Corneal Biomechanical Properties after Small Incision Lenticule Extraction Surgery on Thin Cornea

Purpose To explore the biomechanical changes in thin corneas after the small incision lenticule extraction (SMILE) surgery.Methods This prospective survey screened patients scheduled for the SMILE surgery from November 2017 to March 2018. Patients with thin corneas (central corneal thickness [CCT] ≤500 μm) and those with normal corneal thickness (CCT > 500 μm) were enrolled. Corneal biomechanics were examined by the ocular response analyser and Corvis ST, preoperatively and at 1 day, 3 weeks, and 3 months postoperatively.Results Twenty-seven patients (46 eyes) with mean spherical equivalent of −5.1 ± 1.7 D were assigned to the thin cornea group, and 28 (45 eyes) cases with mean spherical equation of −5.3 ± 1.8 D were assigned to the control group. Safety and efficacy did not differ between the two age- and refraction-matched groups (safety index, 1.15 ± 0.14 vs 1.14 ± 0.17 [P = .7]; efficacy index, 1.11 ± 0.13 vs 1.16 ± 0.22 [P = .2]). All biomechanical parameters changed significantly after SMILE. The thin cornea group showed less decrease in the second applanation time (A2 Time), stiffness parameters at first applanation (SP-A1). Pooling data from two groups, lower CCT was correlated with less A2 Time change (r = 0.37, P < .01) and less SP-A1 change (r = −0.33, P < .01). Less bIOP change was correlated with higher residual stromal thickness index and thicker CCT (P = .003, R2 = 0.12).Conclusion Over a short-term observation period, less corneal biomechanic deterioration may have contributed to the safety of SMILE on thin cornea.