Evolution of corneal thickness and optical density after laser in situ keratomileusis versus small incision lenticule extraction for myopia correction

Abstract

Purpose To compare the thickness and stromal optical density (OD) evolution of the flap and cap after femtosecond (FS-LASIK; Visumax) and small incision lenticule extraction (SMILE), respectively, for myopia correction. Methods A prospective study comprising 78 eyes (n=39 per group) was performed. Anterior segment optical coherence tomography (MS39) images were collected at each postoperative visit (1 day, 1 week and 1 and 3 months) and flap/cap thicknesses were measured. Using ImageJ software, three regions were defined to measure OD (in grayscale units: flap/cap stroma, residual stromal bed (RSB) and all stroma (including flap/capstroma, surgical interface and RSB). Results Mean central thickness significantly increased during the 3-month follow-up in both LASIK (5.43 ±4.23 μm; p=0.0118) and SMILE (2.76±5.61 μm; p=0.0118), with differences between both techniques statistically significant (p=0.020). All OD values showed a satatistically significant reduction during SMILE follow-up: cap 48.96±5.55 versus 44.95±5.41 (p=0.19×10−6), RSB 50.26±7.06 versus 45.42±7.53 (p=0.00005), total stroma 50.34±6.49 versus 45.46±6.96 (p=0.5×10−5) at 1 day and 3 months, respectively, whereas no statistically significant changes were found during LASIK follow-up. No significant differences were observed in OD values between both techniques at any time point, although SMILE showed a tendency for higher OD values than LASIK within the first postoperative month. Visumax presented a tendency for thicker caps than target (11.48±7.85 μm), but not for flaps (2.73±8.93 μm) p=0.00003). Conclusions Both LASIK flaps and SMILE caps show a significant postoperative rethickening. SMILE corneas present higher optical densities than LASIK corneas in the early postoperative period, with a significant decrease thereafter and up to 3 months. These findings correlate with the delayed visual recovery observed after SMILE.a FS laser dissected lenticule is extracted through a small corneal incision.1 The latter has the potential advantages of less iatrogenic dry eye,2 lower laser energy requirements,3 fewer induced higher-order aberrations,4 reduced corneal inflammation and keratocyte damage,5 6 and lower suction intraocular pressure during the FS laser procedure.7 On the other hand, most studies report a slower visual recovery after SMILE when compared with LASIK.1 8 This could be related to the increased trauma to the surrounding tissue during manual lamellar dissection within the cornea during SMILE, and also due to a possible difference on the smoothness of the resultant residual stromal bed (RSB). Visumax FS laser platform (Zeiss, Germany), in contrast to other FS laser platforms, uses a curved coupling contact glass interface with the cornea, leading to very little corneal distortion when securing the eye. However, these low-pressure curved patient interfaces (instead of high-pressure flat ones) have been associated with an early transient increase in corneal optical density (OD), cornea oedema and slightly thicker LASIK flaps than target,9 what could also justify the slower visual recovery after SMILE. The purpose of the current study is to prospectively analyse the evolution of the OD and thickness of LASIK flaps and SMILE caps created with the Visumax FS laser platform in order to evaluate the influence from its curved low-pressure glass interface and the one from the extrasurgical manipulation during SMILE cases.