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

This study investigated whether the direction of preoperative corneal astigmatism is associated with directional mechanical anisotropy in human corneal stromal tissue obtained during small-incision lenticule extraction (SMILE). Fifty-three spherical SMILE-derived stromal lenticules from patients receiving no programmed cylinder correction were classified according to with-the-rule (WTR) or against-the-rule (ATR) preoperative corneal astigmatism. The authors mechanically tested each lenticule along the superior-inferior and nasal-temporal directions and calculated tangent moduli at an engineering stress of 0.03 MPa. Across the entire cohort, the superior-inferior tangent modulus was higher on average than the nasal-temporal modulus, but stratification by astigmatism direction revealed opposite mechanical patterns. WTR lenticules predominantly showed greater superior-inferior stiffness, with mean tangent moduli of 2.82 ± 0.67 MPa in the SI direction and 2.32 ± 0.55 MPa in the NT direction. ATR lenticules predominantly showed the opposite pattern, with mean NT modulus of 2.62 ± 0.67 MPa compared with 2.34 ± 0.61 MPa in the SI direction. The direction of greater modulus agreed with WTR or ATR classification in 46 of 53 lenticules. A coordinate-matched vertical-horizontal astigmatic component was strongly correlated with the signed difference between SI and NT moduli, and corneal astigmatism magnitude was also positively associated with the magnitude of mechanical anisotropy. The authors conclude that preoperative corneal astigmatic direction may be an important stratification variable when interpreting corneal stromal mechanical anisotropy, although the cross-sectional ex vivo findings do not establish causality and require validation in larger independent cohorts.