Small incision lenticule extraction (SMILE), which has not yet been approved for refractive correction in the United States, may be less accurate and produce more irregular astigmatism than LASIK or PRK because of the imprecision resulting from the two incisions needed to create the lenticule. In addition, the technique for potential enhancements needs improvement. However, the treatment is promising and may stand the test of time as the technology advances.
Take-home message: Small incision lenticule extraction (SMILE), which has not yet been approved for refractive correction in the United States, may be less accurate and produce more irregular astigmatism than LASIK or PRK because of the imprecision resulting from the two incisions needed to create the lenticule. In addition, the technique for potential enhancements needs improvement. However, the treatment is promising and may stand the test of time as the technology advances.
By Nancy Groves; Reviewed by Steven E. Wilson, MD
Cleveland -Small incision lenticule extraction (SMILE) is a promising new approach for refractive correction but has its limitations as currently performed, said Steven E. Wilson, MD.
During a SMILE procedure, the femtosecond laser makes two lamellar passes across the cornea that intersect in the peripheral cornea to produce a lenticule with precise dimensions to correct the patient’s refractive error, including myopia, hyperopia, and astigmatism. These two cuts are one of the main reasons why Dr. Wilson has reservations about the technique, although he is not opposed to it.
“It’s important to point out its potential advantages and potential limitations relative to the other procedures that we’re already using, LASIK and PRK,” said Dr. Wilson, professor of ophthalmology, director of corneal research, and cornea and refractive surgery fellowship director, the Cole Eye Institute, Cleveland Clinic.
The precision of the cuts made with the femtosecond laser versus those made with an excimer laser is what concerns him most.
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“We have been using the femtosecond laser for many years to make flaps for LASIK, and we know that no two cuts are the same,” Dr. Wilson said. “They all have different OBL (opaque bubble layer) patterns.”
In LASIK when only one cut is made with that laser, there is a lot of forgiveness for those imperfections because afterwards, the flap is lifted and the ablation is done on the bed. When the flap is replaced back down, those imperfections still line up as long as you align the flap back in its original position because there is only one interface, Dr. Wilson explained.
Next: Lack of precision
“But when you make two cuts with the femtosecond laser and those two cuts are never the same, then when the two surfaces become opposed to each other, there’s going to be some irregularity,” Dr. Wilson continued. “Some of that is going to be transmitted to the anterior surface of the cornea. That translates into a certain level of irregular astigmatism inherent in that procedure.”
This lack of precision could produce results that are less accurate than those achieved with LASIK and PRK, Dr. Wilson said.
He referred to an evaluation of the safety and complications of SMILE (Iversen et al., Ophthalmology. 2014;121:822-828) as one of the best studies to date on this topic.
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In this study of 922 patients treated with SMILE at a center in Denmark, the mean preoperative spherical equivalent refraction was -7.25 D ± 1.84 D. The average postoperative refraction was -0.28D ± 0.52 D, meaning that about 5% of eyes were expected to be more than 1 D from average, and the mean error of treatment was -0.15D ± 0.50 D.
By 3 months, 86% (1,346 eyes) had unchanged or improved best-corrected distance visual acuity. A loss of 2 or more lines was observed in 1.5% of eyes; however, at a late follow-up visit, corrected distance visual acuity was within 1 line of the preoperative level in all eyes.
The authors of this study concluded that SMILE had an acceptable safety profile and that patient satisfaction was high.
Dr. Wilson suggested that the precision of the SMILE procedure could be substantially improved by using either more pulses or pulses distributed closer together when making the two lamellar cuts. This would reduce or eliminate the step of inserting a spatula to break the adhesions between the lenticule and the anterior and posterior stroma.
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“There will always be some limitations because no two femtosecond cuts are the same, but I think you can make them more precise,” he added.
Next: Second issue
A second issue Dr. Wilson raised was that some patients treated with SMILE would need enhancements, as is true with any refractive surgery procedure because of variability in the wound-healing response between individual patients.
“Assuming that’s going to be somewhere between 5% and 15% of patients, how will those enhancements be performed?” he asked. “There’s a risk in trying to recut the lenticule and do another SMILE-type procedure because those two procedures could intersect each other. You’ll end up with fragments of stroma not attached anywhere.”
PRK with mitomycin C seems to be the safest method if retreatment is needed, Dr. Wilson said. While enhancement with PRK is an acceptable solution, it seems to defeat the concept of SMILE to resort to the surface technique to achieve the desired results, he added.
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However, a modification that would enable surgeons to make side cuts into the interface, converting the SMILE procedure into LASIK, is being evaluated.
Citing a potential advantage of the SMILE technique, Dr. Wilson noted that it might cause less LASIK-induced dry eye if more of the anterior nerve trunks are spared. It may also have biomechanical advantages, such as maintaining the strength of the anterior stroma, which provides a significant proportion of corneal rigidity.
However, surgeons should be cautious when treating patients with traditional risk factors for keratoconus or low biomechanical rigidity.
“If a patient has irregular corneal topography or decreased corneal thickness or both, you have to be very careful in applying SMILE,” Dr. Wilson said, adding that several cases of ectasia associated with this technique have been reported.
Although the SMILE procedure is performed in more than 30 countries, it is still undergoing clinical trials in the United States prior to regulatory approval.
“We’re at the beginning,” Dr. Wilson said. “Like many procedures, it may improve over time. We still don’t know with SMILE.”
Steven E. Wilson, MD
This article was adapted from Dr. Wilson’s presentation during the 2015 meeting of the American Academy of Ophthalmology. Dr. Wilson does not have any commercial interests related to the topic; however, he has been a trainer for VISX.