Small incision lenticule extraction (SMILE) results in similar or better refractive outcomes compared with LASIK for the treatment of myopia and with a number of advantages.
Take-home message: Small incision lenticule extraction (SMILE) results in similar or better refractive outcomes compared with LASIK for the treatment of myopia and with a number of advantages.
By Cheryl Guttman Krader; Reviewed by Dan Z. Reinstein, MD
Accumulating data demonstrate that for patients with myopia seeking refractive surgery, there are many reasons to consider small incision lenticule extraction (SMILE) performed with a proprietary femtosecond laser (VisuMax, Carl Zeiss Meditec).
“The refractive outcomes for treating myopia and cylinder are equivalent if not better with SMILE compared with LASIK, and the results are more predictable with SMILE when treating myopia greater than –8 D,” said Dan Z. Reinstein, MD, MA (Cantab), FRCSC, DABO, FRCOphth, FEBO.
“In addition, SMILE avoids flap-related concerns and variables affecting excimer laser treatment delivery, which may explain while SMILE has greater accuracy for correcting high myopia,” said Dr. Reinstein, medical director, London Vision Clinic, London, and clinical professor of ophthalmology, Columbia University Medical Center, New York.
SMILE also has less biomechanical impact on the cornea, provides higher optical quality, and is associated with less neurotrophic epitheliopathy, he noted.
Discussing procedure effects on corneal biomechanical integrity, Dr. Reinstein said that the difference between SMILE and LASIK is understandable knowing that the flap side-cut, not the delamination, is responsible for the reduction in cornea tensile strength after LASIK and recognizing that the anterior stroma is twice as strong as the posterior stroma.
“SMILE is a flapless procedure in which the anterior stromal lamellae remain uncut,” he explained.
Evidence that SMILE has an advantage over LASIK in its effect on corneal biomechanics was first demonstrated in a mathematical model developed by Dr. Reinstein and colleagues. Subsequently, they applied the model to actual clinical cases and found that the model predicted that corneal tensile strength was about 30% greater in eyes that underwent SMILE compared with an age-matched LASIK group across the entire range of myopia treated (up to –8 D).
Dr. Reinstein pointed out that the difference favoring SMILE occurs despite the fact that for a given correction, SMILE removes more tissue than LASIK because he had chosen to use a larger optical zone in order to reduce the induction of spherical aberration.
“With SMILE there is about 65% less spherical aberration induced than with a wavefront-optimized LASIK procedure due to the ability to use a larger optical zone,” Dr. Reinstein said. “However, SMILE still reduces corneal tensile strength less than LASIK. Furthermore, the predictability of the spherical aberration change is also much better for SMILE than LASIK.
“Since SMILE induces less spherical aberration than LASIK, it is irrelevant that there is not a SMILE wavefront-guided procedure,” he added.
Eyetracker performance and the need to control environmental conditions are also moot issues with SMILE. The flapless nature of SMILE is an attractive feature for patients and has positive implications for minimizing postoperative keratocyte apoptosis and inflammation.
In addition, published data on postoperative corneal sensitivity and tear parameters support the conclusion that SMILE causes less neurotrophic epitheliopathy than LASIK, Dr. Reinstein said.
Dr. Reinstein noted that both enhancements and hyperopic corrections are possible with SMILE. The technique used for an enhancement depends on cap thickness. In eyes with a thicker cap (≥135 µm), a thin-flap LASIK procedure with a 100-µm flap is performed within the cap itself. Otherwise, in eyes with a thin cap, LASIK is done using the circle software that converts the cap into a flap with a larger diameter.
Finally, there is always the option of PRK if the surgeon prefers.
“The enhancement in eyes with a thicker cap is just like a primary LASIK, and so it is much safer than an enhancement after LASIK, for which there is an increased risk of epithelial ingrowth,” Dr. Reinstein said.
Two studies are currently under way investigating SMILE for hyperopia. Dr. Reinstein is working with Kishore Pradhan, MD, in Kathmandu, Nepal, while another study is being run by Walter Sekundo, MD, in Marburg, Germany.
Dan Z. Reinstein, MD
This article was adapted from Dr. Reinstein’s presentation during the 2014 meeting of the American Academy of Ophthalmology. Dr. Reinstein is a consultant to Carl Zeiss Meditec.