UVA/riboflavin collagen crosslinking stiffening effects on anterior and posterior corneal flaps.

The UVA/riboflavin collagen crosslinking (CXL) is one of the treatment procedure for stopping the progression of keratoconus. The inclusion criterion for this procedure is a minimum corneal thickness of 400 μm, which is not often met in patients with advanced keratoconus. Preoperatively swelling thin corneas was shown to stabilize the keratectasia without any postoperative endothelial damage. Recently, we have shown that swelling porcine corneas prior to the CXL treatment had no significant effect on the resulting improvement in their tensile properties. In the present study, we extended this previous study and characterized the stiffening effects of CXL on anterior and posterior flaps as a function of their hydration. A DSAEK system was used to excise 10 mm corneal flaps from 80 porcine corneas. Individual flaps were crosslinked at different initial hydration levels by using riboflavin solutions composed of different dextran concentrations; the thickness was taken as a measure of flap hydration. A DMA machine was used to measure the tensile properties either immediately after the CXL treatment or after the thickness (hydration) of the crosslinked samples was brought down to a specific value. The average thickness of anterior groups was 670 μm, 540 μm, and 410 μm, and the average thickness of posterior groups was 845 μm, 650 μm, and 440 μm. It was found that although CXL significantly increased the tensile properties of all anterior groups, it had an insignificant effect on the stiffness of posterior flaps. Furthermore, except for the posterior flaps in 845 μm and 650 μm thickness groups, decreasing the hydration significantly increased the tensile modulus (p < 0.05). Finally, the anterior flaps that were crosslinked at higher hydration, i.e. swollen before CXL, showed significantly less amount of stiffening in comparison with those crosslinked at lower hydration when the tensile property measurement was done at similar hydration (p < 0.05).