APR 17, 2017
Cornea/External Disease, Refractive Mgmt/Intervention
This prospective randomized trial compared cap thickness in small incision lenticule extraction (SMILE) in patients with thick corneas, and found that visual outcomes are similar for both thin and thick flaps, but a thicker cap conferred better corneal recovery.
SMILE (Visumax, Carl Zeiss Meditec) is a minimally invasive, flapless procedure approved in September 2016 to treat myopia of -1.00 to -8.00 D, with ≤-0.50 D cylinder and MRSE -8.25 D. Despite its novelty, some physicians already consider SMILE superior to LASIK. However, there is not yet a unified standard for ideal cap thickness. Based on their findings in thick corneas, the authors recommend that surgeons should not increase the thickness of the residual stroma solely to decrease the risk of ectasia while ignoring the increased morphologic changes of the cornea in a thin cap.
In this study, 40 patients with central corneal thickness of more than 560 µm were randomized to receive SMILE with a 120-µm cap thickness in 1 eye and 140-µm cap thickness in the other. At 3 months postop, refractive outcomes, uncorrected and corrected distance visual acuity, contrast sensitivity and higher-order aberration changes were similar between both groups.
Analysis using 3D OCT, in vivo confocal microscopy and scanning electron microscopy indicated that eyes with the 140-µm cap showed better recovery, lower-level wound-healing response and smoother reflective lenticules. By month 3, the hyperreflectivity line at the interface layer had almost completely disappeared in eyes with the thicker cap, but persisted in 43% of the eyes with the 120-µm cap.
Furthermore, the study authors found that it was harder to dissect the 120-μm cap in eyes with thick corneas. The authors postulate that the anterior cornea, which has more integrated collagen fibers and compact Bowman membrane, may provide stronger resistance and therefore increase the difficulty of dissecting corneal tissues after femtosecond laser treatment.
Though there was no difference in visual outcomes, the authors wonder whether the thicker flap may result in other unknown benefits. Further research will be required to evaluate biomechanical changes over time in the cornea with the thinner flaps.