SMILE procedure brings potential advantages to hyperopic correction

Reviewed by Dan Z. Reinstein, MD

Hyperopic small-incision lenticule extraction (SMILE) performed using a proprietary femtosecond laser (VisuMax, Carl Zeiss Meditec) is feasible and, in early experience, associated with refractive and visual acuity results similar to those achieved with LASIK, said Dan Z. Reinstein, MD.

“Compared with LASIK, SMILE brings potential advantages to hyperopic correction, including maintaining integrity of the corneal nerve plexus, which is particularly important considering that hyperopes tend to be older patients,” said Dr. Reinstein, medical director, London Vision Clinic, London, and adjunct professor of ophthalmology, Columbia University Medical Center, New York.

The original attempt at treating hyperopia using femtosecond lenticule extraction was associated with poor safety and significant regression, but the treatment was performed using a 5.5-mm optical zone and no transition zone. Safety and stability were much better in a subsequent small series treated with a 5.75-mm optical zone and a 2-mm transition zone, he noted.

“The lenticule shape for hyperopic SMILE has been further optimized, and initial results are encouraging,” he said. “We are in the process of treating more patients and analyzing contrast sensitivity and other parameters of quality of vision.”

Four phases

Four phases

Dr. Reinstein is participating in a prospective study of hyperopic SMILE at the Tilganga Institute of Ophthalmology, Kathmandu, Nepal. It consists of four phases. The first three phases sequentially enrolled blind eyes, densely amblyopic eyes, and mildly amblyopic eyes with aims of establishing the feasibility of the procedure, optimizing the laser energy settings and lenticule parameters, and evaluating centration.

The study is now in its fourth phase that plans to enroll 200 sighted eyes with best-corrected visual acuity (BCVA) of 20/40 or better, sphere ≤ +7 D, and astigmatism ≤ 6 D.

The hyperopic SMILE lenticule has a doughnut shape, and in the study, is being created with a 6.3- to 6.7-mm optical zone, a 2-mm transition zone, 30-μm minimum thickness, and a 90° side cut.

The cap measures 8.8 mm in diameter and is 120 μm thick, and there is a 0.25-mm clearance between the edge of the lenticule and the periphery. A 2-mm incision is made for removing the lenticule, and an escape incision is created on the opposite side.

“The minimum thickness is set at 30 μm because there is no curvature change in the center of the lenticule, and we wanted to make sure there was no chance of button-holing through the center of the doughnut,” Dr. Reinstein said. “Surgeons performing myopic SMILE will find that the hyperopic procedure is identical and that the lenticule dissection is no more difficult.”

Outcomes

The efficacy and safety of hyperopic SMILE were analyzed in a group of 31 eyes with BCVA of 20/40 or better and compared with outcomes in a cohort of LASIK-treated eyes matched 3:1 for sphere and cylinder (±0.5 D). Mean treated spherical equivalent (SEQ) and cylinder in the SMILE eyes was +5.73 D and 1.15 D, respectively.

The efficacy analysis showed that at 3 months, distance UCVA was the same or better than preoperative BCVA in 47% of SMILE eyes and within 1 line of preoperative BCVA in 93% of SMILE eyes. In the LASIK-treated eyes, distance UCVA was the same or better than preoperative BCVA in 50% of eyes and within 1 line of preoperative BCVA in 81%.

“It is interesting that the vision results are better with SMILE, and we believe this may be explained by the fact that the ocular surface at 3 months is healthier in the SMILE eyes compared with the LASIK eyes,” Dr. Reinstein said.

Safety and predictability

Safety and predictability

The safety analysis showed 32% of SMILE eyes lost 1 line from preoperative BCVA. This finding was expected because of the minification effect of the surgery and similar to the result in the LASIK group where 25% of eyes had a 1-line loss of BCVA, Dr. Reinstein said.

The predictability analysis showed that SEQ was within ±0.5 D of attempted in 65% of SMILE eyes and in 53% of LASIK eyes. In the regression analysis, the r2 value was higher for SMILE than for LASIK, 0.59 versus 0.48, he noted.

“This is interesting, because we would expect to see tighter correlation in an analysis including more eyes,” Dr. Reinstein said. “Therefore, based on this limited comparison, it seems there is less scatter of results with SMILE treatment for higher hyperopia.”

Stability of the SMILE correction was good during the short follow-up. Mean change in SEQ from 1 to 3 months was about 0.1 D in both the SMILE and LASIK groups.

Other analyses compared optical zone centration, topographic optical zone diameter, and spherical aberration induction in a cohort of 60 consecutive SMILE eyes and two LASIK control groups treated at 6.5- and 7-mm optical zones, respectively, and matched 1:1 by treated SEQ [Reinstein DZ, et al. J Refract Surg. 2017;33:150-156. Reinstein DZ, et al. J Refract Surg. 2017;33:370-376].

All eyes had BCVA of 20/40 or worse and the mean optical zone for the SMILE group was 6.37 mm.

The results showed optical zone centration was similar for SMILE and LASIK. Mean topographic optical zone diameter was significantly larger for SMILE compared with both LASIK groups, and mean induced spherical aberration in the SMILE eyes was similar to the mean change in the 7-mm LASIK group and significantly less than after 6.5-mm LASIK.

“The explanation for the optical zone finding is that with the excimer laser procedure, there are projection errors in the periphery of the cornea that are never fully compensated,” Dr. Reinstein said. “SMILE cuts precisely at the location geometrically calculated by the software and gives a truer-sized transition zone. Knowing that transition zone size is key to regression stability and treatment accuracy might also be the explanation for why we saw less scatter in the 3-month predictability outcomes with SMILE.”

Dan Z. Reinstein, MD

E: dzr@londonvisionclinic.com

This article was adapted from Dr. Reinstein’s presentation at the 2017 meeting of the American Society of Cataract and Refractive Surgery. Dr Reinstein is a consultant for Carl Zeiss Meditec.