend to have fewer issues of transient dry eye compared with LASIK patients. In a randomized controlled trial comparing dry eye metrics after SMILE and after LASIK, tear breakup time and Ocular Surface Disease Index scores were significantly worse in the LASIK group than in the SMILE group at 1 month, 3 months, and 6 months postoperative.3

Another benefit to cutting fewer nerves in SMILE is the effect on corneal sensation. In a prospective, nonrandomized, comparative study, corneal hypoesthesia was tested immediately after SMILE and after LASIK.4 SMILE eyes showed less compromised corneal sensation than LASIK eyes in the central, inferior, nasal, and temporal areas at 1 week and 1 month postoperative. In the SMILE group, the inferior, nasal, and temporal quadrants recovered faster than other areas. In the LASIK group, the sensation over the flap did not recover to preoperative levels by postoperative month 6.

It has also been proposed that, because the anterior corneal layers remain intact during SMILE, the procedure may result in greater biomechanical stability than LASIK. In LASIK, there is a large cut through the anterior corneal layers, which are considered the strongest layers of the cornea. During SMILE, the side cut is the size of the corneal hinge in LASIK. There is no large incision and no cut through the anterior corneal layers, theoretically leaving the cornea in a stronger state and reducing the possibility of corneal ectasia. Researchers built a mathematical model to calculate the relative remaining tensile strength in patients after SMILE, PRK, and LASIK. The model predicted that the postoperative total stromal tensile strength is considerably higher after SMILE than after either PRK or LASIK, given that the strongest anterior lamellae remain intact during the SMILE procedure.5

SMILE also has some disadvantages. Treatment parameters under the FDA approval to date are narrower than those for LASIK and PRK. SMILE is FDA-approved only to treat spherical myopia. In Europe, SMILE is approved to treat up to -5.00 D of cylinder, and parameters are expected to be expanded over time. A second disadvantage of SMILE as compared with LASIK or PRK is seen in the rare case that a patient needs an enhancement. At present, when this is necessary we perform a secondary PRK. We take this approach for SMILE patients because the ablation in PRK is superficial and leaves more tissue between the loci of the two surgeries. It is to be hoped that this prevents the secondary procedure from interfering with the original procedure.

CONCLUSION

As with all refractive surgeries, optometrists should make sure that patients’ expectations are in line with what the surgical procedure can provide. In our practice, our optometrists inform nearsighted patients that their options include glasses, contact lenses, and corneal refractive surgery. We are fans of glasses and contact lenses, we tell patients, and many people do well with them. But for those who want to reduce their dependence on these external aids, there are some great surgical options.

We tell patients that we can now offer three surgical options to correct nearsightedness that are accurate and safe: SMILE, LASIK, and PRK. Depending upon the parameters of their particular refractive error, corneal thickness, and corneal curvature and symmetry, and based on the advantages and disadvantages of each surgical option, we can determine which procedure is going to give each patient the best possible results.

1. FDA approves VisuMax Femtosecond Laser to surgically treat nearsightedness [press release]. US Food and Drug Administration. September 13, 2016. http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm520560.htm. Accessed February 1, 2017.

2. Shen Z, Zhu Y, Song X, et al. Dry eye after small incision lenticule extraction (SMILE) versus femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for myopia: a meta-analysis. PLoS One. 2016;11(12):e0168081.

3. Li M, Zhou Z, Shen Y, et al. Comparison of corneal sensation between small incision lenticule extraction (SMILE) and femtosecond laser-assisted LASIK for myopia. J Refract Surg. 2014;30(2):94-100.

4. Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg. 2013;29(7):454-460.

Keith Rasmussen, OD
• Optometric Physician at Vance Thompson Vision, Sioux Falls, South Dakota
keith.rasmussen@vancethompsonvision.com
• Financial interest: none acknowledged

Vance Thompson, MD
• Director of Refractive Surgery, Vance Thompson Vision, Sioux Falls, South Dakota
• Professor of Ophthalmology at the Sanford University of South Dakota School of Medicine
vance.thompson@vancethompsonvision.com
• Financial disclosure: consultant to Carl Zeiss Meditec

Doug Wallin, OD
• Optometric Physician at Vance Thompson Vision, Sioux Falls, South Dakota
douglas.wallin@vancethompsonvision.com
• Financial interest: none acknowledged