Long-term biomechanical properties of rabbit cornea after photodynamic collagen crosslinking

Photodynamic riboflavin/ultraviolet-A (UVA)-induced collagen cross-linking, which increases the biomechanical stiffness of the human cornea by about 300%, has been introduced recently as a possible treatment for progressive keratoconus. The present study was undertaken to evaluate the longterm biomechanical effects of this new cross-linking treatment as a necessary prerequisite to its clinical success.

The corneas of the left eyes of nine male rabbits were cross-linked. The contralateral eyes served as controls. After removal of the central 7 mm of the epithelium, the corneas were treated with the photosensitizer riboflavin and UVA irradiation for 30 mins with an irradiance of 3 mW/cm(2) using a 370-nm UVA double diode. Groups of three animals were killed immediately after treatment and at 3 and 8 months, respectively. Biomechanical stress-strain measurements were performed using a microcomputer-controlled biomaterial tester on 4 x 10-mm corneal strips.

Corneal thickness in the treated rabbit cornea was 408 +/- 20 microm. A constant and significant increase in ultimate stress (of 69.7-106.0%), Young's modulus of elasticity (of 78.4-87.4%) and a decrease in ultimate strain (of 0.57-78.4%) were found over a time period of up to 8 months after cross-linking treatment.

Riboflavin/UVA-induced collagen cross-linking leads to a longterm increase in biomechanical rigidity which remains stable over time. These data support our previous longterm clinical observations and give hope that this new treatment will halt progressive keratoconus definitively.