Biomechanical changes after keratorefractive lenticule extraction with CLEAR and after femtosecond LASIK, correlated with optical coherence tomography findings

The aim of this retrospective, comparative, single-eye study was to assess the biomechanical changes after laser correction of myopia by keratorefractive lenticule extraction (KLEx) and by femtosecond LASIK (FS-LASIK), correlating them with the stromal changes on anterior segment optical coherence tomography. Corneal biomechanical parameters, provided by the high-speed Scheimpflug camera CorVis-ST (Oculus Optikgeräte GmbH) and measured pre-operatively and 1 week post-operatively, were: stiffness parameter at first applanation (SP-A1), stress-strain index (SSI), inverse integrated radius (IIR), deformation amplitude ratio at 2 mm (DA ratio-2mm). A total of 79 eyes undergone KLEx (CLEAR, Ziemer Group) and 93 eyes undergone FS-LASIK were included. The mean residual stromal bed (RSB) was 271.36±17.22 µm after KLEx and 304.21±21.82 µm after FS-LASIK (p=0.00). All parameters in both groups showed statistically significant post-operative changes (p=0.00), except for SSI after FS-LASIK (p=0.39). The percent changes in all parameters were significantly higher in the KLEx group. Even in the eyes with equal RSB (300±5 µm; 19 eyes post-KLEx, 26 eyes post-FS-LASIK), changes were significantly higher after KLEx (SP-A1: -35.9%; SSI: -6.7%) than after FS-LASIK (SP-A1: -29.6%; SSI: -3.8%) (respectively, p=0.02, and p=0.00). In KLEx, reduction in stromal thickness had a weak correlation with reduction in SP-A1 (r=0.39), and poor correlation with reduction in SSI (r=0.26). In conclusion, stiffness parameters were significantly worse after KLEx than after FS-LASIK, even in eyes ending with similar RSB. These findings should be valued cautiously, as the CorVis-ST might not capture all the clinically significant alterations, especially in the post-KLEx bi-layered cornea.