Stress-strain measurements of human and porcine corneas after riboflavin-ultraviolet-A-induced cross-linking

Purpose. To evaluate the biomechanical effect of combined riboflavin-ultraviolet A (UVA) treatment on porcine and human corneas. Setting: Department of Ophthalmology, Technical University of Dresden, Dresden, Germany. Methods: Corneal strips from 5 human enucleated eyes and 20 porcine cadaver corneas were treated with the photosensitizer riboflavin and irradiated with 2 double UVA diodes (370 nm, irradiance = 3 mW/cm(2)) for 30 minutes. After cross-linking, static stress-strain measurements of the treated and untreated corneas were performed using a microcomputer-controlled biomaterial tester with a prestress of 5 x 10(3) Pa. Results: There was a significant increase in corneal rigidity after cross-linking, indicated by a rise in stress in treated porcine corneas (by 71.9%) and human corneas (by 328.9%) and in Young's modulus by the factor 1.8 in porcine corneas and 4.5 in human corneas. The mean central corneal thickness was 850 mum +/- 70 (SD) in porcine corneas and 550 +/- 40 mum in human corneas. Conclusions: Riboflavin-UVA-induced collagen cross-linking led to an increase in mechanical rigidity in porcine corneas and an even greater increase in human corneas. As collagen cross-linking is maximal in the anterior 300 mum of the cornea, the greater stiffening effect in human corneas can be explained by the relatively larger portion of the cornea being cross-linked in the overall thinner human cornea.