Biomechanical Differences Between Femtosecond Lenticule Extraction (FLEx) and Small Incision Lenticule Extraction (SmILE) Tested by 2D-Extensometry in Ex Vivo Porcine Eyes

PURPOSE. To evaluate the biomechanical stability of ex vivo porcine corneas after femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SmILE) refractive surgeries. METHODS. Forty-five porcine eyes were equally divided into three groups: Groups 1 and 2 were treated with FLEx and SmILE procedure, respectively. Group 3 served as control. A refractive correction of -14 diopters (D) with a 7-mm zone using either a 160-mu m flap (FLEx) or a 160 mu m cap (SmILE) was performed. For two-dimensional (2D) elastic and viscoelastic biomechanical characterization, two testing cycles (preconditioning stress-strain curve from 1.27 to 12.5 N, stress-relaxation at 12.5 N during 120 seconds) were conducted. Young's modulus and Prony constants were calculated. RESULTS. At 0.8% of strain, FLEx (370 +/- 36 kPa) could resist a significantly lower stress than SmILE (392 +/- 19 kPa, P = 0.046) and the control group (402 +/- 30 kPa, P = 0.013). Also, FLEx (46.1 +/- 4.5 MPa) had a significantly lower Young's modulus than the control group (50.2 +/- 3.4 MPa, P = 0.008). The Young's modulus of SmILE (48.6 +/- 2.5 MPa) had values situated between untreated corneas and FLEx-treated corneas. When compared to untreated controls, the stress resistance decreased by 8.0% with FLEx and 2.5% with SmILE; Young's modulus decreased by 5.1% with FLEx and 1.04% with SmILE. With a cap-based procedure, both anterior cap and stromal bed carry the intraocular pressure, while in a flap-based procedure, only the stromal bed does. CONCLUSIONS. Compared to flap-based procedures like FLEx, the cap-based technique SmILE can be considered superior in terms of biomechanical stability, when measured experimentally in ex vivo porcine corneas.