Comparison of the Curative Effect after Small Incision Lenticule Extraction Surgery at Different Ablation Depths

Objective: To determine the effect of different ablation depths on the curative effect after small incision lenticule extraction (SMILE). Methods: A total of 116 eyes (58 patients with anisometropia) that underwent SMILE were included in this retrospective study from April 2016 to July 2017 in Ningbo Yinzhou People's Hospital. The deeper ablated eye in each patient was assigned to Group A, while the contralateral eye was assigned to Group B. Uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), spherical equivalent (SE), higher-order aberrations (HOA), and other parameters were recorded preoperatively and at 1 day, 1 week, 1 month, and 6 months postoperatively. The average preoperative corneal curvature, axial length, corrected intraocular pressure, and expected ablation depth were recorded, while the actual ablation depths were calculated. The data were analyzed by repeated measurement ANOVA, paired samples t-tests, Pearson correlation analysis, and linear regression analysis. Results: The mean safety index and the mean effective index were greater than 1.0 in both groups 6 months postoperatively. Mean postoperative SEs were both within the range of ±0.5 diopter (D). The increases of corneal total higher-order aberrations (t=3.747, P=0.001), spherical aberration (t=3.308, P=0.002), and vertical coma (t=3.507, P=0.001) in Group A were all higher than in Group B at 1 month postoperatively. Two cases of diffuse lamellar keratitis occurred in Group A while three occurred in Group B. All were cured by glucocorticoid treatment. The deviations between the expected and actual ablation depths in Group A were greater than that in Group B (t=4.250, P<0.001). The deviation between the expected and actual ablation depths was correlated with the expected ablation depth (r=0.397, P<0.001), axial length (r=0.350, P=0.002), and SE (r=-0.308, P=0.007). Using multiple linear regression, the following relationship was determined: D=0.171×expected ablation depth -8.201, where D was the deviation between the expected and actual ablation depths. Conclusions: Within the range of indications, ablation depth did not affect the safety, stability, or effectiveness of SMILE surgery. However, the deeper the expected ablation depth was, the more corneal total HOA, spherical aberration, and vertical coma were introduced after surgery, and the worse the predictability of ablation depth was. The predictability of ablation depth was weakly correlated with expected ablation depth axial length, and preoperative SE. Key words: small incision lenticule extraction; ablation depth; high-order aberrations; treatment outcomes