Subbasal nerve morphology, corneal sensation, and tear film evaluation after refractive femtosecond laser lenticule extraction.

BACKGROUND: The purpose of this study was to compare corneal subbasal nerve morphology, corneal sensation, and tear film parameters after femtosecond lenticule extraction (FLEX) and small-incision lenticule extraction (SMILE).

METHODS: A prospective, randomized, single-masked, paired-eye design clinical trial of 35 patients treated for moderate to high myopia with FLEX in one eye and SMILE in the other. In both techniques, an intrastromal lenticule was cut by a femtosecond laser and manually extracted. In FLEX, a LASIK-like flap allowed removal of the lenticule, whereas in SMILE, it was removed through a small incision. In-vivo confocal microscopy was used to acquire images of the central corneal subbasal nerve plexus, from which nerve density, total nerve number, and nerve tortuosity were analyzed. Corneal sensation was measured using Cochet-Bonnet esthesiometry. A visual analog scale, tear osmolarity, non-invasive tear film break-up time (keratograph) tear meniscus height (anterior segment OCT), Schirmer's test, and fluorescein tear film break-up time were used to evaluate tear film and ocular surface symptoms. Patients were examined before and 6 months after surgery.

RESULTS: There were no statistically significant differences in baseline parameters between FLEX and SMILE (p > 0.050). With regard to changes from before to 6 months after surgery, mean reduction in subbasal nerve density was 14.22 ± 6.24 mm/mm(2) in FLEX eyes, and 9.21 ± 7.80 mm/mm(2) in SMILE eyes (p < 0.05). The total number of nerves decreased more in FLEX eyes than in SMILE eyes (p < 0.05). No change was found when comparing tortuosity (p > 0.05). Corneal sensation was reduced with 0.38 ± 0.49 cm in FLEX eyes, and 0.10 ± 0.34 cm in SMILE eyes (p < 0.01). No differences were found between FLEX and SMILE in tear film evaluation tests (p > 0.05). Significantly more patients felt postoperative foreign body sensation in the FLEX eye within the first days after surgery, as compared to the SMILE eye.

CONCLUSIONS: Six months after surgery, the less invasive SMILE technique seemed better at sparing the central corneal nerves as compared to FLEX. Corneal sensation was only significantly reduced in FLEX eyes. There were no differences between FLEX and SMILE when comparing tear film evaluation tests 6 months after surgery.