Collagen cross-linking treatment effects on corneal dynamic biomechanical properties.

Cornea is a soft tissue with the principal function of transmitting and refracting light rays. The objective of the current study was to characterize possible effects of the riboflavin/UVA collagen cross-linking on corneal dynamic properties. The original corneal cross-linking protocol was used to induce cross-links in the anterior portion of the bovine cornea. A DMA machine was used to conduct mechanical tensile experiments at different levels of tensile strains. The samples were divided into a control group (n = 5) and a treated group (n = 5). All specimens were first stretched to a strain of 5% and allowed to relax for twenty minutes. After completion of the stress-relaxation experiment, a frequency sweep test with oscillations ranging from 0.01 to 10 Hz was performed. The same procedure was repeated to obtain the stress-relaxation and dynamic properties at 10% strain. It was observed that the collagen cross-linking therapy significantly increased the immediate and equilibrium tensile behavior of the bovine cornea (P < 0.05). Furthermore, for all samples in control and treated groups and throughout the whole range of frequencies, a significantly larger tensile storage modulus was measured at an axial strain of 10% compared to what was obtained at a tensile strain of 5%. Finally, it was noted that although this treatment procedure resulted in a significant increase in the storage and loss modulus at any axial strain and frequency (P < 0.05), it significantly reduced the ratio of the dissipated and stored energy during a single cycle of deformation. Therefore, it was concluded that while the riboflavin/UVA collagen cross-linking increased significantly corneal stiffness, it decreased significantly its damping capability and deformability. This reduced damping ability might adversely interfere with corneal mechanical performance.

Interrelation of Hydration, Collagen Cross-Linking Treatment, and Biomechanical Properties of the Cornea.

PURPOSE: The present study was designed to investigate the effects of hydration and collagen cross-linking treatment on biomechanical properties of the cornea. METHODS: The original corneal collagen cross-linking protocol was used to induce cross-links in bovine corneas. The thickness of samples was used as a measure of their hydration and five different thickness groups (n = 5 each) were considered. The cross-linked corneal strips were allowed to hydrate/dehydrate until their thickness reached 500, 700, 900, 1100, and 1500 µm. The tensile behavior of specimens in each thickness group was characterized by conducting uniaxial tensile experiments. The experiments were done in mineral oil in order to keep the thickness of samples constant and minimize hydration changes. RESULTS: It was observed that collagen cross-linking treatment significantly increased both the maximum tensile stress and the equilibrium (relaxed) stress of the bovine cornea (p < 0.01). Furthermore, with increasing the thickness (hydration) of the collagen cross-linked samples, their tensile stiffness significantly decreased (p < 0.01). An exponential relation and a logarithmic expression successfully represented experimentally measured stress-strain behavior and relaxation response of all groups (r(2 )> 0.99), respectively. CONCLUSION: Hydration and collagen cross-linking treatment concomitantly affect biomechanical properties of the cornea. Therefore, an accurate estimate of stiffening effects of collagen cross-linking treatment option using uniaxial tensile experiments is only possible if the hydration of specimens is fully controlled.