What you should know about SMILE and wavefront guided LASIK

Reviewed by David P. Piñero, PhD 

Take-home: Findings of a literature review highlight the need for more studies to compare SMILE and WFG LASIK for treatment of myopia and myopic astigmatism.

David P. Piñero, PhDProponents of small incision lenticule extraction (SMILE) are advocating this all-femtosecond laser refractive surgery technique for having advantages compared with LASIK. However, examination of the peer review literature shows there is still a lack of high level scientific evidence to support many of the claims, said David P. Piñero, PhD.

“Good clinical results have been reported for SMILE, and compared with wavefront guided (WFG) LASIK, SMILE appears to be associated with less postoperative dry eye and avoids flap-related complications. However, SMILE shares other complications in common with LASIK and published data do not show that SMILE is superior to WFG LASIK in terms of visual, aberrometric, or biomechanical outcomes,” he said.

“Whereas the efficacy and safety of WFG LASIK and enhancement procedures are well established, experience with SMILE is still limited, available follow-up is relatively short-term, and information is needed about methods of retreatment and their efficacy. These issues need to be addressed through future research that ideally will include head-to-head comparison trials. Clearly, more data are needed before refractive surgeons should feel confident adopting SMILE into their practices.”

Dr. Piñero is assistant professor, departments of optics, pharmacology, and anatomy, University of Alicante, Spain. Setting out to compare outcomes of the two refractive surgery procedures when performed with modern technology, he conducted a search of the published literature that was restricted to January 2012 to September 2015.

Efficacy findings

Courtesy of David P. Piñero, PhDThe search identified 72 articles reporting on SMILE and WFG LASIK outcomes for the treatment of myopia or myopic astigmatism. Most eyes were treated for low to moderate myopia (up to -6.00 D), although one study of SMILE included eyes with higher myopia. Data were extracted on uncorrected and best corrected visual acuity (UCVA and BCVA), higher order aberrations (HOAs), dry eye, corneal backscattering, corneal biomechanics, and complications.

In his review, Dr. Piñero found that efficacy, measured by the percentage of eyes achieving distance UCVA of 20/25 or better, was similar for the SMILE and WFG LASIK groups, ranging from about 83% to 100%. Mean postoperative logMAR UCVA values reported in the articles reviewed ranged from 0.02 to -0.17 for SMILE and from -0.04 to -0.18 for WFG FS-LASIK.

There were also no clear differences noted between the two procedures in their safety outcomes, considering an analysis of the percentage of eyes losing one or more lines of BCVA.

“According to the peer-reviewed literature, the percentage of eyes gaining 1 line of CDVA ranged from 0% to 57.10% and from 14% to 51% for SMILE and WFG FS-LASIK techniques, respectively,” Dr. Piñero said.

The review of visual outcomes also showed the expected “wow” effect of LASIK with rapid recovery of excellent visual acuity. However, eyes that underwent SMILE took slightly longer to achieve a high level of uncorrected vision.

Dr. Piñero noted that ultrastructural studies using OCT imaging identify microdistortions in Bowman’s layer as being present after both WFG LASIK and SMILE. However, their number is greater in the SMILE eyes.

“The microdistortions act to increase light backscattering intensity in the anterior stroma, and that phenomenon may be a contributing factor in the slower visual recovery after SMILE,” Dr. Piñero said.

Safety comparision

The safety comparison of the two techniques also considered the development of dry eye. Findings from studies that assessed dry eye based on subjective questionnaires (Ocular Surface Disease Index or the McMonnies questionnaire) showed early worsening after both SMILE and WFG LASIK procedures. The changes were greater after WFG LASIK, but generally returned to preoperative values by 1 to 3 months following both types of surgery.

“The difference in dry eye outcomes may be explained by a less severe decrease in subbasal nerve density after SMILE,” Dr. Piñero said.

Analyses of aberrometry data showed better control of HOAs in eyes that underwent WFG LASIK compared with SMILE. In particular, there was a trend for an increase in coma after SMILE that Dr. Piñero suggested might be explained by a tendency for mild treatment decentration in eyes undergoing that procedure.

Corneal biomechanics data were analyzed because it has been purported that compared with LASIK, SMILE has an advantage for maintaining better corneal biomechanical integrity. “However, the evidence supporting this idea is an estimation derived from a mathematical model,” Dr. Piñero said.

“Results of published studies using different instrumental methods to assess corneal biomechanics provide no clinical evidence that the cornea has better biomechanical behavior after SMILE than after WFG LASIK.”

Review of complications associated with the two procedures showed that SMILE avoids events that can occur with flap creation. However, other complications associated with LASIK-diffuse lamellar keratitis, decentrations, and ectasia-were still reported in the SMILE studies.

“In conclusion, both SMILE and WFG LASIK are effective and safe procedures for the correction of myopia and myopic astigmatism, but more research is still necessary to confirm the potential superiority of one technique over the other,” Dr. Piñero said.

David P. Piñero, PhD 

E: david.pinyero@gcloud.ua.es

This article is based on a presentation given by Dr. Piñero during the XXXIII ESCRS Congress in Barcelona Spain. A paper by Dr. Piñero and Miguel A. Teus, MD, PhD, reporting on the literature review has been accepted for publication in the Journal of Cataract & Refractive Surgery.