The slower visual recovery seen in small incision lenticule extraction (SMILE) surgery as compared with laser-assisted in situ keratomileusis (LASIK), is associated with higher optical densities in the early postoperative period, a study shows. The research shows that the higher optical densities significantly decrease after surgery up to 3 months, whereas LASIK corneas show a nonsignificant decrease in optical densities. This early optical density rise, and subsequent improvement, seen after SMILE might be related to an increased surgical manipulation, according to the study.

Researchers explored the visual recovery in SMILE patients by comparing the thickness and stromal optical density evolution of the flap and cap after each procedure. They found that, up until approximately 1 month postoperative, corneas that underwent SMILE have higher optical densities than those that underwent LASIK.

The investigators examined 78 eyes of 42 consecutive subjects who underwent LASIK (39 eyes) or SMILE (39 eyes) to correct myopia. The femtosecond laser the surgeons used applies a low-pressure, curved coupling contact glass interface instead of a high-pressure, flat one.


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At a series of 4 postoperative examinations (1 day, 1 week, 1 month and 3 months later), they performed anterior segment optical coherence tomography (AS-OCT) imaging and measured the flaps and caps in a section in a horizontal meridian in the center of the cornea. They also measured the mean central thickness, the mean total thickness (MTT) of 17 flap data points in the horizontal meridian, the SD of the MTT and the thickness range and homogeneity of the flap or cap, obtained the optimal AS-OCT image and measured the total optical density in the central 2 mm area in grayscale units.

Eyes that underwent SMILE experienced more relative change from 24 hours to 3 months after the operation in total optical density (-9.69%) and residual stromal bed (RSB) optical density (-9.63%) than those that underwent LASIK (total optical density: -4.79%; RSB optical density: -4.98%). While the 2 optical density measurements were higher in SMILE (RSB optical density: 50.87±6.45 µm; Total optical density: 50.92± 5.86 µm) than in LASIK (RSB optical density: 49.59±6.56 µm; Total optical density: 50.92±5.86 µm) during the first week, that became the inverse in the last follow-up visit. At that point, SMILE’s optical density measurements (RSB optical density: 45.42±7.53 µm; Total optical density: 45.46±6.96 µm) were lower than LASIK’s (RSB OD: 46.79±6.86 µm; Total optical density: 47.15±6.28 µm). MTT increased more in LASIK (5.43±4.23 μm) than in SMILE (2.76±5.61 μm; P =.020) from 1 week to 3 months after the operation.

“While previous evidence could not find significant differences in postoperative corneal densitometry after LASIK and SMILE, concluding that the delayed visual recovery seen in SMILE is not related to changes in corneal density, we have observed a statistically significant reduction of all SMILE OD parameters (about 10% improvement) during the follow-up period,” the researchers wrote. “This finding, not observed in any of LASIK’s optical density parameters (about 5% improvement), could justify SMILE’s delayed visual recovery by reflecting a progressive improvement in corneal stroma transparency.”

The researchers said that their use of different imaging technology in the study may have affected their findings. 

Disclosure: One of the researchers, Jorge Luis Alio del Barrio, is part of the medical advisory board of Costruzione Strumenti Oftalmici.

Reference

Alio del Barrio JL, Parafita-Fernandez A, Canto-Cerdan M, Alio JL, Teus M. Evolution of corneal thickness and optical density after laser in situ keratomileusis versus small incision lenticule extraction for myopia correction. Published online September 6, 2020. Br J Opthalmol. doi:10.1136/ bjophthalmol-2020-316601