Stromal mechanical anisotropy of the SMILE lenticule shows no detectable association with postoperative refractive...

Small-incision lenticule extraction (SMILE) removes an intrastromal lenticule whose stiffness differs between corneal meridians, producing measurable stromal mechanical anisotropy. Whether this meridional anisotropy biases postoperative refractive or astigmatic outcomes-and could therefore guide individualized treatment-remains unestablished. We tested whether lenticule mechanical anisotropy is associated with axis-specific astigmatic error, using an orthogonal negative control to guard against spurious correlation.

In this prospective consecutive cohort, 282 patients undergoing SMILE for the correction of myopia or myopic astigmatism were enrolled, and 279 right eyes with complete 3-month follow-up were analyzed. Excised lenticules underwent orientation-registered biaxial mechanical testing to derive the meridional anisotropy index A_mech = (E_SI - E_NT)/(E_SI + E_NT) and the mean tangent modulus Ē. The pre-specified primary outcome was the association between A_mech and the on-axis astigmatic power-vector error (ΔJ₀); the association between A_mech and the orthogonal oblique error (ΔJ₄₅)-onto which a true meridional effect should not project-served as a pre-specified negative control. The mean modulus was related to spherical-equivalent error (ΔM). Associations were assessed by Spearman correlation and refractive predictability by attempted-versus-achieved analysis.

Refractive outcomes were excellent (spherical-equivalent error + 0.29 ± 0.39 D; 99.3% within ± 1.00 D; attempted-versus-achieved R² = 0.918). The primary association between A_mech and ΔJ₀ was null (ρ = +0.054, P = 0.365), and the orthogonal negative control was likewise null (A_mech-ΔJ₄₅ ρ = +0.033, P = 0.587), indicating the absence-rather than masking-of an axis-specific signal. The mean modulus showed only a borderline relation to ΔM (ρ = +0.117, P = 0.050), and the residual astigmatic vector was centred near the origin (ΔJ₀ +0.03 ± 0.15 D, ΔJ₄₅ -0.02 ± 0.11 D).

Although SMILE lenticules exhibit reproducible meridional mechanical anisotropy, this property did not translate into detectable axis-specific or magnitude-related refractive error in this cohort. The concordant null finding in the orthogonal negative control supports the interpretation that no measurable axis-specific signal was present, rather than a meridian-independent analytical artifact. Lenticule-derived biomechanical metrics therefore appear unlikely, by themselves, to meaningfully refine refractive accuracy through individualized treatment planning in this setting. These conclusions are limited to the ex vivo lenticule-derived metrics evaluated here and should not be generalized to all corneal biomechanical measurements or to their established role in ectasia risk assessment.