Corneal Cross-Linking: Epi-On

When treating corneal ectasias, successful corneal cross-linking (CXL) requires three factors: riboflavin saturation of the corneal stroma, ultraviolet (UV) light, and oxygen. Riboflavin is too large to pass through epithelial tight junctions, so traditionally epithelial debridement is performed before riboflavin is applied making this approach an epithelium-off (epi-off) technique. However, this can result in pain as the epithelium regrows, corneal haze, and an increased infection risk postoperatively, which needs careful management with pharmacotherapy. Epithelium-on (epi-on) CXL should reduce the extent of these issues. Riboflavin can be passed through the epithelium into the stroma either by iontophoresis or with penetration enhancers, however this alone results in suboptimal cross-linking effects, as the epithelium not only absorbs around 20% of incoming UV energy, it also acts as a barrier to oxygen diffusion into the stroma. While it is simple to adjust the UV fluence delivered to the stroma to compensate for the energy lost in the epithelium, compensating for the lack of stromal oxygen is less simple. Several approaches (including oxygen goggles) have been taken to achieve this. However, adding iontophoresis and supplemental oxygen through goggles in the operating theater adds complexities that could be engineered out. Accordingly, the technique has advanced in the laboratory to a point where penetration enhancers, optimized UV irradiation profiles, and atmospheric oxygen can now provide epi-on CXL with the same corneal strengthening efficacy as epi-off CXL, suggesting simple, effective epi-on CXL could soon be in clinical use.