Structural and biomechanical analysis of WST11/NIR scleral stiffening

To characterize structural and biomechanical changes following ex vivo treatment of rabbit sclera with the bacteriochlorophyll derivative WST11 and near-infrared (NIR) light. Enucleated left scleras of 10 rabbits were cross-linked by a 20-min topical application of WST11 solution, followed by 30-min NIR irradiation (10 mW/cm2). Paired right eyes served as untreated controls. Scleral tangent modulus and ultimate tensile strength were obtained from tensile strip testing. Collagen molecular-level nanostructure was measured using wide-angle X-ray scattering (WAXS). Scleral fibrillar-level microstructure was assessed using second harmonic generation (SHG) and two-photon fluorescence (TPF) imaging. WST11/NIR treatment increased scleral tangent modulus by an average of 79% for strains between 1.5% and 3% (p < 0.05), while ultimate tensile strength increased by 140% (control: 1.719 MPa, treated: 4.134 MPa, p < 0.01). WAXS indicated that collagen molecular spacing (control: 1.485 nm/treated: 1.480 nm, p = 0.65), spatial disorder (47.6 a.u./47.6 a.u., p > 0.99), and anisotropy (0.935 a.u./1.157 a.u., p = 0.09) were not significantly affected by treatment. No marked treatment effects on collagen fibril bundle organization were observable by SHG imaging. TPF imaging revealed additional crimped fibrous structures in the outer half of the tissue depth in treated specimens, indicative of enhanced collagen cross-linking. WST11/NIR treatment significantly increased the stiffness of the ex vivo rabbit sclera. The origin of the stiffening effect is consistent with the formation of collagen cross-links that are predominantly extrafibrillar in nature. Further research is needed to establish the full mechanism, efficacy, and safety of the method as a potential myopia treatment.