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

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).

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 E_SI - E_NT.

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.

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.