Corneal Biomechanical Changes After SMILE: Relationship with Residual Stromal Thickness and Percent Tissue Altered

ABSTRACT

Purpose: To investigate the relationship between residual stromal thickness (RST), percent tissue altered (PTA and PTA’), and corneal biomechanical changes after small incision lenticule extraction (SMILE), and to identify dominant predictors of postoperative corneal biomechanics..
Methods: This retrospective study included 186 eyes from 104 patients who underwent SMILE at the Affiliated Hospital of Southwest Medical University between 2023 and 2024. Corneal biomechanical parameters — deformation amplitude ratio (DA Ratio), stiffness parameter at first applanation (SP-A1), and integrated radius (IR) were measured preoperatively and 1 month postoperatively using the Corvis ST. Linear and nonlinear regression analyses were applied to assess the associations of PTA, PTA’, and RST with changes in biomechanical parameters. Stepwise multivariate regression was performed to identify factors associated with postoperative biomechanics, with covariates including gender, age, preoperative mean corneal curvature, spherical equivalent (SE), preoperative biomechanical parameters, and postoperative central corneal thickness (CCT). Preoperative and postoperative data were compared using paired t-tests, and correlations were evaluated using Pearson or Spearman analysis.
Results: At 1 month postoperatively, the mean changes in DA Ratio, SP-A1, and IR were -0.99±0.57 (-23.0%), 19.66±14.46 (17.0%), and -2.54±1.15 (-28.0%), respectively, all statistically significant (t=12.9, 18.53, -30.11; all P<0.01). Fitting curves indicated that changes in DA Ratio, SP-A1, and IR increased with decreasing RST or increasing PTA and PTA’. A marked increase in the slope of the DA Ratio change curve was observed when RST fell within 280~310 μm, PTA rose to 22%~35%, or PTA’ reached 50%~65%.
Multivariate regression showed that postoperative SP-A1 was most strongly influenced by preoperative SP-A1 (Beta=0.755, P<0.01), while postoperative IR was primarily associated with preoperative IR (Beta=0.592, P<0.01). Postoperative CCT was the strongest predictor of postoperative DA Ratio (Beta=0.327, P<0.01) and the second strongest for SP-A1 (Beta=-0.703, P<0.01). Preoperative DA Ratio also significantly influenced postoperative DA Ratio (Beta=-0.326, P<0.01), and preoperative mean corneal curvature was the second influencing factor for postoperative IR (Beta=-0.12, P<0.05).
Conclusions: Corneal biomechanical changes following SMILE, as reflected in DA Ratio, SP-A1, and IR, intensify with lower RST or higher PTA and PTA’. The rate of DA Ratio change accelerates notably within specific ranges: RST of 280~310 μm, PTA of 22%~35%, or PTA’ 50%~65%. Preoperative corneal biomechanical properties and postoperative CCT are the primary determinants of corneal biomechanical behavior after SMILE.