Flapless SMILE procedure provides improved optical quality

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Results of a study comparing matched groups of eyes undergoing myopic correction by small-incision lenticule extraction (SMILE) or LASIK show that the optical quality was better and the cornea retained higher tensile strength after the flapless SMILE procedure.

Dr. Reinstein

By Cheryl Guttman Krader; Reviewed by Dan Z. Reinstein, MD, MA

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New York City- The corneal tensile strength after surgery is much greater after small-incision lenticule extraction (SMILE)- a flapless refractive procedure performed using a proprietary femtosecond laser (VisuMax, Carl Zeiss Meditec) to cut an intrastromal lenticule that is removed through a small, 2-3 mm incision-than LASIK or PRK for an equivalent tissue removal, said Dan Z. Reinstein, MD, MA.

The flapless nature of the procedure means that anterior stromal lamellae remains uncut, and has the added benefit of the anterior stroma being the strongest part of the stroma.

“With its flap and ablation, LASIK severs the stromal lamellae at its strongest region, whereas SMILE leaves the most anterior stromal lamellae intact,” said Dr. Reinstein, medical director, London Vision Clinic, London UK, and clinical professor of ophthalmology, Columbia University Medical Center, New York.

SMILE advantages

In fact, he said the difference in tensile strength is enough that the cornea is still significantly stronger after SMILE than LASIK even when a larger optical zone is used in SMILE (i.e. greater tissue removal)-which enables less spherical aberration induction-therefore better optical quality, according to the results of a comparative study reported by Dr. Reinstein.

“With its benefits for maintaining greater corneal strength postoperatively and inducing less spherical aberration, SMILE allows higher levels of myopia to be treated with greater optical and biomechanical safety compared with LASIK,” he said. “Therefore, we believe SMILE should raise the bar for the level of myopia at which phakic IOL implantation becomes the preferred procedure over laser vision correction.”

Result evaluation

Dr. Reinstein analyzed the relative postoperative total tensile strength (PTTS) after myopic correction with SMILE and LASIK in matched cohorts of 96 eyes each.

The calculations were based on a mathematical model developed by Dr. Reinstein and colleagues (J Refract Surg. 2013; 29[70]: 454-60) that uses published data on cohesive tensile strength as a function of corneal depth to determine the remaining tensile strength following tissue ablation/removal.

To further illustrate the relative effects of the different procedures on corneal total tensile strength, Dr. Reinstein presented data from his published paper which compared the change in tensile strength as a result of removing 100 µm of stroma via ablation (LASIK or PRK) or as a lenticule (SMILE) from a 550-µm thick cornea.

Using the mathematical model, the calculated postoperative PTTS was 75% after SMILE performed with a 130-µm cap, 68% for PRK, and 54% for a thin-flap (100-micron) LASIK procedure.

In a further analysis, the SMILE and LASIK eyes were matched by sphere (±0.25 D), cylinder (±0.25 D), and pachymetry (±20 microns). Mean values for SEQ, cylinder, and pachymetry in both groups were approximately -4.83 D, 0.56 D, and 540 µm, respectively.

The mean ±SD thickness of the cap in the SMILE group was 130±6 µm and the LASIK group had a mean flap thickness of 96±12 µm. Mean OZ diameter was 6.7±0.39 mm for SMILE and 6.08±0.22 mm for LASIK. Mean (range) lenticule thickness was 107 µm (72-149) for SMILE, while the LASIK group had a mean ablation depth of 81 µm (25-134).

Mean PTTS in the SMILE and LASIK groups was 73% (65% - 82%) and 57% (45% - 72%), respectively.

Across the entire range of myopia treated (up to -8.00 D), PTTS was about 16% greater on average in the SMILE eyes compared with the LASIK group.

Analyses of higher order aberration data confirmed that SMILE induced significantly less spherical aberration than LASIK. Mean change from baseline spherical aberration was 0.11±0.16 µm in the SMILE eyes and 0.31±0.12 µm after LASIK.

Dan Z. Reinstein, MD, MA

E: dzr@londonvisionclinic.com

Dr. Reinstein is a consultant for Carl Zeiss Meditec AG. 

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