SMILE May Not Improve Corneal Strength Over LASIK

Christopher J. Rapuano, MD


August 12, 2019

Despite its popularity and safety track record, laser in situ keratomileusis (LASIK) remains associated with the rare but significant complication of corneal ectasia. One theoretical advantage of small incision lenticule extraction (SMILE), a newer refractive technique, is that it may induce less biomechanical weakening of the cornea than LASIK because it requires a much smaller anterior stromal cut,[1] ideally translating into fewer cases of postoperative ectasia.

To analyze the effect of the SMILE cap versus the LASIK flap on corneal biomechanical properties, Khamar and colleagues[2] compared results in the eyes of 24 patients before, during, and after SMILE in one eye and LASIK in the fellow eye. The creation of the SMILE cap and the LASIK flap (but before removal of the stromal tissue) both weakened the cornea, although the weakening was statistically significantly greater in the LASIK eyes.

Postoperative measurements demonstrated evidence of further weakening due to the opening of the SMILE cap and the LASIK flap interfaces and removal of the stromal tissue. However, 1 week and 1 month after surgery, the authors found similar degrees of corneal weakening compared with preoperative measurements in both SMILE and LASIK eyes. They thereby concluded that the stromal tissue removal (including opening the interface) was the primary determinant of the corneal weakening.

Even though the LASIK flap creation appeared to weaken the cornea more than the SMILE flap/cap cut, there did not seem to be any difference in corneal weakening between the two groups at 1 week and 1 month postoperatively. Whether this finding is because the removal of the stromal tissue (which was similar between pairs of eyes in the two groups) was a much greater factor than the flap/cap creation, healing differences between the two surgical procedures evened out the strength of the cornea, or another factor is not known.

In addition, there is currently no way to directly measure "corneal weakness." The Corvis ST (Oculus, Inc .; Wetzlar, Germany) measurement used in this study evaluates numerous variables to assess "corneal weakness," but because there is no gold standard to which to compare the results, how accurately they represent reality is unknown.

One of the hopes of surgeons was that SMILE would not weaken the cornea and increase the risk for ectasia as much as LASIK. Although that may in fact be true, this study does not support that conclusion. The authors end their article by noting that "the safety criteria established for recommending LASIK to patients should also be followed for recommending SMILE."

At this point in time, I totally agree.

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