Biomechanical Stiffening Effect in Ultra-High-Fluence Extracorporeal Cross-Linked Corneal Allogenic Intrastromal Ring Segments

Purpose: To compare the biomechanical stiffening effects of a novel ultra-high fluence corneal cross-linking (CXL) protocol with conventional high-fluence CXL for extracorporeal corneal allogenic intrastromal ring segment optimization (ECO-CAIRS). Setting: ELZA Institute, Zurich, Switzerland. Design: Experimental, randomized, controlled ex vivo laboratory study using porcine corneas. No masking was applied. Methods: Eighty-four porcine corneas were divided into four groups: Group A (control), Group B (10 J/cm 2 ), Group C (30 J/cm 2 ), and Group D (60 J/cm 2 ). Following epithelial debridement, Groups B, C, and D were treated with 0.1% hypo-osmolar riboflavin. UV-A irradiation was delivered at fluences of 10 J/cm 2 (18 mW/cm 2 , 9:15 min) for Group B, 30 J/cm 2 (30 mW/cm 2 , 16:40 min) for Group C, and 60 J/cm 2 (30 mW/cm 2 , 33:18 min) for Group D. Corneal biomechanical properties were assessed via stress-strain extensiometry. Results: Mean elastic modulus (5% strain) values were: Group A (1.39 ± 0.49 N/mm), Group B (1.83 ± 0.48 N/mm), Group C (2.25 ± 0.83 N/mm), and Group D (2.31 ± 0.88 N/mm). Ultra-high fluence groups (C and D) showed significantly greater stiffening than Group B (P = 0.041 and P = 0.015), with no significant difference between Groups C and D (P = 1.00). Group B exhibited significantly greater stiffening than Group A (P = 0.023). Conclusions: Ultra-high fluence CXL enhances biomechanical stiffening compared to conventional high-fluence CXL, supporting its use in ECO-CAIRS. In this extracorporeal context, endothelial safety and haze are irrelevant, while increased stiffness facilitates intrastromal ring insertion and reduces corneal swelling.