Corneal riboflavin gradients and UV-absorption characteristics after topical application of riboflavin in concentrations ranging from 0.1 to 0.5.

Avoiding damage of the endothelial cells, especially in thin corneas, remains a challenge in corneal collagen crosslinking (CXL). Knowledge of the riboflavin gradients and the UV absorption characteristics after topical application of riboflavin in concentrations ranging from 0.1% to 0.5% could optimize the treatment. In this study, we present a model to calculate the UV-intensity depending on the corneal thickness. Ten groups of de-epithelialized porcine corneas were divided into 2 subgroups. Five groups received an imbibition of 10 min and the other five groups for 30 min. The applied riboflavin concentrations were 0.1%, 0.2%, 0.3%, 0.4% and 0.5% diluted in a 15% dextran solution for each subgroup. After the imbibition process, two-photon fluorescence microscopy was used to determine fluorescence intensity, which was compared to samples after saturation, yielding the absolute riboflavin concentration gradient of the cornea. The extinction coefficient of riboflavin solutions was measured using a spectrophotometer. Combining the obtained riboflavin concentrations and the extinction coefficients, a depth-dependent UV-intensity profile was calculated for each group. With increasing corneal depth, the riboflavin concentration decreased for all imbibition solutions and application times. The diffusion coefficients of 10 min imbibition time were higher than for 30 min. A higher RF concentration and a longer imbibition time resulted in higher UV-absorption and a lower UV-intensity in the depth of the cornea. Calculated UV-transmission was 6 percentage points lower compared to the measured transmission. By increasing the riboflavin concentration of the imbibition solution, a substantially higher UV-absorption inside the cornea is achieved. This offers a simple treatment option to control the depth of crosslinking e.g. in thin corneas, resulting in a lower risk of endothelial damage.