Preoperative Corneal Astigmatic Direction is Associated with Stromal Mechanical Anisotropy in SMILE-Derived Lenticules.

PURPOSE: Direction-dependent mechanical behavior exists in the human corneal stroma, but fixed-axis biomechanical testing may be influenced by preoperative astigmatic direction. This study evaluated whether corneal astigmatic direction, a routinely measurable clinical phenotype, corresponds to superior-inferior (SI) vs nasal-temporal (NT) mechanical anisotropy in human stromal lenticules obtained during small-incision lenticule extraction (SMILE). PATIENTS AND METHODS: Fifty-three spherical stromal lenticules with a programmed cylinder correction of 0 diopters were prospectively collected. Preoperative anterior corneal astigmatism was classified as with-the-rule (WTR) or against-the-rule (ATR), and the steep-axis information was decomposed into a vertical-horizontal component matched to the SI-NT mechanical testing coordinates. Equibiaxial testing was performed along the SI and NT axes, and tangent moduli were calculated at 0.03 MPa. The signed meridional difference index (MDI) was defined as ESI - ENT. RESULTS: WTR lenticules predominantly showed higher SI modulus, whereas ATR lenticules predominantly showed higher NT modulus. The vertical-horizontal astigmatic component was strongly correlated with signed MDI (Spearman rho = 0.875, P < 0.001), and this association remained significant after multivariable adjustment (β = 0.673 MPa/diopter, 95% CI 0.537-0.809, P < 0.001). Corneal astigmatism magnitude was also associated with direction-independent anisotropy magnitude. CONCLUSION: Preoperative corneal astigmatic direction may be a clinically measurable stratification variable for interpreting fixed-axis stromal mechanical anisotropy measurements in SMILE-derived lenticules. These findings support incorporating directional clinical phenotypes into corneal biomechanical research and patient-specific modeling, while requiring validation in larger independent samples.