Biomechanical Weakening of Different Re-treatment Options After Small Incision Lenticule Extraction (SMILE)

PURPOSE: To determine the corneal weakening induced by different re-treatment options after small incision lenticule extraction (SMILE) and investigate the potential of corneal cross-linking (CXL) to reestablish the original corneal stress resistance. METHODS: A total of 96 freshly enucleated porcine corneas were used. The initial refractive correction was defined to be −11.00 diopters (D) and the required enhancement to be −3.00 D. Three different re-treatment options were analyzed: −3D Re-SMILE, −3D photorefractive keratectomy (PRK) on top of the SMILE cap, and cap-to-flap conversion and −3D excimer ablation on the stromal bed (LASIK). The control condition did not receive any treatment. Subsequently, accelerated CXL (9 mW/cm 2 , 10 min) was performed in two groups with currently common enhancement techniques: following cap-to-flap conversion (−3D LASIK enhancement) and in controls. Biomechanical properties were measured with stress-strain extensometry ranging from 1.27 to 12.5 N. RESULTS: The Re-SMILE and PRK enhancement did not significantly reduce the overall elastic modulus of the cornea compared to controls (24.7 ± 2.23 and 22.7 ± 2.61 versus 23.8 ± 3.35 MPa, P ≥ .176), whereas LASIK enhancement did (22.2 ± 3.37 MPa, P = .048). CXL treatment significantly increased the elastic modulus compared to all non-cross-linked conditions ( P ≤.001). Refractive surgery decreased the overall elastic modulus by 7%, whereas CXL increased it by 20%. CONCLUSIONS: In enhancement, the corneal biomechanical integrity is less affected with both Re-SMILE and PRK enhancement. Corneal weakening through laser refractive surgery is small compared to the stiffening effect after CXL. [ J Refract Surg. 2017;33(3):193–198.]