Biomechanical Properties of Human Cornea Tested by Two-Dimensional Extensiometry Ex Vivo in Fellow Eyes: PRK Versus SMILE

PURPOSE: To investigate the biomechanical properties of the ex vivo human paired corneas after completion of photorefractive keratectomy (PRK) versus small incision lenticule extraction (SMILE) in the same donor. METHODS: In this experimental study, 13 pairs of human corneas unsuitable for transplantation were equally divided into two groups. Corneal thickness was measured in each eye directly before laser refractive surgery. Corneas from the right eye were treated with PRK and corneas from the left eye with SMILE. All corneas were subjected to a refractive correction of −10.00 diopters (D) sphere and −0.75 D cylinder at 0° with a 7 mm zone, using either surface ablation (PRK) or 130 µm cap (SMILE). For two-dimensional biomechanical measurements, corneoscleral buttons underwent two testing cycles (preconditioning stress-strain curve from 0.03 to 9.0 N and stress-relaxation at 9.0 N during 120 seconds) to analyze the elastic and viscoelastic material properties. The effective elastic modulus was calculated. Statistical analysis was performed with a confidence interval of 95%. RESULTS: In stress-strain measurements, the effective elastic modulus was not significantly different ( P = .081) between SMILE (9.58 ± 4.26 MPa) and PRK (11.9 ± 4.90 MPa). The effect size was medium (Cohen's d = 0.58). In stress-relaxation measurements, the remaining stress was not significantly different ( P = .878) between SMILE (122 ± 33 kPa) and PRK (123 ± 30 kPa). CONCLUSIONS: The lenticule extraction procedure (SMILE) and the surface ablation technique (PRK) may be considered equivalent in terms of biomechanical stability when measured experimentally in ex vivo human fellow eye corneas. [ J Refract Surg. 2019;35(8):501–505.]