Changes in the Anterior Corneal Surface Morphology after SMILE

Objective: To compare the changes in asymmetric indexes and asphericity of the anterior corneal surface of patients with myopia and astigmatism six months after small incision lenticule extraction (SMILE) and sub-Bowman's keratomileusis (SBK). Methods: This was a prospective, non-randomized, controlled study. Patients underwent SMILE (96 eyes, SMILE group) or SBK (96 eyes, SBK group). The anterior corneal surface asphericity indexes and asphericity (Q-value) at a diameter of 6 mm were measured with the Pentacam before surgery and 10 days, 1 month, 3 months, and 6 months after surgery. Data were analyzed with t-tests, repeated measures analysis of variance, and Pearson correlation analysis. Results: The removed corneal tissue in the SMILE group was significantly thicker than that in the SBK group (t=-6.717, P<0.001). There were no differences in the Q-value, or mean of keratoconus index (Km) and minimum ridius (Rmin) values. The postoperative 1 month, 3 months, 6 months, values of the index of surface variance (ISV) were significantly larger than those in the SBK group (t=-2.168, -2.030, -2.628, all P<0.05). The postoperative values index of vertical asymmetry (IVA), index of height asymmetry (IHA), index of height decentration (IHD), and keratoconus index (KI) in the SMILE group were significantly larger than those in the SBK group (all P<0.01). For center keratoconus index (CKI) values, the outcomes in the SMILE group were lower than in the SBK group at 10 days, 1 month and 3 months (t=6.715, 5.003, 2.810, all P<0.01). There was a significant negative linear correlation between the△Q and the spherical equivalent (SE) in the two groups (SMILE: r=-0.750, P<0.001; SBK: r=-0.886, P<0.001). Conclusions: There is no difference between the Q-values in the SMILE and SBK groups. The negative linear correlation between△Q and SE is significant, and the postoperative values of ISV, IVA, IHA, IHD and KI in the SMILE group are greater than those in the SBK group. Key words: small incision lenticule extration; sub-Bowman's keratomileusis; anterior corneal surface; Q value; asymmetric indexes